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 Fm3¯m symmetry for rare-earth monopnictides is found to be dynamically unstable for LaN at zero temperature. Using density functional theory and evolutionary crystal prediction, we discover a new, dynamically stable structure with P1 symmetry at 0 K. This P1-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 P1 to Fm3¯m symmetry, when temperature is increased. Moreover, P1-LaN will transform to a tetragonal structure with P4/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.