超イオン導電体Cu8GeS6およびAg7TaS6の低温相の構造とイオン伝導パス: 六次元解析法による多重双晶の解析

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The structures of the orthorhombic room-temperature phase of Cu8GeS6 (phase II) and the monoclinic low-temperature phase of Ag7TaS6 (phase II) have been successfully refined based on X-ray diffraction data from 12-fold twinned (Cu8GeS6II) and 24-fold twinned (Ag7TaS6II) crystals. Respectively among 6 major and 6 minor twin domains of Cu8GeS6II, or among 12 major and 12 minor twin domains of Ag7TaS6II, the argyrodite-type frameworks, GeS6or TaS6, can be superposed to each other in principle, and only Cu-Cu or Ag-Ag network directions differ. At higher temperature, the crystals were considered to be 2-fold twinned crystals of superionic-conductor phase I with a space groupF43m. On cooling, each domain transforms into 6 domains of orthorhombic Cu8GeS6II or 12 domains of monoclinic Ag7TaS6II. Superposed projections along 6 directions of the structure of Cu8GeS6II and along 12 directions of the structure of Ag7TaS6II seem to show approximate expressions for Cu-ion and Ag-ion conduction paths in superionic-conductor phases, Cu8GeS6I and Ag7TaS6I.