Disorder–order and order–order phase transformations in Ta5C4 phases predicted using the evolutionary algorithm and symmetry analysis

Abstract

A search for stable ordered phases in the nonstoichiometric cubic tantalum carbide TaC_0.8 has been performed by use of the evolutionary algorithm and symmetry analysis. Four stable Ta₅C₄ superstructures with tetragonal, monoclinic, orthorhombic, and triclinic symmetry have been predicted for the first time. The DOS values of these Ta₅C₄ superstructures and stoichiometric TaC_1.00 carbide have been calculated. All the tantalum carbide superstructures and stoichiometric TaC_1.00 carbide have metal conductivity. The disorder–order phase transition channels TaC_y → Ta₅C₄ associated with the formation of the considered model superstructures include superstructural vectors of non-Lifshitz stars {k₁}, {k₂}, and {k₄}. The distribution functions of carbon atoms over the sites of the tetragonal, monoclinic, orthorhombic, and triclinic Ta₅C₄ superstructures have been calculated. For the first time, the physically permissible sequence of disorder–order and order–order phase transitions is established for the detected phases of the Ta₅C₄ family. Based on the formation enthalpy and the cohesion energy magnitudes, the triclinic Ta₅C₄ superstructure is the most favorable among all Ta₅C₄ phases predicted. The composition of the predicted Ta₅C₄ superstructures corresponds to TaC_0.80 which possesses the highest melting temperature and hardness.