Rietveld refinements of the solid solution Li(1�2x)NixTiO(PO4)

(0 � x � 0.50)

B. Manouna,*, A. El Jazoulia, P. Gravereaub, J.P. Chaminadeb

aLaboratoire de Chimie des Materiaux Solides, Faculte des Sciences Ben M’Sik Boulevard Idriss El Harti,

Sidi Othman, B.P. 7955 Casablanca, MoroccobInstitut de Chimie de la Matiere Condensee de Bordeaux, 87, Av. du Dr. Schweitzer,

33608 Pessac Cedex, France

Received 31 May 2004; received in revised form 4 October 2004; accepted 27 October 2004

Abstract

Li(1�2x)NixTiO(PO4) oxyphosphates with 0 � x � 0.10 crystallize in the orthorhombic system with the space

group Pnma, those with 0.10 < x � 0.25 crystallize in the monoclinic system with the space group P21/c and

compositions with 0.25 < x < 0.50 present a mixture of the limit of the solid solution Li0.50Ni0.25TiO(PO4) and

Ni0.50TiO(PO4). The structure of the compositions 0 � x � 0.25 is based on a three-dimensional anionic frame-

work constructed of chains of alternating TiO6 octahedra and PO4 tetrahedra, with the lithium and nickel atoms in

the cavities in the framework. The dominant structural units in the compositions are chains of tilted corner-sharing

TiO6 octahedra running parallel to one of the axis. The oxygen atoms of the shared corners, not implied in (PO4)

tetrahedra, justify the oxyphosphate designation. Titanium atoms are displaced from the geometrical center of the

octahedra resulting in alternating long (�2.25 A) and short (�1.71 A) Ti–O(1) bonds. The four remaining Ti–O

bond distances have intermediate values ranging from 1.91 to 2.06 A.

# 2004 Elsevier Ltd. All rights reserved.

Keyword: C. X-ray diffraction

www.elsevier.com/locate/matresbu

Materials Research Bulletin 40 (2005) 229–238

* Corresponding author. Present address: Center for the Study of Matter at Extreme Conditions, Florida International

University, U.P., VH-140 Miami, FL 33199, USA. Tel.: +1 3053483445; fax: +1 3053483070.

E-mail address: manounb@fiu.edu (B. Manoun).

0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2004.10.025

1. Introduction

Titanium phosphates, both in glass and crystalline forms, have been extensively studied for the interest

in their properties as ionic conductors or non-linear optical materials [1–7]; the most known is potassium

titanium oxide phosphate (KTiOPO4), or KTP, which is an efficient nonlinear optical crystal with large

nonlinear coefficient, in the visible to infrared spectral region at relatively low cost. Nonlinear optic

(NLO) crystals, and in particular the KTP single crystal, are commonly used for effective nonlinear

transformation of laser radiation, the most popular application is for harmonic generation including

frequency doubling (SHG), tripling (3HG) and frequency mixing (SHG). The qualities and the potential

applications of KTP arose great interest in analogues which have partial or complete substitution on K, Ti

and P sites. For example K is replaced by small cations like Li and Ni leading to new materials:

LiTiO(PO4), Ni0.50TiO(PO4) and Li0.50Ni0.25TiO(PO4) [5,6,8,9].

The structure of Ni0.50TiO(PO4) (P21/c space group), Li0.50Ni0.25TiO(PO4) (P21/c space group) and

LiTiO(PO4) (Pnma space group) have been determined from powder for all three compounds [5,8,9] and

single crystal for LiTiO(PO4) [6] from X-ray diffraction data. An eventuality of a solid state solution

between LiTiO(PO4) and Ni0.50TiO(PO4) was envisaged in a way to create cation deficiency in

LiTiO(PO4):Li(1�2x)NixTiO(PO4). This hypothesis will be confirmed during the study of the structure

of intermediate compositions. Indeed, theoretical analyses of the structure of Ni0.50TiO(PO4), showed

that the non-occupied sites (2d, 2c, and 2b in P21/c space group) are not able to accommodate additional

cations such as Ni2+. The aim of this paper is to describe the structural characterization and follow the

structural changes of Li(1�2x)NixTiO(PO4) materials using X-ray diffraction.

2. Experimental

As we mentioned in our previous paper [10], Li(1�2x)NixTiO(PO4) (x = 0, 0.1, 0.2, 0.25, 0.33, 0.4, 0.5)

oxyphosphate powders were prepared from diluted solutions of NiCl2�6H2O (I), Li2CO3 (II),

(NH4)2HPO4 (III) and TiCl4 in ethanol (IV). A slow addition of (IV) in (I) + (II) + (III) mixture, with

constant stirring, at room temperature induces a precipitation. After drying at 100 8C, the resulting

amorphous powder was progressively heated up to 850 8C for 48 h. The final products have been

controlled by X-ray powder diffraction analysis using Cu Ka radiation. The structural refinements were

undertaken from the powder data. Diffraction data were collected at room temperature on a Phillips PW

3040 (u–u) diffractometer: Bragg-Brentano geometry; diffracted-beam graphite monochromator; Cu Ka

radiation (40 kV, 40 mA); Soller slits of 0.02 rad on incident and diffracted beams; divergence slit of 18;antiscatter slit of 18; receiving slit of 0.05 mm; holder surface was corrected with a razor blade; with

sample spinner; steps of 0.028 (2u) over a large angular range with a fixed counting time of 30 s.

Rietveld’s profile analysis method was employed for refinements using the program FULLPROF [11].

3. Results and discussion

The X-ray diffraction pattern for the title phases was assigned to an orthorhombic symmetry for

LiTiO(PO4) [5,6] and to a monoclinic symmetry for Ni0.50TiO(PO4) and Li0.50Ni0.25TiO(PO4) [8,9]. The

structural refinements of Li(1�2x)NixTiO(PO4) (0 � x � 0.1) were done starting from the data of

B. Manoun et al. / Materials Research Bulletin 40 (2005) 229–238230

Robertson et al. [5] (Table 1), and the calculations were performed in the Pnma orthorhombic space

group. For 0.10 < x � 0.25, the refinements were done starting from the data of Li0.80Ni0.10TiO(PO4) in

Pnma space group considering that Ni0.50TiO(PO4) exists as an impurity (the peak observed at

2u � 13.858 is not compatible with Pnma space group. In the first hypothesis, it was assigned to the

phosphate Ni0.50TiO(PO4) (Table 1)). The results show, especially for the peak observed at 2u � 22.128, a

difference between positions of the observed and calculated reflections (Fig. 1). The shift in positions

become more important for x = 0.25 and did not appear for x = 0.10 and knowing that this shift in position

cannot be assigned to an impurity, the refinement of Li0.60Ni0.20TiO(PO4) was then done starting from

Li0.50Ni0.25TiO(PO4) model [9].

B. Manoun et al. / Materials Research Bulletin 40 (2005) 229–238 231

Fig. 1. Shift between calculated and observed peaks around 2u � 22.128, observed in the Rietveld refinement plot (Pnma space

group) for the compositions x = 0.20 and 0.25.

Table 1

Results obtained from Rietveld’s profile analysis method for Li(1�2x)NixTiO(PO4) in Pnma space group

Prepared composition Orthorhombic

phase percent

Monoclinic phase

Ni0.5TiO(PO4)

Orthorhombic phase

formula

LiTiO(PO4) 100 0 LiTiO(PO4)

Li0.80Ni0.10TiO(PO4) 100 0 Li0.80Ni0.10TiO(PO4)

Li0.60Ni0.20TiO(PO4) 97.1 2.9 Li0.62Ni0.19TiO(PO4)

Li0.50Ni0.25TiO(PO4) 96.0 4.0 Li0.52Ni0.24TiO(PO4)

Li0.33Ni0.33TiO(PO4) 65.1 34.9 Li0.52Ni0.24TiO(PO4)

Li0.20Ni0.40TiO(PO4) 38.9 61.1 Li0.52Ni0.24TiO(PO4)

In the Li(1�2x)NixTiO(PO4) series, the X-ray patterns of the compositions 0.25 < x < 0.50 show a

mixture of two monoclinic phases, viz., Li0.50Ni0.25TiO(PO4) structure type whose composition has to be

calculated and Ni0.50TiO(PO4). Both the phases are taken into consideration in the Rietveld refinements.

The method of refinement in this case is as follows:

Lið1�2xÞNixTiOðPO4Þ! yLið1�2x0ÞNix0TiOðPO4Þ þ ð1 � yÞNi0:50TiOðPO4Þ

where y is the weight percent of the Li0.50Ni0.25TiO(PO4) type.

In this case structural characteristics of both phases are included in the refinement program, MZ (M:

molar mass; Z: formulae number per unit cell) were also included for each phase. The molar percent of

Li0.50Ni0.25TiO(PO4) type is obtained by:

y0 ¼ 1 þ 1

y�1

� �� �Mm1

Mm2

� ��1

where Mm1 is the molar mass of Li0:50Ni0:25TiOðPO4Þ type;Mm2 the molar mass of Ni0:50TiOðPO4Þ type.

Then, the value x0 is equal to (2x + y � 1)/2y.

For 0.25 < x < 0.50 the refinements were done starting from the structural data of Li0.50Ni0.25-

TiO(PO4) (P21/c) and Ni0.50TiO(PO4) (P21/c); the refinements showed a mixture of these two phases

and gave the quantity of each compound (see Table 2), concluding that Li0.50Ni0.25TiO(PO4) is the limit

of the solid solution. Some of these results were confirmed by other refinements using neutron

diffraction. The X-ray powder patterns were fitted to the calculated ones using a full-profile analysis

B. Manoun et al. / Materials Research Bulletin 40 (2005) 229–238232

Fig. 2. Final Rietveld plot of LiTiO(PO4) (a), Li0.80Ni0.10TiO(PO4) (b) and Li0.60Ni0.20TiO(PO4) (c). In each plot the upper

symbols illustrate the observed data (cross) and calculated pattern (solid line). The vertical markers show calculated positions of

Bragg reflexions. The lower curve is the difference diagram.

B.

Ma

no

un

eta

l./Ma

terials

Resea

rchB

ulletin

40

(20

05

)2

29

–2

38

23

3

Table 2

Refinement conditions (X-ray powder data) for Li(1�2x)NixTiO(PO4)

x = 0 x = 0.10 x = 0.20 x = 0.25 x = 0.33 x = 0.40

Zero point (8, 2u) �0.011 (2) 0.013 (2) �0.011 (2) �0.011 (2) �0.0002 (2) �0.005 (2)

Rp 0.08 0.138 0.078 0.074 0.074 0.068

Rwp 0.116 0.191 0.110 0.100 0.101 0.095

cRp 0.098 0.176 0.111 0.102 0.100 0.090

cRwp 0.135 0.230 0.141 0.128 0.128 0.118

X2 2.22 1.55 1.63 1.76 1.78 1.62

Phase LiTiO(PO4) Li0.80Ni0.10TiO (PO4) Li0.60Ni0.20TiO (PO4) Li0.50Ni0.25TiO (PO4) Li0.50Ni0.25TiO (PO4) Li0.50Ni0.25TiO (PO4)

Percentage 100 100 100 100 68.67 38.42

No. of reflections 562 564 709 1021 1022 1021

No. of refined parameters 35 35 44 44 44 21

Space group Pnma Pnma P21/c P21/c P21/c P21/c

a (A) 7.4015(2) 7.3892(2) 6.3904(2) 6.3954(2) 6.3971(3) 6.3950(2)

b (A) 6.3756(2) 6.3832(2) 7.2532(3) 7.2599(2) 7.2641(4) 7.2667(3)

c (A) 7.2350(2) 7.2429(2) 7.3759(3) 7.3700(2) 7.3691(3) 7.3646(5)

b (8) – – 90.154(2) 90.266(2) 90.273(3) 90.262(6)

V (A3) 341.4 341.6 341.88 342.18 342.44 342.29

Z 4 4 4 4 4 4

Atom number 7 7 9 9 9 9

RF 0.029 0.059 0.031 0.021 0.024 0.024

RB 0.041 0.071 0.041 0.036 0.038 0.037

Monoclinic phase – – – – Ni0.50TiO(PO4) Ni0.50TiO(PO4)

Percentage 0 0 0 0 31.33 61.58

B. Manoun et al. / Materials Research Bulletin 40 (2005) 229–238234

Table 3

Refined structural parameters for Li(1�2x)NixTiO(PO4)

Atom x = 0 x = 0.10 x = 0.20 x = 0.25

Ti x/a 0.3221(3) 0.3328(4) 0.7539(5) 0.7564(5)

y/b 0.75 0.75 0.2210(4) 0.2216(4)

z/c 0.2197(3) 0.2206(3) 0.3344(5) 0.3346(5)

B (A2) 0.64(8) 0.5(1) 0.5(1) 0.3(1)

Li1 x/a 0 0 0 0

y/b 0 0 0 0

z/c 0 0 0 0

B (A2) 0.90(5) 1.4(3) 0.4(3) 2.8(3)

Ni1 x/a 0.3743(4) 0 0 0.5

y/b 0.25 0 0 0

z/c 0.1276(5) 0 0 0

B (A2) 0.63(8) 1.4(3) 0.4(3) 0.18(3)

Li2 x/a 0.1121(9) 0.3749(5) 0.5 0.2527(7)

y/b 0.75 0.25 0 0.1280(7)

z/c 0.1504(10) 0.1282(6) 0 0.3743(8)

B (A2) 0.6(1) 0.4(1) 0.4(3) 0.4(1)

Ni2 x/a 0.7917(10) 0.1126(12) 0.5 0.7448(8)

y/b 0.75 0.75 0 0.1535(10)

z/c �0.0017(9) �0.1516(13) 0 0.1126(16)

B (A2) 0.6(1) 0.7(1) 0.4(3) 0.4(1)

P x/a 0.0477(10) 0.7898(13) 0.2520(7) 0.7356(16)

y/b 0.25 0.75 0.1285(7) �0.0001(8)

z/c 0.4842(9) �0.0014(11) 0.3739(8) 0.7921(14)

B (A2) 0.6(1) 0.7(1) 0.5(1) 0.4(1)

O(1) x/a 0.8715(6) 0.0448(13) 0.7490(8) 0.2465(16)

y/b 0.4460(7) 0.25 0.1544(9) 0.4864(15)

z/c 0.2479(7) 0.4852(12) 0.1110(16) 0.0481(15)

B (A2) 0.6(1) 0.7(1) 0.3(1) 0.4(1)

O(2) x/a 0.7355(16) 0.4424(9)

y/b �0.0004(8) 0.2427(13)

z/c 0.7933(14) 0.8724(11)

B (A2) 0.3(1) 0.4(1)

O(3) x/a 0.2450(16) 0.9437(9)

y/b 0.4863(15) 0.7466(13)

z/c 0.0471(15) 0.1341(11)

B (A2) 0.3(1) 0.4(1)

O(4) or O(41) x/a 0.4428(9) 0.2216(4)

y/b 0.2420(13) 0.3346(5)

z/c 0.8717(11) 0.3(1)

B (A2) 0.3(1)

O(5) or O(42) x/a 0.9453(9) 0

y/b 0.7472(13) 0

z/c 0.1320(11) 2.8(3)

B (A2) 0.4(1)

program (Rietveld method, FULLPROF program [11]) to minimize the profile discrepancy factor Rp.

Table 2 lists the refinements conditions, cell parameters and reliability factors. Fig. 2 displays the

agreement obtained between the observed and the calculated diffraction profiles. The resulting data

for atomic coordinates and isotropic temperature factors with their estimated deviation (e.s.d.) are

listed in Table 3. Standard deviations have been multiplied by Berar factor to correct local correlations

[12].

All the observed reflections for the compositions x � 0.10 could be indexed in the space group Pnma

(no. 62) in which phosphorous cations were located on the tetragonal sites (4c), the titanium cations

were located on the octahedral sites (4c), nickel and lithium cations were located on the octahedral

sites (4a).

B. Manoun et al. / Materials Research Bulletin 40 (2005) 229–238 235

Fig. 3. Ti–O–Ti–O– chain.

Fig. 4. Titanium atoms are displaced from the geometrical center of the octahedra in LiTiO(PO4).

All the observed reflections for the compositions 0.10 < x � 0.25 could be indexed in the space group

P21/c (no. 14) in which phosphorous cations were located on the tetragonal sites (4e), the titanium cations

were located on the octahedral sites (4e), nickel and lithium cations were located on the octahedral sites

(2b) and (2a).

The structure of the compositions 0 � x � 0.25 is based on a three-dimensional anionic framework

constructed of chains of alternating TiO6 octahedra and PO4 tetrahedra, with the lithium and nickel atoms

B. Manoun et al. / Materials Research Bulletin 40 (2005) 229–238236

Fig. 5. (a) Illustration of NiO6 octahedron in Li0.50Ni0.25TiO(PO4). (b) Illustration of LiO6 octahedron in Li0.50Ni0.25TiO(PO4).

in the cavities in the framework. The dominant structural units in the compositions are chains of tilted

corner-sharing TiO6 octahedra running parallel to one of the axis (Fig. 3). The oxygen atoms of the shared

corners, not implied in (PO4) tetrahedra, justify the oxyphosphate designation. Titanium atoms are

displaced from the geometrical center of the octahedra (Fig. 4) resulting in alternating long (�2.25 A)

and short (�1.71 A) Ti–O(1) bonds. The four remaining Ti–O bond distances have intermediate values

ranging between 1.91 and 2.06 A. Fig. 5 shows the illustrations of NiO6 and LiO6 octahedra in

Li0.50Ni0.25TiO(PO4).

In the compositions x = 0 and 0.10 infinite chains of TiO6 octahedra run parallel to a axis and (Li/

Ni)O6 chains (sharing bridges) run parallel to b axis, Li and Ni are distributed statistically in 4a site for

x = 0.10. In the compositions 0.10 < x � 0.25 infinite chains of TiO6 octahedra run parallel to c axis and

LiO6 and NiO6 chains (sharing edges) run parallel to a axis. The structure of Li0.50Ni0.25TiO(PO4) has

been described by Manoun et al. [9]. The structure of Li0.60Ni0.20TiO(PO4) is similar to that of

Li0.50Ni0.25TiO(PO4). The only difference between these compounds is the distribution of Li+ and

Ni2+ in 2a and 2b sites in P21/c space group. In Li0.50Ni0.25TiO(PO4): [Li0.50]2a[Ni0.25&0.25]2bTiO(PO4),

Ni2+ and & are distributed randomly and in Li0.60Ni0.20TiO(PO4): [Li0.48Ni0.02]2a[Li0.12Ni0.18&0.20]2b-

TiO(PO4), lithium and nickel are distributed statistically in 2a and 2b sites.

We can see clearly that Bragg factor for LiTiO(PO4) is very significant (RB = 4.1%) unlike the one

found by Robertson et al. [5] (RB = 17.50%). In this refinement the atomic positions and interatomic

distances of LiTiO(PO4) are very close to those found by Nagornoy et al. [6].

4. Conclusion

Li(1�2x)NixTiO(PO4) oxyphosphates have been prepared and characterized by X-ray diffraction. Their

structures have been determined on powder by the Rietveld method. This study shows the existence of a

solid solution in the domain 0 � x � 0.25 with the orthorhombic symmetry for 0 � x � 0.10 and with the

monoclinic symmetry for 0.10 < x � 0.25. Compositions with 0.25 < x < 0.50 present a mixture of the

limit of the solid solution Li0.50Ni0.25TiO(PO4) and Ni0.50TiO(PO4).

Acknowledgements

B.M. would like to thank the ICMCB institute (France) for their support.

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