Recently, copper selenides have shown to be promising thermoelectric materials due to their possible
superionic character resulting from mobile copper cations. Inspired by this recent development in the
class of quaternary copper selenides we have focused on the structure-to-property relationships in the
solid solution series Cu_2ZnGeSe_(4-x)S_x. The material exhibits an insulator-to-metal transition at higher
temperatures, with a transition temperature dependent on the sulfur content. However, the lattice
parameters show linear thermal expansion at elevated temperatures only and therefore no indication of
a structural phase transformation. ^(63)Cu nuclear magnetic resonance shows clear indications of Cu
located on at least two distinct sites, which eventually merge into one (apparent) site above the phase
transformation. In this manuscript the temperature dependent lattice parameters and electronic
properties of the solid solution Cu_2ZnGeSe_(4-x)S_x are reported in combination with ^(63)Cu NMR, and an
attempt will be made to relate the nature of the electronic phase transformation to a superionic phase
transformation and a changing covalent character of the lattice upon anion substitution in this class of
materials
