Pressure-Induced Locking of Methylammonium Cations versus Amorphization in Hybrid Lead Iodide Perovskites

Abstract

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. The single crystals were specially grown with the final thickness needed for pressure experiments, retaining their high quality due to a less invasive preparation procedure, which avoids sample thinning. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4, 2.7, and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in the line width 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. 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 spectrum 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, the sharp Raman features observed at high pressures do not support amorphization of MAPbI3 with onset at 3 GPa, as claimed by most of the high-pressure (X-ray) literature. We interpret this apparent discrepancy in terms of the degree of disorder introduced at different length scales in the perovskite lattice by the pressure-induced freeze-out of the methylammonium cation motion.

Accelerated Hot-Carrier Cooling in MAPbI3 Perovskite by Pressure-Induced Lattice Compression

“…The variational polaron method predicts a polaron relaxation time constant of ∼0.1 ps 40 for this material, which is consistent with our observation of a very short HCC time. This long-range dielectric coupling is not expected to be significantly affected by pressure, 41 in agreement with our observations.…”

Pressure-induced transformation of CH3NH3PbI3: the role of the noble-gas pressure transmitting media

“…S3). This is consistent with the firstorder phase transition, which was reported before between 0.3 GPa and 0.4 GPa (Capitani et al, 2016;Szafrań ski & Katrusiak, 2016;Francisco-Ló pez et al, 2018). However, for the pseudo-cubic phase, we found the space group R 3 3, which differs from the cubic Im 3 3 and orthorhombic Imm2 reported previously by Szafrań ski & Katrusiak (2016) and Capitani et al (2016), respectively.…”

Spectroscopic ellipsometry study of FAxMA1−xPbI3 hybrid perovskite single crystals

“…The exact compositions of the segregated phases cannot be reliably determined because of the strain induced by defects in these regions and the strain-dependence of the PL energy positions. 11 The non-monotonous dependence of the band gap energy with temperature is influenced by different contributions, in particular for intermediate…”

“…9,10 Although for applications thin films are employed, studies on single crystals allow for an improved understanding of many properties, among them the optical constants, due to the better quality of samples for structural and optical studies. 11 Here, we focus on the optical spectra of FA x MA 1−x PbI 3 solid solution single crystals by SE.…”