  The crystal structure of CH3NH3PbBr3 perovskite has been investigated under
high-pressure by synchrotron-based powder X-ray diffraction. We found that
after the previously reported phase transitions in CH3NH3PbBr3
(Pm-3m->Im-3->Pmn21), 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 7.6 GPa, the highest pressure covered by
experiments. The new high-pressure phase is also described by the space group
Pmn21, but 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 and visual observations and by the fact that
changes induced by pressure 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.
