Low-temperature phases (phase II) of ionic conductors Ag7TaS6 and Ag7NbS6

The crystal structures of low-temperature forms of argyrodite-type superion ic conductors, namely Ag7TaS6 II (between approx. 280 K and approx. 170 K) and Ag7NbS6 II (between approx. 280 K and approx. 140 K), analyzed based on powder X-ray diffraction data have been compared with each other. The crys tal structure of Ag7TaS6 II could be described on the basis of a space grou p Pc with the lattice constants a = 7.453, b = 7.403, c = 12.806 Angstrom a nd beta = 124.62 degrees. In the Rietveld analysis of Ag7NbS6 II, a commens urately modulated structure approach was adopted to obtain a smooth converg ence. The crystal data of Ag7NbS6 II are expressed as a(1) = 7.368, a(2) = 14.770, a(3) = 12.768 Angstrom, sigma = (0.25 0 0), beta = 124.28 degrees i n a four-dimensional formalism with a superspace-group Pc(alpha 1/2 gamma), that is A = 29.47, B = 14.770, C = 12.768 Angstrom, beta = 124.28 degrees in a three-dimensional formalism with a space group Cc. In Ag7TaS6 II and A g7NbS6 II, almost a half of Ag ions are in distorted tetrahedrons and the o ther Ag ions are in triangular faces shared by two tetrahedrons. A split mo del of mobile Ag ions in the room-temperature phase I is discussed in conne ction with Ag sites in the low-temperature phase II. (C) 1998 Elsevier Scie nce B.V. All rights reserved.