X-RAY AND ELECTRON-DIFFRACTION STUDY OF THE TETRAGONAL SODIUM TUNGSTEN BRONZE, NA0.10WO3, WITH DISTORTED PEROVSKITE STRUCTURE

The crystal structure of the tetragonal sodium tungsten bronze, Na0.10 WO3, was investigated by single-crystal X-ray and electron diffraction methods. The average structure has space group P-4/nmm and cell const ants a(av)=5.2492(5); Angstrom, c(av)=,, 3.8953(4); Angstrom, and Z = 2. The superstructure has space group very probably P4 and cell consta nts a(s) = 7.423(3) Angstrom, c(s) = 7.791(1) Angstrom, and Z = 8. Ful l-matrix least-squares refinements resulted in a conventional R value of 0.041 [86 observed unique reflections, I > 2 sigma(I)] for the aver age structure and an R value of 0.086 [537 observed unique reflections , I > 2 sigma(I)] for the superstructure. The general features of the average structure, which is of distorted perovskite type, are the same as those reported earlier (A. Magneli, 1951. Acta Chem Scand. 5, 670) . The two axial bonds of the WO6 octahedron are not equal (1.95; Angst rom) as observed in the previous work but have significantly different lengths [1.63(6) and 2.27(6) Angstrom]. The superstructure, reported for the first time, results from the average structure by the combined effects of tilting of the WO6 octahedra about c leading to a doubling of this axis (c(s) = 2c(av),,) and of shape deformation of these octa hedra in the ab plane leading to a supercell in this plane with a(s) = root 2a(av). (C) 1997 Academic Press.