Is CH3NH3PbI3 Polar?,
Sharada G, Pratibha Mahale, Bhushan P. Kore, Somdutta Mukherjee, Mysore S. Pavan, Chandan De, Somnath Ghara,
A. Sundaresan, Anshu Pandey, Tayur N. Guru Row, and D. D. Sarma,
J. Phys. Chem. Lett., 7 (13), 2412, 2016.
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.
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