Anomalous mass dependence of phonon thermal transport in lanthanum monopnictides and its origin in the nature of chemical bonding

Abstract

In certain compounds of binary lanthanum monopnictides exhibiting a rocksalt crystal structure, a phenomenon arises: larger average atomic mass, smaller acoustic phonon velocities, and smaller acoustic optical frequency gap do not necessarily result in lower lattice thermal conductivity. In this study, we present an analysis of the lattice thermal conductivity of both LaP and LaBi compounds using a first-principles approach to calculate force constants and to solve the linearized phonon Boltzmann transport equation. Through a comparison of phonon dispersions, interatomic force constants, and Grüneisen parameters, we infer that long-range interactions stemming from the electronic polarizability in the lighter compound, LaP, are stronger than those in LaBi. This leads to heightened anharmonicity and phonon scattering of transverse optical (TO) modes in LaP, which is the primary contributor to its lower lattice thermal conductivity. Additionally, we examined the electron density distribution and the anti-bonding character of the valence bands in order to gain a better chemical understanding of the increased anharmonicity and bond softening in the case of LaP. Our findings show that the lighter compound, LaP, with a more delocalized electron density distribution due to the stronger antibonding character of La-p and P-p orbitals, exhibits softer and more anharmonic TO modes, thereby resulting in weakened bonds and lower lattice thermal conductivity compared to LaBi. Next, it was found that the polarizability, Born charges and the Grüneisen parameters of LaP have a much stronger dependence on pressure compared to LaBi. These are signatures of the metavalent nature of the bonds in LaP, leading to its lower lattice thermal conductivity.