Phonon lifetimes and mode softening in cubic Cs2AgBiBr6

<h2>Summary</h2> Metal halide perovskites (MHPs) have emerged as noteworthy candidates for photovoltaic applications in recent years. Their high power-conversion efficiency is largely attributed to the structure-function relationship, which is not well understood. In this study, we examine the low-frequency phonons of the double perovskite <mml:math><mml:mrow><mml:mrow>Cs</mml:mrow><mml:mn>2</mml:mn>AgBiBr<mml:mn>6</mml:mn></mml:mrow></mml:math> using neutron inelastic scattering. We find that the acoustic phonon lifetimes decrease from 16 to 3 ps along <mml:math><mml:mrow><mml:mi>Γ</mml:mi></mml:mrow></mml:math> to <mml:math><mml:mrow><mml:mi>X</mml:mi></mml:mrow></mml:math>, which is indicative of significant anharmonicity that contributes to the ultralow thermal conductivity. Additionally, we observe a linear temperature dependence of the square of the zone-center soft optical phonon energy, which is consistent with a weakly first-order displacive cubic-tetragonal structural phase transition. These results provide a deeper understanding of the lattice dynamics and phase transitions in MHPs as well as the effects of anharmonicity by comparison with prototypical hybrid organic-inorganic MHPs, (<mml:math><mml:mrow>CH<mml:mn>3</mml:mn>NH<mml:mn>3</mml:mn></mml:mrow></mml:math>) <mml:math><mml:mrow><mml:mrow>PbX</mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:math> (X = Br, I).