Low-temperature phases (phase II) of ionic conductors Ag 7TaS 6 and Ag 7NbS 6

Abstract

The crystal structures of low-temperature forms of argyrodite-type superionic conductors, namely Ag 7TaS 6 II (between approx. 280 K and approx. 170 K) and Ag 7NbS 6 II (between approx. 280 K and approx. 140 K), analyzed based on powder X-ray diffraction data have been compared with each other. The crystal structure of Ag 7TaS 6 II could be described on the basis of a space group Pc with the lattice constants a=7.453, b=7.403, c=12.806 Å and β=124.62°. In the Rietveld analysis of Ag 7NbS 6 II, a commensurately modulated structure approach was adopted to obtain a smooth convergence. The crystal data of Ag 7NbS 6 II are expressed as a 1=7.368, a 2=14.770, a 3=12.768 Å, σ=(0.25 0 0), β=124.28° in a four-dimensional formalism with a superspace-group Pc( α 1/2 γ), that is A=29.47, B=14.770, C=12.768 Å, β=124.28° in a three-dimensional formalism with a space group Cc. In Ag 7TaS 6 II and Ag 7NbS 6 II, almost a half of Ag ions are in distorted tetrahedrons and the other Ag ions are in triangular faces shared by two tetrahedrons. A split model of mobile Ag ions in the room-temperature phase I is discussed in connection with Ag sites in the low-temperature phase II.