Phase transitions in copper tellurides

Abstract

This paper presents the results of model calculations of the structure of the Cu-Te system at low temperatures, as well as the results of a study of the structure and phase transitions of synthesized non-stoichiometric compounds of copper tellurides. For the first time, using the USPEX evolutionary algorithm, model computer calculations of the search for stable phases of copper tellurides Cu n Te m (n,m = 1÷10) were carried out. At temperature T = 0 K and pressure p = 1 atm. As stable structures, the compositions Cu₅Te₄, Cu₃Te₂, and Cu₇Te₄ were identified, which are indicated in the triclinic and monoclinic syngonies. Based on the study of the phase diagram, calorimetric measurements and X-ray diffraction studies of experimentally synthesized samples of copper telluride, it was found that the non-stoichiometric compositions Cu_1.96Te, Cu_1.85Te, Cu_1.80Te and Cu_1.75Te at room temperature are single-phase. The Cu_1.85Te, Cu_1.80Te, and Cu_1.75Te compounds are described by hexagonal crystalline superstructures obtained on the basis of the Novotny phase for Cu₂Te with different degrees of unit cell parameter multiplicity. Cu1.96Te at room temperature is indicated in an orthorhombic phase with lattice parameters that are also multiples of the unit cell parameters of the Novotny phase. At high temperatures, all compositions of Cu_2−x Te (x=0.04, 0.15, 0.20, 0.25) transform into high-temperature disordered FCC structures that exist up to their melting point. It is shown that in these compounds the transition of a low-temperature hexagonal and orthorhombic structure to a high-temperature FCC phase occurs through a series of polymorphic phase transformations.