Phonon Lifetimes and Mode Softening in Cubic Cs2AgBiBr6*

The best-performing Hybrid organic-inorganic metal halide perovskite (HOIP) devices contain toxic lead, resulting in a search for lead-free perovskite formulations. One promising lead-free archetype is the double perovskite A2BIBIIIX6 formulation, with Cs2AgBiBr6 reported to have a device-relevant 1.95 eV bandgap. [1,2,3] Recent anharmonic lattice dynamics calculations have suggested that the phase transition in Cs2AgBiBr6 proceeds by the condensation of a soft, zone-center optical phonon. [4] Raman scattering studies have not identified this phonon in the cubic phase, therefore no experimental confirmation of the phenomenon exists. [5] 

In this work, we use inelastic neutron scattering to characterize the acoustic and low-energy optical phonons modes. The acoustic phonon energies identified from constant Q-scans match well with room temperature phonon dispersions calculated from fully anharmonic first-principles lattice dynamics. [4] Compared to the prototypical HOIP, MAPI, Cs2AgBiBr6 acoustic modes exhibit a larger group velocity and longer lifetime, resulting in a longer phonon mean free path (MFP). We propose that the acoustic modes are the dominant contribution to the thermal conductivity in Cs2AgBiBr6. We also confirm the presence of a zone-center optical soft mode and track the temperature dependence. Ultimately, this work helps to understand the free charge carrier properties in double-perovskite CsAgBiBr6 which are largely governed by electron-phonon interactions.