Dielectric
measurements on formamidinium lead halide perovskites,
FAPbCl3 and FAPbBr3, compared to those of MAPbCl3 and previously reported MAPbBr3, reveal the strongly
suppressed temperature dependence of dielectric constants in FA compounds
in the temperature range of approximately 140–300 K. Although
the behavior of dielectric constants of FA compounds for temperatures
<140 K resembles that of the MAPbX3 system, the absence
of any strong temperature dependence in sharp contrast to MA analogues
in the higher temperature range up to room temperature suggests that
the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike
the MA dipoles that rotate nearly freely in the temperature range
relevant for any photovoltaic application. This observation is further
supported by the temperature-dependent single-crystal X-ray diffraction
(XRD) results.
Pyro-current measurements have been widely used to study ferroelectric properties in multiferroic materials. However, determination of intrinsic polarization by this method is not straightforward because of leakage current and trapped charge carriers. Here, we demonstrate the formation of internal electric field due to thermally stimulated charge carriers and its influence on ferroelectric polarization in a polycrystalline sample of the well known multiferroic TbMnO 3 . While an electric field (E ext ) poling across the ferroelectric transition (T C ∼ 26 K) is essential to obtain depolarization current at T C , the sample poled only in the paraelectric state (T pole = 130 − 50 K) also exhibits a pyro-current peak at T C but with the same polarity (− I pyro ) as that of the external field (− E ext ).We demonstrate that these unusual behavior of pyro-current are caused by a positive internal electric field (+ E int ) which in turn is created by thermally stimulated free charge carriers during the poling process in the paraelectric state. We also show that a combination of DC-biased current and pyro-current measurements is a promising method to study the intrinsic ferroelectric properties in multiferroic materials.
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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