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Student’s signature:......................................................…… Date…………………………… Revised July 2014

Investigation of Mixed Metal

Polyoxometalates as

Precursors to Niobate and

Tantalate Materials

Written and Submitted by

Akina Marissa Carey

In fulfillment for the degree of

Master of Science (by Research) in Chemistry

University of Warwick, Department of Chemistry

September 2014

Table of Contents

1.1 POLYOXOMETALATE CHEMISTRY ......................................................................................... 3

1.1.1 Historical Background ......................................................................................... 3

1.1.2 Structural Background......................................................................................... 4

1.1.3 Lindqvist Structure .............................................................................................. 6

1.2 PEROVSKITES ..................................................................................................................... 7


1.2.2 Structural Background......................................................................................... 7

1.3 MICROSCOPY OF CARBON NANOTUBES ............................................................................... 9


1.3.2 Structural Background....................................................................................... 10

3.1 MATERIALS ....................................................................................................................... 14

3.2 INSTRUMENTATION ............................................................................................................ 14

3.2.1 Infrared Spectroscopy ....................................................................................... 14

3.2.2 Single Crystal X-ray Diffraction ......................................................................... 14

3.2.3 Powder X-ray Diffraction ................................................................................... 14

3.2.4 High-Resolution Powder X-ray Diffraction ........................................................ 15

List of Figures ................................................................................................................ i

List of Tables ................................................................................................................ iii

Acknowledgements ....................................................................................................... iii

Declaration ................................................................................................................... iv

Abstract ........................................................................................................................ v

Abbreviations ................................................................................................................ 1

1 Introduction ................................................................................................................ 3

2 Motivation and Objective .......................................................................................... 12

3 Experimental Procedure .......................................................................................... 14

3.2.5 Thermogravimetric Analysis .............................................................................. 15

3.2.6 High-Resolution Transmission Electron Microscopy ........................................ 15

3.3 PEROXIDE SYNTHESIS ....................................................................................................... 15

K3Nb(O2)4 ................................................................................................................... 15

K3Ta(O2)4 .................................................................................................................... 16

3.4 POM SYNTHESIS .............................................................................................................. 16

K7Na[Nb6O19] • 10 H2O and K8[Nb6O19] • x H2O ......................................................... 16

K7Na[Nb4Ta2O19] • 13 H2O ......................................................................................... 16

K7Na[Nb3Ta3O19] • 12 H2O and K8[Nb3Ta3O19] • x H2O.............................................. 16

K7Na[Nb2Ta4O19] • 11 H2O ......................................................................................... 17

K7Na[Ta6O19] • 12 H2O .............................................................................................. 17

TMA salt of [Ta6O19]8-

• x H2O .................................................................................... 17

TBA salt of [Nb2W4O19]4-

• x H2O ................................................................................. 18

3.5 PEROVSKITE SYNTHESIS ................................................................................................... 18

KNbO3 and NaNbO3 using Chlorides ......................................................................... 18

KNbO3 and NaNbO3 using [Nb6O19]8-

......................................................................... 18

KNb0.5Ta0.5O3 and NaNb0.5Ta0.5O3 using Chlorides ................................................... 19

KNb0.5Ta0.5O3 and NaNb0.5Ta0.5O3 using [Nb3Ta3O19]8-

.............................................. 19


........................................ 19

KTaO3 and NaTaO3 using Chlorides .......................................................................... 19

KTaO3 and NaTaO3 using [Ta6O19]8-

.......................................................................... 20

3.6 ATTEMPTED FILLING OF CARBON NANOTUBES .................................................................... 20

K7Na[Nb4Ta2O19] • 13 H2O in DWNT ......................................................................... 20

TMA salt of [Ta6O19]8-

• x H2O in DWNT .................................................................... 20


x H2O in DWNT .................................................................. 21


x H2O in SWNT .................................................................. 21

4.1 K3NB(O2)4 AND K3TA(O2)4 ................................................................................................ 22

4.2 NIOBIUM- AND TANTALUM-CONTAINING POLYOXOMETALATE IONS ....................................... 24

4.3 PEROVSKITE MATERIALS ................................................................................................... 31

4 Results and Discussion ............................................................................................ 22

4.4 FILLING OF CARBON NANOTUBES....................................................................................... 36

APPENDIX A: TABLES OF CONDUCTED EXPERIMENTS ............................................................... 45

APPENDIX B: IR SPECTRA OF K3NB(O2)4 AND K3TA(O2)4; FULL AND CLOSEUP ........................... 49

APPENDIX C: TGA MEASUREMENT AND COMPARISON BETWEEN K3NB(O2)4 AND K3TA(O2)4 ....... 50

APPENDIX D: PXRD COMPARISON OF OBSERVED (A) K8[NB6O19] AND (B) K8[NB3TA3O19]........... 51

APPENDIX E: BOND LENGTHS IN [NB6O19]8-

, [TA6O19]8-

, AND K7NA[NB4TA2O19] • 13 H2O_

SINGLE CRYSTAL ..................................................................................................................... 52

APPENDIX F: TGA COMPARISON BETWEEN K7NA[NB6O19] AND K7NA[TA6O19] ........................... 53

APPENDIX G: TGA COMPARISON BETWEEN MIXED-METAL POMS .............................................. 54

APPENDIX H: OUTLINE OF ROUTES CONSIDERED FOR PEROVSKITE SYNTHESIS ........................... 55

APPENDIX I: COMPARISON OF SAMPLES MADE FROM CHLORIDES AND MADE FROM POMS ........... 56

APPENDIX J: 93

NB SOLID-STATE NMR OF PEROVSKITE SAMPLES ............................................... 60

5 Conclusion ............................................................................................................... 42

6 Future Work ............................................................................................................. 43

Appendix .................................................................................................................... 45

References ................................................................................................................. 61

i

List of Figures

Figure 1: Table mapping most common POM structures with their respective possible

compositions ................................................................................................................. 5

Figure 2: Structures of a) Lindqvist anion and b) Keggin anion found for

polyoxoniobates and polyoxotantalates ........................................................................ 6

Figure 3: Lindqvist anion [M6O19]8- ................................................................................ 7

Figure 4: Possible perovskite representations and arrangements ................................. 8

Figure 5: Orientations of carbon nanotubes ................................................................ 11

Figure 6: Outline of the Synthetic Chemistry Proposed for the Project ........................ 13

Figure 7: PXRD patterns of (a) K3Nb(O2)4_observed, (b) K3Nb(O2)4_reported ............ 22

Figure 8: Structure of [M(O2)4]3- ,(M= Nb or Ta) ........................................................... 23

Figure 9: PXRD pattern comparison for series of POMs (a) {K7Na[Nb6O19]} ............... 24

Figure 10: PXRD comparison of (a) K8[Nb6O19], (b) K7H[Nb6O19], (c) Na7H[Nb6O19], (d)

{K7Na[Nb6O19]}, (e) {K7Na[Nb4Ta2O19]}, (f) {K7Na[Nb3Ta3O19]}, (g) {K7Na[Nb2Ta4O19]},

(h) {K7Na[Ta6O19]}, (i) K8[Ta6O19], (j) K7Na[Ta6O19], (k) Na8[Ta6O19]............................. 25

Figure 11: PXRD pattern comparison for (a) K8[Nb6O19]_reported .............................. 26

Figure 12: Structure of K7Na[Nb4Ta2O19] • 13H2O determined by single crystal XRD .. 27

Figure 13: Comparison of PXRD of (a) {K7Na[Nb3Ta3O19]}, (b) simulated pattern from

single-crystal data of K7Na[Nb4Ta2O19] • 13H2O, and (c) {K7Na[Nb4Ta2O19]} ............... 30

Figure 14: HRPXRD of Potassium Perovskites ........................................................... 33

Figure 15: HRPXRD of Sodium Perovskites (a) NaNbO3_chloride, (b) NaNbO3_POM

(c) NaNb0.5Ta0.5O3_chloride, (d) NaNb0.5Ta0.5O3_POM, (e) NaNb0.33Ta0.67O3_POM (f)

NaTaO3_chloride, (g) NaTaO3_POM .......................................................................... 34

Figure 16: HRTEM of {K7Na[Nb4Ta2O19]} clusters surrounding a multi-walled carbon

nanotube .................................................................................................................... 37

Figure 17: PXRD comparison of (a) TMA salt of [Ta6O19]8- and (b) K8[Ta6O19] ............. 38

Figure 18: Sequence of images showing the TMA salt of [Ta6O19]8- degrading walls of

E. Flahaut’s DWNTs ................................................................................................... 39

ii

Figure 19: PXRD comparison of synthesized TBA-[Nb2W4O19]4- and TMA-[Ta6O19]

8- .. 40

Figure 20: HRTEM of clusters of TBA salt of [Nb2W4O19]4- inside double- and multi-

walled nanotubes ........................................................................................................ 41

Figure 21: HRTEM of clusters and possible single ions of TBA salt of [Nb2W4O19]4-

inside single and multi-walled nanotubes .................................................................... 41

iii

List of Tables

Table 1: Crystal data for K7Na[Nb4Ta2O19] • 13H2O .................................................... 28

Table 2: Occupancy of Nb and Ta of Addendum Metal Sites in K7Na[Nb4Ta2O19] •

13H2O ......................................................................................................................... 29

Table 3: Table of established niobate and tantalate perovskite space groups and lattice

parameters ................................................................................................................. 32

Table 4: Space groups and refined lattice parameters of K-perovskites ...................... 34

Table 5: Space groups and lattice parameters of Na-perovskites ............................... 35

Acknowledgements

I would like to thank my supervisor, Professor Richard Walton, and supporting

supervisor, Professor Jeremy Sloan, for their continued support and motivation

throughout my project. Both professors have been wonderful role models and

exceedingly patient when explaining new material.

I am also grateful for the support and help from current members of the research group

Luke Daniels, Craig Hiley, David Burnett, Dan Cook, Matthew Breeze, and Dr. Juliana

Fonseca de Lima.

In particular, I would like to thank Luke Daniels for HRPXRD and TGA, Dr. Guy

Clarkson for single crystal XRD, Andrew Rankin from University of St. Andrews for

solid-state NMR.

I am thankful to the United States and Luxembourg governments, and NATO for

financial support.

Finally, I would like to thank my family, friends, and particularly my boyfriend for the

emotional support and encouragement during my project.

iv

Declaration

I hereby declare that this thesis is solely my work unless otherwise stated. The project

was carried out in support of the degree of MSc (by research) in Chemistry, at the

University of Warwick, Department of Chemistry. The section “Introduction,

Polyxometalates: Historical Background, Polyoxometalates: Structural Background” is

based on text in the author’s Bachelor Thesis at Jacobs University, Bremen, Germany,

submitted May 24, 2013 titled “Titanium-Containing Tungstoarsenates”.

______________________________ ___________________________

Authors Signature Date

v

Abstract

[NbxTa6-xO19]8- with varying ratios of niobium and tantalum and countercations were

synthesized. The samples were characterized by powder X-ray diffraction and

thermogravimetric analysis. K7Na[Nb3Ta3O19] • 12H2O produced crystals which were

analysed using single-crystal X-ray diffraction. The structure gave a formula of

K7Na[Nb4Ta2O19]. When comparing the simulated powder pattern of K7Na[Nb4Ta2O19]

and comparing it to those of K7Na[Nb3Ta3O19] • 12H2O and K7Na[Nb4Ta2O19] • 13H2O,

none equated, inferring the batch samples were a mixture of various ratios of

K7Na[NbxTa6-xO19].

Perovskites synthesised by hydrothermally reacting the polyoxometalates, NbCl5, and

TaCl5 were investigated and analysed using high-resolution X-ray diffraction

(HRPXRD). It was observed that for the potassium perovskites, KNbO3 using the POM

was less crystalline, and KNb0.5Ta0.5O3 from the chlorides produce a phase impure

sample. All sodium perovskites were orthorhombic as expected. Preliminary solid-state

NMR showed both samples of NaNbO3 have comparatively similar Nb environments, a

decrease in intensities for decreasing niobium content for the potassium niobium-

tantalum samples, and a significant difference in location and peak width between both

samples of KNbO3.

When inserting K7Na[Nb4Ta2O19] into double-walled nanotubes, the anions were only

visible surrounding the nanotubes. The countercation was substituted to

tetrabutylammonium and [Ta6O19]8- was used, this resulted in the destruction of the

carbon walls. This could be due to the high charge of the anion as well as the electron-

beam-induced reaction between Ta and the carbon walls. TBA-[Nb2W4O19]4- was used

instead resulting in the insertion of [Nb2W4O19]4- clusters. To decrease the nanotube

diameter, single-walled nanotubes with a diameter of 1.2-1.7 nm were used, resulting

in smaller clusters with possible single-ion-like structures being observed.

1

Abbreviations

{K7Na[Nb6O19]}: K7Na[Nb6O19] • 10 H2O

{K7Na[Nb4Ta2O19]}: K7Na[Nb4Ta2O19] • 13 H2O



{K7Na[Ta6O19]}: K7Na[Ta6O19] • 12 H2O

POM: Polyoxometalate

CNT: Carbon nanotubes

SWNT: Single-walled nanotube

DWNT: Double-walled nanotube

TEM: Transmission Electron Microscopy

HRTEM: High-Resolution Transmission Electron Microscopy

EDAX: Energy-Dispersive X-ray Analysis

SEM: Scanning Electron Microscopy

XRD: X-ray Diffraction

PXRD: Powder X-ray Diffraction

HRPXRD: High-resolution powder x-ray diffraction

EDXRD: Energy-DispersiveX-ray Diffraction

IR: Infrared Spectroscopy

TGA: Thermogravimetric Analysis

Temp: Temperature

RT: Room temperature

2

A.U.: arbitrary unit

NI: NanoIntegris

SWeNT: SouthWest Nanotubes

MeOH: Methanol

EtOH: Ethanol

TMAOH: Tetramethylammonium hydroxide

TEAOH: Tetraethylammonium hydroxide

TPAOH: Tetrapropylammonium hydroxide

TBAOH: Tetrabutylammonium hydroxide

3

1 Introduction

1.1 Polyoxometalate Chemistry

1.1.1 Historical Background

Polyoxometalates (POMs) are a subset of metal oxide clusters, generally classified as

polyanions, and have a range of physical and structural properties. (1) The first POM

was synthesized by Swedish chemist, Jöns Jakob Berzelius, in 1826. (2) He discovered

that when adding ammonium molybdate to phosphoric acid, a yellow precipitate formed

leading to the now known ammonium 12-molybdophosphate, (NH4)3PMo12O40. (3) Its

structure was investigated in 1848 by Svanberg and Struve. The next significant step

came in 1862 when Marignac discovered tungstosilicic acids and their corresponding

salts (4) leading to Werner creating his coordination theory to explain the compositions

and structures of heteropolyanions. (5) His understanding and theory resulted in him

receiving a Nobel Prize in Chemistry in 1913, later becoming the foundation for modern

coordination chemistry. Werner’s achievements were later advanced by Miolati and

Pizzighelli in 1908 and even further by Rosenheim. From this, the Miolati-Rosenheim

theory was introduced stating that heteropolyacids were based on 6-coordinate

heteroatoms with a MO42- or M2O7

2- anions as ligands or bridging groups.

This was criticized in 1929 by Pauling when he recognized that the ionic radii of Mo6+

and W6+ were suitable for an octahedral coordination by corner oxygen. Pauling

suggested that each of the MO6 (M = Mo, W) encapsulated a central tetrahedron, XO4

(X = heteroatom), for a 12:1 complex. This gave a more accurate explanation for the

observed basicity than what would be observed using the Miolati-Rosenheim theory.

Pauling’s criticism was later corrected by Keggin when he revealed the structure of the

polyanion H3[PW12O40] • 5H2O by use of XRD. The crystal structure indicated that the

octahedral structure by WO6 was actually due to the linkage of both corner- and edge-

sharing between octahedra. (6) Reports of isomorphous complexes of the “Keggin ion”

4

followed this correction, resulting in the reports of the structures of Evans-Anderson,

Lindqvist, and Wells-Dawson anions. (5)

1.1.2 Structural Background

POMs are commonly synthesized using aqueous solutions, resulting in the

condensation of octahedral MO6 units which are linked via three types of sharing; edge,

corner, and, less frequently, face. (7) POMs are usually composed of early transition

metal addendum atoms in MO6 octahedra (M (addendum) = W6+,Mo6+, V5+, Nb5+,Ta5+)

with heteroatoms in the XO4 tetrahedra (X = P, Si, etc). Since an octahedral

coordination is formed by the metal atom, the maximum coordination number of these

atoms needs to be six to fulfil the structural requirements. In addition to the

coordination, the early transition metal atoms which can be used in such structures are

limited based on their ionic radius and charge, and the ability to accept pπ electrons

from oxygen to form stable dπ-pπ M-O bonds. This, therefore, reduces the number of

metal atoms which can be used. There are no such restrictions of the heteroatom. (5)

POMs are separated into two sub-categories: isopolyanions and heteropolyanions.

Isopolyanions consist only of the addenda atoms, M, in its highest oxidation state and

bridged via oxygen, while, heteropolyanions contain a heteroatom, X.

These polyanions are formulated as follows:

Isopolyanions: [MmOy]p- M = Mo, W, V, Nb, Ta

Heteropolyanions: [XxMnOy]q- X = P, Si (x ≤ m)

Figure 1 shows an outline of the commonly known POMs in all possible structural

orientations.

5

Figure 1: Table mapping most common POM structures with their respective possible compositions taken from Long, D.-L., Tsunashima, R. and Cronin, L.

Chem. Int. Ed. 49. 2010. 1736–1758. (1)

If niobium or tantalum are the addendum atom, the anion is referred to as being

polyoxoniobates and polyoxotantalates, respectively. These isopolyanions have not

been highly investigated compared to other POMs, and are less evolved due to the

ions being stable only in basic solutions. Polyoxoniobates have recently been

developed in the form of a heteropolyanion and isolated as a Keggin structure. (8) The

two structures of polyoxoniobates and tantalates, which can be observed, are shown in

Figure 2.

Since this thesis focuses on the synthesis, characterization, and reaction of niobium,

tantalum, and tungsten containing Lindqvist isopolyanions, the structure of the

Lindqvist anion will be discussed further below.

6

Figure 2: Structures of a) Lindqvist anion and b) Keggin anion found for

polyoxoniobates and polyoxotantalates

1.1.3 Lindqvist Structure

The Lindqvist anion, [M6O19]8-, is named after its discoverer I. Lindqvist. The structure

was first realized in 1952 when fusing Nb2O5 with excess metal hydroxides or

carbonates and then dissolving the melt in water. This resulted in crystals of

Na7HNb6O19 • 16H2O. (9) In parallel to this discovery, Lindqvist and Aronsson

characterized the structure of K8Ta6O19 • 16H2O. (10) Since these findings, very little

have been investigated regarding the Lindqvist anions in comparison to other POMs. It

is also possible to synthesize these structures via a solution route, and not only

hydrothermally. (11)

The Lindqvist anion is a super-octahedron containing 6 edge-sharing MO6 octahedra.

Each octahedral metal is bonded to the central oxygen, resulting in 6 terminal oxygens

(Figure 3), which can, and have been, used for introducing functional ligands allowing

for greater control over the POM.

-MO6 octahedron -Heteroatom

7

Figure 3: Lindqvist anion [M6O19]8-

Despite the lack of interest in this isopolyanion, they do have appealing physical

properties. For example, they have high overall charges and very basic oxygen

surfaces which could be used for further use as building blocks for extended solid

structures. (1)

1.2 Perovskites


The perovskite mineral (CaTiO3) is found naturally around the world and was first

discovered by Gustav Rose in 1839 in the Ural Mountains. He then named it after the

Russian mineralogist, Count Lev Aleksevich von Perovski. (12) Unfortunately, research

on perovskites did not make a significant increase until the mid-1940s in which the first

crystal structure a CaTiO3 type orthorhombic perovskite was discovered and reported

by Helen Dick in 1945. Interest toward solid-state research, focusing on ferroelectric

materials, is attributed to this increase. Since this increase, applications of perovskite

materials have ranged from use in sensors and memory devices to superconductors

and solid oxide fuel cells. (13)


The perovskite structure has an ideal formula of ABX3 with a space group of Pm ̅m.

The A-site cations are larger in size than the B-site cations, however, the latter are of

M

O

8

similar size to the X-site anions. In this ideal structure, the A-cations are bound by

twelve anions in the cubo-octahedral coordination while the B-cations by six anions in

the octahedral coordination, (Figure 4a). The structure is best represented by SrTiO3 as

it exhibits the closest resemblance to the ideal. (14)

The standard cubic perovskite structure has a Pm ̅m space group, and with 2 common

representations of the unit cell. In the cubic A-cell structure, the B-cations are located

at each of the corners, A-cation in the centre of the cube, and X-anions at the edge-

centred position, (Figure 4b1).This differs from the cubic B-cell structure where the A-

cations are at each corner, B-cation in the centre of the cube, and X-anions at the face-

centred positions, (Figure 4b2). (13)

Figure 4: Possible perovskite representations and arrangements a) Ideal structure b1) Cubic A-cell structure b2) Cubic B-cell

c) Orthorhombic structure

B

X

A

9

In order to achieve this aforementioned arrangement, the relative size of the A-cation,

B-cation, and X-anion must be of a specific ratio in relation to each other. If this ratio is

not achieved, other orientations can occur. Most notably, the CaTiO3 mineral exhibits

an orthorhombic structure, with a space group of Pnma or Pbnm, (Figure 4c). In this

structure the octahedra at the corners of the cubic structure become tilted, but are still

corner-sharing. This occurs when the A-cation is too small within the polyhedral

framework causing the centre-vertical edge X-anions to compress inwards. (14)

Other structures have been reported such as tetragonal, rhombohedral, and various

distortions of these. As this study focused on MNbO3, MTaO3, and MNbxTa1-xO3 (M=

Na or K), only cubic and orthorhombic structures have been shown since these are the

crystal symmetries expected for these materials.

1.3 Microscopy of Carbon Nanotubes


Microscopes have been in existence since the late 1500s however with the introduction

of the transmission electron microscopy in 1931 by Ernst Ruska and Max Knoll began

a surge of advancements. The scanning electron microscope (SEM) was introduced in

1942 and since, the improvements of these microscopes have improved dramatically.

(15)

Microscopy was not used to view nanotubes until 1991 when Sumio Iijima made and

imaged the first carbon nanotubes using transmission electron microscopy. This was

done when synthesising fullerenes by an arc-discharge evaporation method. (16) Carbon

nanotubes (CNTs) have been of increase interest for their applications toward energy

storage, sensors, and encapsulation. (17)

Filling of nanotubes have varied significantly since the 1990s when the research was

dominated by inserting fullerenes into single- and multi-walled CNTs resulting in

“peapod” structures. (18) More recently, CNTs have been filled with various metals,

10

alloys, ions and clusters. For example, polymeric iodine chains in one-dimensional

crystal structures, polyhedral chains of lanthanide trihalides, as well as HgTe has been

observed in single- and/or double-walled carbon nanotubes (SWNTs or DWNTs). (19) (20)

In addition to this, POM ions have been observed in DWNTs since 2008 when Sloan

and co-workers reported encapsulating polyoxotungstates using high-resolution

transmission electron microscopy. (19) (21) (22)


CNTs are tubes several microns in length consisting of single, double, or multiple walls

made of carbon atoms. They are synthesized by arc discharge and laser ablation, and

chemical vapour deposition (thermal and plasma-enhanced). These methods however

can leave residue, catalyst particles, and contamination behind so a cleansing

preparation should be considered before use. (17) Due to the lack of complete control

over the formation of the nanotubes, each sample will have a small percentage of

nanotubes with various number of walls. For example, if a single-walled nanotube

sample was observed, there would be a small presence of double- and multi-walled

nanotubes within the sample.

CNTs are generally seen as hollow cylinders which are formed by rolling graphene

sheets. With this comes curvature resulting in a rehybridization of the σ bonds which

have now been pushed slightly outwards. This causes the π-orbitals to become more

delocalised, therefore making the tubes mechanically stronger, more conductive

thermally and electrically, and more chemically active. Due to the hybridisation of

carbon atoms, the preferred orientation would be in a hexagonal structure, but at the

ends of the tubes this cannot occur resulting in a single pentagon. This does cause

defects in the overall structure as the more incorporation of pentagons and heptagons,

the more likely the tube will be bent, helical, or capped. If a tube has more than one

layer of carbons stacked over each other, then these are named double or multi-walled

tubes. (17)

11

CNTs can also have different orientations, such as zig-zag, armchair, and chiral. This is

done by rolling the graphite sheet in different directions, (Figure 5).

Figure 5: Orientations of carbon nanotubes taken from Galano, A. Nanoscale 2(3).2010. 373-80. (23)

In addition to the various orientations of CNTs, each type can vary in length and

diameter. Typically, single-walled nanotubes have a diameter of <2 nm, while double

and multi-walled nanotubes have an average diameter of 1-4 nm and >3 nm,

respectively. The range of diameters is due to the different methods of synthesizing

CNTs and control over the exact growth have not yet been identified with great

accuracy.

12

2 Motivation and Objective

The interest in in perovskite materials have dramatically increased in the last 50 years

with applications towards sensors and fuel cells. (13) In 2008, the Walton research group

published findings of a Lindqvist-structured polyoxoniobate intermediate in the

hydrothermal synthesis of sodium niobates using Nb2O5. By using in situ energy-

dispersive X-ray diffraction (EDXRD), the group could observe its transient appearance

during the reaction. (24)

This observation was used in the work described here to see if it was possible to

control the perovskite by starting from the POM in synthesis. The aim of this study was

to synthesize a mixed niobium-tantalum containing POM in the Lindqvist structure, and

further convert it to mixed niobium-tantalum perovskites. Niobate and tantalate

perovskites have ferroelectric properties and there is a need to make new mixed oxides

with controlled composition. Usually high temperatures are used in the synthesis and

solution hydrothermal methods could provide a way of controlling the composition and

crystal form of the product.

Simultaneous to this work, the group led by Jeremy Sloan published a paper on

encapsulating Lindqvist ions containing tungsten inside of carbon nanotubes. A click-

move-click movement was observed within the nanotubes. (19) (21) (22) The work in this

thesis continues the finding by attempting to insert mixed-metal Lindqvist ions and

observing the position of each metal atom within the ion as well as the ion inside the

nanotube. The outline of the synthetic chemistry in the project is shown in Figure 6.

13

Chlorides Peroxides POMs Perovskites POMs in NTs

Figure 6: Outline of the Synthetic Chemistry Proposed for the Project

[NbxTa6-xO19]8-

In: H2O H2O/EtOH

EtOH DWNT

[NbxW6-xO19](2+x)-

In: EtOH DWNT SWNT

NbCl5

[Nb6O19]8-

KNbO3

K3Nb(O2)4

[NbxTa6-xO19]8-

KNbxTa1-xO3

K3NbxTa1-x(O2)4

TaCl5

[Ta6O19]8-

KTaO3

K3Ta(O2)4

14

3 Experimental Procedure

See Appendix A for full description of reactions done

3.1 Materials

The following chemicals were obtained from Sigma Aldrich; Na2WO4 • 2H2O (99%),

HNaSO3 (40%), TBAOH • 30H2O (98.0%), carbon nanotube, double walled (≤10%

Metal Oxide), carbon nanotube, single-walled, purified (>95% carbon, <1% catalyst,

supplied by NanoIntegris). NbCl5 (99%) was supplied by Alfa Aesar, TaCl5 (99.90%) by

AcrosOrganics, carbon nanotube, double-walled by Emmanuel Flahaut, carbon

nanotube, single-walled by SWeNT, and lacey and holey Cu support grids (carbon-

coated, 3.05 mm) by Agar Scientific.

3.2 Instrumentation

3.2.1 Infrared Spectroscopy

Infrared spectroscopy of powder samples was done on the powder samples using a

PerkinElmer Spectrum100 FT-IR instrument.

3.2.2 Single Crystal X-ray Diffraction

Single-crystal X-ray diffraction was used on a Gemini R diffractometer from Oxford

Diffraction equipped with an Oxford Cryosystems Cobra. The structures were solved by

direct methods using SHELXS and refined using SHELXL 97. This was performed by

Dr. Guy Clarkson.

3.2.3 Powder X-ray Diffraction

A Siemens D5000 diffractometer equipped with Cu Kα radiation was used for the

preliminary characterisation. The data were collected over a range of 2θ, 8-60°, using a

step size of 0.02° and 1.1 second/step measurement.

15

3.2.4 High-Resolution Powder X-ray Diffraction

High-resolution data were collected using a Panalytical X’Pert Pro MPD that was

equipped with a monochromatic Cu Kα1 radiation using a PIXcel solid state detector.

The data were collected over a range of 2θ, 20-100°, and spun at a rate of 4

revolutions/second.

3.2.5 Thermogravimetric Analysis

Thermal analysis was done using a Mettler Toledo Systems TGA/DSC 1 instrument.

Alumina crucibles with a constant flow of N2 at 50 mL/minute were used as well as the

temperature ranging from room temperature to 1000°C at a heating rate of 10°C

/minute.

3.2.6 High-Resolution Transmission Electron Microscopy

A JEM-ARM 200F microscope (at 80 kV) equipped with a CEOS aberration correction

and a Gatan SC1000 ORIUS camera with a 4008 2672 pixel charge-coupled device

(CCD) was used. Dispersions of POM and SWNT/DWNT nanocomposites were drop-

casted onto 3.05 mm Cu lacey or holey carbon-coated support grids.

3.3 Peroxide Synthesis

The following syntheses are based a publication by Nyman and co-workers. (11)

K3Nb(O2)4

NbCl5 (6.60 g, 24.4 mmol) was added to 75 mL 30% H2O2 in an ice bath with moderate

stirring. KOH (65 mL, 4 M) added to solution in 1mL aliquots. 150 mL of MeOH was

added and allowed to cool to 5-8°C. A further 100 mL of MeOH was added and allowed

to stir for 5 minutes. The product was filtered, washed with 200 mL MeOH, collected

and air dried at room temperature.

16

K3Ta(O2)4

TaCl5 (8.75 g, 24.4 mmol) was added to 75 mL 30% H2O2 in an ice bath with moderate

stirring. KOH (65 mL, 4 M) added to solution in 1 mL aliquots. 150 mL of MeOH was

added and allowed to cool to 5-8°C. A further 100 mL of MeOH was added and allowed

to stir for 5 minutes. The product was filtered, washed with 200 mL MeOH, collected

and air dried at room temperature.

3.4 POM Synthesis

The following syntheses are based on a publication by Nyman and co-workers (11)

K7Na[Nb6O19] • 10 H2O and K8[Nb6O19] • x H2O

K3Nb(O2)4 (3.18 g, 9.40 mmol), KOH (3.82 g, 68.1 mmol), and Na3VO4 (0.110 g, 0.598

mmol) was added to 15 mL H2O. The suspension was refluxed for 2 hours at 115°C.

The solution was then filtered, allowed to evaporate slowly in air at room temperature,

periodically collecting the product as it crystallized. To synthesize K8[Nb6O19], Na3VO4

was substituted with K3VO4 (0.137 g, 0.590 mmol).

{K7Na[Nb6O19]} will be used to reference this product in future sections.

K7Na[Nb4Ta2O19] • 13 H2O

K3Nb(O2)4 (2.12 g, 6.27 mmol), K3Ta(O2)4 (1.34 g, 3.14 mmol), KOH (3.82 g, 68.1

mmol), and Na3VO4 (0.110 g, 0.598 mmol) was added to 15 mL H2O. The suspension

was refluxed for 2 hours at 115°C. The solution was then filtered, allowed to evaporate

slowly in air at room temperature, periodically collecting the product as it crystallized.

{K7Na[Nb4Ta2O19]} will be used to reference this product in future sections.

K7Na[Nb3Ta3O19] • 12 H2O and K8[Nb3Ta3O19] • x H2O

K3Nb(O2)4 (1.59 g, 4.70 mmol), K3Ta(O2)4 (2.00 g, 4.69 mmol), KOH (3.82 g,

68.1 mmol), and Na3VO4 (0.110 g, 0.598 mmol) was added to 15 mL H2O. The

suspension was refluxed for 2 hours at 115°C. The solution was then filtered, allowed

17

to evaporate slowly in air at room temperature, periodically collecting the product as it

crystallized. To synthesize K8[Nb3Ta3O19], Na3VO4 was substituted with K3VO4 (0.137

g, 0.590 mmol).


K7Na[Nb2Ta4O19] • 11 H2O

K3Nb(O2)4 (1.06 g, 3.13 mmol), K3Ta(O2)4 (2.67 g, 6.26 mmol), KOH (3.82 g, 68.1

mmol), and Na3VO4 (0.110 g, 0598 mmol) was added to 15 mL H2O. The suspension

was refluxed for 2 hours at 115°C. The solution was then filtered, allowed to evaporate

slowly in air at room temperature, periodically collecting the product as it crystallized.


K7Na[Ta6O19] • 12 H2O

K3Ta(O2)4 (4.00 g, 9.38 mmol), KOH (3.82 g, 68.1 mmol), and Na3VO4 (0.110 g, 0.598

mmol) was added to 15 mL H2O. The suspension was refluxed for 2 hours at 115°C.

The solution was then filtered, allowed to evaporate slowly in air at room temperature,

periodically collecting the product as it crystallized.

{K7Na[Ta6O19]} will be used to reference this product in future sections.

TMA salt of [Ta6O19]8- • x H2O

K3Ta(O2)4 (2.00 g, 4.69 mmol), TMAOH (6.16 g, 67.6 mmol), and Na3VO4 (0.0550 g,

0.299 mmol) was added to 15 mL H2O. The suspension was refluxed for 2 hours at

115°C. The solution was then filtered, allowed to evaporate slowly in air at room

temperature, periodically collecting the product as it crystallized.

18

TBA salt of [Nb2W4O19]4-• x H2O

The following syntheses are based on a publication by Dabbabi and Boyer. (25)

[Nb6O19]8- (10 mL, 0.04 M) solution was pre-heated and added to Na2WO4 • 2H2O (10

mL, 0.5 M) and H2O2 (0.11 mL, 0.5 M). The mixture was acidified to pH 5.5 using 0.5

mL conc. Acetic Acid, and refluxed for 2 hours at 80°C. After 2 hours, 1 mL NaHSO3

was added and solution allowed to cool. Once cooled, TBAOH (1.50 g, 1.87 mmol) was

added, and resulting solution filtered. 100 mL EtOH was added and product filtered,

collected and dried in air at room temperature.

3.5 Perovskite Synthesis

KNbO3 and NaNbO3 using Chlorides

NbCl5 (500 mg, 1.85 mmol) and 12 mL 20 M KOH were added and stirred together in

an autoclave with inner volume of ~20 mL. After 5 minutes of stirring, the autoclave

was placed in an oven at 240°C for 24 hours. After 24 hours, the autoclave was

removed from the oven and left to cool to ambient temperature. Once cooled, the

product was filtered, washed generously with H2O, collected and air dried at room

temperature. For NaNbO3, KOH was replaced with 12 mL 20 M NaOH and left in oven

for 48 hours.

KNbO3 and NaNbO3 using [Nb6O19]8-

500 mg [Nb6O19]8- and 12 mL 20 M KOH were added and stirred together in an

autoclave with inner volume of ~20 mL. After 5 minutes of stirring, the autoclave was

placed in an oven at 240°C for 5 days. After 5 days, the autoclave was removed from

the oven and left to cool to ambient temperature. Once cooled, the product was filtered,

washed generously with H2O, collected and air dried at room temperature. For

NaNbO3, KOH was replaced with 12 mL 20 M NaOH.

19

KNb0.5Ta0.5O3 and NaNb0.5Ta0.5O3 using Chlorides

NbCl5 (100 mg, 0.370 mmol), TaCl5 (132 mg, 0.368 mmol), and 12 mL 20 M KOH were

added and stirred together in an autoclave with inner volume of ~20 mL. After 5

minutes of stirring, the autoclave was placed in an oven at 240°C for 24 hours. After 24

hours, the autoclave was removed from the oven and left to cool to ambient

temperature. Once cooled, the product was filtered, washed generously with H2O,

collected and air dried at room temperature. For NaNb0.5Ta0.5O3, KOH was replaced

with 12 mL 20M NaOH and left in the oven for 48 hours.


500 mg [Nb3Ta3O19]8- and 12 mL 20 M KOH were added and stirred together in an


placed in an oven at 240°C for 5 days. After 5 days the autoclave was removed from



NaNb0.5Ta0.5O3, KOH was replaced with 12 mL 20 M NaOH.


500 mg [Nb2Ta4O19]8- and 12 mL 20 M KOH were added and stirred together in an





NaNb0.33Ta0.67O3, KOH was replaced with 12 mL 20 M NaOH.

KTaO3 and NaTaO3 using Chlorides

TaCl5 (500 mg, 1.85 mmol) and 12 mL 20 M KOH were added and stirred together in

an autoclave with inner volume of ~20 mL. After 5 minutes of stirring, the autoclave

was placed in an oven at 240°C for 24 hours. After 24 hours, the autoclave was

20

removed from the oven and left to cool to ambient temperature. Once cooled, the

product was filtered, washed generously with H2O, collected and air dried at room

temperature. For NaTaO3, KOH was replaced with 12 mL 20 M NaOH and left in oven

for 48 hours.

KTaO3 and NaTaO3 using [Ta6O19]8-

500 mg [Ta6O19]8- and 12 mL 20 M KOH were added and stirred together in an





NaTaO3, KOH was replaced with 12 mL 20 M NaOH.

3.6 Attempted filling of Carbon Nanotubes

K7Na[Nb4Ta2O19] • 13 H2O in DWNT

70 mg {K7Na[Nb4Ta2O19]} and 1 mg DWNT (E. Flahaut) were dispersed in 5 mL EtOH

each using a sonic probe for 5 minutes (2 seconds pulse on, 2 seconds pulse off;

amplitude 20%). Each sample was dispersed separately, added together after

dispersion, and allowed to stir. The sample was then drop-casted on a lacey or holely

carbon grid.

TMA salt of [Ta6O19]8- • x H2O in DWNT

70 mg TMA salt of [Ta6O19]8- and 1 mg DWNT (Sigma Aldrich) were dispersed in 5 mL

EtOH each using a sonic probe for 5 minutes (2 seconds pulse on, 2 seconds pulse off;



carbon grid.

21

TBA salt of [Nb2W4O19]4- x H2O in DWNT

70 mg TBA salt of [Nb2W4O19]4- and 1 mg DWNT (Sigma Aldrich) were dispersed in 5

mL EtOH each using a sonic probe for 5 minutes (2 seconds pulse on, 2 seconds pulse

off; amplitude 20%). Each sample was dispersed separately, added together after


carbon grid.

TBA salt of [Nb2W4O19]4- x H2O in SWNT

5 mg SWNT (Sigma Aldrich/NI) was dispersed in chloroform using a sonic probe for 10

minutes (2 seconds pulse on, 2 seconds pulse off; amplitude 20%). The nanotubes

were then lightly filtered, collected and dried. Once dried, they were put in a tube

furnace at 400°C for 24 hours.

70 mg TBA salt of [Nb2W4O19]4- and 1 mg SWNT (heat treated) were dispersed in 5 mL

EtOH each using a sonic probe for 5 minutes (2 seconds pulse on, 2 seconds pulse off;



carbon grid.

22

4 Results and Discussion

4.1 K3Nb(O2)4 and K3Ta(O2)4

K3Nb(O2)4 and K3Ta(O2)4 were each synthesized by adding 6.61 mmol NbCl5 and

TaCl5, respectively, to 65 mL 4 M KOH solution, followed by the addition of MeOH.

Once the mixture cooled and a further addition of MeOH, the white product was

washed with MeOH, collected and air dried at room temperature.

Both K3Nb(O2)4 and K3Ta(O2)4 were then analysed using PXRD and compared to

patterns simulated from crystal structures taken from the ICSD database. This

comparison showed that the intended product was formed, (Figure 7). Peaks at ~38.3

and ~44.6 correspond to aluminium peaks from the sample holder and should be

disregarded.

Figure 7: PXRD patterns of (a) K3Nb(O2)4_observed, (b) K3Nb(O2)4_reported, (c) K3Ta(O2)4_observed, (d) K3Ta(O2)4_reported; Al peaks labelled *

2θ / deg

Inte

nsit

y (

a.u

.)

a

b

c

d

* *

* *

23

K3Nb(O2)4 and K3Ta(O2)4 structurally contain the metal (Nb or Ta) surrounded by four

bidentate peroxo groups bound to the centre metal therefore forming an overall

distorted dodecahedral arrangement. The structural orientation is represented in Figure

8.

Figure 8: Structure of [M(O2)4]3- ,(M= Nb or Ta) taken from Bayot, D., and M. Devillers. Coordination Chemistry Reviews. 250. 19-20. 2006. 2610-626. (26)

In addition to PXRD, IR spectroscopy and thermal analysis were done to observe the

stretching of the peroxo groups as well as the mass loss of decomposition respectively.

The characteristic vibrational bands for K3Nb(O2)4 and K3Ta(O2)4 are the O – O

stretching of the peroxo groups at ~812 cm-1 and ~806 cm-1 respectively.

Decomposition begins at approximately 419°C for both K3Nb(O2)4 and K3Ta(O2)4 and

exhibits a percentage weight loss of 19.9% and 15.8% respectively. These values are

comparable to the expected weight loss values of 19.0% and 15.0% respectively,

corresponding to the loss of 2 peroxy groups resulting in the following decomposition

representation: (27)

K3Nb(O2)4 (s) K3NbO4 (s) + 2 O2 (g)

K3Ta(O2)4 (s) K3TaO4 (s) + 2 O2 (g)

Refer to Appendix B and C for the respective IR and TGA spectra of both peroxide

compounds.

O

Nb or Ta

24

4.2 Niobium- and Tantalum-Containing Polyoxometalate Ions

All Nb- and Ta-containing POMs were synthesized by refluxing K3Nb(O2)4, K3Ta(O2)4,

or varying ratios of K3Nb(O2)4 and K3Ta(O2)4 with 68 mmol KOH and 0.3 mmol of either

Na3VO4 or K3VO4 in H2O for 2 hours. The solution was filtered and allowed to

evaporate slowly at room temperature until crystallization process is complete. The

resulting colourless crystals were collected and air dried at room temperature.

Figure 9: PXRD pattern comparison for series of POMs (a) {K7Na[Nb6O19]}, (b) {K7Na[Nb4Ta2O19]}, (c) {K7Na[Nb3Ta3O19]}, (d) {K7Na[Nb2Ta4O19]}, (e)

{K7Na[Ta6O19]}

Figure 9 is an overlay of the PXRD patterns for the K7Na salts of Nb and Ta POMs with

varying ratios. None of the patterns match, which would imply that each sample made

contained a different product, whether it contained the correct and expected ratio of Nb

and Ta or not. It is likely therefore that the size of the countercation (whether Na+ or K+)

and the amount of crystal water affects the crystal packing and therefore the size and

a

b

c

d

e

Inte

nsit

y (

a.u

.)

2θ / deg

25

shape of the unit cell. As these POMs have not been previously reported, therefore, in

order to confirm whether a POM had actually been synthesized various comparisons to

reported powder patterns were done, (Figure 10).

Figure 10: PXRD comparison of (a) K8[Nb6O19], (b) K7H[Nb6O19], (c) Na7H[Nb6O19], (d) {K7Na[Nb6O19]}, (e) {K7Na[Nb4Ta2O19]}, (f) {K7Na[Nb3Ta3O19]}, (g) {K7Na[Nb2Ta4O19]},

(h) {K7Na[Ta6O19]}, (i) K8[Ta6O19], (j) K7Na[Ta6O19], (k) Na8[Ta6O19]; Observed patterns are highlighted in yellow while reported values are not highlighted

When comparing the observed and reported patterns, similarities can be seen. For

example, the pair of peaks at approximately 9° and 9.5° are present in reported

patterns (a) K8[Nb6O19], and (i) K8[Ta6O19] as well as observed patterns (d)

{K7Na[Nb6O19]}, (f) {K7Na[Nb3Ta3O19]}, (g) {K7Na[Nb2Ta4O19]}, and (h)

{K7Na[Ta6O19]}. Peaks at approximately 10° and 11° for observed pattern (e)

{K7Na[Nb4Ta2O19]} are similar to those present in pattern (j) K7Na[Ta6O19]. Patterns

a

b

c

d

e

Inte

nsit

y (

a.u

)

2θ / deg

f

g

h

i

j

k

26

from K8[Nb6O19], {K7Na[Nb3Ta3O19]}, and K8[Ta6O19] have been overlaid to emphasize

this comparison, (Figure 11).

Figure 11: PXRD pattern comparison for (a) K8[Nb6O19]_reported, (b) {K7Na[Nb3Ta3O19]}_observed, (c) K8[Ta6O19]_reported

Comparable peaks are present in all synthesized samples to make a convincing

assumption that the POMs synthesized are indeed POMs. When comparing the PXRD

patterns of these POMs, an increase in the number of peaks between 20 and 60° 2θ is

observed with increasing tantalum content.

A trend is also observed after crystallisation. For structures that are niobium-rich the

crystallites are large, however with increasing tantalum content the crystallite size

decreases dramatically.

a

b

c

Inte

nsit

y (

a.u

.)

2θ / deg

27

PXRD patterns of these POMs but containing only potassium countercations can be

found in Appendix D.

Selective single crystal XRD, (Table 1), was done on one crystal taken from a batch of

{K7Na[Nb3Ta3O19]} which gave the actual formula is K7Na[Nb4Ta2O19] rather than the

expected 1:1 Nb/Ta ratio. Each of the metal sites in the POM is occupied by both Nb

and Ta and Table 2 shows the refined site occupancies. The data also showed the

number of crystal waters resulting in a formula of K7Na[Nb4Ta2O19] • 13H2O. The 13

crystal waters present is consistent with the 13 crystal waters observed using TGA for

{K7Na[Nb4Ta2O19]} Figure 12 shows the structure of the ion.

Figure 12: Structure of K7Na[Nb4Ta2O19] • 13H2O determined by single crystal XRD

Nb/Ta

O

28

Table 1: Crystal data for K7Na[Nb4Ta2O19] • 13H2O

Formula H80Nb15.106Ta8.894O132

Crystal System Monoclinic

Space Group P21/c (14)

Cell Length (Å) a 12.6562(3) b 10.7052(2) c 24.3067 (5)

Cell Angles (°) α 90.000 β 92.100 (2) γ 90.000

Cell Volume (Å3) 3291.03 (10)

Cell Ratios a/b= 1.1822 b/c= 0.4404 c/a= 1.9205

The bond distances for the structure are consistent with occupancy data indicating the

two sites which have slightly higher amounts of tantalum, therefore decreasing the M-O

terminal bond length. The terminal M-O bonds in the measured structure were

compared with terminal M-O in both the [Nb6O19]8- and [Ta6O19]

8- structures to

determine the likelihood of the location of the metal atoms. See Appendix E for bond

length comparison data.

29

Table 2: Occupancy of Nb and Ta of Addendum Metal Sites in K7Na[Nb4Ta2O19] • 13H2O

Atom Site occupancy factor Atom Site occupancy factor

Nb1 0.659 Nb4 0.661

Ta1 0.341 Ta4 0.339

Nb2 0.653 Nb5 0.621

Ta2 0.347 Ta5 0.379

Nb3 0.661 Nb6 0.521

Ta3 0.339 Ta6 0.479

It should be noted that the single-crystal data are from only one crystal; this cannot be

representative for the whole sample batch. To compare the purity {K7Na[Nb3Ta3O19]}

and {K7Na[Nb4Ta2O19]}, their corresponding PXRD patterns were plotted against the

simulated pattern of the single-crystal. As shown in Figure 13, the patterns do not

match, therefore suggesting that not only is {K7Na[Nb3Ta3O19]} a mixture of various

ratios of POMs with an overall composition of {K7Na[Nb3Ta3O19]}, but it is also true for

{K7Na[Nb4Ta2O19]}. Furthermore, the different crystal packing in the PXRD-measured

samples would adjust the pattern compared to the simulated pattern from the single

crystal data. It should be noted that this is only a hypothesis and further research is

being conducted to justify this theory.

30

Figure 13: Comparison of PXRD of (a) {K7Na[Nb3Ta3O19]}, (b) simulated pattern from single-crystal data of K7Na[Nb4Ta2O19] • 13H2O, and (c) {K7Na[Nb4Ta2O19]}

TGA was performed on all Nb and/or Ta containing POMs and can be seen in

Appendix F and G. It was observed that for {K7Na[Nb6O19]} and {K7Na[Nb4Ta2O19]}

there are two stages of water loss, suggesting they are two types of water molecules

within the crystal structure. The following are the resulting formula including waters of

crystallisation as well as their corresponding percentage mass loss: K7Na[Nb6O19] •

10H2O (15%), K7Na[Nab4Ta2O19] • 13H2O (17%), K7Na[Nb3Ta3O19] • 12H2O (16%),

K7Na[Nb2Ta4O19] • 11H2O (13%), K7Na[Ta6O19] • 12H2O (13%).

It should be noted that an assumption that the countercations for each sample is ‘K7Na’

due to the synthesis carried out being identical, being reported by Nyman and co-

workers, (11) as well as being shown in the single crystal measurements.

a

b

c

2θ / deg

Inte

nsit

y (

a.u

.)

31

4.3 Perovskite Materials

All perovskite samples were synthesized by hydrothermal synthesis. Either the Nb-

and/or Ta-containing POMs or NbCl5 and/or TaCl5 were placed in an autoclave at

240°C for either 1, 2, or 5 days, along with either KOH or NaOH solution. The resulting

white powder was washed with generous amounts of H2O, collected and air dried at

room temperature for collection. PXRD was initially done to determine the purity of the

samples, however HRPXRD was subsequently done to determine space groups and

crystal structures. Figures 14 and 15 show the HRPXRD patterns of various

perovskites synthesized using both chlorides and POMs as starting materials. Figure

14 illustrates the potassium perovskites while Figure 15 the sodium perovskites. The

overall outline of possible perovskite synthesis done in this thesis can be seen in

Appendix H.

The space groups and lattice structures of the potassium and sodium niobates and

tantalates have been widely studied and published for materials prepared by solid state

synthesis. The following table shows the established structures and corresponding

lattice parameters of K- and Na- perovskites:

32

Table 3: Table of established niobate and tantalate perovskite space groups and lattice parameters

Perovskite Crystal

System

Space

Group

a/ Å b/ Å c/ Å V/ Å3 Reference

KNbO3 Orthorhombic Amm2 3.975 5.692 5.719 129.266 (28)

KTaO3 Cubic Pm ̅m 3.988

(2)

3.988

(2)

3.988(2

)

63.44 (29)

NaNbO3 Orthorhombic Pbcm

or

P21ma

5.504(1)

5.571(1)

5.570

(1)

7.766

(1)

15.517

(1)

5.513

(1)

475.709

238.517

(30)

NaTaO3 Orthorhombic Pbnm 5.476(1) 5.521

(1)

7.789

(2)

235.528 (31)

In order to determine and compare the space group and lattice parameters, the

samples were first compared to the established data. Refer to Appendix I for direct

single comparisons between products synthesized from only chlorides as well as only

POMs.

33

As seen in Table 3, the perovskites formed are mostly orthorhombic, with only KTaO3

being cubic as expected. KTaO3 and KNb0.5Ta0.5O3 from the chloride require further

investigation as it appears that the former can be fitted by two cubic perovskites while

the latter appears phase impure.

Figure 14: HRPXRD of Potassium Perovskites

(a) KNbO3_chloride, (b) KNbO3_POM (c) KNb0.5Ta0.5O3_chloride, (d)

KNb0.5Ta0.5O3_POM, (e) KNb0.33Ta0.67O3_POM (f) KTaO3_chloride, (g)

KTaO3_POM; zoomed plot represents peaks at 55.5 to 56.8°.

2θ / deg

Inte

nsit

y (

a.u

)

a

b

c

d

f

g

e

34

Table 4: Space groups and refined lattice parameters of K-perovskites

Sample Space

Group

a/ Å b/ Å c/ Å V/ Å3 Rwp /%

KNbO3 –

chloride

Amm2 3.976(7) 5.694

(13)

5.717

(12)

129.481(5) 20.665

KTaO3 – POM Pm ̅m 3.990(1) - - 63.522(4) 10.297

KTa0.67Nb0.33O3

– POM

Amm2 3.994(5) 5.6678(9) 5.653

(10)

128.011(2) 13.191

KTa0.50Nb0.50O3

– POM

Amm2 4.027(8) 5.6574(18) 5.661

(20)

128.991(7) 13.547

KNbO3 – POM Amm2 4.036(17) 5.654(48) 5.677

(46)

129.563(10) 18.839

Figure 15: HRPXRD of Sodium Perovskites (a) NaNbO3_chloride, (b)

NaNbO3_POM (c) NaNb0.5Ta0.5O3_chloride, (d) NaNb0.5Ta0.5O3_POM, (e)

NaNb0.33Ta0.67O3_POM (f) NaTaO3_chloride, (g) NaTaO3_POM; zoomed plots

represent peaks from 57.0 to 59.0°.

2θ / deg

b

c

g

d

e

f

Inte

nsit

y (

a.u

)

a

35

The Na- perovskites formed are all orthorhombic, (Table 5), with NaNbO3 from both the

chlorides and POM are distinctly Pbcm rather than Pbnm, as expected. With the

sample NaNb0.5Ta0.5O3 from the chloride, the space group is most likely one of the two

rather than any other possible space group, however neither perfectly fit. Further

analysis is required on these samples.

Table 5: Space groups and lattice parameters of Na-perovskites

Sample Space

Group

a/ Å b/ Å c/ Å V/ Å3 Rwp /%

NaTaO3 –

chloride

Pbnm 5.482(4) 5.524(5) 7.795(6) 236.120(3) 11.444

NaTa0.5Nb0.5O3

– chloride

Pbnm 5.511(21) 5.558(26) 7.789(19) 238.671(6) 16.603

Pbcm 5.510(26) 5.557(24) 15.572(43) 476.896

(13)

14.955

NaNbO3 –

chloride

Pbcm 5.511(8) 5.571(5) 15.545(19) 477.360(9) 11.461

NaTaO3 – POM Pbnm 5.488(10) 5.528(12) 7.797(15) 236.589(8) 10.053

NaTa0.67Nb0.33O

3 – POM

Pbnm 5.492(4) 5.533(5) 7.807(6) 237.301(3) 16.196

NaTa0.5Nb0.5O3

– POM

Pbnm 5.496(7) 5.534(9) 7.808(10) 237.514(6) 10.604

NaNbO3 – POM Pbcm 5.508(9) 5.566(6) 15.546(19) 476.690(6) 16.183

Preliminary investigation of sodium containing samples suggest that both the chloride

and POM routes produce crystals with the expected space groups while the potassium

samples appear to not be phase pure when using the POMs, and KNbO3 synthesized

from [Nb6O19]8- being less crystalline than its chloride counterpart.

36

Solid-state 93Nb NMR has been preliminarily conducted, which suggests an agreement

with the HRPXRD data. Both samples of NaNbO3 are comparatively similar in the local

Nb environment. The perovskites containing mixed Nb and Ta show a Nb signal

located at approximately -1100 ppm with increasing peak intensities. KNb0.33Ta0.67O3

from the POM has a large peak at around -1050 ppm, KNb0.5Ta0.5O3 from the POM is

approximately half as intense while KNb0.5Ta0.5O3 using chlorides has a slightly less

intense peak. KNbO3 from the chlorides and POMs are unexpectedly different. The

peak for the POM synthesised perovskite is located at around -1050 ppm with a slightly

narrower peak compared to that synthesized using the chlorides which is located at

around -1075 ppm with a very broad peak. A more in-depth analysis is currently being

conducted on all synthesized samples at the University of St Andrews. Appendix J

shows the 93Nb solid-state NMR spectra.

4.4 Filling of Carbon Nanotubes

The attempted filling of carbon nanotubes was done using solution filling. Initially

{K7Na[Nb4Ta2O19]} and Emmanuel Flahaut’s double-walled nanotubes were used. In

order to allow for maximum dissolution and dispersion, {K7Na[Nb4Ta2O19]} was

dispersed in 5 mL H2O or EtOH by stirring while the DWNTs were dispersed in 5 mL

EtOH using a sonic probe. Two individual solutions were mixed and allowed to stir. The

mixture was drop-casted onto lacey and holey Cu, carbon coated grids for analysis.

Emmanuel’s DWNTs have an inner diameter of 0.65 – 2 nm which is within the

expected range for DWNTs. When imaged, only the nanotubes were seen with clusters

of the POM located on the outside of the nanotubes, (Figure 16).

37

Figure 16: HRTEM of {K7Na[Nb4Ta2O19]} clusters surrounding a multi-walled carbon nanotube

It was also observed that the tips of the nanotubes, as well as inside, were covered in

obstructive material, which would prevent the insertion of the POM. To correct this, the

nanotube sample was pre-treated in H2O2. (32) It was reported that the use of H2O2

mildly oxidizes the nanotubes, unlike another known method which uses an acid for

strong oxidation. This prevents large loss of product seen with the harsher conditions of

the acid, and can be done at room temperature. However, when this was observed, no

difference could be seen.

It was then decided that in order for further maximizing of dissolution and dispersion,

alkylammonium salts may be beneficial, as had been used in the previous insertion of

[W6O19] into DWNT. (19) (21) (22) With this in mind, the TMA salt of [Ta6O19]8- was

synthesized, using the procedures for the synthesis of POMs in this project. Since the

structure is not known, a comparison of the TMA salt of [Ta6O19]8- and the reported

K8[Ta6O19] was made, (Figure 17).

38

Figure 17: PXRD comparison of (a) TMA salt of [Ta6O19]8- and (b) K8[Ta6O19]

It can be seen that the POM synthesized is indeed a POM and is mostly likely that of

[Ta6O19]8- due to the similarity in peak positions to the reported K8[Ta6O19].

When attempting to insert this POM into Emmanuel Flahaut’s DWNTs, it was observed

that the anions were corroding and degrading the carbon walls of the nanotube, (Figure

18). This could be an electron-beam-induced reaction between the Ta of the POM and

carbon of the nanotube walls. The lack of insertion, however, could be due to the high

charge of the [Ta6O19]8- anion compared to the previously reported [W6O19]

2- anion.

Inte

nsit

y (

a.u

.)

2θ / deg

39

Figure 18: Sequence of images showing the TMA salt of [Ta6O19]8- degrading walls of E. Flahaut’s DWNTs

The project then turned towards previously reported results, which did show the

successful insertion of the TBA salt of [W6O19]2-. (19) (21) (22) The TBA salt of

[Nb2W4O19]4- was thus synthesized to lower the charge of the ion, but keeping the idea

of mixtures of metals. This was done in a multi-step solution synthesis, following the

procedure of Dabbabi and Boyer. (25) A hot solution of [Nb6O19]8- was added to a

solution of Na2WO4 • 2 H2O and H2O2. Concentrated acetic acid was added to adjust

the pH to 5.5 and resulting solution was refluxed for 2 hours at 80°C. NaHSO3 was

added and then allowed to cool, in which TBAOH was added, solution filtered, and

EtOH added to precipitate the product. The product was filtered, collected, and air dried

at room temperature. When the PXRD patterns were compared between the previously

1

4

2

3

40

synthesized TMA salt of [Ta6O19]8-, the patterns show similarities, which would suggest

the formation of a POM, (Figure 19). Recrystallization methods are needed to obtain a

defined composition.

Figure 19: PXRD comparison of synthesized TBA-[Nb2W4O19]4- and TMA-[Ta6O19]8-

The insertion of [Nb2Ta4O19]8- was then attempted with DWNTs supplied by Sigma

Aldrich, having an average diameter of 3.5 nm. Once dispersed and drop casted onto a

carbon grid, the sample was then analysed and it was observed that some clusters had

been inserted, but no evidence for the single POM, (Figure 20). It was concluded that

the diameter of the carbon nanotubes was too large to allow for single ions to exist;

therefore single walled nanotubes were then to be investigated.

2θ / deg

Inte

nsit

y (

a.u

.)

41

Figure 20: HRTEM of clusters of TBA salt of [Nb2W4O19]4- inside double- and multi-walled nanotubes

SWNTs supplied by Sigma Aldrich (synthesized by NanoIntegris) were then used to

decrease the inner diameter to 1.2 – 1.7 nm. Clusters were again seen, however

evidence of possible single ions were observed but were only visible in multi-walled

nanotubes within the SWNT sample, (Figure 21). This would suggest the removal of

tantalum and decrease in nanotube diameter has a substantial effect on the

relationship between the anion and the nanotube.

Figure 21: HRTEM of clusters and possible single ions of TBA salt of [Nb2W4O19]4-

inside single and multi-walled nanotubes

42

The presence of possible single ions and clusters of the POM imply the diameter of the

nanotubes is still slightly too large to accommodate single ions. Single ion-like

structures were only observed in multi-walled nanotubes within the single-walled

nanotube sample which further suggests the incorrect diameter of the nanotubes.

5 Conclusion

Salts of [NbxTa6-xO19]8- with varying Nb and Ta content have been synthesized and

studied using powder and single-crystal X-ray diffraction. Single-crystal XRD was

performed on [Nb3Ta3O19]8- which indicated the composition of the crystal studied was

K7Na[Nb4Ta2O19] • 13 H2O , leading to the conclusion that the sample was most likely a

mixture of various [NbxTa6-xO19]8- ions. This was supported when the simulated powder

pattern of the bulk sample for which the crystal was taken did not match the observed

pattern for [Nb4Ta2O19]8-.

Potassium and sodium perovskite samples were hydrothermally synthesized using the

POM samples as well as using NbCl5 and TaCl5 to study the effect on perovskite

structure and purity. It was observed by using HRPXRD that it is possible to control the

perovskite but resulting in less crystalline materials with a longer reaction time when

POMs were used. Solid-state NMR is in progress which will aid in determining the

composition of the samples, but preliminary data suggest the composition using

various methods differ.

Double-walled nanotubes were first used to be filled by (K,Na)8[NbxTa6-xO19] using a

solution insertion method, however the ions were only visible as clusters outside of the

tubes. In order to increase dissolution of the POM in EtOH as well as observe if any

change would occur due to the ion being an organic salt, TMA-[Ta6O19]8- was used and

the carbon walls disintegrated in the electron beam, however, the high charge of the

POM could also be preventing its insertion. TBA-[Nb2W4O19]4- was then used, which

resulted in the observation of clusters inside the double-walled nanotubes. Single-

43

walled nanotubes were then used to decrease the inner diameter of the nanotubes to

prevent clusters from forming allowing single ions to be seen. Smaller clusters and

possible single ions were observed after this alteration however the presence of the

clusters implied the diameter of the tubes was still too large. These first observations

are a significant new result and suggest that with future work, imagining of mixed-metal

POMs within nanotubes should prove possible.

6 Future Work

Future recrystallization and single-crystal investigation should be done to determine

correctly the composition of each POM sample, the corresponding HRPXRD patterns,

and mass spectroscopy should also be measured to determine their compositions,

metal distribution, and structural information. Once this has been clarified, the synthetic

process for the POMs should be considered in order to attempt to further control the

exact composition of each POM to which would then aid in controlling the product when

converting them to perovskites. Additionally, solid-state NMR studies should be

performed for both POM and perovskite samples in order to determine their

compositions and structural trends with varying niobium and tantalum content.

HRPXRD data will be analysed in greater detail to conclude whether the perovskites

synthesized from the POMs have similar or different characteristics regarding structure,

orientation, and crystallinity. TEM should be done on both POM and perovskite

samples to observe the crystal formation. Energy-dispersive X-ray analysis (EDAX)

would be useful to analyse the composition of both the POMs and perovskites, and to

determine the distribution of elements in the materials.

Finally, smaller diameters of carbon nanotubes, as well as nanotubes supplied by

various sources, must be investigated to insert niobium-tungsten-containing POMs into

nanotubes. SWNTs supplied by SWeNT, with a nanotube diameter of 0.7 – 1.1 nm, will

first be used. Various methods of treatment of the nanotubes to increase the number of

single and open nanotubes may be needed, as well as recrystallizing [Nb2W4O19]4- to

44

obtain a pure sample. This may then lead to the observation of a chain of anions within

a nanotube allowing for behavioural properties and location of each atom within the

structures to be observed.

45

Appendix

Appendix A: Tables of Conducted Experiments

Reactant Amount Product

NbCl5 6.6g K3Nb(O2)4

30% H2O2 75mL

4M KOH 65mL

MeOH 250mL

TaCl5 6.6g K3Ta(O2)4

30% H2O2 75mL

4M KOH 65mL

MeOH 250mL

NbCl5 0.88g K3Nb0.67Ta0.33(O2)4

TaCl5 0.58g

30% H2O2 15mL

4M KOH 13mL

MeOH 550mL

NbCl5 0.66g K3Nb0.5Ta0.5(O2)4

TaCl5 0.875g

30% H2O2 15mL

4M KOH 13mL

MeOH 550mL

NbCl5 0.44g K3Nb0.33Ta0.67(O2)4

TaCl5 1.17g

30% H2O2 15mL

4M KOH 13mL

MeOH 550mL

Reactant Mass Catalyst Amount Solution Amount Product

K3Nb(O2)4 3.18 g Na3VO4 0.11 g H2O 15 mL K7Na[Nb6O19]

KOH 3.82 g

K3Ta(O2)4 4 g Na3VO4 0.11 g H2O 15 mL K7Na[Ta6O19]

KOH 3.82 g

K3Nb(O2)4 3.18 g K3VO4 0.137 g H2O 15 mL K8[Nb6O19]

KOH 3.82 g

K3Nb(O2)4 2.12 g Na3VO4 0.11 g H2O 15 mL K7Na[Nb4Ta2O19]

K3Ta(O2)4 1.34 g

KOH 3.82 g


K3Ta(O2)4 2 g

KOH 3.82 g

K3Nb(O2)4 1.59 g K3VO4 0.137 g H2O 15 mL K8[Nb3Ta3O19]

K3Ta(O2)4 2 g

KOH 3.82 g

46

Reactant Amount Catalyst Amount Solution Amount Product


K3Ta(O2)4 2.67 g

KOH 3.82 g

K3Ta(O2)4 2 g Na3VO4 0.055 g H2O 15 mL TMA-[Ta6O19]8-

TMAOH 6.16 g

K3Nb(O2)4 3.18 g Na3VO4 0.11 g H2O 15 mL TMA-[Nb6O19]8-

TMAOH 12.32 g

K3Nb(O2)4 3.18 g Na3VO4 0.11 g H2O 15 mL TEA-[Nb6O19]8-

TEAOH 4.89 mL

K3Nb(O2)4 3.18 g Na3VO4 0.11 g H2O 20 mL TPA-[Nb6O19]8-

TPAOH 3.05 mL

K3Nb(O2)4 3.18 g Na3VO4 0.11 g H2O 30 mL TBA-[Nb6O19]8-

TBAOH 27.19 g

0.04M [Nb6O19]

8- 10 mL ----------- ----------

Conc. Acetic acid 0.5 mL

TBA-Nb2W4O19]

4- 0.5M Na2W4• 2H2O 10 mL NaHSO3 1 mL

TBAOH 1.5 g EtOH 100 mL

Reactant Amount Solution Amount Time Temp Product

K7Na[Nb3Ta3O19] 500 mg KOH 12 mL 24 hours

200°C KNb0.5Ta0.5O3

K7Na[Nb3Ta3O19] 500 mg KOH 12 mL 24 hours

240°C KNb0.5Ta0.5O3

K7Na[Nb3Ta3O19] 500 mg KOH 12 mL 3 days 240°C KNb0.5Ta0.5O3



K7Na[Nb3Ta3O19] 500 mg NaOH 12 mL 5 days 240°C NaNb0.5Ta0.5O3

K8[Nb3Ta3O19] 500 mg KOH 12 mL 5 days 240°C KNb0.5Ta0.5O3

K8[Nb3Ta3O19] 500 mg NaOH 12 mL 5 days 240°C NaNb0.5Ta0.5O3

K7Na[Nb6O19] 500 mg KOH 12 mL 5 days 240°C KNbO3

K7Na[Nb6O19] 500 mg NaOH 12 mL 5 days 240°C NaNbO3

K7Na[Ta6O19] 500 mg KOH 12 mL 5 days 240°C KTaO3

K7Na[Ta6O19] 500 mg NaOH 12 mL 5 days 240°C NaTaO3

K8[Nb6O19] 500 mg KOH 12 mL 5 days 240°C KNbO3

K8[Nb6O19] 500 mg NaOH 12 mL 5 days 240°C NaNbO3


K7Na[Nb2Ta4O19] 500 mg NaOH 12 mL 5 days 240°C NaNb0.33Ta0.67O3

47

Reactant Amount Solution Amount Time Temp Product

NbCl5 500mg KOH 12 mL 24 hours

240°C KNbO3

NbCl5 500mg NaOH 12 mL 24 hours

240°C NaNbO3

TaCl5 500mg KOH 12 mL 24 hours

240°C KTaO3

TaCl5 500mg NaOH 12 mL 24 hours

240°C NaTaO3

TaCl5 500mg KOH 12 mL 48 hours

240°C KTaO3

TaCl5 500mg NaOH 12 mL 48 hours

240°C NaTaO3

NbCl5 362 mg KOH 12 mL 24 hours

240°C KNb0.67Ta0.33O3

TaCl5 240 mg

NbCl5 362 mg NaOH 12 mL 48 hours

240°C NaNb0.67Ta0.33O3

TaCl5 240 mg


240°C KNb0.5Ta0.5O3

TaCl5 132 mg


240°C NaNb0.5Ta0.5O3

TaCl5 132 mg


240°C KNb0.33Ta0.67O3

TaCl5 480 mg


240°C NaNb0.33Ta0.67O3

TaCl5 480 mg

48

Successful

Unsuccessful

Pending

POM Solvent SWNT/DWNT Source Solvent

K7Na[Nb3Ta3O19] solution DWNT E. Flahaut H2O

K7Na[Nb3Ta3O19] H2O DWNT Sigma EtOH

K7Na[Nb3Ta3O19] EtOH DWNT Sigma EtOH

K7Na[Nb3Ta3O19] EtOH DWNT (H2O2 treated) Sigma EtOH

TMA-[Ta6O19]8- EtOH DWNT (H2O2 treated) Sigma EtOH

TBA-[Nb2W4O19]4- EtOH DWNT (H2O2 treated) Sigma EtOH

TBA-[Nb2W4O19]4- EtOH SWNT NI EtOH

TBA-[Nb2W4O19]4- EtOH SWNT (heat treated) NI EtOH

TBA-[Nb2W4O19]4- EtOH SWNT SWeNT EtOH

49

Appendix B: IR Spectra of K3Nb(O2)4 and K3Ta(O2)4; full and closeup

Inte

nsit

y (

a.u

.)

Inte

nsit

y (

a.u

.)

Wavenumber (cm-1)

Wavenumber (cm-1)

50

Appendix C: TGA Measurement and comparison between K3Nb(O2)4

and K3Ta(O2)4

51

Appendix D: PXRD comparison of observed (a) K8[Nb6O19] and (b)

K8[Nb3Ta3O19]

Inte

nsit

y (

a.u

)

2θ / deg

a

b

52

Appendix E: Bond lengths in [Nb6O19]8-, [Ta6O19]

8-, and

K7Na[Nb4Ta2O19] • 13 H2O_single crystal

[Nb6O19]8- [Ta6O19]

8- K7Na[Nb4Ta2O19]_single crystal

Nb1 O3 1.7903 Ta1 O8 1.8392 Nb1|Ta1

O10 1.799

Nb4|Ta4 O6 1.7879

O6 1.9786 O4 1.9284 O14 1.986 O16 1.9932

O7 1.9893 O7 1.9542 O13 1.989 O12 1.9937

O10 1.9935 O5 1.9904 O12 1.992 O7 1.994

O9 1.999 O6 2.006 O11 1.996 O5 1.9985

O1 2.3782 O1 2.3776 O1 2.388 O1 2.3595

Nb2 O2 1.7947 Ta2 O9 1.7146 Nb2|Ta2 O4 1.795

Nb5|Ta5 O2 1.8164

O8 1.9749 O2 1.9391 O17 1.981 O3 1.9673

O4 1.9795 O3 1.9727 O3 1.988 O18 1.9887

O7 1.997 O6 1.9963 O13 1.992 O14 1.9938

O10 1.997 O4 2.0327 O5 2.005 O9 1.9942

O1 2.3434 O1 2.3457 O1 2.382 O1 2.3603

Nb3 O5 1.796 Ta3 O10 1.8 Nb3|Ta3 O8 1.792

Nb6|Ta6 O19 1.8314

O8 1.9794 O2 2.0288 O9 1.984 O17 1.9476

O9 1.9831 O3 2.0343 O15 1.993 O18 1.9729

O4 1.9881 O7 2.0494 O7 2.001 O16 1.9903

O6 1.9984 O5 2.0616 O11 2.009 O15 2.0194

O1 2.3721 O1 2.3735 O1 2.357 O1 2.3169

Yellow: [Nb6O19]8- Orange: [Ta6O19]

8- Blue: K7Na[Nb4Ta2O19] • 13 H2O

Green values are terminal oxygens bound to each addendum atom.

*All values are in Ångström.

53

Appendix F: TGA comparison between K7Na[Nb6O19] and

K7Na[Ta6O19]

54

Appendix G: TGA comparison between mixed-metal POMs

55

Appendix H: Outline of routes considered for perovskite synthesis

K7Na[Nb4Ta2O19]

KNb0.67Ta0.33O3

K7Na[Nb2Ta4O19]

KNb0.33Ta0.67O3

NbCl5 KNbO3 NbCl5

NaNbO3

NbCl5+TaCl5 2:1

KNb0.67Ta0.33O3

NbCl5+TaCl5 2:1

NaNb0.67Ta0.33

O3

K8[Nb6O19] NaNbO3 K8[Nb6O19] KNbO3

K7Na[Nb3Ta3O19]

KNb0.5Ta0.5O3

K7Na[Nb4Ta2O19]

NaNb0.67Ta0.33O3

K7Na[Nb3Ta3O19]

NaNb0.5Ta0.5O3

K7Na[Nb2Ta4O19]

NaNb0.33Ta0.67

O3

K7Na[Ta6O19]

KTaO3

K7Na[Ta6O19]

NaTaO3

NbCl5+TaCl5 1:1

KNb0.5Ta0.5O3

NbCl5+TaCl5 1:1

NaNb0.5Ta0.5O3

NbCl5+TaCl5 1:2

KNb0.33Ta0.67O3

TaCl5 KTaO3

NbCl5+TaCl5 1:2

NaNb0.33Ta0.67

O3

TaCl5 NaTaO3

56

Appendix I: Comparison of samples made from chlorides and made

from POMs

K-salts from Chlorides:

(a) KTaO3, (b) KNb0.5Ta0.5O3, (c) KNbO3

Inte

nsit

y (

a.u

.)

2θ / deg

a

b

c

57

K-salt from POMs:

(a) KNbO3, (b) KNb0.5Ta0.5O3, (c) KNb0.5Ta0.5O3, (d) KTaO3

a

b

c

d

Inte

nsit

y (

a.u

.)

2θ / deg

58

Na-salt from Chlorides

(a)NaTaO3, (b) NaNb0.5Ta0.5O3, (c) NaNbO3

a

b

c

Inte

nsit

y (

a.u

.)

2θ / deg

59

Na-salt from POMs:

(a) NaNbO3, (b) NaNb0.5Ta0.5O3, (c) NaNb0.5Ta0.5O3, (d) NaTaO3

Inte

nsit

y (

a.u

.)

a

b

c

d

2θ / deg

60

Appendix J: 93Nb solid-state NMR of perovskite samples

AC7= NaNbO3 from POM

AC6= NaNbO3 from chloride

AC5= KNb0.5Ta0.5O3 from chloride

AC4= KNb0.5Ta0.5O3 from POM

AC3= KNb0.33Ta0.67O3 from POM

AC2= KNbO3 from POM

AC1= KNbO3 from chloride

61

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