CRYSTAL-STRUCTURE AND CONDUCTIVITY OF THE NEW SUPERIONIC CONDUCTORS AG4ZR3S8 AND AG3.8SN3S8

The new superionic conductors Ag4Zr3S8 and Ag3.8Sn3S8 crystallize both in cubic space groups with lattice parameters a 10.9427(3) Angstrom (P4(3)32, Z = 4, and rho = 4.87 g cm(-3)) and a = 10.8013(4) Angstrom (P4(1)32, Z = 4, and rho = 5.39 g cm(-3)), respectively. These compounds have similar sulfur packing consisting of sulfur octahedra sharing common edges. In both cases the M cations, either Zr or Sn, are located inside the sulfur octahedra (12d sites). The main crystallographic difference between the two structures is the silver packing. In Ag4Zr3S8 the silver ions are located in three different tetrahedral sites, two of them being partially occupied. For Ag3.8Sn3S8 the silver ions are exclusively situated in statistically occupied octahedral sites (12d and 4b sites) giving a rock-salt-type lattice. From the properties point of view, they are both mixed conductors exhibiting a relatively high ionic conductivity. The ionic sigma(Ag+) and total conductivities were measured from room temperature to 205 degrees C. (C) 1995 Academic Press, Inc.