A composite modulated structure mechanism for Ag+ fast ion conduction in pearceite and polybasite mineral solid electrolytes

A careful electron diffraction investigation of pearceite-Tac and polybasite-Tac samples in the Ag+ fast ion conducting state reveals that the average hexagonal [AgI9]9+ sub-structure in these mineral solid electrolyte systems is mutually incommensurable with respect to the remaining [(AgI,CuI)6(AsIII,SbIII)2S7]2−[CuIS4]7− framework sub-structure. It is shown how the mutual incommensurability of the two component sub-structures can be compatible with crystal chemical commonsense, with previously reported average structure refinements of these phases as well as providing an insight into their Ag+ fast ion conductivity at such low temperatures. In addition, Ag+/Cu+ ion ordering (and associated displacive shifts of the centreing S2− ions) in the S2−(Ag,Cu)6 octahedral sites is shown to provide a mechanism for low temperature superstructure ordering.