Distinct anharmonic characteristics of phonon-driven lattice thermal conductivity and thermal expansion in bulk MoSe₂ and WSe₂

Using inelastic neutron scattering and X-ray diffraction measurements, together with ab initio and machine-learning molecular dynamics simulations, we bring out the distinct nature of anharmonicity in the phonon spectra of MoSe2 and WSe2 relevant to thermal transport and thermal expansion behaviour. We show that the perturbation method, including 4th-order force constants, is insufficient to capture the temperature-dependent explicit anharmonicity. The Green-Kubo method captures the explicit anharmonicity and reproduces the thermal conductivity (kappa(l)) with high fidelity. Our mode-resolved calculation reveals that the major contribution (similar to 90%) to kappa(l) is attributed to a small explicit anharmonicity of low-energy phonons. In contrast, these modes exhibit large positive Gruneisen parameters (implicit anharmonicity), causing the large thermal expansion of the material.