Anharmonic Acoustic Phonons and A-site Critical Fluctuations in the Lead Halide Perovskite CsPbBr3
M. Songvilay, Nathan Giles-Donovan, Guangyong NMN Xu, Peter M. Gehring, K. Schmalzl, William D. Ratcliff, Sandy Cochran, C. Stock
We apply neutron spectroscopy to investigate the transverse acoustic phonons in the lead halide CsPbBr3 near the three zone boundaries of the cubic Pm3m Brillouin zone at QX = (2,1/2,0), QM = (3/2,1/2,0), and QR = (1/2,1/2,3/2). CsPbBr3 undergoes two structural phase transitions, from a high temperature cubic phase to a tetragonal unit cell at 410 K, and to a low temperature orthorhombic symmetry at 360 K. Elastic scattering, sampling the static response, is used to confirm that the cubic - tetragonal transition is associated with a new Bragg peak formed at QM = (3/2,1/2,0) and the tetragonal - orthorhombic transition by Bragg peaks at QX = (2,1/2,0) and QR = (1/2,1/2,3/2). The dynamics driving the 410 K cubic - tetragonal distortion is characterized by scattering that is extended both in energy and momentum near QM = (3/2,1/2,0), indicative of excitations with a shortened lifetime and weak spatial correlations. No underdamped soft phonon is observed at QM. In contrast, the 360 K tetragonal - orthorhombic transition displays scattering which is critical both in momentum and energy near QR = (1/2,1/2,3/2). On cooling through the transition to an orthorhombic cell the energy and momentum linewidths sharpen and a broadening of the zone boundary phonon is measured near QX = (2,1/2,0). Based on selection rules of neutron scattering, these results suggests the presence of fluctuations arising from the displacements of the Cs atoms rather than tilt distortions of the PbBr6 octahedra. The momentum and energy broadened nature of the zone boundary dynamics is indicative of disorder and relaxational dynamics on the Cs A-site in CsPbBr3.