Lone-Pair Electrons Lead to Strong Phonon Anharmonicity and Anomalous Strain Enhancement of Thermal Conductivity

Manipulating heat conduction is an appealing thermophysical problem with enormous practical implications, which requires insight into the lattice dynamics. Lone-pair electrons have long been proposed to induce strong phonon anharmonicity. However, no direct evidence is available from a fundamental point of view and the electronic origin still remains untouched. In this Letter, by performing comparative study of thermal transport in two-dimensional group III-nitrides (h-BN, h-AlN, h-GaN) and graphene, we establish a microscopic picture to provide direct evidence for the interaction between lone-pair non-bonding electrons and covalently bonding electrons based on the analysis of orbital-projected electronic structures, which demonstrates how nonlinear restoring forces arise from atomic motions. The microscopic picture of lone-pair electrons driving strong phonon anharmonicity not only provides coherent understanding of the diverse thermal transport properties of the monolayer group III-nitrides compared to graphene, but also successfully explains their anomalous positive response of thermal conductivity to the external tensile strain.