Low-Frequency In-Plane Phonons Dominate Thermal Transport in Montmorillonite: Machine-Learning Molecular Dynamics...

Montmorillonite (MMT), a foundational constituent of permafrost systems, plays a critical role in determining the thermophysical properties of frozen soils through its thermal transport behavior. The current understanding of these properties remains limited because of the experimental challenges associated with cryogenic conditions and the inherent limitations of classical molecular dynamics potentials in accurately capturing low-temperature phonon interactions. In this study, we integrate the neuroevolution potential (NEP) framework with active learning to develop a high-precision, data-driven model for evaluating the thermal conductivity of MMT. Nonequilibrium molecular dynamics, homogeneous nonequilibrium molecular dynamics, and spectral heat current are employed as complementary computational approaches to systematically simulate and rigorously compare the thermal conductivity of MMT. The results show that the thermal conductivity of MMT reaches 4.28 W m^-1 K^-1 at 300 K, with thermal transport predominantly governed by in-plane phonons (>85% contribution) and low-frequency phonon modes (<20 THz). Furthermore, temperature-dependent analyses indicate that increasing temperature enhances phonon-phonon scattering, thereby reducing thermal conductivity. In contrast, out-of-plane phonons exhibit markedly lower temperature sensitivity with changes in thermal conductivity not exceeding 0.15 W m^-1 K^-1. Complementary phonon density of states, phonon lifetime, and group velocity analyses confirm that thermal transport is dominated by dispersive, long-lived, low-frequency in-plane phonons, whereas high-frequency vibrations are strongly localized and contribute minimally. This work validates the accuracy of the NEP model for complex minerals and elucidates the thermal transport mechanisms of MMT, thereby providing a theoretical foundation for permafrost thermal stability and cold-region engineering applications.