An expansive library of structurally complex two-dimensional (2D) and three-dimensional (3D) lead halide perovskites has emerged over the past decade, finding applications in various aspects of photon management: photovoltaics, photodetection, light emission, and nonlinear optics. Needless to say, the highest degree of structural plasticity enjoys the former group, offering a rich playground for modifications of relevant optoelectronic parameters such as exciton energy. Structural tailorability is reflected in the ease of modification of the chemistry of the organic layers residing between inorganic slabs. In this vein, we show that the introduction of methylhydrazinium cation (MHy⁺, CH₃NH₂NH₂⁺) into 2D perovskite gives a material with a record low separation of the inorganic layers (8.91 Å at 300 K). Optical studies showed that MHy₂PbBr₄ features the most red-shifted excitonic absorption among all known A₂PbBr₄ compounds as well as a small exciton binding energy of 99.9 meV. MHy₂PbBr₄ crystallizes in polar Pmn2₁ symmetry at room emperature (phase III) and at 351 K undergoes a phase transition to modulated Pmnm phase (II) followed by another phase transition at 371 K to Pmnm phase (I). The ferroelectric property of room-temperature phase III is inferred from switching of the pyrocurrent, dielectric measurements, and optical birefringence results. MHy₂PbBr₄ exhibits multiple nonlinear optical phenomena such as second-harmonic generation, third-harmonic generation, two-photon excited luminescence, and multiphoton excited luminescence. Analysis of MHy₂PbBr₄ single-crystal luminescence spectra obtained through linear and nonlinear optical excitation pathways indicates that free exciton emission is readily probed by the ultraviolet excitation, whereas crumpled exciton emission is detected under two- and multiphoton excitation conditions. Overall, our results demonstrate that incorporation of MHy⁺ into the organic layer is an emergent strategy for obtaining a 2D perovskite with polar character and multifunctional properties.

We have studied the electrocaloric response of the archetypal antiferroelectric $\mathrm{PbZr}{\mathrm{O}}_3$ as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of −3.5 K to an electroheating of $+5.5\mathrm{K}$, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics.

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO₃ and AgNbO₃) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity.

We present an X-ray absorption spectroscopy study on Fe-doped SrTiO₃ thin films grown by pulsed laser deposition. The Fe L_2,3 edge spectra are recorded for doping concentrations from 0–5% after several annealing steps at moderate temperatures. The Fe valence state is determined by comparison with an ilmenite reference sample and calculations according to the charge transfer multiplet model. We found clear evidence of Fe²⁺ and Fe³⁺ oxidation states independently of the doping concentration. The Fe²⁺ signal is enhanced at the surface and increases after annealing. The Fe²⁺ configuration is in contrast to the mixed Fe³⁺/Fe⁴⁺ valence state in bulk material and must be explained by the specific defect structure of the thin films due to the kinetically limited growth which induces a high concentration of oxygen vacancies.

Epitaxial thin films of Fe doped SrTiO₃ have been studied by the use of resonant photoemission. This technique allowed us to identify contributions of the Fe and Ti originating electronic states to the valence band. Two valence states of iron Fe²⁺ and Fe³⁺, detected on the base of x-ray absorption studies spectra, appeared to form quite different contributions to the valence band of SrTiO₃. The electronic states within the in-gap region can be attributed to Fe and Ti ions. The Fe²⁺ originating states which can be connected to the presence of oxygen vacancies form a broad band reaching binding energies of about 0.5 eV below the conduction band, while Fe³⁺ states form in the gap a sharp feature localized just above the top of the valence band. These structures were also confirmed by calculations performed with the use of the FP-LAPW/APW+lo method including Coulomb correlations within the d shell. It has been shown that Fe doping induced Ti originating states in the energy gap which can be related to the hybridization of Ti and Fe 3d orbitals.