Low-Temperature Form (Phase II) of Ionic Conductor Ag7TaS6 Analyzed using High-Resolution Synchrotron X-Ray Powder Diffraction Data

Abstract

The ternary sulfide Ag₇TaS₆ displays high Ag-ion conductivity at ambient temperature, and the framework TaS₆ and Ag ions can be respectively regarded as the host structure and guest ions. Two low-temperature phases, i.e. Ag₇TaS₆ II and Ag₇TaS₆ III, were found. The Rietveld analysis of Ag₇TaS₆ II has been performed based on the X-ray data measured at 240K using a wave length 0.64997Å by a high-resolution powder diffraction instrument combined with the beam line BMlB at the European Synchrotron Radiation Facilities. Smooth convergence has been obtained (R_WP=0.066), and the crystal structure was described based on a space group Pn with monoclinic lattice constants a=7.4462, b=7.4013, c=10.5296Å and β=90.069°. The narrow widths of reflections even at high angle have made possible the analysis of individual thermal parameters for Ag ions. In Ag₇TaS₆ II, Ag-Ag linkage rather than Ag-S coordination seem to affect the thermal parameters of Ag ions. The guest-guest interaction can be important for mobility of guest ions in the ionic conductor.