Anharmonic Acoustic Phonons and A-site Critical Fluctuations in the Lead Halide Perovskite CsPbBr3

Published

September 17, 2019

Author(s)

M. Songvilay, Nathan Giles-Donovan, M. Bari, Z.-G. Ye, J. L. Minns, M. A. Green, Guangyong Xu, Peter M. Gehring, K. Schmalzl, William D. Ratcliff, Craig Brown, D. Chernyshov, W. van Beek, Sandy Cochran, Christopher Stock

Abstract

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), Q_M = (3/2,1/2,0), and Q_R = (1/2,1/2,3/2). CsPbB_r3 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 Q_M = (3/2,1/2,0) and the tetragonal - orthorhombic transition by Bragg peaks at Q_X = (2,1/2,0) and Q_R = (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 Q_M = (3/2,1/2,0), indicative of excitations with a shortened lifetime and weak spatial correlations. No underdamped soft phonon is observed at Q_M. In contrast, the 360 K tetragonal - orthorhombic transition displays scattering which is critical both in momentum and energy near Q_R = (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 Q_X = (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.

Citation

Physical Review Materials

Volume

3

Issue

9

Pub Type

Journals

Download Paper

https://doi.org/10.1103/PhysRevMaterials.3.093602

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Keywords

anharmonicity, lattice dynamics, acoustic phonons, perovskites, photovoltaics, neutron scattering

Citation

Songvilay, M. , Giles-Donovan, N. , Bari, M. , Ye, Z. , Minns, J. , Green, M. , Xu, G. , Gehring, P. , Schmalzl, K. , Ratcliff, W. , Brown, C. , Chernyshov, D. , van Beek, W. , Cochran, S. and Stock, C. (2019), Anharmonic Acoustic Phonons and A-site Critical Fluctuations in the Lead Halide Perovskite CsPbBr₃, Physical Review Materials, [online], https://doi.org/10.1103/PhysRevMaterials.3.093602, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=927869 (Accessed December 11, 2024)