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-degrees-C for 4 days. The compound Ag7TaS6 crystallizes in the cubic system with the unit cell dimension a = 10.5139 +/- 0.0003 angstrom. 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 F43mBAR 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-sigma-Ag+ reaches 1.8 x 10(-1) OMEGA-1 cm-1 at 200-degrees-C.