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The crystal structures of low-temperature forms of argyrodite-type ionic conductors, namely Ag7TaS6 II (between approx. 280 and 170 K) and Ag7NbS6 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 Ag7TaS6 II could be described on the basis of a space group Pc with the lattice constant a = 7.453 A, b = 7.403 A, c = 12.806 A and β = 124.62°. In the Rietveld analysis of Ag7NbS6 II, a commensurately modulated structure approach was adopted to obtain a smooth convergence. The crystal data are expressed as a1 = 7.368 A, a2 = 14.770 A, a3 = 12.768 A, 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 A, B = 14.770 A, C = 12.768 A, β = 124.28° in a three-dimensional formalism with a space group C. In Ag7TaS6 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 Ag7NbS6 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. |
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