First-principles study of phase stability of Ti2N under pressure

Abstract

A first-principles study of phase stability of various phases of Ti${}_{2}$N under normal conditions and as a function of pressure was carried out. Among the \ensuremath{\epsilon} and \ensuremath{\delta}\ensuremath{'}phases of Ti${}_{2}$N that are observed experimentally, \ensuremath{\epsilon}-Ti${}_{2}$N is the most stable. The \ensuremath{\delta}\ensuremath{'} 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 \ensuremath{\epsilon} and \ensuremath{\delta}\ensuremath{'} 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 Ti${}_{2}$N under pressure is predicted: \ensuremath{\epsilon}-Ti${}_{2}$N (space group P4/mnm), $P$ $=$ 77.5 GPa \ensuremath{\rightarrow} Au${}_{2}$Te type (space group C2/m), $P$ $=$ 86.7 GPa \ensuremath{\rightarrow} Al${}_{2}$Cu 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.