Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.076
Publication date: 6th July 2018
The structural phase behavior of high quality single crystals of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) was revisited by combining Raman scattering and photoluminescence (PL) measurements under high hydrostatic pressure up to ca. 10 GPa. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4 GPa, 2.7 GPa and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in linewidth of the phonon modes of the inorganic cage, similar to the changes observed at the tetragonal-to-orthorhombic phase transition occurring at around 160 K but ambient pressure [1]. This behavior is interpreted as evidence for the locking of the organic cations in the cage voids above 2.7 GPa, due to the reduced volume and symmetry of the unit cell. At the third phase transition, reported here for the first time, the PL is greatly affected, whereas the Raman experiences only subtle changes related to a splitting of some of the peaks. This behavior may indicate a change mostly in the electronic structure with little effect on the crystal structure. Strikingly, no amorphization of the sample was observed up to the highest pressure which reached close to 10 GPa, in frank discrepancy with most of the high-pressure (x-ray) data of the literature [2], which established an onset of 3 GPa for the set in of an amorphous phase in MAPI3.
[1] Leguy, A.M.A. et al., Phys. Chem. Chem. Phys. 2016, 18, 27051–27066.
[2] Postorino, P. & Malavasi, L., J. Phys. Chem. Lett. 2017, 8, 2613–2622 and references therein.
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