LaN Structural and Topological Transitions Driven by Temperature and Pressure

Abstract

We study lanthanum mononitride LaN by first-principles calculations. The commonly reported rock-salt structure of F m 3m symmetry for rare-earth monopnictides is found dynamically unstable for LaN at zero temperature. Using density functional theory and evolutionary crystal prediction, we discover a new, dynamically stable structure with P 1 symmetry at 0 K. This P 1-LaN exhibits spontaneous electric polarization. Our ab initio molecular dynamics simulations of finite-temperature phonon spectra further suggest that LaN will undergo ferroelectric and structural transitions from P 1 to F m 3m symmetry, when temperature is increased. Moreover, P 1-LaN will transform to a tetragonal structure with P 4/nmm symmetry at a critical pressure P = 18 GPa at 0 K. Electronic structures computed with an advanced hybrid functional show that the high-temperature rock-salt LaN can change from a trivial insulator to a strong topological insulator at P ∼ 14 GPa. Together, our results indicate that when P = 14 − 18 GPa, LaN can show simultaneous temperature-induced structural, ferroelectric, and topological transitions. Lanthanum monopnictides thereby provide a rich playground for exploring novel phases and phase transitions driven by temperature and pressure.