Phase transition and crystal structure of silver-ion conductor Ag12−nM+nS6 (M=Ti, Nb, Ta)

Phase transition of Ag-ion conductor of argyrodite sulfides, Ag12−nM+nS6 (M=Ti, Nb or Ta), has been investigated through thermal analyses and the low-temperature X-ray diffraction methods. It was found that all the room-temperature phase with the cubic F 4̄3m structure transforms into the several less symmetric forms at lower temperatures than 293 K. In Ag8TiS6, the low temperature modification of type β crystallizes in the orthorhombic symmetry with a=15.095 Å, b=7.462 Å, c=10.641 Å, space group Pna21 (No. 33), Z=4. The structure was determined from the Rietveld refinement of powder X-ray diffraction data and the final R factors were Rwp=0.13 and RF=0.048. The structure of Ag8TiS6 contains four tetrahedral TiS4 units and Ag and S atoms. The coordination of the Ag atoms by sulfur is distorted tetrahedral (Ag–S, 2.28–2.79 Å), trigonal planar (Ag–S, 2.36–2.89 Å), or linear (Ag–S, 2.46–2.64 Å).