Band Gap Engineering in MASnBr₃ and CsSnBr₃ Perovskites: Mechanistic Insights through the Application of Pressure

Here we report on the first structural and optical high-pressure investigation of MASnBr₃ (MA = [CH₃NH₃]⁺) and CsSnBr₃ halide perovskites. A massive red shift of 0.4 eV for MASnBr₃ and 0.2 eV for CsSnBr₃ is observed within 1.3 to 1.5 GPa from absorption spectroscopy, followed by a huge blue shift of 0.3 and 0.5 eV, respectively. Synchrotron powder diffraction allowed us to correlate the upturn in the optical properties trend (onset of blue shift) with structural phase transitions from cubic to orthorhombic in MASnBr₃ and from tetragonal to monoclinic for CsSnBr₃. Density functional theory calculations indicate a different underlying mechanism affecting the band gap evolution with pressure, a key role of metal-halide bond lengths for CsSnBr₃ and cation orientation for MASnBr₃, thus showing the impact of a different A-cation on the pressure response. Finally, the investigated phases, differently from the analogous Pb-based counterparts, are robust against amorphization showing defined diffraction up to the maximum pressure used in the experiments.