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 1.3-1.5 GPa from absorption spectroscopy, followed by a huge blue-shift of 0.3 and 0.5 eV, respectively. Synchrotron single crystal diffraction allowed 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. DFT calculations indicate a different underlying mechanism affecting the bandgap 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.