Exploring the temperature-dependent phase transitions of the solid electrolytes copper and silver tetraiodomercurates (Cu₂, Ag₂) HgI₄: a study of thermal and electrical conductivities

Copper and silver tetraiodomercurates (Cu₂HgI₄, Ag₂HgI₄) are thermochromic materials whose color changes result from a crystalline phase transition, affecting their electrical and thermal conductivities. Both materials, defined as superionic solids, are solid electrolytes where the metallic cations are the charge carriers in the higher temperature phase, which occurs at 50 °C for Ag₂HgI₄ and at 69 °C for Cu₂HgI₄. In this work, we present the thermal characterization of these materials by measuring the thermal diffusivity as a function of temperature, intending to elucidate the influence of randomly moving cations on thermal transport and their interactions with the phonons produced in the anion sublattice. The electrical conductivity characterization enabled us to contrast their different behavior as the phase transition occurs due to temperature changes. Thermal and electrical transport performance characterization of these materials opens the possibility of using them in different applications, such as solid-state batteries, optical devices for recording media, active materials for thermally controlled systems, temperature sensing devices, and fillers for manufacturing smart composites, among many others.