Jean-Paul Randin Hydro-Quebec Institute of Research Varennes, P. Q., Canado
I I
Sodium Tungsten Bronzes Oxides with metallic character
The sodium tungsten bronzes are nonstoichiometric compounds of the general formula Na,W03, where 0 < x < 1. In the approximate composition range 0.49 < x < 0.93 Na,W03 has a cubic structure intermediate between the hypothetical Na,W03 and undistorted WOa structures and with a fraction (1 - x) of the sodium atoms missing from the cube comers of the Na,W03 unit cell (see the figure). The composition x = 1 is never reached, but if it were the structure would be a perovskite. The reason for this failure to achieve a full complement of atoms is not a t all clear. The perovskite structure (see the figure) is found in mixed oxides ABOa, where one cation is much larger than the other. Oxygen ions and large cations (Na+ in NaW03) form a cubic close-packed array with the small cations occupying octahedral interstices. In the defective phase, Na,W03, there are (1 x) WV' atoms and (1 - X) unoccupied Na sites. As the sodium content of the bronze decreases, the structure goes through various lower symmetry modifications (tetragonal, orthorhombic, and monoclinic), all based on the lattice of WOa. At all sodium concentrations the tungsten atoms are equivalent. Ionization of the sodium atoms produces electrons which are evenly distributed among all the tungsten atoms. As evidence of this, sodium tungsten bronzes in which x > 0.25 behave as metallic conductors between 4 and 800°K. with Hall and Seebeck effects suegestine that each sodium atom contributes one free electron to the conduction band. Further evidence that the sodium atoms lose their valence electrons completely into a conduction band that extends only across the tungsten atoms comes from nuclear magnetic resonance experiments, which show that the 23Na resonance. has an extremely small Knieht shift. The nrooosed conduction band arises nredom&antly from the overlapping of the W, 5d (tl,) a n d 0.. ZD.,., orbitals (1). A striking feature of i\ia,WOa is its color which changes from golden yellow a t x = 0.9, through orange (x = 0.8), red (x = OX), and purple (x = 0.5), to deep blue a t x = 0.4. The intense color of the bronzes is a consequence of the resonance interaction of the two different oxidation states of tungsten in the molecule (2). For the same reason, the presence of Fe(I1) and Fe(1II) in minerals leads to deep colors, e.g., Prussian blue F e P [FelI (CN)&. The change of color of bronze with the x-value could result from interference effects (3). The sodium tungsten bronzes are very resistant towards acids but are readily oxidized to tungstate in the presence of alkali and oxygen (or oxidizing agents), ~h~ inertness of tungsten bronzes has been attributed to either the hieh energv of activation for diffusion of the sodium the oxide matrix (51.8 kcal/mole in Na0.7sWOs metal ( 4 ) )or to the formation of a protective layer of W 0 3 (5). For a more complete description of the properties of bronzes the reader is referred to various recent reviews on the subject (6, 7). The preparation of tungsten bronzes has been the subject of several studies (8); detailed instructions may be obtained from the author.
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Perovskite structure AB03
@A
OB
0
0
Unit c d for Nawo3 (perovskite structure).
ApplicationsofTungsten Catalvsis The use of tungsten bronzes as heterogeneous catalysts in certain reactions has been proposed (9). It has been reported that tungsten bronzes containing -400 ppm P t show electrocatalytic activity similar to platinum towards the cathodic reduction of oxygen (10). This behavior makes these. compounds attractive electrode material in . .. Or
Electroanalvtical Chemistrv .. . The tungsten bronzes have recently been used as indicating electrodes in several widely differing electrochemical systems. It has been shown (11) that electrodes made of the bronzes may be used to determine pH, p[Metal] for certain reducible species such as Ag (I) and Hg (II), and to follow potential changes involved in some oxidationreduction systems. Even more significant, with respect to possible applications in pollution, is the fact that they appear to function as oxygen sensitive electrodes (12). Solid-state Measurements The reversibilit~ to sodium ions of sodium tungsten bronzes with the tetragonal I structure has been used measure the diffusion of sodium ion in beta-alumina (13). Acknowledgment The author is grateful to Dr. A. K. ~ i j hwho , made helpful Literature Cited 11) F c n m e h . J. B.. Bull. Sor. Chim h.. lZW (19651; Sienko. M. J., in"N6n-Stoi"hlornetrieCompounds," ',Ad". in Chemistry Series:' NO 39. A~FI. Chem. SW., washington.1963. D . 224.
12) Robin, M. 8.. and Day, P., in "Advance I o o g . Chcm. and Radioehem;' 10, (Editom: Emclcua, H. J.. and Shame A. G.1 Aoedmd. Prras. New York, L96?, p. 247. (3) Taylor, G.H.,J. SolidSt~feChpm.,I..%9(1969). (4) Smith, J. F., and Danielson, G. C., J Chem. Phya., 12.266 (1954). (61 Straumanls. M. E., J.Am*,. Chpm. Soc.. 71.67911949). I61 Diekcn8.P. G.,andWhittingham,M. S., Quart. Re", 22,30(1968). (71 Hsgonmuller, P., in "Pmgr. W i d . Sfa* Chem.," 5, (Editor: Roiss. H.1 Pergamon Pmss.NcarYork, 1971, p71. (8) Weller, P. F., Taylor, B. E., and Mohler, R. L., Mat. Re. BUN.,5. 465 (1910): S h a h , M. R., J . Clyrtol Gmwch 13-14. 433 (1972); Fmdlein, R. A,, and
Damjanauic, A,, J. Solid Sfofe Chem.. 4, 94 (19721; Randi, J. P., J. Electm. chem. Sor.. lM.132511973). 19) Diekena,P.G.,andWhittingham. M.S., h n s . FamdoySoc., 61,1226, (1965). (10) Boeluis, J. O'M.,andMeHsrdy, J.,J. Electmchpm. Soe., IM,61(19731. (11) Wechter, M. A., Shanks, H. R.,Carter, G Ehert. G. M., Guglieimino. R., and Voirt, A. F.. A w l . Cham., 44, 890 119721. Wochter, M. A,, Hahn, P. B., E k n , G.M., Mmtoya, P. R., and Vaight, A. F . A w 1 . Chpm.. 46.1267 (1973). (121 Hshn, P. B., Wcchfer. M. A.. Johnson. D. C., and Voight, A. F., A w l . Chpm., 15, 1016 11973). (13) Whittingham, M. 8.. aodHuggina,R.A..J Chem.Phys.. 51.414(1971).
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Volume 51. Number 1, January 1974
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