Local and Long Range Structure in CH3NH3PbBr3: Evidence for Significant Anharmonicity
Nicholas Weadock a, Cameron MacKeen b, Louis Waquier a, Yevgeny Rakita c, Julian Vigil a d, Hemamala Karunadasa d e, Frank Bridges b, Michael Toney f
a SLAC National Accelerator Laboratory, Stanford University, Stanford, CA, USA, United States
b UC Santa Cruz, 1156 High Street, Santa Cruz, 95064, United States
c Columbia University, US, Broadway, 3000, New York, United States
d Department of Chemistry, Stanford University, Stanford, 94305, United States
e Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, 94025, United States
f Department of Chemical and Biological Engineering, University of Colorado Boulder
Proceedings of Atomic-level characterization of hybrid perovskites (HPATOM)
Online, Spain, 2021 January 26th - 28th
Organizers: Dominik Kubicki and Amita Ummadisingu
Poster, Nicholas Weadock, 026
Publication date: 14th January 2021

The hybrid metal halide perovskite (MHP) family exhibits large structural fluctuations which are believed to contribute to the optoelectronic properties. Additionally, these fluctuations have been shown to cause local deviations from the average bulk structure in the three-dimensional MHPs, such as CH3NH3PbBr3 (herein MAPB). [1–3] MAPB has three well defined crystal structures; orthorhombic, tetragonal, and cubic, with phase transitions identified at 141 K and 235 K. An additional incommensurate phase bridges the orthogonal and tetragonal phases. [4] These phase transitions proceed as an unlocking of the organic cation and rotational modes of the PbBr6 octahedra. Several studies have characterized the dynamics of the organic cation, the PbBr6 octahedra, and their interactions, however the dynamics of the Pb-Br bond have received less attention. [4–7]

We investigate the temperature dependence of correlated motions and anharmonicity of the Pb-Br bond with a combined extended X-ray absorption fine structure (EXAFS) and single crystal X-ray diffraction (XRD) study of MAPB. EXAFS provides a snapshot of the local environment around the probe atom, rather than the time- and space-averaged structure obtained from XRD. Fitting the EXAFS spectra to a cumulant expansion reveals that the Pb-Br bond exhibits anharmonicity even in the low-temperature orthorhombic phase. We also examine the correlated motion of the Pb and Br atoms by comparing the mean squared displacement obtained from XRD with the mean squared relative displacement obtained from EXAFS. [8,9] In this picture, we find that Pb and Br motions are correlated along the bond direction; perpendicular to the bond direction, however, the motion trends towards anti-correlated. An in-depth understanding of anharmonicity and the phase transitions will allow future researchers to design structurally robust MHP devices.

NJW and MFT gratefully acknowledge full support from the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, Office of Science within the U.S. Department of Energy through contract number DE-AC36-08G028308.

© Fundació Scito
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