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

Johan Klarbring; Olle Hellman; Igor A. Abrikosov; Sergei I. Simak

doi:10.1103/PhysRevLett.125.045701

paper

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

arXiv:1912.05351 · doi:10.1103/PhysRevLett.125.045701

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.