Structural and optical properties of methylhydrazinium lead bromide perovskites under pressure

Abstract

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.