Crystal growth and electrical conductivity of Ag7PS6 and Ag8GeS6 argyrodites

Abstract

Single crystals of ternary argyrodites Ag7PS6 and Ag8GeS6 were grown by the method of directional crystallization from the melt. Large samples with size l = 30 - 40 mm and d = 12 mm were obtained. Grown crystals were investigated by powder XRD, DTA, SEM and EIS methods. The crystal structure of the electron-ionic conductor Ag7PS6 was studied by the Rietveld method. The low-temperature modification of Ag7PS6 crystallizes in a primitive cubic cell SG P213 (No 198) with lattice parameters a = 10.3917 Å and Z = 4. It was established by the SEM method that the studied ternary sulfides Ag7PS6 and Ag8GeS6 are characterized by a defect microstructure. The frequency (1 × 101 – 3 × 105 Hz) and temperature (293–383 K) behavior of electrical conductivity of single crystalline samples were studied. Using the EEC, the contributions of ionic and electronic conductivity were determined and the corresponding activation energies for Ag7PS6 and Ag8GeS6 single crystals were calculated. The influence of structural parameters of Ag7PS6 and Ag8GeS6 on electrical parameters of single crystals is established.