A structural differentiation of quaternary copper argyrodites: Structure - Property relations of high temperature ion conductors

The crystal structures of 12 argyrodite type copper compounds with the general formula Cu_(12-n)Bⁿ⁺Q^2- _6-yX^-_y (B = P, As, Si, Ge; Q = S, Se; X = Cl, Br, I) were refined. The positions of the copper atoms were refined by using a non-harmonic approach. All polymorphic argyrodites were investigated in their cubic high temperature modification crystallizing in spacegroup F43m (No. 216). A comprehensible way to describe the complex structures was developed based on a topological description of the rigid anion- and B-cation substructure as an arrangement of Frank-Kasper polyhedra. An analysis of the joint probability density function and of the one particle potentials for the copper atoms was performed to get a detailed insight in the copper distribution in these argyrodites. They can be divided into four types based on their different distribution of copper. This classification corresponds to the physical properties of the argyrodites, especially to their ionic conductivities, which show a significant dependence on the composition.