Study of structural and optical properties of the thermochromic silver and copper tetraiodomercurates (Ag₂, Cu₂) HgI₄ ceramics

Powders of silver (Ag₂HgI₄) and copper (Cu₂HgI₄) tetraiodomercurates were synthesized by a simple co-precipitation method. Their phase transition behavior was investigated by differential scanning calorimetry, X-ray diffraction (XRD), as well as diffuse reflectance to determine its reversibility and stability during heating–cooling cycles. Calorimetry showed a sharp transition at 51 °C when heating, and at 40 °C when cooling for Ag₂HgI₄ samples. For Cu₂HgI₄ samples in the first cycle, one peak was observed when heating at 69.6 and 49.7 °C when cooling. However, in these samples, the first peak splits into two peaks when heating during the second and third cycles (67.5 and 57.8 °C), indicating a probable loss of crystallinity. XRD and diffuse reflectance showed that the transition starts almost since the beginning of the warming-up process and is more evident around 40 °C for Ag₂HgI₄ and around 60 °C for Cu₂HgI₄. The energy bandgap indicates that Ag₂HgI₄ and Cu₂HgI₄ behave differently during the transition. Copper tetraiodomercurate follows almost the same path for heating than for cooling. Its reversible thermochromic and superionic properties are promising for several applications and open the possibility of using them as inclusions in smart composite systems.