Structural characteristics, mechanical properties, electronic behavior, and lattice thermal conductivities for novel LaX phases (X = P, As, Sb)

Rare-earth-metal monopnictides have rich high-pressure structures, magnetism, and topological properties, but there is still relatively little research related to them, especially in recent years. We here predict a new group of LaX (X = P, As, Sb) phases (HR-type) by means of the particle swarm optimization method. Their stabilities can be verified by dynamic, thermodynamic, and mechanical calculations. By utilizing the density functional and semiclassical Boltzmann transport theory, we investigate their mechanic, electronic, and thermal transport properties. These HR-type LaX are rather soft with lower than 6 GPa of Vickers hardness, and exhibit the obvious mechanical anisotropy. These HR-type LaX also exhibit amazing topological properties, owning a mostly flat surface band between two bulk Dirac cones. Based on the calculations of gaps in energy dispersion planes and Z2 invariants, we prove that these HR-type LaX should be the nodal-line semimetals. We compute their lattice thermal conductivities, and find that HR-type LaAs own the largest lattice thermal conductivity of 6.34 (22.34) W/mK along x (y) direction at 300 K. This mean free path of the pertinent heat-carrying phonons in the HR-type LaX are also predicted. Our research might offer new perspectives on the various allotropes of rare-earth-metal monopnictides.