Coupling of phase transition, anharmonicity, and thermal transport in CaSnF$_6$

TL;DR

This study uses advanced simulations to explore how phase transitions and anharmonic vibrations in CaSnF$_6$ influence its thermal transport properties, revealing a non-monotonic thermal conductivity anomaly near the phase transition.

Contribution

It introduces a machine-learned neuroevolution potential combined with large-scale molecular dynamics to accurately model phase transition effects on thermal transport in CaSnF$_6$.

Findings

Negative thermal expansion caused by rigid unit modes.

Strong anharmonicity suppresses lattice thermal conductivity.

Non-monotonic thermal conductivity anomaly observed near phase transition.

Abstract

Understanding the coupling between structural phase transitions and thermal transport is essential for designing functional materials with tunable properties. Here, we investigate this interplay in CaSnF$_6$ by combining first-principles calculations with a machine-learned neuroevolution potential that enables large-scale molecular dynamics simulations across a wide temperature range. The simulations accurately capture the first-order structural phase transition and associated lattice dynamics. We show that the negative thermal expansion originates from low-energy rigid unit modes involving cooperative rotations of corner-sharing [CaF$_6$]$^{4-}$ octahedra, which induce bond-angle bending and volume contraction. At the same time, strong anharmonicity, dominated by four-phonon scattering, plays a central role in suppressing lattice thermal conductivity ($\kappa_L$). Crucially, non-equilibrium simulations reveal a pronounced non-monotonic anomaly in $\kappa_L$ near the phase transition, deviating from the conventional $\sim 1/T^{\alpha}$ behavior and providing direct transport evidence of lattice reconstruction. These results establish a unified mechanism linking lattice geometry, anharmonic vibrational dynamics, and thermal transport, and highlight the potential of machine-learned potentials for bridging atomic-scale phase transitions with macroscopic transport properties.