LaN Structural and Topological Phase Transitions Driven by Pressure and Temperature*

Lanthanum monopnictides (LaX, X = N, P, As, Sb, Bi) have attracted much attention due to their unusual electronic and topological properties. In this study, we examine the stabilities of all rock-salt structured LaX, and show that LaN exhibits soft or negative phonon modes, and is thereby dynamically unstable. Using evolutionary algorithm, we predict a stable structure of LaN with lower symmetry and lower energy, compared to the face-centered cubic phase. The lower-symmetry phase possesses spontaneous polarization, and therefore LaN can be a potential ferroelectric material at low temperature. We also study how temperature and pressure change the structural and electronic properties of LaN, and demonstrate that topological phase transitions in this material can be driven by both effects.