Currently, the electrocaloric effect (ECE) is considered for applications in solid-state refrigeration. Direct semi-classical first-principles-based simulation to the case of ultra-thin PbZrO 3 films in order to unveil the unusual electrocaloric potential of antiferroelectrics with phase competition are applied. Strong enhancement of the ECE in the vicinity of the antiferroelectric-ferroelectric phase transition, the coexistence of electric-field tunable positive and negative ECE in antiferroelectrics, and a large ECE in the vicinity of lossless antipolar-polar phase transitions are predicted. Direct simulations demonstrate that phase switching, which occurs with hysteretic losses, gives rise to the irreversible heating which is explained using basic thermodynamics. A refrigeration cycle that tames the irreversible heating is proposed, tested in direct simulations, and found to outperform the conventional cycle based on fully reversible processes. Furthermore, direct simulations reveal critical misconceptions in the application of the controversial indirect approach to the case of ferroics with electric hysteresis. The route to overcoming the uncertainties with this dominant ECE investigation approach is proposed.

Inverse-hybrid perovskites (IHPs) with large polarization have recently been predicted from first-principles computations. We use one representative from the IHP class of materials, (CH3NH3)3OI (MA3OI), to propose a route to the first-principles prediction of structural and electrical properties, such as polarization, polarization reversibility, and the associated coercive field for hybrid organic–inorganic perovskites. The route relies on the construction of the polarization reversal path that models experimental measurements. Such a path was found to play an important role in the ground-state search as well as in the identification of competing structural variations. The latter is believed to be the origin of the structural disorder that is characteristic of hybrid organic–inorganic perovskites. The application of such an approach to MA3OI leads to the prediction of several structural variations that are expected to result in a structurally disordered phase above 766 K and of the polar ground state with a polarization of 25.3 μC/cm2 that is reversible with the application of an electric field. The upper estimate for the coercive field associated with homogeneous polarization reversal is 6.9 GV/m. The piezoelectric constants of MA3OI are predicted to be an order of magnitude smaller in comparison with a prototypical inorganic ferroelectric PbTiO3; however, the low symmetry of the MA3OI structure yields finite values for all components of the piezoelectric tensor. The polarization in MA3OI is tunable by the epitaxial strain (11.5% change under 5% epitaxial strain), although less so as compared with PbTiO3.

Spatial symmetries occur in combination with temporal symmetries in a wide range of physical systems in nature, including time-periodic quantum systems typically described by the Floquet formalism. In this context, groups formed by three-dimensional point group symmetry operations in combination with time translation operations are discussed in this work. The derivation of these 'spatio-temporal' groups from conventional point groups and their irreducible representations is outlined, followed by a complete listing. The groups are presented in a template similar to space group operations, and are visualized using a modified version of conventional stereographic projections. Simple examples of physical processes that simultaneously exhibit symmetry in space and time are identified and used to illustrate the application of spatio-temporal groups.

Atomistic first-principles-based simulations are used to investigate polarization reversal in ferroelectrics in both the intrinsic and extrinsic regimes in order to determine the origin of nearly an order of magnitude difference in the coercive field predicted theoretically and observed in experiments. We find that the residual depolarizing field that is routinely ignored from considerations is responsible for the drastic reduction of the coercive field. The depolarizing field stabilizes a polydomain phase which allows for counterintuitive cooperation with the applied field to achieve polarization reversal in an energy efficient way. Contrary to the common belief that low coercive field necessitates polarization reversal via domains formation and propagation, we predict that the same fields could be achieved if the residual depolarizing field is taken into account. An efficient way to incorporate such depolarizing field in any type of atomistic simulations is proposed, which is expected to resolve the long-standing issue of overestimation of fields in simulations of ferroelectrics.