Three-State Dielectric Switching within a Narrow Temperature Range in Isopropylammonium Lead Iodide, a One-Dimensional Perovskite with Polar Phase.

The phenomenon of dielectric switching has garnered considerable attention due to its potential applications in electronic and photonic devices. Typically, hybrid organic-inorganic perovskites, HOIPs, exhibit a binary (low-high) dielectric state transition, which, while useful, represents only the tip of the iceberg in terms of functional relevance. One way to boost the versatility of applications is the discovery of materials capable of nonbinary switching schemes, such as three-state dielectric switching. The ideal candidate for that task would exhibit a trio of attributes: two reversible, first-order phase transitions across three distinct crystal phases, minimal thermal hysteresis, and pronounced, step-like variations in dielectric permittivity, with a substantial change in its real part. Here, we demonstrate a one-dimensional lead halide perovskite with the formula (CH3)2C(H)NH3)PbI3, abbreviated as ISOPrPbI3, that fulfills these criteria and demonstrates three-state dielectric switching within a narrow temperature range of ca. 45 K. Studies on ISOPrPbI3 also revealed the polar nature of the low-temperature phase III below 266 K through pyrocurrent experiments, and the noncentrosymmetric character of the intermediate phase II and low-temperature phase III is confirmed via second harmonic generation measurements. Additionally, luminescence studies of ISOPrPbI3 have demonstrated combined broadband and narrow emission properties. The introduction of ISOPrPbI3 as a three-state dielectric switch not only addresses the limitations posed by the wide thermal gap between dielectric states in previous materials but also opens new avenues for the development of nonbinary dielectric switchable materials.

Revisiting a (001)-oriented layered lead chloride templated by 1,2,4-triazolium: structural phase transitions, lattice dynamics and broadband photoluminescence.

This study revisits a (001)-oriented layered lead chloride templated by 1,2,4-triazolium, Tz2PbCl4, which recently has been an object of intense research but still suffers from gaps in characterization. Indeed, the divergent reports on the crystal structures of Tz2PbCl4 at various temperatures, devoid of independent verification of chiral phases through second harmonic generation (SHG), have led to an unresolved debate regarding the existence of a low-temperature phase transition (PT) and the noncentrosymmetric nature of the low-temperature phase. Now, by combining differential scanning calorimetry, single-crystal X-ray diffraction, dielectric, as well as linear and nonlinear optical spectroscopies on Tz2PbCl4, we reveal a sequence of reversible PTs at T1 = 361 K (phase I-II), T2 = 339 K (phase II-III), and T3 = 280 K (phase III-IV). No SHG activity could be registered for any of the four crystal phases, as checked by wide-temperature range SHG screening, supporting their centrosymmetry. The dipole relaxation processes indicate a decrease in activation energy with increasing temperature, from 0.60, 0.38, to 0.24 eV observed for phase IV (space group P21/c), phase III (Pnma), and phase II (Cmcm), respectively. This change is interpreted as a result of the diminishing strength of H-bonds as the system transforms from phase IV to III and subsequently to II. The weaker H-bonds facilitate the reorientation of Tz+ cations in the presence of an external electric field. The photoluminescence spectra of Tz2PbCl4 reveal an intriguing interplay of narrow and broadband emission, linked respectively to free excitons and excitons trapped on defects. Notably, as the temperature decreases from 300 K to 16 K, both the emission bands exhibit distinctive blue and red shifts, indicative of increased in-plane octahedral distortion. This dynamic behaviour transforms the photoluminescence of Tz2PbCl4 from greenish-blue at 300 K to yellowish-green at 13 K, enriching our understanding of 2D lead halide perovskites and highlighting the optoelectronic potential of Tz2PbCl4.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites.

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Phase Transitions, Dielectric Response, and Nonlinear Optical Properties of Aziridinium Lead Halide Perovskites.

Hybrid organic-inorganic lead halide perovskites are promising candidates for next-generation solar cells, light-emitting diodes, photodetectors, and lasers. The structural, dynamic, and phase-transition properties play a key role in the performance of these materials. In this work, we use a multitechnique experimental (thermal, X-ray diffraction, Raman scattering, dielectric, nonlinear optical) and theoretical (machine-learning force field) approach to map the phase diagrams and obtain information on molecular dynamics and mechanism of the structural phase transitions in novel 3D AZRPbX3 perovskites (AZR = aziridinium; X = Cl, Br, I). Our work reveals that all perovskites undergo order-disorder phase transitions at low temperatures, which significantly affect the structural, dielectric, phonon, and nonlinear optical properties of these compounds. The desirable cubic phases of AZRPbX3 remain stable at lower temperatures (132, 145, and 162 K for I, Br, and Cl) compared to the methylammonium and formamidinium analogues. Similar to other 3D-connected hybrid perovskites, the dielectric response reveals a rather high dielectric permittivity, an important feature for defect tolerance. We further show that AZRPbBr3 and AZRPbI3 exhibit strong nonlinear optical absorption. The high two-photon brightness of AZRPbI3 emission stands out among lead perovskites emitting in the near-infrared region.

Probing Methyl Group Tunneling in [(CH3)2NH2][Zn(HCOO)3] Hybrid Perovskite Using Co2+ EPR.

At low temperature, methyl groups act as hindered quantum rotors exhibiting rotational quantum tunneling, which is highly sensitive to a local methyl group environment. Recently, we observed this effect using pulsed electron paramagnetic resonance (EPR) in two dimethylammonium-containing hybrid perovskites doped with paramagnetic Mn2+ ions. Here, we investigate the feasibility of using an alternative fast-relaxing Co2+ paramagnetic center to study the methyl group tunneling, and, as a model compound, we use dimethylammonium zinc formate [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite. Our multifrequency (X-, Q- and W-band) EPR experiments reveal a high-spin state of the incorporated Co2+ center, which exhibits fast spin-lattice relaxation and electron spin decoherence. Our pulsed EPR experiments reveal magnetic field independent electron spin echo envelope modulation (ESEEM) signals, which are assigned to the methyl group tunneling. We use density operator simulations to extract the tunnel frequency of 1.84 MHz from the experimental data, which is then used to calculate the rotational barrier of the methyl groups. We compare our results with the previously reported Mn2+ case showing that our approach can detect very small changes in the local methyl group environment in hybrid perovskites and related materials.

Synthesis, Photoluminescence and Vibrational Properties of Aziridinium Lead Halide Perovskites.

Three-dimensional lead halide perovskites are known for their excellent optoelectronic properties, making them suitable for photovoltaic and light-emitting applications. Here, we report for the first time the Raman spectra and photoluminescent (PL) properties of recently discovered three-dimensional aziridinium lead halide perovskites (AZPbX3, X = Cl, Br, I), as well as assignment of vibrational modes. We also report diffuse reflection data, which revealed an extended absorption of light of AZPbX3 compared to the MA and FA counterparts and are beneficial for solar cell application. We demonstrated that this behavior is correlated with the size of the organic cation, i.e., the energy band gap of the cubic lead halide perovskites decreases with the increasing size of the organic cation. All compounds show intense PL, which weakens on heating and shifts toward higher energies. This PL is red shifted compared to the FA and MA counterparts. An analysis of the PL data revealed the small exciton binding energy of AZPbX3 compounds (29-56 meV). Overall, the properties of AZPbX3 are very similar to those of the well-known MAPbX3 and FAPbX3 perovskites, indicating that the aziridinium analogues are also attractive materials for light-emitting and solar cell applications.

[Methylhydrazinium]2PbCl4, a Two-Dimensional Perovskite with Polar and Modulated Phases.

Two-dimensional (2D) lead halide perovskites are a family of materials at the heart of solar cell, light-emitting diode, and photodetector technologies. This perspective leads to a number of synthetic efforts toward materials of this class, including those with prescribed polar architectures. The methylhydrazinium (MHy+) cation was recently presumed to have an unusual capacity to generate non-centrosymmetric perovskite phases, despite its intrinsically nonchiral structure. Here, we witness this effect once again in the case of the Ruddlesden-Popper perovskite phase of formula MHy2PbCl4. MHy2PbCl4 features three temperature-dependent crystal phases, with two first-order phase transitions at T1 = 338.2 K (331.8 K) and T2 = 224.0 K (205.2 K) observed in the heating (cooling) modes, respectively. Observed transitions involve a transformation from high-temperature orthorhombic phase I, with the centrosymmetric space group Pmmn, through the room-temperature modulated phase II, with the average structure being isostructural to I, to the low-temperature monoclinic phase III, with non-centrosymmetric space group P21. The intermediate phase II is a rare example of a modulated structure in 2D perovskites, with Pmmn(00γ)s00 superspace symmetry and modulation vector q ≅ 0.25c*. MHy2PbCl4 beats the previous record of MHy2PbBr4 in terms of the shortest inorganic interlayer distance in 2D perovskites (8.79 Šat 350 K vs 8.66 Šat 295 K, respectively). The characteristics of phase transitions are explored with differential scanning calorimetry, dielectric, and Raman spectroscopies. The non-centrosymmetry of phase III is confirmed with second harmonic generation (SHG) measurements, and polarity is demonstrated by the pyroelectric effect. MHy2PbCl4 also exhibits thermochromism, with the photoluminescence (PL) color changing from purplish-blue at 80 K to bluish-green at 230 K. The demonstration of polar characteristics for one more member of the methylhydrazinium perovskites settles a debate about whether this approach can present value for the crystal engineering of acentric solids similar to that which was recently adopted by a so-called fluorine substitution effect.

Effect of Dimensionality on Photoluminescence and Dielectric Properties of Imidazolium Lead Bromides.

Hybrid organic-inorganic lead halide perovskites have emerged as promising materials for various applications, including solar cells, light-emitting devices, dielectrics, and optical switches. In this work, we report the synthesis, crystal structures, and linear and nonlinear optical as well as dielectric properties of three imidazolium lead bromides, IMPbBr3, IM2PbBr4, and IM3PbBr5 (IM+ = imidazolium). We show that these compounds exhibit three distinct structure types. IMPbBr3 crystallizes in the 4H-hexagonal perovskite structure with face- and corner-shared PbBr6 octahedra (space group P63/mmc at 295 K), IM2PbBr4 adopts a one-dimensional (1D) double-chain structure with edge-shared octahedra (space group P1̅ at 295 K), while IM3PbBr5 crystallizes in the 1D single-chain structure with corner-shared PbBr6 octahedra (space group P1̅ at 295 K). All compounds exhibit two structural phase transitions, and the lowest temperature phases of IMPbBr3 and IM3PbBr5 are noncentrosymmetric (space groups Pna21 at 190 K and P1 at 100 K, respectively), as confirmed by measurements of second-harmonic generation (SHG) activity. X-ray diffraction and thermal and Raman studies demonstrate that the phase transitions feature an order-disorder mechanism. The only exception is the isostructural P1̅ to P1̅ phase transition at 141 K in IM2PbBr4, which is of a displacive type. Dielectric studies reveal that IMPbBr3 is a switchable dielectric material, whereas IM3PbBr5 is an improper ferroelectric. All compounds exhibit broadband, highly shifted Stokes emissions. Features of these emissions, i.e., band gap and excitonic absorption, are discussed in relation to the different structures of each composition.

Structural, magnetic and photoluminescence properties of new hybrid hypophosphites: discovery of the first noncentrosymmetric and two cobalt-based members.

Hybrid organic-inorganic perovskites comprising hypophosphite ligands are emerging functional materials exhibiting magnetic, photoluminescence, negative thermal expansion and negative linear compressibility behaviours. This work reports five novel hypophosphite perovskites, [A]M(H2POO)3 (A = pyrrolidinium (PYR+), guanidinium (GUA+) and imidazolium (IM+); M = Cd2+ and Co2+). [GUA]Cd(H2POO)3, [IM]Cd(H2POO)3, [GUA]Co(H2POO)3 and [IM]Co(H2POO)3 belong to the centrosymmetric trigonal R3̄c, monoclinic P21/c, monoclinic I2/m, and orthorhombic Pbca space groups, respectively, while [PYR]Cd(H2POO)3 crystallizes in the noncentrosymmetric orthorhombic space group Aea2. The polar order of PYR+ cations was confirmed by observation of moderate second harmonic generation (SHG) activity. Magnetic studies reveal that [GUA]Co(H2POO)3 and [IM]Co(H2POO)3 are weak ferromagnets with the ordering temperatures higher compared to their manganese analogues. Upon ultraviolet excitation, the cadmium counterparts exhibit purplish-blue emissions at low temperatures, which decrease on heating. Analysis of the photoluminescence data reveals that the emission quenching decreases with decreasing distortion of the cadmium-hypophosphite framework. Discovery of the new hypophosphites exhibiting magnetic or polar order and photoluminescence properties shows that hypophosphite perovskites offer a promising platform for generating new functional materials, including those that are light emitting, ferroelectric and multiferroic.

Quantification of Exciton Fine Structure Splitting in a Two-Dimensional Perovskite Compound.

Applications of two-dimensional (2D) perovskites have significantly outpaced the understanding of many fundamental aspects of their photophysics. The optical response of 2D lead halide perovskites is dominated by strongly bound excitonic states. However, a comprehensive experimental verification of the exciton fine structure splitting and associated transition symmetries remains elusive. Here we employ low temperature magneto-optical spectroscopy to reveal the exciton fine structure of (PEA)2PbI4 (here PEA is phenylethylammonium) single crystals. We observe two orthogonally polarized bright in-plane free exciton (FX) states, both accompanied by a manifold of phonon-dressed states that preserve the polarization of the corresponding FX state. Introducing a magnetic field perpendicular to the 2D plane, we resolve the lowest energy dark exciton state, which although theoretically predicted, has systematically escaped experimental observation (in Faraday configuration) until now. These results corroborate standard multiband, effective-mass theories for the exciton fine structure in 2D perovskites and provide valuable quantification of the fine structure splitting in (PEA)2PbI4.

Multiple rotor modes and how to trigger them: complex cation ordering in the family of relaxing hybrid formates.

A combination of structural, dielectric and calorimetric studies is used to describe a highly atypical behaviour of novel hybrid formate [NH3(CH2)3NH2(CH2)3NH3][Mn(HCOO)3]3, incorporating large triprotonated molecular cations. Two successive phase transitions, switching between fast multiple rotor modes, and the surprising probable coexistence of static and dynamic disorder are discussed for this compound.

The cation-dependent structural, magnetic and optical properties of a family of hypophosphite hybrid perovskites.

Hypophosphite hybrid perovskites have recently received widespread attention due to their diverse structural and magnetic properties, negative thermal expansion and photoluminescence behaviour. Herein, we report two new three-dimensional hybrid perovskites containing unusually large organic cations, pyrrolidinium and 2-hydroxyethylammonium. We report the crystal structures of these new manganese-hypophosphite frameworks and their magnetic and optical properties. We also report the magnetic and optical properties of two previously discovered analogues, dimethylammonium and imidazolium manganese hypophosphites. Both new compounds crystallize in a monoclinic structure, space group P21/n, with ordered organic cations at room temperature. Magnetic studies show that all studied compounds are examples of canted antiferromagnets but the weak ferromagnetic contribution and the ordering temperature are significantly modulated by the type of organic cation located in the cavity of the framework. We discuss the origin of this behaviour. Upon ultraviolet excitation, all compounds exhibit broadband photoluminescence associated with the 4T1g(G) → 6A1g(S) transition of octahedrally coordinated Mn2+ ions. The position of the PL band depends on the type of organic cation, being the most blue-shifted for the imidazolium analogue (646 nm) and the most red-shifted for the pyrrolidinium counterpart (689 nm). The most interesting property of the studied hypophosphites is, however, the strong temperature dependence of the photoluminescence intensity, suggesting the possible application of these compounds in non-contact optical thermometry.

Benzyltrimethylammonium cadmium dicyanamide with polar order in multiple phases and prospects for linear and nonlinear optical temperature sensing.

Coordination polymers with multiple non-centrosymmetric phases have sparked substantial research efforts in the materials community. We report the synthesis and properties of a hitherto unknown cadmium dicyanamide coordination polymer comprising benzyltrimethylammonium cations (BeTriMe+). The room-temperature (RT) crystal structure of [BeTriMe][Cd(N(CN)2)3] (BeTriMeCd) is composed of Cd centers linked together by triple dca-bridges to form one-dimensional chains with BeTriMe+ cations located in void spaces between the chains. The structure is polar, the space group is Cmc21, and the spontaneous polarization in the c-direction is induced by an arrangement of BeTriMe+ dipoles. BeTriMeCd undergoes a second-order phase transition (PT) at T1 = 268 K to a monoclinic polar phase P21. Much more drastic structural changes occur at the first-order PT observed in DSC at T2 = 391 K. Raman data prove that the PT at T2 leads to extensive rearrangement of the Cd-dca coordination sphere and pronounced disorder of both dca and BeTriMe+. On cooling, the HT polymorph transforms at T3 = 266 K to another phase of unknown symmetry. Temperature-resolved second harmonic generation (TR-SHG) studies at 800 nm confirm the structural non-centrosymmetry of all the phases detected. Optical studies indicate that BeTriMeCd exhibits at low temperatures an intense emission under 266 nm excitation. Strong temperature dependence of both one-photon excited luminescence and SHG response allowed for the demonstration of two disparate modes of optical thermometry for a single material. One is the classic ratiometric luminescence thermometry employing linear excitation in the ultraviolet region while the other is single-band SHG thermometry, a thus far unprecedented subtype of nonlinear optical thermometry. Thus, BeTriMeCd is a rare example of a dicyanamide framework exhibiting polar order, SHG activity, photoluminescence properties and linear and nonlinear optical temperature sensing capability.

Cadmium and manganese hypophosphite perovskites templated by formamidinium cations: dielectric, optical and magnetic properties.

The recently discovered hypophosphite perovskites are promising functional materials. This contribution is devoted to the structural, thermal, dielectric, Raman and optical studies of a new hybrid organic-inorganic perovskite, [FA]Cd(H2POO)3 (FA = formamidinium, NH2CHNH2+). We also report the thermal, magnetic, dielectric and optical properties of the known perovskite [FA]Mn(H2POO)3. [FA]Cd(H2POO)3 crystallizes in a monoclinic structure, with the space group C2/c, and transforms at 190 K to another monoclinic structure, with the space group P21/n. For both compounds, the FA+ is disordered through the two-fold axis in the high-temperature (HT) phases. However, lowering of the temperature of [FA]Mn(H2POO)3 results in the complete ordering of FA+, while the organic cations still occupy two positions in [FA]Cd(H2POO)3. Raman data provide strong evidence that both FA+ cations have the same or a very similar structure and that the phase transition is triggered by an ordering of the FA+ cations. The dielectric studies confirm the order-disorder nature of the phase transition and reveal the presence of two dipolar relaxations observed in the 320-220 K range and near the phase transition temperature. The low-temperature (LT) process exhibits the classical Arrhenius-type behaviour, whereas the behaviour of the HT relaxation suggests glass-like behaviour. Magnetic studies show that [FA]Mn(H2POO)3 is an example of a canted antiferromagnet with a low ordering temperature of 2.4 K. Diffuse reflectance studies show that the Mn (Cd) hypophosphite is a wide bandgap material with Eg = 5.20 eV (5.42 eV). Upon ultraviolet excitation (266 nm), [FA]Mn(H2POO)3 exhibits reddish-orange emission at 656 nm associated with the 4T1g(G) →6A1g(S) transition of octahedrally coordinated Mn2+ ions. [FA]Cd(H2POO)3 shows significantly weaker purplish-blue emission at low temperatures composed of two bands at 425 and 443 nm, which are almost completely quenched at 160 K. For both compounds, CIE chromaticity coordinates show negligible change with temperature. Furthermore, the intensity of the observed emissions decreases quickly with increasing temperature, suggesting the possible application of [FA]Mn(H2POO)3 in non-contact optical thermometry.

Vibrational Properties and DFT Calculations of Perovskite-Type Methylhydrazinium Manganese Hypophosphite.

Recently discovered hybrid perovskites based on hypophosphite ligands are a promising class of compounds exhibiting unusual structural properties and providing opportunities for construction of novel functional materials. Here, we report for the first time the detailed studies of phonon properties of manganese hypophosphite templated with methylhydrazinium cations ([CH3NH2NH2][Mn(H2PO2)3]). Its room temperature vibrational spectra were recorded for both polycrystalline sample and a single crystal. The proposed assignment based on Density Functional Theory (DFT) calculations of the observed vibrational modes is also presented. It is worth noting this is first report on polarized Raman measurements in this class of hybrid perovskites.

Suppression of phase transitions and glass phase signatures in mixed cation halide perovskites.

Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles.

Pyrrolidinium-Based Cyanides: Unusual Architecture and Dielectric Switchability Triggered by Order-Disorder Process.

Two three-dimensional metal-organic compounds of the formula Pyr2KM(CN)6, where M = Co, Fe and Pyr = pyrrolidinium ((CH2)4NH2+), have been found to crystallize at room temperature in a monoclinic structure, space group P21/c. They are cyano-bridged compounds with an unprecedented type of architecture containing pyrrolidinium cations in the voids. The materials have been investigated by X-ray diffraction, dielectric, and spectroscopic methods as a function of temperature in order to determine their properties and the mechanism of the reversible phase transitions occurring at ca. 345-370 K. The phase transitions in both crystals are first order and are associated with a symmetry increase to a rhombohedral structure (space group R3̅m) as well as a significant disorder of organic cations above Tc. On the basis of Raman scattering and IR spectroscopy it has been assumed that the phase transition in both crystals is triggered by thermally induced pseudorotation of the organic cation and large out-of-plane motions of its atoms followed by a "click-in" of the cyanide bridges. The materials have been proposed as possible switchable dielectrics due to their respective high differences in dielectric permittivities across the phase transition.

Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn2+ doped [NH4][Zn(HCOO)3] hybrid formate framework.

We present an X- and Q-band continuous wave (CW) and pulse electron paramagnetic resonance (EPR) study of a manganese doped [NH4][Zn(HCOO)3] hybrid framework, which exhibits a ferroelectric structural phase transition at 190 K. The CW EPR spectra obtained at different temperatures exhibit clear changes at the phase transition temperature. This suggests a successful substitution of the Zn2+ ions by the paramagnetic Mn2+ centers, which is further confirmed by the pulse EPR and 1H ENDOR experiments. Spectral simulations of the CW EPR spectra are used to obtain the temperature dependence of the Mn2+ zero-field splitting, which indicates a gradual deformation of the MnO6 octahedra indicating a continuous character of the transition. The determined data allow us to extract the critical exponent of the order parameter (ß = 0.12), which suggests a quasi two-dimensional ordering in [NH4][Zn(HCOO)3]. The experimental EPR results are supported by the density functional theory calculations of the zero-field splitting parameters. Relaxation time measurements of the Mn2+ centers indicate that the longitudinal relaxation is mainly driven by the optical phonons, which correspond to the vibrations of the metal-oxygen octahedra. The temperature behavior of the transverse relaxation indicates a dynamic process in the ordered ferroelectric phase.

Novel hypophosphite hybrid perovskites of [CH3NH2NH2][Mn(H2POO)3] and [CH3NH2NH2][Mn(H2POO)2.83(HCOO)0.17] exhibiting antiferromagnetic order and red photoluminescence.

Hybrid perovskites based on hypophosphite ligands constitute an emerging family of compounds exhibiting unusual structures and offering a platform for construction of novel functional materials. We report the synthesis, crystal structure, and magnetic and optical properties of novel undoped and HCOO--doped manganese hypophosphite frameworks templated by methylhydrazinium cations. The undoped compound crystallizes in a three-dimensional perovskite-like orthorhombic structure, space group Pnma, with ordered organic cations located in windows between the perovskite cages expanding along the a-direction. Both conventional anti-phase tilting, unconventional in-phase tilting and columnar shifts in the a-direction are present. Doping with HCOO- ions has a insignificant influence on the crystal structure but leads to a decrease of the unit cell volume. Magnetic studies indicate that these compounds order antiferromagnetically at T N = 6.5 K. Optical studies indicate that they exhibit red photoluminescence under 266 nm excitation with the activation energy for thermal quenching of 98 and 65 meV for the undoped and doped sample, respectively. For the undoped sample, the emission lifetime reaches 5.05 ms at 77 K but it decreases to 62.26 µs at 300 K. The low value of the activation energy and huge temperature dependence of photoluminescence intensity suggest a high potential of these hypophosphites for non-contact temperature sensing.

Crystal growth, IR specular reflectance and polarized Raman studies of LiNa5Mo9O30 polar single crystal.

A non-centrosymmetric, polar LiNa5Mo9O30 single crystal was grown using Kyropoulos technique and characterized by polarized Raman scattering and polarized infrared specular reflection spectroscopy at room temperature. The comparison to the polycrystalline spectra has been made and assignment of all observed modes to the respective vibrations and their symmetry has been proposed. Based on the four parameter model the infrared reflection spectra were analyzed and the LO-TO splitting has been determined for the observed modes. Stronger anharmonicity of bands assigned to stretching vibrations of the Mo-O⋯Mo bridges has been evidenced. Our results show that this crystal is attractive and promising SRS-active nonlinear optical material for up- and down-Raman laser-frequency converters with the most intense lasing lines at 947 and 884 cm-1. Dielectric permittivity studies in the frequency domain exhibit two anomalies that are assigned to thermally activated relaxation and conductivity processes. The observed increase in electrical conductivity above 400 K is dominated by ionic contribution and change of the conductivity mechanism.