First-principles study of phase stability of Ti2N under pressure

A first-principles study of phase stability of various phases of Ti2N under normal conditions and as a function of pressure was carried out. Among the epsilon and delta' phases of Ti2N that are observed experimentally, epsilon-Ti2N is the most stable. The delta' phase can only exist at high temperature due to the soft acoustic modes at the X point. The origin of the tetragonal structure of both the epsilon and delta' phases is supposed to be caused by the tetragonal local lattice distortion around a nitrogen vacancy. Based on the results of the total-energy and phonon-spectrum calculations at zero temperature, the following sequence of phase transformation in Ti2N under pressure is predicted: epsilon-Ti2N (space group P4/mnm), P = 77.5 GPa -> Au2Te type (space group C2/m), P = 86.7 GPa -> Al2Cu type (space group I4/mcm). The present study shows that, to correctly predict relative phase stability, the peculiarities of the phonon spectra of the materials under investigation have to be properly accounted for.