Topotaxy in the Oxidation of Valentinite, Sb

Pram~n.a, Voi. 3, No. 5, 1974, pp. 277-285. © Printed in India.

Topotaxy in the oxidation of vaientinite, Sb~Os, to eervantlte, Sb~04

P S GOPALAKR1SHNAN* and H MANOHAR Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012

MS received 22 August 1974

Abstcact. The oxidation of orthorhombic SbzOs, valentinite, to orthorhombic Sb~O4, cervantite, has been shown by single crystal x-ray diffraction techniques to be a topotactic reaction. The orientation relationships between the two lattices have been determined by making use of a hybrid crystal. It has been found that the individual axes in the two oxides are parallel. The two crystal structures have been compared in the appropriate orientation and their close similarity has been established. "lhe shifts of the individual atoms in valentinite during the process of oxidation have been calculated to be not more than 0.6A. It has been established that the reduction of cervantite to valentinite also takes place topotactically.

1. Introduction

During studies on the polymorphic transformation in antimony trioxide, Sb2Os, a chance observation was made by the authors that a single crystal of valentinite, Sb2Os, could be oxidised to a single crystal of cervantite, Sb20~. This appeared to be an exceedingly interesting result because, firstly, it gave a method of preparing single crystals of cervantite, which had not been obtained earlier, and secondly, it suggested some structural similarity between the two oxides, on which no studies appeared to have been made. This type of a reaction in which a single crystal of the starting material is converted into a single crystal of the product, or a polycrystalline aggregate with definite preferred orientation, and there exist certain definite three-dimensional orientation relationships between the original and transformed lattices is known as a topotactic reaction (Bernal 1960, Brindley 1963, Lotgering 1959, Dent Glasser etal 1962, Bernal and Mackay 1965). Antimony trioxide, Sb2Oa, exists in two crystalline modifications, the low temperature cubic form called senarmontite and the high temperature orthorhombic form called valentinite, with a transition temperature of about 570°C (Roberts and Fenwick 1928). Valentinite can however exist at room temperature as a metastable phase. The common modification of antimony tetroxide is the ortho-

* Present address: Materials Science Division, National Aeronautical Laboratory, Baagalore 560017.

277 P--I 278 P S Gopalakrishnan and H Manohar

rhombic cervantite, ~-Sb204 (Dihlstrom 1938). However, recently, a high temperature monoclinic form, B-SbzO4, has been identified (Rogers and Skapski 1964). In this paper we establish that the oxidation of valentinite to cervantinite, as also the reverse reduction process, is a topotactic reaction.

2. Crystal structures of valenOnite and cervantite

The crystal structure of valentinite (Buerger and Hendricks 1937) consists of infinitely long chains of Sb406 groups in which each trivalent antimony atom is bonded to three oxygen atoms and each oxygen atom to two antimony atoms. The chains extend in the direction of the c-axis, alternate chains in the unit cell running antiparallel. The Sb-O distances are all equal to 2.0 A. There are also secondary weak bonds of 2" 51 A between an antimony atom of one chain and oxygen of a neighbouring chain. These bonds perhaps hold the chains together in the crystal. Crystal data for valentinite are given in table 1 and projectibns of the structures down [100] and [001] are shown in figures 1 a and 2 a respectively. The structure of cervantite had not been determined earlier for lack of single crystals (Rogers and Skapski 1964). However, the topotactic nature of the oxidation of valentinite to cervantite leads to a method of preparing single crystals of cervantite. Its structure has been determined by single crystal x-ray diffraction techniques (Gopalakrishnan and Manohar 1974a). The structure is a three- dimensional network in which each pentavalent antimony atom is bonded to six oxygen atoms at the corners of a distorted octahedron. These Sb (V)-O octahedra are linked together by sharing edges to form corrugated sheets running parallel to (010). The oxygen atoms of adjacent sheets are bridged through trivalent antimony atoms so that the latter have a one-sided four-fold coordination of oxygen atoms. The Sb-O distances range between 1.93 and 2.26 A. Crystal data for cervantite are given in table 1 and projections of the structure down [100] and [001] are shown in figures 1 b and 2 b respectively.

3. Experimental 3.1. Preparation of valentinite single crystals Antimony trioxide of semiconductor grade purity (supplied by Koch-Light Labora- tories Limited) was used for these studies. X-ray powder patterns showed that the sample consisted of pure valentinite. Antimony trioxide reacts with most materials at high temperatures. Therefore the powder was enclosed in a capsule made of platinum foil. This capsule was kept inside a snugly fitting silica tube which was evacuated to a pressure of 1 mm of mercury and sealed. The material was then heated at 640 ° C, a temperature close to its melting point, in a tubular furnace for about 10 hr and cooled to room temperature. This heat treatment yielded a cluster of colourless, long platy crystals tabular in habit, of approximate dimensions 4 × 0' 5 × 0.1 mm. Debyc-Scherrer and single crystal rotation and Weissenberg photographs confirmed that the crystals were of valentinite. From the photographs it could also be deduced that the c axis of valentinite coincides with the needle axis, the a axis perpendicular to the plane of the plate and the b axis in the plane of the plate. ~t 3r 4 ~' .

,'3 I 5

Q ~~10

? b

(a) (o)

3 Jll I I

~/ I\~ J . II:~,2 ~ U' L~/I \L,, p,, ,o'Q,

I ~ /\ " ~/-" I o z~ I ~ I I , I • ANTIMONY ( v ) ~ ANTIMONY(,j) O OXYGEN • ANTIM(:~IY (v~ ~ ANTIMONY(m) O OXYGEN I (b) (b) I'O Figure 1. [100] projections of valcntinitc (a) and ccrvantitc (b). Vigure 2. [001] projections of valcntinitc (a) and cervantitc (b). 280 P S Gopalakrishnan and H Manohar

3.2. Oxidation of single co,stals of valentinite For the oxidation of valentinite single crystals, a temperature of 490 ° C was found to be most suitable since the oxidation proceeds at a conveniently slow rate at this temperature. A large number of single crystals of valentinite were heated for different lengths of time in platinum capsules kept exposed to air in a tubular furnace maintained at this temperature. The capsules were then taken out of the furnace and cooled to room temperature. It was interesting to note that the crystals retained their external morphology after the heat treatment suggesting the possiblity of their remaining as single crystals even at that stage. Indeed this surmise was confirmed when the product crystals were examined by single crystal x-ray diffraction techniques. Photographs of crystals heated for less than 8 hours showed some additional sharp spots apart from those due to valentinite. Those spots were later identified to be due to cervantite. This meant that there had been no breakup of the lattice during the oxidation and what had been obtained was a 'hybrid' crystal in which both phases coexist. X-ray rotation and zero layer Weissenberg photographs of a crystal of valentinite rotated about the c (needle) axis before and after heating for 4 hr at 490 ° C are shown in figures 3 and 4.

3.3. Orientation relationships between the axes of valentinite and cervantite From the photographs of crystals heated for different lengths of time, it was evident that the amount of conversion depended on the time of heating. In fact, crystals of valentinite heated for more than 8 hr at 49ff~C were completely oxidised to cervantite, identification of which was made using Debye-Scherrer and single crystal X-ray patterns. From an analysis of the rotation and zero layer Weissenberg photographs of the hybrid crystal, the following orientation relationships were deduced between the axes of valentinite and cervantite.

[100],. II [100],. [010], ]l [010]¢ and [001], II [001k The subscripts v and c represent valentinite and cervantite respectively. The parallelism holds good within the limits of experimental error of ea ± 30 min. The close values of corresponding cell dimensions can be observed in table 1, which also gives the changes in the parameters during the oxidation.

3.4. Reduction of single crystals of cervantite It has been reported in the literature (Durrant and Durrant 1970) that Sb2Oi on heating in air above 900°C decomposes to Sb203. However, this method did not appear to be feasible as the temperature required is far above the melting point of Sb2Oz. Therefore, attempts were made to decompose Sb204 by heating at lower temperatures, of the order of 5000 C, under continuous evacuation. The method, though successful, yielded only polycrystalline Sb203 in small quantity. However it was interesting to note from x-ray diffraction patterns that the product was valentinite and not senarmontite, even though the latter is the stable form under these temperature conditions. P s q~palakrishnan attdlt Manahar Pram~ina, Vol. 3, No. 5, 1974, pp. 277-285

r a

Figure 3. X-ray rotation photograph~ (Cu-K~ ra:liation) of a crystal of valvntinit¢ rotated about the c axis before (above) and after heating for 4 hr at 490 ° C (below).

(facing page 280) P S Gopatakrishnan attd H Manohar Pramina, V.oL 3) No~ 5, 1974, pp 277-285

Figare 4. Zero layer Woissenberg photographs (Cu-Ka radiation) of a crystal of valerttinito rotated about the c axis before (above) and after heating for 4 hr at 4900 C (below).

(facing page 281) Topotaxy in the oxidation of valentinite to cervantite 281 Table 1. Crystallographic data for valentinite and cervantite and the changes in cell parameters during the oxidation of valentinite to cervantite.

Parameter Sb, O~ a-Sb:O~* Relative change valentinite cervantite

a 4. 914 A 4" 810 A Contractionof 2.1~ b 12.468 A 11.76 A Contractionof 5. 770 c 5.421 A 5.436 A Expansionof 0.28~.o/ V 332.1 Aa 307.5 ./k~ Contractionof 7.47o Space group Pccn Pc21n * The crystallographic axes given by the authors (Gopalakrishnan and Manohar 1974 a) have been interchanged to facilitate comparison with valentinite.

Single crystal conversion of cervantite to valentinite was achieved by decompo- sition under an atmosphere of hydrogen at temperatures around 40if' C. For example, at 430 c~ C under one atmospheric pressure of hydrogen, complete reduction to valentinite took place in about 4 hr. ' Hybrid ' crystals could also be obtained by partial reduction. X-ray rotation and Weissenberg photographs of these were identical with those obtained during the oxidation studies.

4. Relationship between the structures of valentinite and cervantite The relationship between the two structures can be deduced by comparing the individual projections down [100] and [001] respectively in the appropriate orien- tations as shown in figures 1 and 2. It is then possible to identify the Sb406 groups of valentinite in the cervantite structure. One such group, designated Sb(l) O(2) Sb(3) O(4) Sb(5) O(6) Sb(7) O(8) O(9) O (10) in valentinite (figures l a and 2 a), is clearly seen in both projections (figures 1 b and 2 b) of cervantite. It is thus evident that the relative position of atoms in these groups are almost identical in both structures. The only two additional oxygen atoms in cervantite are O (ll) and O (12). As the [001] projection clearly shows, the valentinite structure has empty channels between the (Sb406)~ chains parallel to the c axis and these oxygen atoms take up positions along the channels close to the antimony atoms. Each oxygen atom bridges two antimony atoms of neighbouring chains along [100] direction. For example, O (11) bonds with Sb (1) and Sb (3'") and O (12) with Sb (3'") and Sb (1") of the neighbouring cell. As a result the Sb (l)-Sb (3'") distance, which is 3.96 A in valentinite, reduces to 3-63 A, in cervantite. Pentavalent antimony is normally found to exhibit an octahedral coordination of oxygen (Wells 1962). In the present case, the secondary weak bond of 2"51 A in valentinite from the antimony atom of one chain to the oxygen of a neigfibouring antiparaUel chain along [010] direction gets reduced to 2.02 A. Thus as a result of the additional bonds Sb (1)-O (2') and Sb (3'")-0 (9'), Sb (1) and Sb (3) acquire a distorted octahedral coordination. The contraction in the a and b dimensions, as a result of the oxidation, is thus easily understood on the basis of a closing up of the chains along these directions. No change in the c dimension, which is parallel to the length of the chains, is to be expected and this is indeed observed. 282 P S Gopalakrishnan and H Manohar

The conversion of the chain structure of valentinite to the three-dimensional structure of cervantite also follows from the above arguments. Tivalent antimony atoms are bonded to four oxygen atoms in cervantite as compared to three in valentinite. It was seen above that Sb (V) atoms (1) and (Y") belong- ing to different chains in valentinite come closer to each other in cervantite. In other words, they move slightly in opposite directions along [100]. Correspondingly the Sb (l)-Sb (3) distance in the same chain increases from 3.38 A in valentinite to 3.63/k in cervantite. As a result, oxygen atoms O (8) and O (4) attached to Sb (1) and Sb (3) respectively also show small movements away from each other along [100]. This is clearly seen in figures 2 a and 2 b. Thus O (8) approaches Sb (5) and O (4), Sb (7"). The contraction in the b axis dimension may also be responsible for reducing these distances along [010]. These two factors facilitate the formation of a fourth bond by trivalent antimony. The Sb (5)-0 (8) and Sb (7")-0 (4) distances are each reduced from 2.63 A in valentinite to 2"26 A in cervantite. Exactly similar changes take place in the other half of the unit cell because of the additional oxygen atoms O (11') and O (12'). The following additional observations can be made with regard to the topotactic relationship between valentinite and cervantite: (1) The addition of the extra oxygen atoms O (11) and O (12), O (I 1') and O (12') to the valentinite structure during the oxidation reduces the symmetry of the lattice, the space group changing from centrosymmetric P2~/c 2Jc 2In to non-centric Pc2~n. (2) It may be noted that the row of antimony atoms Sb (3), Sb (1) and Sb (3'") are oxidised to the pentavalent state. It is of interest to speculate whether the other set of antimony atoms Sb (7), Sb (5) and Sb (7'"), etc., could undergo the oxidation instead. A comparison of the structures in two projections, as was done earlier, shows that this requires far greater distortion of the Sb2Oa structure. Therefore this alter- native appears improbable. (3) The fact that the reduction process is also a topotactic reaction leads to the conclusion that the oxygen atoms in positions O (11), O (12), etc., which entered the lattice during the oxidation are removed during the reduction. This result has an important bearing on the mechanism of the oxidation (Gopalakrishnan and Manohar 1974 b).

5. Atomic shifts involved during the oxidation Having established the close structural relationship between valentinite and cervantite, it was considered worthwhile to calculate the shifts of the individual atoms of Sb~O~ during the course of the reaction. Since the two space groups are different, it was first necessary to refer the atoms in the two structures to a common origin. The procedure adopted was to find the location of the origin of cervantite in the valentinite structure such that the atomic shifts are a minimum. It should be mentioned here that in view of the change in cell parameters during the oxidation, only the shifts of atoms in one unit cell of valentinite have been considered. Suppose xl, Yx, zl ; x~, Y2, z~; ... ; xn, Yn, zn are the fractional coordinates of the atoms in valentinite with reference to its origin 0 and xl', y~', zx' ; x/, Y2', z2' • • • ; x/, y/, z/ are the coordinates of corresponding atoms in the cervantite structure with respect to its origin 0'. When the best fit between the coordinates of the atoms in the two structures is obtained, suppose O' has coordinates p, q, r with respect to O. According to the principle of least squares, when Topotaxy in the oxidation of valentinite to cervantite 283

/1\ . \ \" .x. , I ~1 ,/

i / ~.9

,b \ ~/" ,7 (a)

q I_~'. ~_ .AK.jo....-~)~z3 "c, - ""

/ + *-° "%...... J t-~_., b " o 2~ I_ , I (b)

0 ATOMSIN Sb203 • ATOMSIN s b~O4 Figure 5. Supcrposad projections of valentinita (Sb~O~) and cervantite (Sb~O~) along [100l (above) and [00]] (below). the.best fit is obtained the sum of the squares of the shifts, say S, will be a minimum. Thus

S -- ,~ {[x, -- (x;' + p)]~ + [y, - (y~' + q)]~ + [z, -- (zz' Jr r)] ~} l=1 is a minimum. Therefore 8S O, 8S 3S 3x = ~y=O and~zz =0 On simplification one obtains

fl t II p = (l/n) l (x, -- xt ), q = (I/n) I(Y, -- y,') |=I |~1 and

r = (l/n) I (z, - z,') [--1 284 P S Gopalakrishnan and H Manohar

Table 2. Magnitudes and directions of atomic shifts during the oxidation of valentinite to cervantite

Direction cosines Magnitude Atom of atomic X Y Z shift in A

Sb 1 0.5662 0'2668 -0.7800 0.39 O 2 -0.4898 0.6755 -0.5522 0.36 Sb 3 -0.4559 0.2817 -0.8441 0.37 O 4 -0.6385 0.6620 0"3921 0.60 Sb 5 0.0182 -0'0126 0.9750 0.27 O 6 -0.4937 0.6597 0.5667 0.27 Sb 7 0.0069 -0'1242 0.9920 0.28 O 8 0.6829 0.6233 0.3809 0.64 O 9 0.5809 0"6233 -0.5264 0-40 O 10 -0.3341 0.7299 0.5965 0.25 Sb 13 0.3898 --0.5854 0.7113 0.43 O 14 --0.7061 --0.3313 0.6261 0.33 Sb 15 --0.4798 --0.5495 0.6840 0-46 O 16 --0.8792 0.0860 --0.4691 0.49 Sb 17 --0.0041 --0.8281 --0.5604 0-47 O 18 0.3341 -0.7144 -0.6149 0-25 Sb 19 -0.0105 -0.8162 -0.5681 0.48 O 20 0.8442 0.0938 -0.5283 0.45 O 21 --0.4935 -0.6454 -0.5831 0.27 O 22 0.6220 -0.3770 0.6862 0.29

Table 3. Values of corresponding interatomic distances and bond angles in the two structures of valentinite and cervantite

Interatomic Valentinite Cervantite Bond angles Valentinite Cervantite distances

Sb (1)- 0 (2) 2.00A 2.00A Sb (1)- O (2)-Sb (3) 116 ° 130.9 ° O (2)-Sb (3) 2.00 A 2.00 A O (2)-Sb (3)- O (4) 99 ° 96,4 ° Sb (3)- 0 (4) 2.00A 1.99A Sb (3)- O (4)-Sb (5) 132 ° 136.3 ° O(4)-Sb (5) 2.00A 2.01A O(4)-Sb (5)- O (6) 99 ° 87.6 ° Sb (5)- 0 (6) 2.00 A 2.04A Sb (5)- O (6)-Sb (7) 116 ° 106.9 °

O(6)-Sb (7) 2-00A 2.21A O(6)-Sb (7)- O (8) 81 ° 74.3 ° Sb (7)- 0 (8) 2.ooA 2.01A .Sb (7)- O (8)-Sb (1) 132° 136.3 ~ O(8)-Sb (1) 2.00A 1.99A O(8)-Sb (1)- O (2) 81 ° 92.8 °

O (8)-Sb (5) 2.63 A 2.26 A Sb (3)- 0(9') 2.51A 2.03A Topotaxy in the oxidation of valentinite to cervantite 285

In the present case, the values of p, q, r obtained were d- 0.052 A, - 1.276 A and - 1.365 A respectively. The origin of cervantite was shifted to these coordinates with respect to the origin of valentinite and superposed projections down [100] and [001] were made. They are shown in figure 5 a and b respectively. The coordinates of the atoms in cervantite were then read off using the common origin and mOvements of corresponding atoms, magnitudes as well as direction cosines, were calculated. The results are given in table 2. It can be seen that the magnitudes of the atomic shifts involved are small, as expected. They lie between 0.24 and 0.64 ~. Values of corresponding interatomic distances and bond angles in the structures of valentinite and cervantite are compared in table 3. Here again the close agreement in values is evident.

Acknowledgement We thank Mr R Padmanabhan for useful discussions. One of us (PSG) thanks CSIR, New Delhi, for financial support.

Note added in proof : Since submission of this paper, the authors have seen a paper by C. Svensson, [Acta Crystallogr. (1974) B 30, 458] where the structure of valentinite has been accurately refined using diffractometer data. His results are in good agreement with the earlier structure determination of Buerger and Hendricks. The argu. ments given in our paper are, therefore, in no way altered.

References

Bernal J D 1960 Schw. Archiv. Jahrb. 26 69 Bernal J D and Mackay A L 1965 Tscher. Min. Petr. Mitt. 10 331 Brindley G W 1963 in Progress in Ceramic Science, 3 1 (Oxford: Pergamon Press) Buerger M J and Hendricks S B 1937 Z. Kristallogr. 98 1 Dent Glasser L S, Glasser F P and Taylor H F W 1962 Quart Rev. 16 343 Dihlstrom K 1938 Z. Anorg. Chem. 239 57 Durrant P J and Durrant B 1970 Introduction to Advanced Inorganic Chemistry (London: Longmans) p 773 Gopalakrishnan P S and Manohar H 1974 a Cryst. Str. Commun. (in press) Gopalakrishnan P S and Manohar H 1974 b Under publication Lotgering F K 1959 J. Inorg. Nucl. Chem. 9 113 Roberts E J and Fenwick F 1928 J. Amer. Chem. Soc. 50 2125 Rogers D and Skapski A C 1964 Prec. Chem. Soc. p 400 Wells A F 1962 Structural inorganic chemistry, 3rd edn. (Oxford: Clarendon Press) p 678

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