Anharmonicity and Ultra-Low Thermal Conductivity in Lead-Free Halide Double Perovskites

TL;DR

This study uses first-principles calculations to reveal that lead-free halide double perovskites like Cs₂AgBiBr₆ exhibit high anharmonicity and ultra-low thermal conductivity, making them promising for stable, non-toxic applications.

Contribution

It provides a detailed first-principles analysis of lattice dynamics and thermal properties in Cs₂AgBiBr₆, highlighting the role of anharmonicity and phase transition mechanisms.

Findings

Highly anharmonic lattice vibrations due to octahedral tilting.

Collapse of a soft phonon branch causes phase transformation.

Ultra-low thermal conductivity observed in the cubic structure.

Abstract

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs$_2$AgBiBr$_6$ , the prototypical compound in this class, and show that the lattice dynamics of Cs$_2$AgBiBr$_6$ is highly anharmonic, largely in regards to tilting of AgBr$_6$ and BiBr$_6$ octahedra. Using an energy and temperature dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs$_2$AgBiBr$_6$ yield an ultra-low thermal conductivity, unexpected of high symmetry cubic structures.