Temperature-driven Phase Transition of Ti2CN from First-Principles Calculations

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 R3̄m 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 R3̄m 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.