Three-dimensional (3D) lead halide perovskites have emerged as a promising class of coordination polymers for solar cells, photodetectors, and light-emitting devices. These compounds thus far comprise methylammonium, formamidinium, or cesium as cations. In this work, we introduce a new methylhydrazinium 3D perovskite, CH3NH2NH2PbBr3, that crystallizes in the polar P21 structure at room temperature and undergoes a phase transition to the cubic Pm3̅m phase at 418 K. This perovskite exhibits strong second-harmonic generation activity, features switchable dielectric behavior, thermochromism, and two-photon energy upconversion under 800 nm excitation.

We report the synthesis, crystal structure, thermal, dielectric, Raman, infrared, and magnetic properties of hydrogen and deuterated divalent metal formates, [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3], where M = Ni, Mn. On the basis of Raman and IR data, assignment of the observed modes to respective vibrations of atoms is proposed. The thermal studies show that for the Ni compounds deuteration leads to a decrease of the phase transition temperature Tc by 5.6 K, whereas it has a negligible effect on Tc in the Mn analogues. This behavior excludes the possibility of proton (deuteron) movement along the N-H···O (N-D···O) bonds as the microscopic origin of the first-order phase transition observed in these crystals below 190 K. According to single-crystal X-ray diffraction, the dimethylammonium (DMA) cations are dynamically disordered at room temperature, because the hydrogen bonds between the NH2 (ND2) groups and the metal-formate framework are disordered. The highly dynamic nature of hydrogen bonds in the high-temperature phases manifests in the Raman and IR spectra through very large bandwidth of modes involving vibrations of the NH2 (ND2) groups. The abrupt decrease in the bandwidth and shifts of modes near Tc signifies the ordering of hydrogen bonds and DMA(+) cations as well as significant distortion of the metal-formate framework across the phase transition. However, some amount of motion is retained by the DMA(+) cation in the ferroelectric phase and a complete freezing-in of this motion occurs below 100 K. The dielectric studies reveal pronounced dielectric dispersion that can be attributed to slow dynamics of large DMA(+) cations. The low-temperature studies also show that magnetic properties of the studied compounds can be explained assuming that they are ordered ferrimagnetically with nearly compensated magnetic moments of Ni and Mn. IR data reveal weak anomalies below 40 K that arise due to spin-phonon coupling. Our results also show that due to structural phase transition more significant distortion of the metal-formate framework occurs for the deuterated samples.

Formation of noncentrosymmetric three-dimensional (3D) lead halide perovskites has been a widely sought after goal because the polar structure opens up new vistas to properties of these materials, e.g., improved charge separation for photovoltaics arising from ferroelectric order. Here, we report growth and unique properties of a new highly distorted 3D perovskite, methylhydrazinium lead chloride (CH3NH2NH2PbCl3, MHyPbCl 3 ). This perovskite crystallizes in polar P21 structure at room temperature, which consists of two types of PbCl6 octahedra: one weakly and another strongly deformed. The unusual deformation of every second perovskite layer is forced by the large size of methylhydrazinium cations and the ability of NH2 + terminal groups of methylhydrazinium cations to form coordination bonds with Pb2+ metal centers. On heating, MHyPbCl 3 undergoes a phase transition at 342 K into another polar Pb21 m phase with ordered organic cations. Temperature-resolved second-harmonic generation (TR-SHG) measurements confirm acentricity of both phases and show that second-harmonic response is enhanced for the high-temperature Pb21 m phase. This intriguing property of MHyPbCl 3 has been employed to demonstrate an unprecedented kind of quadratic nonlinear optical switching in which a second-harmonic response is switched between a room-temperature, low-SHG state and a high-temperature, high-SHG state. X-ray diffraction shows that enhancement of polar properties is due to rearrangement of the perovskite’s organic substructure. There is a clear pyrocurrent peak, but switching of the electric polarization could not be observed. Optical studies showed that MHyPbCl 3 is a wide-bandgap material with a bandgap of 3.4 eV (365 nm). At low temperatures, it exhibits weak UV emissions at 362 and 369 nm as well as a strong broadband white emission.

We report the synthesis, crystal structure, thermal, dielectric, IR, and Raman studies of [NH4][Mg(HCOO)3] formate. Single-crystal X-ray diffraction shows that it crystallizes in the hexagonal space group P6322, with orientationally disordered NH4(+) ions located in the cages of the network. Upon cooling, [NH4][Mg(HCOO)3] undergoes a phase transition at around 255 K to the ferroelectric P63 structure. Raman and IR spectra show a strong increase in intensity of the N-H stretching bands as well as narrowing of the bands related to the NH4(+) ions upon cooling. These changes indicate that the phase transition is due to orientational ordering of the NH4(+) ions. Analysis of the Raman data show, however, that the rotational and translational motions of NH4(+) do not freeze completely at the phase transition but exhibit further slowing down below 255 K, and the motional freezing becomes nearly complete below 140 K.

We report the synthesis of four perovskite-type metal formate frameworks, [CH3NH2NH2]­[M­(HCOO)3] (MHyM) with M = Mn, Mg, Fe, and Zn. These compounds exhibit two structural phase transitions. The first transition temperature depends weakly on a type of divalent metal and is observed at 310–327 K on heating. X-ray diffraction, DSC, and vibrational studies revealed that it has a second-order character. It is associated with partial ordering of the methylhydrazinium (MHy+) cations and change of symmetry from nonpolar R3̅c to polar R3c. Pyroelectric measurements suggest the ferroelectric nature of the room-temperature phase. The second, low-temperature phase transition has a first-order character and is associated with further ordering of the MHy+ cations and distortion of the metal formate framework. Magnetic susceptibility data show that MHyMn and MHyFe exhibit ferromagnetic-like phase transitions at 9 and 21 K, respectively. Since the low-temperature phase is polar, these compounds are possible multiferroic materials. MHyFe shows additional magnetic anomaly in the magnetically ordered state, which most likely manifests some blocking of magnetic moments. We also report high-pressure Raman scattering studies of MHyMn that revealed a pressure-induced reversible phase transition between 4.8 and 5.5 GPa. Analysis of the data indicates that the transition leads to significant changes in both the manganese formate framework and the MHy+ structure.

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 report the synthesis, crystal structure, thermal, dielectric, Raman, infrared, and magnetic properties of [(CH3)2NH2][Na(0.5)Fe(0.5)(HCOO)3] (DMNaFe), the first metal formate framework templated by organic cations, presenting an ABO3 perovskite architecture with NaO6 octahedra as building blocks of the framework. On the basis of Raman and IR data, assignment of the observed modes to respective vibrations of atoms is proposed. We have found that DMNaFe undergoes a structural phase transition at 167 K on cooling. According to the X-ray diffraction, the compound shows R3[combining macron] symmetry at 293 K and triclinic P1[combining macron] symmetry at 110 K. The DMA(+) cations are dynamically disordered in the high-temperature phase and the phase transition is associated with ordering of the DMA(+) cations and distortion of the metal formate framework. The dielectric studies reveal pronounced dielectric dispersion that can be attributed to slow dynamics of the DMA(+) cations. Based on the low-temperature magnetic studies, this compound is a weak ferromagnet with a critical temperature 8.5 K.

Layered lead halides offer potential for lightemitting and photovoltaic devices. Here, we report the synthesis of two-dimensional (2D) lead iodide with single-octahedra slabs separated by the smallest organic cation used to date in the construction of A 2 PbI 4 compounds. This compound, MHy 2 PbI 4 , where MHy + denotes methylhydrazinium cation, shows exceptionally short separation of adjacent lead iodide layers and strong N− H•••I hydrogen bonds as well as unusually small interoctahedral tilting of PbI 6 octahedra, leading to exceptionally small band gap of 2.20 eV. Photoluminescence (PL) studies show that this compound exhibits at room temperature two emission bands at 561 and 610 nm related to free exciton and bound exciton. The latter one exhibits blue shift on cooling by around 37 nm in the 80− 300 K range. As a result, PL of MHy 2 PbI 4 changes strongly with temperature from yellowish pink at 300 K to yellow-green at 80 K. MHy 2 PbI 4 undergoes three structural phase transitions. The first phase transition occurs at 320 K, and it does not show any evident change of Pmmn symmetry. The second phase transition that occurs at 298 K is associated with ordering of the MHy + cations and change of symmetry to Pccn. This phase transition leads also to pronounced steplike change of dielectric permittivity. Structural investigation indicates that major reorientation of MHy + cations and tilts of PbI 6 octahedra contribute to this switchable dielectric property. The third phase transition observed at 262 K (233 K) on cooling (heating) leads to distortion of the structure to triclinic, space group P1̅ . MHy 2 PbI 4 shows a low value of direct current conductivity σ DC at low temperatures, but the conductivity increases rapidly with increasing temperature and reaches 1.02 × 10 −7 S m −1 at 298 K. We also report temperature-dependent Raman scattering of MHy 2 PbI 4 . Analysis of Raman data revealed clear shifts and changes in bandwidths at the phase transitions that provided deeper insight into mechanisms of the structural phase transitions in this material. An interesting feature of the studied perovskite is also unusually large broadening of many bands on heating the sample from 80 to 300 K that is much larger than typically observed in A 2 PbI 4 compounds. This behavior proves that lattice dynamic effects play very important role in MHy 2 PbI 4 and that the nature of organic cation has a great impact on the lattice dynamics of 2D perovskites.