Rotational Soft Modes and Octahedral Distortion as Design Principles for Ultralow Thermal Conductivity in Halide Materials

TL;DR

This paper identifies rotational soft modes and static octahedral distortions as key mechanisms for achieving ultralow thermal conductivity in halide materials, and introduces a screening strategy to discover new low-kappa compounds.

Contribution

It reveals two distinct mechanisms that reduce thermal conductivity in halide perovskites and develops a high-throughput screening method to find materials with ultralow thermal conductivity.

Findings

Rotational soft modes reshape the phonon spectrum and increase phonon scattering.

Static octahedral distortions enhance anharmonicity and further suppress thermal conductivity.

The screening strategy identified TaGaI8 with ultralow thermal conductivity.

Abstract

We establish that ultralow lattice thermal conductivity in halide perovskites and related octahedral framework materials arises from two distinct and complementary mechanisms: (i) halogen-halogen-enabled rotational soft modes that reshape the low-frequency spectrum and intensify phonon scattering, and (ii) static octahedral distortions that further enhance anharmonicity and reduce phonon lifetimes. Using first-principles calculations on CsPbBr3, we demonstrate that Br-Br interactions induce rotational soft modes that decongest the phonon spectrum and enhance three- and four-phonon scattering, strongly suppressing particle-like thermal conductivity (kappa_p). Independently, static octahedral distortions further reduce kappa_p by amplifying anharmonicity while leaving wave-like conductivity (kappa_c) intact. Based on these mechanistic insights, we introduce a geometric distortion factor rho and perform a high-throughput screening that first selects materials with halogen-coordinated octahedral building blocks-ensuring the presence of rotational soft modes-and then identifies those with pronounced distortion. This strategy uncovers TaGaI8 with an ultralow kappa_L = 0.11 W/mK at room temperature. This work establishes halogen-halogen-enabled rotational soft modes and octahedral distortions as transferable design principles for octahedra-containing halides, spanning both extended frameworks and molecular-cluster motifs, for discovering ultralow-kappa_L materials.