Common acoustic phonon lifetimes in inorganic and hybrid lead halide perovskites

TL;DR

This study demonstrates that short acoustic phonon lifetimes are common in both inorganic and hybrid lead halide perovskites, likely due to large atomic displacements or disorder, affecting thermal properties.

Contribution

It reveals that phonon lifetime broadening occurs in inorganic CsPbBr3 similarly to hybrid perovskites, challenging the idea that organic molecules are necessary for phonon damping.

Findings

Short phonon lifetimes are observed in CsPbBr3, an inorganic perovskite.

Large thermal displacements are confirmed at Cs and Br sites.

Damping of acoustic phonons may be driven by atomic displacements or disorder.

Abstract

The acoustic phonons in the organic-inorganic lead halide perovskites have been reported to have anomalously short lifetimes over a large part of the Brillouin zone. The resulting shortened mean free paths of the phonons have been implicated as the origin of the low thermal conductivity. We apply neutron spectroscopy to show that the same acoustic phonon energy linewidth broadening (corresponding to shortened lifetimes) occurs in the fully inorganic CsPbBr$_{3}$ by comparing the results on the organic-inorganic CH$_{3}$NH$_{3}$PbCl$_{3}$. We investigate the critical dynamics near the three zone boundaries of the cubic $Pm\overline{3}m$ Brillouin zone of CsPbBr$_{3}$ and find energy and momentum broadened dynamics at momentum points where the Cs-site ($A$-site) motions contribute to the cross section. Neutron diffraction is used to confirm that both the Cs and Br sites have unusually large thermal displacements with an anisotropy that mirrors the low temperature structural distortions. The presence of an organic molecule is not necessary to disrupt the low-energy acoustic phonons at momentum transfers located away from the zone center in the lead halide perovskites and such damping may be driven by the large displacements or possibly disorder on the $A$ site.