Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CH₃NH₃PbBr₃ ferroelastic single crystals

Abstract

Accumulation of CH3NH3+ and Br− ionic species at the ferroelastic domain boundaries creates a polar electret state in the hybrid perovskite CH3NH3PbBr3 that mimics a ferroelectric P(E) hysteresis loop.

Deciphering the Nature of Temperature-Induced Phases of MAPbBr3 by Ab Initio Molecular Dynamics

“…Our conclusion agrees with several X-ray diffraction experiments ,, and with more recent reports of a lack of ferroelectric switching in the orthorhombic phase. It has been suggested that there may exist local minima slightly higher in energy than the Pnma structure differing from it by small relative rotations and displacements of the MA ions . While these were ruled out in our 0 K optimization studies, we considered their possible occurrence at higher temperatures through our 0 GPa AIMD simulations at 40, 180, and 300 K (see SI).…”

“…In particular, 96-atom supercells used previously along with the Γ-point only approximation led to a spurious polar minimum in MAPbBr 3 (see SI). A unimodal distribution of the dihedral angle θ 12 between MA 1 and 2 under the v or w group confirms the absence of any local (intermediate polar) energy minima close to the nonferroelectric/antipolar arrangement (θ 12 = 180°) (see Figure S4) and is in agreement with observations made in a recent MD study by Bokdam et al The occurrence of such intermediate-polar minima at 0 K in previous studies is an artifact of fixed cell optimizations, a constraint not imposed by our variable-cell simulations (see SI). While a polar arrangement of MA (θ 12 = 0°) with slightly higher energy (≈2 meV/atom) as the antipolar one is also possible (see SI), it was not accessed in the 40 K simulations.…”

“…Our density functional theory (DFT)-based optimizations employing the van der Waals (vdW) interaction corrected the PBE + D2 exchange−correlation (xc) functional 52,53 yielded the Pnma structure for MAPbBr 3 at 0 K in agreement with previous theoretical findings. 37,54,55 The nonferroelectric ground state thus predicted is robust to variation of the vdW scheme employed or use of hybrid functionals (see the Supporting Information(SI)). Our conclusion agrees with several X-ray diffraction experiments 30,33,34 and with more recent reports of a lack of ferroelectric switching 37 in the orthorhombic phase.…”

“…The transitions are associated with increasing disorder of MA. , The exact nature of the orientational order in the δ (<149.5 K) and β (155.1–236.9 K) phase has been under debate, with some studies assigning nonpolar space groups to these structures, ,, while others claiming them to be polar. ,, Notably, a recent study employing a combination of dielectric measurements, pyroelectric current, and positive-up negative-down measurements claimed the presence of polar ferroelectric behavior along ⟨001⟩ in the β phase, ruling out the assignment of the nonpolar I 4/ mcm space group . However, this claim has been subsequently challenged, and the observation of pyroelectric current was attributed to the presence of metastable polar states. On the other hand, although the high-temperature α phase (>236.9 K) is described as cubic Pm 3̅ m , ,,, recent studies , identified the presence of short-range octahedral distortion at room temperature leading to breaking of cubic symmetry.…”

Remanent Polarization and Strong Photoluminescence Modulation by an External Electric Field in Epitaxial CsPbBr3 Nanowires

“…That is, PL modulation can be in-phase (for sinewave V initial > 0, Figure 2b) or antiphase (for π-shifted sinewave V initial < 0, Figure 2c) with the oscillating field, which appears to indicate a rapidly formed internal field in the nanowires, opposing the direction of the very initial external Efield applied. Furthermore, once the response direction is set, it cannot be flipped (on a measured time scale of several weeks), even when applying opposite DC fields of a higher magnitude (up to 2 × 10 7 V/m), meaning the system does not behave as a ferroelectric, 12 but rather resembles an electret, 13 a material exhibiting a dynamically stable electric dipole. 14 We note that more than 80% of the 54 measured nanowires showed PL modulation above the noise level, all responding asymmetrically, out of which about 70% followed the direction of the initial external field.…”