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 [Ag^I ₉]⁹⁺ sub-structure in these mineral solid electrolyte systems is mutually incommensurable with respect to the remaining [(Ag^I,Cu^I)₆(As^III,Sb^III)₂S₇]^2-[Cu^IS₄]^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 S^2- ions) in the S^2-(Ag,Cu)₆ octahedral sites is shown to provide a mechanism for low temperature superstructure ordering.