Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites.

Johan Klarbring,Igor A Abrikosov,Olle Hellman,Sergei I Simak

doi:10.1103/physrevlett.125.045701

Johan Klarbring, Igor A Abrikosov + Show 2 more

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https://doi.org/10.1103/physrevlett.125.045701

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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 ultralow thermal conductivity, unexpected of high-symmetry cubic structures.

Highlights

Novel complex materials envisioned for future energy technologies are often highly anharmonic; i.e., their atomic vibrations strongly deviate from those described by a set of uncoupled harmonic oscillators, which makes accurate theoretical modeling challenging [1,2,3]
We reveal that the softness and anharmonicity of Cs2AgBiBr6 yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures
The electron-phonon coupling appears to be strong in these systems [21,22], which makes a thorough characterization of the nature of the anharmonic lattice vibrations highly desirable in these materials

Summary

Introduction

Novel complex materials envisioned for future energy technologies are often highly anharmonic; i.e., their atomic vibrations strongly deviate from those described by a set of uncoupled harmonic oscillators, which makes accurate theoretical modeling challenging [1,2,3]. Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites We reveal that the softness and anharmonicity of Cs2AgBiBr6 yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures.

Results

Conclusion