Crystal Growth, Structural Phase Transitions, and Optical Gap Evolution of CH₃NH₃Pb(Br_1–<i>x</i>Cl_<i>x</i>)₃ Perovskites

Chemically tuned inorganic–organic hybrid halide perovskites based on bromide and chloride anions CH3NH3Pb­(Br1–xClx)3 have been crystallized and investigated by synchrotron X-ray diffraction (SXRD), scanning electron microscopy, and UV–vis spectroscopy. CH3NH3PbBr3 and CH3NH3PbCl3 experience successive phase transitions upon cooling, which are suppressed for intermediate compositions probably due to compositional disorder. For CH3NH3PbCl3, a transient phase, formerly described as tetragonal, was identified at 167.5 K; the analysis of SXRD data demonstrated that it is indeed orthorhombic, with space group Pnma, and a ≈ √2ap; b ≈ 2ap; c ≈ √2ap (ap is the ideal cubic perovskite unit-cell parameter). The band gap engineering brought about by the chemical management of CH3NH3Pb­(Br1–xClx)3 perovskites can be controllably tuned: the gap progressively increases with the concentration of Cl ions from 2.2 to 2.9 eV, and shows a concomitant variation with the unit-cell parameters of the cubic phases at 295 K. This study provides an improved understanding of the structural and optical properties of the mixed CH3NH3Pb­(Br1–xClx)3 perovskites