Pressure-dependent polymorphism and band-gap tuning of methylammonium lead iodide perovskite

We report the pressure-induced crystallographic transitions and optical behavior of MAPbI₃ (MA=methylammonium) using in situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin-orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO₃-type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase-mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction. Band aid: Through the determination of methylammonium lead iodide perovskite crystallographic transitions under pressure, in situ laser-excited photoluminescence suggests a reduction in band gap in the tetragonal phase on increasing the pressure up to about 0.3 GPa and then an increase in band gap in ReO₃-type cubic phase up to a pressure of 2.7 GPa, a result that is supported by DFT calculations.