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.

Behavior of Methylammonium Dipoles in MAPbX3 (X = Br and I)

“…This can be attributed to the dipoles ordered antiferroelectrically because there has been no evidence of a P – E loop within this low-temperature phase to suggest a ferroelectric state and the crystal structure has been shown in the past to be centrosymmetric. 11 There is clear evidence of a large temperature-dependent contribution from dipoles within the tetragonal phase of MAPbI 3 over the temperature range of 162–300 K in Figure 1b, similar to what is found in the case of MAPbBr 3 . Further, the dielectric constant in MAPbI 3 does not show any discontinuous or even any significant change across the tetragonal to cubic phase transition (similar to ref (20)), indicating a smooth behavior of the dipoles across this phase transition, consistent with our interpretation, as discussed later, of essentially freely rotating dipoles already in the tetragonal phase (see Figure S2 in the Supporting Information).…”

“…The room-temperature structure of MAPbI 3 is, on average, centrosymmetric, as shown by space- and time-averaged XRD, P – E loop, as well as second harmonic generation (SHG) efficiency measurements. 11 However, this time-averaged description is consistent with many different dynamic behaviors, such as MA + units (i) rotating freely and independently, (ii) rotating in a correlated manner, and (iii) not rotating at all but being frozen in an uncorrelated/random manner giving rise to a glassy state. Thus, it becomes crucial to understand the time scale of rotations of these dipoles and the extent of correlation between different MA + units.…”

“…The details of the experimental setup are described elsewhere. 11 Polycrystalline samples of MAPbI 3 , MAPbBr 3 , CsPbBr 3 , and urea were ground into fine powder and pressed into pellets for this measurement. A time delay stage was used to change the delay between the pump and probe pulses.…”

CH 3 NH 3 PbI 3 perovskites: Ferroelasticity revealed

“…The ferroelastic orientation state in a material is determined by its unique spontaneous strain tensor, similar to how spontaneous polarization leads to ferroelectricity. According to the available data ( 17 ), MAPbI 3 exists in three crystallographic phases: cubic ($\mathrm{italicP}\mathrm{italicm}\overline{3}\mathrm{italicm}$), above 327 K; tetragonal ( I 4/ mcm ), between 327 and 162 K; and orthorhombic ( Pnma ), below 162 K. The cubic-tetragonal transition, which lowers the symmetry from the m 3 m to the 4/ mmm centrosymmetric point group, is a purely ferroelastic transition according to Aizu’s classification (that is, the resulting tetragonal phase can be ferroelastic but not ferroelectric) ( 23 , 24 ). …”

“…This hypothesis prompted the fascinating idea that the internal field, due to spontaneous polarization, might be at the origin of the high PCE in OIP solar cells by aiding the separation of photoexcited electron-hole pairs and thus reducing charge recombination ( 16 ). However, solid experimental evidence of ferroelectricity of OIP materials is still lacking, and recent publications ( 17 , 18 ) even question the very polarity of the CH 3 NH 3 PbI 3 (MAPbI 3 ) structure, a necessary requirement for ferroelectricity. In addition, several recent studies conclusively showed that ion migration and the associated charge doping are responsible for the observed hysteretic effects and excluded ferroelectricity as a possible origin ( 19 ).…”