Thermal Contributions to the Local and Long-Range Structural Disorder in CH3NH3PbBr3

The hybrid lead halide perovskite (LHP) family exhibits large structural fluctuations that contribute to the remarkable properties of LHP-based optoelectronic devices. In three-dimensional LHPs such as CH3NH3PbBr3, the structural phase transitions have been well characterized; changes in local structure and origins of structural disorder, however, have received less attention. We investigate the temperature dependence of the Pb - Br bond distribution, effective spring constant, and dynamic correlations with a combined extended x-ray-absorption fine structure (EXAFS), single-crystal x-ray diffraction (SXRD) and ab initio molecular dynamics (AIMD) study. EXAFS provides a snapshot of the local environment around the probe atom, rather than the time- and space-averaged structure obtained from SXRD. Molecular librations are observed below the orthorhombic-tetragonal transition temperature, and across this transition we find an anomalous increase in the Pb-Br effective spring constant. The x-ray absorption near edge structure reveals only subtle changes in the electronic structure; therefore, we propose that the increase in bond strength is the result of a redistribution of the charge density, concomitant with the loss of persistent Br...H bonding. Furthermore, the temperature dependence of the Pb - Br bond asymmetry indicates that the structural disorder in CH3NH3PbBr3 is primarily driven by thermally activated anharmonic dynamics rather than static disorder. Our results describe the local structure responsible for the optoelectronic performance of LHP devices and bring new insights into modifying halide bonding to prevent halide migration.