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

In this study, we theoretically investigate the primary origins of the anomalous behavior in lattice thermal conductivity observed in binary lanthanum monopnictides with a rocksalt structure, specifically LaP and LaBi. We explore the surprising phenomenon where LaBi, despite having a larger average atomic mass, smaller acoustic phonon velocities, and smaller acoustic-optical frequency gap, does not exhibit lower lattice thermal conductivity compared to LaP. Our findings show that in LaP, longer-range interaction due to higher electronic polarizability leads to significant anharmonicity and phonon scattering, particularly of the transverse optical (TO) modes, resulting in lower thermal conductivity. The origin of strong anharmonicity is related to the stronger antibonding character of La-p and P-p orbitals, compared to LaBi. Additionally, the polarizability, Born effective charges, optical dielectric constant, transverse phonon frequency, and Grüneisen parameters of LaP are larger and furthermore show a stronger dependence on pressure compared to LaBi. This supports the hypothesis that LaP has a stronger metavalent bonding character, leading to its lower lattice thermal conductivity.