Polymorphic transformations and thermal expansion in AgCuSe$_{0.5}$(S,Te)$_{0.5}$ crystals

The high-temperature X-ray diffraction technique is used to study AgCuSe$_{0.5}$(S,Te)$_{0.5}$ crystals. It is shown that, at room temperature, the AgCuSe$_{0.5}$S$_{0.5}$ crystal is composed of Cu$_{1.96}$S and AgCuSe phases. At a temperature of 695 K, these phases transform into a single face-centered cubic (fcc) phase. The transformation is reversible. The AgCuSe$_{0.5}$Te$_{0.5}$ composition consists of three phases, specifically, Cu$_{2}$Te, AgCuSe, and a cubic phase. At 444 K, both orthorhombic phases simultaneously transform into a diamond-like cubic phase. In this transformation, the cubic phase plays the role of a seed. From the temperature dependence of the lattice parameters, the thermal-expansion coefficients of the phases involved in both compositions are calculated for the main crystallographic directions.