RESUMO
The high-pressure crystal structure evolution of CH3NH3PbBr3 (MAPbBr3) perovskite has been investigated by single-crystal X-ray diffraction and synchrotron-based powder X-ray diffraction. Single-crystal X-ray diffraction reveals that the crystal structure of MAPbBr3 undergoes two phase transitions following the space-group sequence: Pm3Ì m â Im3Ì â Pmn21, unveiling the occurrence of a nonpolar/polar transition (Im3Ì â Pmn21). The transitions take place at around 0.8 and 1.8 GPa, respectively. This result contradicts the previously reported phase transition sequence: Pm3Ì m â Im3Ì âPnma. In this work, the crystal structures of each of the three phases are determined from single-crystal X-ray diffraction analysis, which is later supported by Rietveld refinement of powder X-ray diffraction patterns. The pressure dependence of the crystal lattice parameters and unit-cell volumes are determined from the two aforementioned techniques, as well as the bulk moduli for each phase. The bandgap behavior of MAPbBr3 has been studied up to around 4 GPa, by means of single-crystal optical absorption experiments. The evolution of the bandgap has been well explained using the pressure dependence of the Pb-Br bond distance and Pb-Br-Pb angles as determined from single-crystal X-ray diffraction experiments.
RESUMO
The crystal structure of the CH3NH3PbBr3 perovskite has been investigated under high-pressure conditions by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm3Ì mâIm3Ì âPmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies, which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to at least 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21; however, the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments, by visual observations, and by the fact that pressure-induced changes up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy, which is discussed using the structural information obtained from X-ray diffraction.