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Article

Infrared Spectra of the CH3NH3PbI3 Hybrid Perovskite: Signatures of Phase Transitions and of Organic Cation Dynamics

Journal of Physical Chemistry C. 2020. Vol. 124. No. 42. P. 23307-23316.
Kirill N. Boldyrev, Vasilisa E. Anikeeva, Marina N. Popova, Semenova O. I.

The high-resolution terahertz reflection and mid- and near-infrared transmission spectroscopic studies of CH3NH3PbI3 hybrid perovskite single crystals are performed for the temperature range from 5 to 350 K. The observation of the torsional mode of the molecular cation (at 306 cm–1) and of 12 other low-frequency modes not observed before is reported. Mode Grüneisen parameters for the strongest inorganic-cage modes are negative, which points to their interaction with a soft mode. Sharp peculiarities and the hysteresis take place in the multiphonon spectra at the phase transition from the tetragonal to the orthorhombic phase (∼160 K). Moreover, pronounced spectral changes are observed at ∼70 K; they correlate with the transition to tunneling dynamics. A spectral line splitting at this temperature is tentatively attributed to the tunneling splitting.