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 reduc- tion 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 pre- paring 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 tempe- rature 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 tempe- rature 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, t~ 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 b 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 4 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.