Microscopic Origins of the Ferroelectric and Ferroelastic Effects in Hybrid Halide Perovskites

The rapid progress in creating high-efficiency photovoltaic devices from hybrid and all-inorganic halide perovskites in the last decade has seen increasing interest in the physical properties of these materials. Major differences are observed with respect to conventional semiconductors in terms of the dynamical lattice properties, with these novel processes thought to underlie some of the novel optoelectronic properties reported. Thermal lattice fluctuations, known as phonons, are soft and diverge from the classical harmonic picture, and thus effectively act as a previously unseen source of structural fluctuations. The organic cation with finite dipole moment, an integral part of the hybrid perovskite lattice, exhibits stochastic reorientation introducing additional perturbations that change the potential energy landscape experienced by charge carriers.<br/>To allow for the engineering of novel halide perovskite materials in the quest for further performance improvements, it is necessary to enable a high control of their lattice dynamics. To deepen our understanding, this study utilizes X-ray, Brillouin and inelastic neutron scattering techniques to cover a wide dynamical range in energy and reciprocal space.<br/>It is crucial to experimentally disentangle the two above-mentioned entropy sources, cation reorientations and anharmonic fluctuations. This was achieved by inducing structural phase transitions by lowering the temperature of the lattice, and by varying the composition at both A-site and X-site in the perovskite ABX3 lattice.<br/>This study demonstrates that while formamidinium based perovskites have the acoustic phonon properties attributed to classical semiconductors, the inclusion of methylammonium cations results in strong coupling between the cations and the inorganic lattice, leading to acoustic phonon softening near the zone centre. The combination of methylammonium at A-site and bromine at X site results in local centro-symmetry breaking, as detected through the observation of the piezoelectric coupling between methylammonium and acoustic phonons near the zone centre.<br/>Diffuse scattering rods stretching across the Brillouin zone edges are observed at low temperatures through single crystal X-ray diffraction in the MAPbBr1.5Cl1.5 mixed halide perovskite. The elastic neutron scattering confirms the presence of diffuse scattering signal along the R-M Brillouin zone edge line in MAPbBr3 and FAPbBr3 perovskites in high temperature cubic and tetragonal phases. This is direct evidence for the presence of short range correlated nano-domains, a manifestation of the dynamically disordered halide perovskite lattice.<br/>In summary, this work investigates how the anion and cation composition influences whether domains possess ferroelastic or ferroelectric character. The insights gained can be exploited for further improvements in perovskite based solar cells, either through harvesting ferroelectric effects or achieving better strain control.