23 P h a s e T r a n s i t i o n s in S o d i u m Tungsten B r o n z e s A. S. RIBNICK, B. POST, and E. BANKS Departments of Chemistry and Physics, Polytechnic Institute of Brooklyn, Brooklyn, Ν. Y.
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The reported structures of tungsten bronzes in clude cubic, pseudocubic, tetragonal, orthorhombic, and monoclinic distortions of the perovskitetype structure; well-characterized tetragonal and hexagonal structures, as found in the series K WO ; and some unit cells which are reported for isolated phases. This work surveys some of the phases found for various alkali tungsten bronzes. In the sodium bronzes there is a pro gression of distorted perovskites from x = 0 to 0.15, in the same order as the thermal tran x sitions of WO (monoclinic --> orthorhombic --> tetragonal). High-temperature x-ray diffractometry shows these phases to go through the same thermal transitions as WO , except that twox
3
3
3
phase regions intervene. Between x = 0.15 and 0.28, two tetragonal phases coexist, the second having a range of homogeneity from x = 0.28 to 0.38. This is followed by a cubic (or pseudocubic) range from x = 0.43 to 0.95.
Tungsten bronzes have been the subject of many investigations in recent years, because of their unusual electrical properties (5, 7, 8, 10, 29), the occurrence of a number of crystallographic modifications (13, 17), and the existence of broad homogeneity ranges (4, 8, 28), which make these phases ideal subjects for the study of metallic behavior under conditions of controlled electron concen tration. Other oxide "bronzes" are known—for example, the vanadium bronzes (19, 30) and niobium bronzes (22). Tungsten bronzes of the alkali metals (3, 8, 12, 16), copper (6), lead (2, 25), silver (26"), and thallium (25) have been prepared, and electrical, magnetic, and structural data reported, but by far the most widely investigated members of this group are the alkali bronzes, particularly those of sodium. The sodium bronzes are the only series where there appears to be a continuous sequence of phases, all having compositions which can be described by a formula of the general type, M WO , so that they may be considered as solid solutions of the metal in some form of W O , whether it is ordinarily a stable phase or not. A l l of the known bronzes can be described in this fashion, although they usually have x
s
s
246 In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.
23. RIBNICK ET AL.
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Sodium Tungsten Bronzes
narrower regions of existence than is the case w i t h the sodium bronzes. A l t h o u g h a number of different structures have been reported, their ranges of homogeneity have not been completely explored, even i n the case of the sodium bronzes. This paper presents the results of a systematic survey of the structure-composition-temperature relationships for the sodium bronzes. T h e present work was an outgrowth of the studies reported from this laboratory on the thermal behavior of tungsten(VI) oxide (20, 23) and the high-sodium bronzes (24). I n the latter work, transitions i n the coefficient of thermal expansion of " c u b i c " bronzes were observed between 150° and 250° C . It appeared that, b y following the phase transitions i n the complete solid solution series, beginning at W O , it might be possible to understand the behavior of the "cubic" bronzes. As it developed, w e were unable to correlate these changes with the behavior of the phases of lower symmetry. However, we have been able to delineate the ranges of composition and temperature over w h i c h various sodium tungsten bronzes exist, at least under the experimental conditions employed.
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s
Experimental In all of this work, the samples were prepared b y "chemical reduction," under conditions where no l i q u i d phase was present and the reaction mixture was completely transformed into a bronze of one or two phases. According to Ingold and de Vries (11), such preparations should lie on the pseudobinary join, W 0 N a W 0 , whereas bronzes prepared b y methods such as electrolytic reduction m a y contain oxygen i n excess of an oxygen-tungsten ratio of 3.0. C h e m i c a l analyses for sodium and tungsten at several compositions indicated that our samples have an O / W ratio of 3.0 ± 0.15. Precision lattice constants of those samples w h i c h fell i n the cubic range (x > 0.40), lay on the same lattice constant-nominal composition plot as reported b y B r o w n and Banks (4). 3
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Powdered samples were prepared i n sealed, evacuated silica tubes (4) at temperatures where no l i q u i d phase was observed. Reaction temperatures were 500°, 650°, or 750° C , at heating periods of several days to a week. T h e samples of higher sodium content required higher temperatures to effect complete reaction. Sodium was determined b y the method of Spitzin a n d Kaschtanov (27). The bronzes are decomposed i n a stream of oxygen and hydrogen chloride at 600° C . T h e residue of sodium chloride is titrated w i t h standard silver nitrate solution, giving the sodium content of the sample. Tungsten is determined b y precipitation of tungstic acid and ignition to tungsten ( V I ) oxide i n the residue from the above treatment. Before analysis a l l of the samples analyzed were leached successively w i t h concentrated ammonia and hydrochloric and hydrofluoric acids. This treatment removes unreacted N a W 0 , W 0 , and tungsten metal, as w e l l as silica, from the reaction tube. Since a l l analyses agreed w i t h the " n o m i n a l " compositions, this was taken as evidence of completeness of the reactions. A n other evidence was the fact that the x-ray patterns of a l l samples showed the sharp lines of one phase when the composition fell i n a homogeneous range, and of two phases otherwise. W h e n several samples fell into the same two-phase region, the positions of the peaks remained constant, but the relative intensities of the two sets of peaks varied continuously over the range. 2
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X - r a y diffractometer patterns were taken w i t h a Norelco diffractometer, using nickel-filtered copper radiation. L o w temperature patterns were taken b y flowing air or nitrogen, cooled b y passage through copper coils immersed i n l i q u i d nitrogen or dry ice-acetone, over the sample. This technique has been described b y Post, In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.
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ADVANCES IN CHEMISTRY SERIES
Schwartz, and Fankuchen (21). Above room temperature, the diffractometer at tachment described b y Perri, Banks, and Post (20) was employed, using a flowing atmosphere of dried helium to protect the sample from oxidation. W i t h this i n strument, it was possible to observe rapid reversible phase changes at temperatures u p to about 700° C . i n samples of very low sodium content (x < 0.10). Above this composition, the phase changes occurred at temperatures where sodium vapor is lost rapidly, or transitions are sluggish, or both. F o r the remainder of the com position range, the transitions were followed b y quenching sealed samples from the furnace into ice water. The x-ray patterns of quenched samples were then meas ured at room temperature.
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Results In a l l samples studied, the x-ray patterns were sharp and clear; i t was pos sible to identify a l l phases present; minor phases could be detected i n quantities below 5 % (better than 1%, i n the case of α-tungsten). F i v e different phases appear, all of w h i c h have been described previously, either for W O itself or for sodium bronzes. T h e "triclinic" (18), monoclinic, and orthorhombic (1, 20, 23) structures have been described only for W O . T h e phase w e designate as "tetrag onal I " was first reported b y Magnéli for N a W O , while that designated "tetragonal I I " was first described b y Hàgg and Magnéli ( 9 ) . Its structure was reported b y Magnéli (15). Figure 1 shows the relations among the cubic, monoclinic, orthorhombic, and tetragonal I unit cells. A l l are based upon the perovskite structure, w i t h distortion progressively increasing as the sodium content decreases. T h e projection on (001) of the tetragonal I I structure is shown i n F i g u r e 2, w h i c h is taken from the work of Magnéli. W e have reversed the designations I and I I from those used b y Magnéli ( 1 6 ) . H i s designations signified the historical order of discovery; we prefer to label these phases i n order of increasing sodium content. s
s
0 1 0
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c (TETRAGONAL)
Figure 1.
Unit cells of cubic, orthorhombic, (I) phases of Na WO œ
and
tetragonal
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Figure 3 shows the results of the x-ray patterns taken at various temperatures from samples i n the composition range 0 ^ χ
