Is CH₃NH₃PbI₃ Polar?

Abstract

In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH₃NH₃PbI₃, we report the first investigation of a time-resolved pump–probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P–E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH₃NH₃PbI₃ are consistent with a centrosymmetric, nonpolar structure.