Crystal structure and conductivity of a new compound, Ag7TaS6

Silver-tantalum with the composition Ag₇TaS₆ has been prepared by a sealed silica tube method at 500 °C for 4 days. The compound Ag₇TaS₆ crystallizes in the cubic system with the unit cell dimension $\text{a = 10.5139 ± 0.0003}\text{A}\text{̊}$. The observed density is D_obs = 6.55 ± 0.03 g cm⁻³. The unit cell contains four formula units of Ag₇TaS₆. The space group $\text{F}\text{4}\text{̄}\text{3m}$ is proposed by analogy with the structure of the similar group of silver-containing ternary chalcogenides. Refinement was performed by using X-ray powder diffraction data, the positional parameters for Ag₇TaS₆ being determined. In the structure of Ag₇TaS₆ the sulphur atoms form centred, interpenetrating, distorted icosahedra and the 11.23, 9.77 and 7.00 silver atoms are statistically distributed over 96i and two 48h sites respectively. This compound is a mixed conductor. The ionic conductivity σ_ag+ reaches $\text{1.8 × 10}{}^{\mathrm{-1}}\text{Ω}{}^{\mathrm{-1}}\text{cm}{}^{\mathrm{-1}}$ at 200 °C.