  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: Pm-3m-to-Im3-to-Pmn21. The transitions take
place at around 0.8 and 1.8 GPa, respectively. This result is contradicting the
previously reported phase transition sequence: Pm-3m-to-Im3-to-Pnma. 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 behaviour of MAPbBr3 has been studied up to around 4 GPa, by the 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.
