Anharmonic lattice dynamics study of phonon transport in layered and molecular-crystal indium iodides

TL;DR

This study uses first-principles anharmonic lattice dynamics calculations to analyze phonon transport in indium iodides, revealing low thermal conductivity and the significance of wave-like phonon contributions depending on composition.

Contribution

It provides a systematic first-principles analysis of phonon transport in indium iodides, highlighting the role of wave-like phonons and structural stacking effects on thermal conductivity.

Findings

Lattice thermal conductivities remain below 1 W/m-K across temperatures.

Wave-like phonon contributions are significant in InI3 but negligible in InI.

Stacking sequence variations have minimal impact on thermal transport.

Abstract

Indium iodides, which adopt layered or molecular-crystal-like arrangements depending on composition, are expected to exhibit low lattice thermal conductivity because of their heavy constituent atoms and weak In-I bonding. In this work, we employed first-principles anharmonic lattice dynamics calculations to systematically investigate phonon transport in indium iodides from particle- and wave-like perspectives. The calculated lattice thermal conductivities of both materials remained below 1 W/m-K over a broad temperature range. Notably, the influence of wave-like phonon transport differed by composition: in InI3, the wave-like contribution became comparable to the particle-like Peierls contribution, whereas it remained negligible in InI. We also investigated the thermal transport properties of the experimentally reported high-pressure phase of InI3. Motivated by experimental indications of stacking faults and partial disorder in indium site occupancy within the rhombohedral phase, we constructed several ordered structural models with different stacking sequences. These stacking sequences exhibited no significant energetic preference and had similar lattice thermal conductivities, suggesting that in-plane thermal transport is largely governed by the vibrational properties of the In2I6 layers themselves rather than by the specific stacking sequence. These findings provide insight into phonon transport in layered and molecular-crystal systems with structural complexity and contribute to a broader understanding of thermal transport mechanisms in layered and molecular-crystal-like materials.