Temperature-driven phase transition of Ti2CN from first-principles calculations

Abstract

First-principles evolutionary simulations are used to predict the stable compound of Ti₂CN. Body-centered tetragonal I4₁/amd-Ti₂CN is found to be more energetically favorable than the other Ti₂CN compounds at 0 K. The phase stability as a function of temperature for all relevant competing Ti₂CN phases is investigated by means of first-principles calculations and quasi-harmonic approximation. Our calculations predict that I4₁/amd-Ti₂CN undergoes a phase transition to P4₂/mmc at 1698 K and then to Rm at 1872 K. The different effects from the harmonic, electronic and quasi-harmonic contributions to the Gibbs free energy for I4₁/amd, P4₂/mmc and Rm phases are compared and analyzed. It is found that both the electronic and quasi-harmonic contributions to the Gibbs free energies significantly affect the phase transition curve of Ti₂CN. The calculated temperature-dependent lattice parameter is carefully compared with the previous experimental results. We also provide important thermodynamic quantities as the volumetric expansion coefficient and isothermal bulk modulus and discuss their temperature dependence.