Pushing thermal conductivity to its lower limit in crystals with simple structures

TL;DR

This study demonstrates that a simple crystal structure material, AgTlI2, can achieve extremely low thermal conductivity due to strong anharmonic lattice vibrations and unique atomic arrangements, challenging the notion that complex structures are necessary for low thermal conductivity.

Contribution

The paper reveals that simple crystal structures can exhibit ultralow thermal conductivity through strong anharmonicity and atomic rattling, supported by experimental and theoretical analysis.

Findings

AgTlI2 has an ultralow thermal conductivity of 0.25 W/mK at room temperature.

Strong anharmonicity and atomic rattling lead to non-traditional phonon behavior.

Unusual coexistence of ultralow propagative and diffusive thermal conductivities was observed.

Abstract

Materials with low thermal conductivity usually have complex crystal structures. Herein we experimentally find that a simple crystal structure material AgTlI2 (I4/mcm) owns an extremely low thermal conductivity of 0.25 W/mK at room temperature. To understand this anomaly, we perform in-depth theoretical studies based on ab initio molecular dynamics simulations and anharmonic lattice dynamics. We find that the unique atomic arrangement and weak chemical bonding provide a permissive environment for strong oscillations of Ag atoms, leading to a considerable rattling behavior and giant lattice anharmonicity. This feature is also verified by the experimental probability density function refinement of single-crystal diffraction. The particularly strong anharmonicity breaks down the conventional phonon gas model, giving rise to non-negligible wavelike phonon behaviors in AgTlI2 at 300 K. Intriguingly, unlike many strongly anharmonic materials where a small propagative thermal conductivity is often accompanied by a large diffusive thermal conductivity, we find an unusual coexistence of ultralow propagative and diffusive thermal conductivities in AgTlI2 based on the thermal transport unified theory. This study underscores the potential of simple crystal structures in achieving low thermal conductivity and encourages further experimental research to enrich the family of materials with ultralow thermal conductivity.