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 ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$, the prototypical compound in this class and show that the lattice dynamics of ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ is highly anharmonic, largely in regards to tilting of ${\mathrm{AgBr}}_6$ and ${\mathrm{BiBr}}_6$ 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 ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$ yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures.

• Received 11 December 2019
• Accepted 22 May 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.045701

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Published by the American Physical Society