Structural and optical properties of methylhydrazinium lead bromide perovskites under pressure

Three-dimensional (3D) MHyPbBr3 and two-dimensional (2D) MHy2PbBr4 perovskites containing methylhydrazinium cations (MHy+) have been studied under high pressure using single-crystal X-ray diffraction and optical spectroscopy. Pb-NH2 coordinate bonds contribute to the strong distortion of the inorganic framework and stabilise the monoclinic structure of MHyPbBr3 over a wide pressure range. The compression progressively narrows the bandgap up to 2 GPa, where the transition to the orthorhombic phase, associated with a huge bandgap widening by 0.53 eV, occurs. The relatively large MHy+ cation is responsible for lower compressibility and higher resistance to amorphization compared to that of analogous hybrid perovskites. The coordinate bonds Pb-NH2 are also formed under pressure in 2D MHy2PbBr4 and substantially contribute to the distortions of the inorganic layers. The crystal undergoes a sequence of phase transitions: at 4 GPa the Pmn21 phase transforms to the monoclinic phase P21 which further transforms at around 4.4 GPa to the nonpolar phase P212121. The unprecedented evolution of the absorption spectrum, which is not related to a structural phase transition, has been revealed in the high-pressure orthorhombic phase. The evolution of the bandgap of both materials across the phases observed has been correlated with the pressure dependence of the Pb-Br bond distances and Pb-Br-Pb angles. Pb-NH2 coordinate bonds contribute to the strong distortions of the inorganic frameworks of methylhydrazinium led bromide perovskites, which translate into massive changes in their optical properties under hydrostatic pressure.