CRYSTAL-STRUCTURE AND CONDUCTIVITY OF THE NEW SUPERIONIC CONDUCTOR AG4HF3S8

Ag4Hf3S8, prepared from a Ag2S, Hf, and S mixture heated for 1 week at 650 degrees C, crystallizes in the cubic system (space group P4(3)32) with lattice constant alpha = 10.9051(2) Angstrom (Z = 4, rho = 6.27 g cm(-3)). The crystal structure refinement from four-circle X-ray data yields a reliability factor R of 4.2%. The 32 S atoms, distributed over two different positions, 24e and 8c, produce 32 octahedra in the unit cell, each one being connected three-dimensionally with 12 other octahedra by sharing edges. The Hf atoms (12d sites) are located in sulfur octahedra. The Ag atoms are distrib uted over three different sites: Ag1 in the 8c positions and Ag2 and Ag3 in two general 24e positions, each of them being occupied statistically (occupancy = 1/6). Ag4Hf3S8 is a mixed conductor exhibiting a relatively high ionic conductivity. This compound becomes more conductive above 137 degrees C due to the transition from a low-temperature ordered phase to a high-temperature disordered one (sigma(Ag)+ reaches 2.27 x 10(-2) Omega(-1) cm(-1) at 198 degrees C). (C) 1995 Academic Press, Inc.