CRYSTAL-STRUCTURES AND SILVER IONIC CONDUCTIVITIES OF THE NEW COMPOUNDS AG7NBS6, AG7TASE6 AND AG7-XTASE6-XIX (0.1-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.5)

The new series of argyrodite compounds with the formula Ag7-xMX6-xIx (M = Ta, Nb; X = S, Se) have been prepared by a sealed silica tube method at 500-550-degrees-C for 4 days. The compounds Ag7NbS6, Ag6.5TaSer5.5I0.5 and Ag7TaSe6 crystallize in the cubic system with the unit-cell dimensions of a = 10.5001 +/- 0.0006 angstrom, a = 10.8422 +/- 0.0009 angstrom and a = 10.8277 +/- 0.0005 angstrom, respectively. The observed densities are 5.90 g cm-3 for Ag7NbS6, 7.02 g cm-3 for Ag6.5TaSe5.5I0.5 and 7.33 g cm-3 for Ag7TaSe6. The unit cell contains four formula units. The X-ray diffraction patterns of the former two compounds are very similar to that of Ag7TaS6 (space group F43mBAR). Refinement was performed using X-ray powder diffraction data, the positional parameters for Ag7NbS6 being determined. These compounds are mixed conductors. The ionic conductivity sigma-Ag+ and transport number were measured at temperatures from room temperature to 220-degrees-C.