Crystal growth, structural phase transitions and optical gap evolution of CH3NH3Pb(Br1-xClx)3 powders

摘要： 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.