Pergamon Materials Research Bulletin, Vol. 29, No. 8, pp. 871-879, 1994

Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved

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AN O R I G I N A L O X I D E OF A N T I M O N Y A N D T U N G S T E N R E L A T E D T O A U R I V I L L I U S P H A S E S

A. CASTRO, P. MILl,AN, Instituto de Ciencia de Materiales de Madrid, CSIC,

Serrano 113, 28006 Madrid, Spain.

R. ENJALBERT, E. SNOECK and J. GALY, Centre d'Elaboration de Mattriaux et d~Etudes Structurales, CNRS,

29, rue J. Marvig, B,P. 4347, 31055 Toulouse Cedex, France.

(Received March 21, 1994; Communicated by E.F. Ber taut )

ABSTRACT

AuriviUius's phases exhibit original [Bi202]n layers ; up to now such phases exist only with bismuth. The aim of this paper is to report new related phase, containing only antimony, which is promissing because its ferroelastic properties. Synthesis, crystal growth and structure determination of the original double oxide Sb2WO6 has been performed. It crystallizes in the triclinic system, space group P1, with a=5.554(1), b=4.941(2), c=9.209(3)/~, ¢t=90.05(3), 13=96.98(2), ~/=90.20(2) °, V=250.8(7)/~3. TEM and optical microscopy investigations allow to evidence a ferroelastic behaviour at room temperature with a phase transition in the range 300-400°C. The crystal structure of Sb2WO6 is built up by [WO4]n layers of W06 octahedra sharing corners sandwiched by two [Sb202]n layers; it can be described as [WO2Sb204]n thick layers held together by weak Sb...O interactions. The antimony atoms exhibit typical 3+1 one sided coordination. Via structural relationships with Sb203, VOSb204 and Bi2WO6, Sb2WO6 is related with the Aurivillius phases announcing how [Bi202]n layers with the shape encountered in red PbO can be formed or distorted.

MATERIALS INDEX : antimony, tungsten, oxygen

INTRODUCTION

Since a long time, solid state and structural chemistry of oxides containing both transition an 2 • M d lone pair ns M metals, have been of central interest in our research (1 to 8). The

stereochemical influence of the lone pairs, designed by E, has been particularly emphasized (9,10).

871

872 A. CASTRO et al. Vol. 29, No. 8

B. Aurivillius, in 1949, reported the synthesis and the crystal structure of CaNb2Bi209 which was a member of a large family Bi202(Mem-lRmO3m+l)(1 I); the structures are built up by quadratic Bi202 layers alternating with perovskite layers, in which Me and R are respectively the 12 and the 6 coordinated atoms and m (an integer) designing the thickness of these layers (12). The f~rst member, i.e. m=l, Bi2NbO5F, together with isomorphous oxide fluorides compounds, were reported in 1952 (13); in its structure the perovskite layers are reduced to simple Nb(O,F)4 layers containing comer-linked Nb(O,F)6 octahedra.

The oxides having the Aurivillius structure type correspond to the general formula Bi2MO6 (M=Mo,W). Bi2WO6 is the most representative material of this family ; it has been studied in great detail for its physical properties (strong ferroelectricity, high transition temperature, large spontaneous polarization) (14 to 16). This oxide shows two reversible polymorphic transitions at respectively 662 and 962°C(17,18). While the room temperature crystal structure has been determined and refined from single crystal X-ray diffraction data (16,19), electron microscopy (19) and neutron powder diffraction methods (20), the determination of the high temperature structure modification, up to now, has been unsuccessful.

(Bi202)n sheets have been considered as unalterable. The works carried out in our laboratories, have demonstrated that M* cations with a stereochemical active lone pair of electrons, like Pb 2+, Sb 3+ or Te 4+, substitute for Bi 3+ without destroying the crystal network (21 to 23). For the first time also, the authors have prepared and determined the crystal structure of Bi2VO5, very similar with these simplest Aurivillius oxides, with a M cation, formally V 4+, possessing unfilled electronic levels ; in the structure, (Bi202)n sheets alternate with (VO3)n layers built up by VO5 square pyramids sharing basal comers (24).

Otherwise, the synthesis and structural study of VOSb204 compound have allowed to precise the configuration and behaviour of Sb 3+ in layer structures with (SbO2E)n strings (3).

This paper reports the synthesis and structural characterization of an original oxide, Sb2WO6, related with these phases. The crystal structure is compared with those of Sb203, Sb2VO5 and Bi2WO6, and structural relationships based on topological transformation are proposed.

SYNTHESIS

The double oxide Sb2WO6 has been synthesized from a mixture of the Sb203 and WO3 oxides in the molar ratio 4:1. The mixture, carefully grounded, was placed in silica ampoules

sealed under a primary vacuum (10-3mm Hg) in order to avoid any partial oxidation of Sb 3+ into Sb 5+. Heated at 750°C for 1 hr, then at 800°C during 24 hrs, the ampoules were gently cooled down to 400°C at a speed of 5°C/hr and then to room temperature at 50°C/hr. This protocole drives to the formation of pale yellow crystals of Sb2WO6.

ANALYSIS

* DTA The Sb2WO6 behaviour versus temperature 25°C -

500°C, has been followed using a Stanton Redcroft STA 781 apparatus.

Figure I : Bright field micrograph of the Sb2WO 6ferroelastic domains.

* MICROSCOPY Optical microscopy observations have been made

with the technique of cross polarizers. Transmission electron microscopy (TEM) has been realized on a Philips CM 20 machine.

Vol. 29, No. 8 AURIVILLIUS PHASES 873

* X-RAY DIFFRACTION The powder pattern of Sb2WO6 has been recorded with a D 501 goniometer set up onto a

Siemens Kristalloflex generator (CuKct radiation ). The single crystal was selected by subsequent studies on a Nonius precession camera and the structural data registered with a CAD 4 Enraf Nonius (MoKct radiation).

MICROSCOPIC AND DTA STUDIES

Sb2WO6 single crystals exhibit domains easily observable both by using TEM or optical microscopy. In the figure 1 is reported a bright field micrograph of these domains. The regular rectilinear interfaces corresponding to the (001) planes are clearly seen. Taking into account that the ferroelectric phase Bi2WO6 presents a high transition temperature, Sb2WO6 single crystals have been heated on the stage of the optical microscope in order to verify if a similar behaviour occurs. The phase transition has been succesfully observed; it is reversible.

The transition is characterized by the total disappearance of the domain at high temperature, needle tips appearing under cooling. The shape of domain growth suggests that the transition is ferroelastic. The estimated temperature of the transition using this in-lab holder, is in between 300 and 400°C. DTA shows an endothermic transformation around 300°C rather extended on the temperature scale (250-350°C).

STRUCTURAL STUDY

Figure 2a : Precession diagramm.

The collected intensity data have been corrected by the Lorentz polarization and absorption factors (25). Scattering factors of the elements with their anomalous contribution (26) have been used. The calculations and drawing were performed with SHELXS86 (27) and ORTEP (28) programs using a vectorial superminicomputer AUiant VFX 80.

A. CASTRO et al. Vol. 29, No. 8 874

Table I

Crystal data of Sb2WO6.

, ,,, I , I

,/,/,,/,/, I t I ~ l I I I l I t

l.. ~ t ~ ,

I ",1 ,V,/ I I I i I | t I t ~ I I I I~ I I

a 8 x 8 ^ a A 8 A ~ ^ B ^ a S ^ ~ A S ~ B ^

Figure 2b : Schematic representation of the hOl plane of the reciprocal lattice of Sb2WO 6 .

Triclinic system Space group P1

a --- 5.554(1) A ct = 90.05(3) ° b --- 4.941(2) A 13 = 96.98(2) ° c = 9.209(3)A ~t = 90.20(2) ° V -- 250.8(7)A 3 px = 6.93g/cm 3 Z = 2 The structure was determined using Pat terson funct ion and subsequent difference Fourier synthesis and refined by full least squares treatments. The final solution is given in the non centric space group P1 in agreement with the ferroelastic properties (note that the statistical tests were in favour o f such a result) (Table I/). The final calculation does not give a good R factor; R = 0.12 for 1228 reflections hkl and 80 variables.

The temperature factors have been kept isotropic; tentatives to use anisotropic factors appeared not reasonable. The intensity measurements seem to have suffered from overlap problems not completely resolved. Anyhow, at this point, it was decided to utilize this obtained average structure which appeared to have an atomic architecture well enough defined to be useful for a discussion of the structural features of Sb2WO6 and to allow a suitable comparison with other double oxides (Table HI).

Table II : Positional parameters and thermal factors in Sb2WO6.

Atom x y z Beq* or Biso (A 2)

W1 0 0 0 2.1(2)* W2 0.53 I(2) 0.468(2) 0.0002(8) 1.5(1)* Sb 1 0.144(2) 0.536(2) 0.3344(9) 1.12(9) Sb2 0.640(2) 0.937(2) 0.3339(9) 0.95(8) Sb3 0.374(2) 0.030(2) 0.6680(8) 0.45(6) Sb4 0.869(2) 0.439(2) 0.6704(7) 0.29(6) O 1 0.307(6) 0.210(6) 0.067(4) ~ 0.2(4) 02 0.792(6) 0.281 (6) 0.051(5) 0.5(4) 03 0.717(6) 0.791 (6) -0.043(4) 0.8(4) 04 0.202(7) 0.720(7) -0.076(6) 1.5(5) 05 -0.002(7) 0.147(7) -0.183(5) 0.8(5) 06 0.030(7) 0.858(7) 0.191 (5) 0.8(5) 07 0.499(7) 0.350(7) -0.181(6) 1.3(5) 08 0.533(6) 0.637(7) 0.184(5) 0.6(5) 09 0.370(7) 0.811 (7) 0.437(6) 1.7(6) 010 0.873(6) 0.692(7) 0.435(5) 0.5(4) 011 0.143(7) 0.326(7) 0.567(6) 1.3(5) O12 0.639(6) 0.188(7) 0.561(5) 0.6(5)

A justification to use this structure is the good fit between calculated and experimental powder patterns (Fig. 3). The experimental one shows two peaks indicating a light pollution of the Sb2WO6 compound by very slight amounts of SiO2 (20 = 22 °) and WO3 (20 = 23°).

V o l . 2 9 , N o . 8 A U R I V I L L I U S P H A S E S 875

10 20 30 40 50

o ,?. N o

o o

o o

Sb2WO 6

,i calc.

° o ~ ~

o o

Z0 lO ZO 3O

exp.

i " , i i i i i " , i i , i i i i i t i ¢ I i L I

40 50

F i g u r e 3 : Calculated and experimental powder patterns of S b 2 W O 6"

T a b l e I I I : S e l e c t e d d i s t a n c e s ( ,~) a n d a n g l e s (o) in S b 2 W O 6

E n v i r o n m e n t o f t u n g s t e n a t o m s

W l - O1 2.024(8) W1- O4c 1.965(9) W 1 - O 2 a 1.901(8) W1- 0 5 1.830(9) W1- O3b 1.878(8) W1- O6c 1.886(9) <W1-O> 1.914 O-WI-O : 12 angles between 84 ° and 97 °

3 angles between 173 ° and 175 °

W2- O1 1.930(8) W2- O6c 1.886(9) W 2 - O 2 1.739(8) W2- 07 1.758(10) W2- O3 1.961(8) W2- O8 1.889(9) <W2 - O> 1.922 O-W2-O: 12 angles between 83 ° and 96 °

3 angles between 166 ° and 177 °

E n v i r o n m e n t o f a n t i m o n y a t o m s

Sbl - O6 2.112(10) Sbl - O9 2.000(9) Sbl - O10a 2.019(9) Sbl - O l l 2.379(10) O6-Sbl- 09 84.6(5) O6-Sbl- O10a 79.3(5) O6-Sbl- O l l 148.9(5) O9-Sbl- O10a 89.0(5) O9-Sbl- O11 86.7(6) O10a-Sbl- O11 70.7(4) Sb4 - O5e 2.047(9) Sb4 - O l l f 1.975(9) O5e-Sb4- O l l f 83.7(7) O1 lf-Sb4- O12 93.7(6) Distances Sbfoasal oxygen plane Sbl/O6 - 0 9 - O10a 1.28(9) Sb3/O7d - O11 - O12 1.32(5)

Sb2 - 0 8 2.059(8) Sb3 - O7d 2.158(9) Sb2 - 0 9 1.966(9) Sb3 - O11 2.090(8) Sb2 - O10 1.931(8) Sb3 - O12 2.023(9) Sb2 - O12 2.430(9) Sb3 - O9 2.386(10) O8-Sb2- O9 85.5(6) O7d-Sb3- O l l 84.6(6) O8-Sb2- O10 89.7(4) OTd-Sb3- O12 80.7(6) O8-Sb2- O12 155.7(5) O7d-Sb3- O9 150.7(6) O9-Sb2- O10 94.4(6) Ol l -Sb3- O12 87.6(6) O9-Sb2- O12 70.4(6) O11-Sb3- O9 88.9(6) O10-Sb2- O12 88.6(5) O12-Sb3- O9 70.5(5) Sb4 - O12 1.965(8) Sb4 - O10 2.503(9) O5e-Sb4- O12 93.0(6) 152.2 O1 lf-Sb4- O10 68.7(6) 86.8(6)

Sb2/O8 - 0 9 - O10 1.14(5) Sb4/O5e - O11f - O12 1.15(6)

O5e-Sb4- O10 O12-Sb4- O10

a : x - l , y, z; b : x - l , y - l , z; c : x , y - l , z; d : x , y, z + l ; e : x + l , y, z + l ; d : x + l , y, z.

876 A. CASTRO et al. Vol. 29, No. 8

S T R U C T U R E D E S C R I P T I O N AND DISCUSSION

A projection of the structure onto the (010) plane is given in the figure 4. Tungsten atoms are octahedrally surrounded by oxygen atoms (CN6). The W-O interatomic distances and O-W-O bond angles are in good agreement with those found in WO3 (29,30) and in a similar compound Bi2WO6 (20). The WO6 octahedra share comers in both [100] and [010] direction forming a [WO4]n layer.

The antimony atoms in the valence state 1/1 exhibit the typical one sided coordination of the atoms having a stereoactivelone pair (3). They are slrongly bonded to three oxygen atoms (Sb-O between 1,931 and 2,158A) a fourth oxygen being at a longer distance (2,379 to 2,503A) according to a typical coordination 3+1.

15 35 15

z /

Figure 4 : Ball and spoke drawing of the projection of Sb2WO6 onto the (OlO) plane . The heights of the atoms in the [010] direction are xl00 .

The oxygen atoms together with the lone pair in the equatorial plane form a distorted trigonal bipyramid. A fast row of antimony atoms, Sbl and Sb2, parallel to [I00] are linked in

that direction by oxygens 09 and O10 and stuck onto the [WO4]n layer by the strong bonds Sbl- 06 and Sb2-O8. Similar situations occur with Sb3 and Sb4, the puckered row ...O11-Sb3-O12-

Vol. 29, No. 8 AURIVILLIUS PHASES 877

Sb4-Oll.. . being linked to the [WO4]n by the oxygens O5 and 07 of the tungsten oxygen octahedra.

At last this atomic architecture can be described as a thick layer [Sb2WO6]n constituted by the layer [WO4]n of the WO6 octahedra sandwiched by the two layers (Sb202)n (built up by antimony atoms in their strong threefold coordination). These layers parallel to the (1301) plane are held together along the [001] direction by the network of the weak Sb-O bonds (Sbl-O11, Sb2- O12, Sb3-O9, Sb4-O10)(Table 111).

The Sb2WO6 structure is very similar to Sb2VO5 (3). In this last compound, containing vanadium (IV), layers of square pyramids VO5 substitute to the ones formed by WO6 octahedra. Its filiation with the orthorhombic form of Sb203 has been extensively described (3). Coordination of Sb atoms with their lone pair (see Sb-O bond lengths in table III) is very similar in Sb203 (31), Sb2VO5, Sb2WO6 and (Bi-O) in Bi2WO6 (20).

Slightly idealized drawings of these structures are given in the figure 5. In Sb203, the double layer of SbO3 trigonal pyramids sharing comers shows an interspace of 1.6,/~ between the bordering oxygen planes; in this structure the weak Sb-O bonds which assumes the stability of the network have a length of 2.62A. In Sb2VO5, these long Sb-O bonds are still around 2.62A and the interspace between [VOSb204]n layers remains close to 1.7,~. In this new compound Sb2WO6 the separation between the layers [WO2Sb204]n comes down to 1.2/~. This fact is due to the marked coordination CN = 3+1 of Sb(11I) in this structure correlated with the shortening of the long Sb-O bond (average value around 2.425A against 2.62./k in the previous ones).

One assists then from one structure to another to a progressive collapsing of the two oxygen planes, this process being achieved in the phase Bi2WO6 (20). The layer of oxygen form a pseudo square net. The two independant Bi atoms have then four strong bonds (Bil-O = 2.172, 2.255, 2.322, 2.517/I, and Bi2-O= 2.180, 2.222, 2.345, 2.501,~) tothis oxygen plane and two extra bond Bil-O ='2.494 and 2.537/~ and Bi2-O = 2.440 and 2.585A with the apices of WO6 octahedra. The lone pair completes the polyhedron around each Bi making with the oxygens a distorted trigonal prism. By the way the characteristic [Bi202]2n-n layei~ depicted for the first time by Aurivillius (11) analogous to the [PbO]n layer in the red form of lead 11 oxide, is t-tartly attached in this phase to the [WO4] layer.

Sb2WO6 or WO2Sb204 appears as an interesting model to imagine the possible ways for the [Bi202]2n-n layers distortion. A better refinement of the structure of the Sb2WO6 twinned crystals using a new procedure will be attempted as well as structural studies at high temperature. The ferroelastic properties of Sb2WO6 presently under investigation, will also be correlated with its atomic structure.

CO NC LUS ION

The aim of this paper was to prepare a double oxide of antimony and tungsten with a formula analogous to Bi2WO6 the bismuth's homologue. The synthesis of the original triclinic Sb2WO6 compound has been succesfully performed by solid state reaction. Its structure determined by X-ray single crystal techniques exhibits [Sb202]n layers sandwiched by ['WO4]n layers of WO6 octahedra sharing corners. Comparison with the AuriviUius's structure type Bi2NbO5F has permitted to emphasize the role of the lone pair E brought by Sb III. Its place among phases like Bi2WO6 or Sb2VO5 allows to depict the distortion affecting layers of [Sb(or Bi)202]n type. This point is rather important in the perspective to develop chemistry around the substitution of lone pair element like Sb III, Te IV, Pb II .... into the Aurivillius's phases and to understand structure/properties evolution.

Acknowledgements : Authors wish to thank the CSIC of Spain and the CN-RS of France for the aid under their reciprocal International Program of Scientific Cooperation (PICS). A.C. and P.M. thank too for the financial support of the DGICYT of Spain (Project PB92 - 1088).

878 A. CASTRO et al. Vol. 29, No. 8

Sb203

VOSb204

y

WO2Bi204

WO2Sb204

Figure 5 : Structural relationships between the Sb203, VOSb204, WO2Bi204 and WO2Sb204 phases.

Vol. 29, No. 8 AURIVILLIUS PHASES 879

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