Structural and mechanical properties of predicted vacancy ordered tantalum carbide phases

A search for ordered TanCm phases in nonstoichiometric tantalum carbide has been performed with the use of an evolutionary algorithm. Three thermodynamically stable phases Ta2C, Ta6C5, and TaC with trigo-nal, monoclinic and cubic symmetry as well as metastable Ta3C2, Ta4C3, Ta7C5, Ta9C7, Ta10C7, Ta5C4, Ta7C6, Ta8C7, Ta10C9, and Ta9C8 phases have been predicted in the composition range TaC0.5-TaC1.0. The disorder-order phase transition channels Ta2Cy -> Ta2C and TaCy -> Ta6C5 associated with the formation of stable trigonal and monoclinic superstructures have been determined for the first time. The distribution func-tions of carbon atoms over the sites of these trigonal Ta2C and monoclinic Ta6C5 superstructures have been calculated. The elastic stiffness constants c(ij) of all the predicted TanCm phases have been also calcu-lated. All the predicted TanCm tantalum carbides are mechanically stable. The calculated values of inverse Pugh's ratio k = G/B values for all the predicted TanCm carbides (except Ta2C) are greater than 0.57 indi-cating that the considered materials are brittle. The calculated Debye temperatures theta(D) of all TanCm phases grow almost monotonically with decreasing concentration of vacancies in TanCm (TaCy with y = m/n) car-bides. The calculated heat capacity of all TanCm phases (except the trigonal Ta3C2 phase) decrease almost monotonically when the concentration of vacancies in TanCm carbides lowers. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The study investigated various thermodynamically stable and metastable phases in nonstoichiometric tantalum carbide, along with phase transition channels and carbon atom distribution functions. It calculated the elastic stiffness constants for all phases and analyzed the mechanical stability of the materials, as well as the relationship between Debye temperatures and heat capacity with varying vacancy concentration.