From soft harmonic phonons to fast relaxational dynamics inCH3NH3PbBr3

Abstract

The lead-halide perovskites, including CH3NH3PbBr3, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. CH3NH3PbBr3 undergoes a series of structural transitions associated with orientational order of the CH3NH3 (MA) molecular cation and tilting of the PbBr3 host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the PbBr3 framework and the quasistatic CH3NH3 dynamics at low energy transfers. At higher energy transfers, we observe a PbBr6 octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to ∼ 50-100 meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.Organic-inorganic, hybrid perovskites (OIPs) are materials based upon an inorganic perovskite host framework with an organic molecular cation occupying the interstitial space. These materials have been studied for quite some time [1], but interest has recently surged owing to their use in photovoltaic devices and to possible ferroelectricity.[2] While earlier work centered on Snbased OIPs as possible sensor materials [3, 4], recent interest has been focused towards Pb-based OIPs, due to their potential advantages in inexpensive photovoltaic devices, with efficiencies of the order of 20%.[5] We apply neutron scattering to study the coupled dynamics of the host PbBr 3 framework and the methylamonium (MA) cation in CH 3 NH 3 PbBr 3 , and discuss the possible relation with the photovoltaic properties. We map out the soft phonons and low-temperature quasistatic molecular rotations, and show that the low-temperature, harmonic fluctuations cross over to temporally overdamped dynamics at high temperature.The OIPs are composed of two sublattices ( Fig. 1 (a) for CH 3 NH 3 PbBr 3 ): the inorganic sublattice, consisting of a fully corner-bonded framework of octahedra (PbBr − 6 ); and the organic sublattice consisting of the MA molecular cation, (CH 3 NH + 3 ). At high temperatures, the structure of CH 3 NH 3 PbBr 3 is cubic (space group Pm3m), and below 235 K has a tetragonal structure in symmetry I4/mcm. [6,7] At 150 K, a transition to an unknown structure (believed to be incommensurate [8]) occurs followed by further distortion to orthorhombic (Pnma) at 148 K. [9-12] Neutron diffraction measurements have suggested that these transitions originate from tilting of the PbBr 6 octahedra and orientational ordering of the MA cation. [6,13] We note that despite minor differences in the phase diagrams, for all MAPbX 3 (X=Cl,Br,I), a transition to an ordered phase, in which the octahedra are tilted and the MA cations have relatively well-defined orientations, occurs in the temperatu...

Interplay between organic cations and inorganic framework and incommensurability in hybrid lead-halide perovskite CH3NH3PbBr3

“…The X-ray diffraction spots at 160 K shown in figure 3a are labeled for the conventional settings of the tetragonal I4/mcm structure (one antiphase octahedral tilt [23]). On the other hand, results obtained at 140 K shown in Fig 3c indicate that the ordered low temperature phase exhibits an orthorhombic Pnma symmetry, in agreement with a previous result by neutron powder diffraction by Swainson et al [24] The X-ray diffraction map at 150 K shown in figure 3b exhibits additional satellite reflections at a fixed wave vectors q away from each main reflection. Interestingly, these new satellites cannot be described by rational numbers of the reciprocal unit cell vectors.…”

“…This rapid softening behavior is drastically different compared to the peaks observed in the orthorhomic phase. Interestingly, both the frequency and its strong temperature dependence of this mode match very well with the quickly softening mode observed within the low temperature orthorhombic phase using inelastic neutron scattering [24], shown in Figure 4c. In the orthorhombic phase, this softening mode was associated purely with the tilting of the PbBr 3 octahedral framework, likely away from Brillouin zone center.…”

“…Our observation is consistent with the recent neutron scattering study by Swainson and coworkers, reporting an intermediate phase in CH 3 NH 3 PbBr 3 speculated to be an incommensurate structure. [24] The average structure of the incommensurate phase is orthorhombic, and belongs to the body-centered space group Imma, which is also a standard perovskite structure described by two equal antiphase tilts. [23] Notably, neither of these two phase transitions (tetragonal to incommensurate and incommensurate to orthorhombic) occur through an apparent group-subgroup relation, and therefore they are first-order transitions as previously described by Knop et al [25] The incommensurate phase is also twinned by a twofold rotation around hkl=(-1 0 1).…”

Giant Rashba Splitting inCH3NH3PbBr3Organic-Inorganic Perovskite

“…Since ARPES is a surface-sensitive technique, the reduced symmetry at the surface may enhance non-centrosymmetric fields and the measured Rashba splitting. In the bulk, CH 3 NH 3 PbBr 3 forms an ordered orthorhombic structure of P nma symmetry at low temperature [41,42], and a disordered cubic structure with an average P m3m symmetry at room temperature [29,36,43]. Both structures are centrosymmetric after averaging over a length scale probed by x-ray and neutron diffraction, with no Rashba-type spin-splitting expected.…”

“…It collects two-dimensional angular photoelectron distribution patterns from an acceptance cone covering −35 • to 45 • both along and perpendicular to the plane of light incidence without the need to move sample or analyzer. During cooling, we perform in situ using photoluminescence spectroscopy reproducing the phase transitions reported in literature [29], see figure S2 (b) [27].…”