Crystal structures of 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 ionic conductors, namely Ag 7TaS 6 II (between approx. 280 and 170 K) and Ag 7NbS 6 II (between approx. 280 and 140 K), have been analyzed and compared to each other using powder X-ray diffraction data. The crystal structure of Ag 7TaS 6 II could be described on the basis of a space group Pc with the lattice constant a = 7.453 Å, b = 7.403 Å, 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 are expressed as a 1 = 7.368 Å, a 2 = 14.770 Å, 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 Å, B = 14.770 Å, C = 12.768 Å, β = 124.28° in a three-dimensional formalism with a space group C. In Ag 7TaS 6 II, three-sevenths of Ag ions are in the distorted S-S tetrahedrons and the residual four-sevenths are in the triangles shared by two tetrahedrons. On the other hand in the model of Ag 7NbS 6 II, most of a half of 28 independent Ag ions are in distorted tetrahedrons and most of the other half of Ag ions are in triangular faces shared by two tetrahedrons.