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.

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.

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.

[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.

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.

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.

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.

A Critical Review of Existing Criteria for the Prediction of Pyrochlore Formation and Stability.

Depending on intrinsic (e.g., radius ratio rule rLn/ rZr) and extrinsic factors (e.g., processing conditions), pyrochlore-type Ln2Zr2O7 oxides achieve variable degrees of structural disorder. We report on a systematic study of the structural and microstructural characteristics of the Gd2- xLn xZr2O7 system, exploring the effect of replacing Gd with a wide range of homovalent lanthanide ions (Ln = Nd, Sm, Dy, Ho, Y, and Er; x = 0.20 and 0.80). All compositions were prepared via a mechanochemical reaction between the corresponding oxides and characterized by X-ray diffraction (standard and synchrotron sources) using the Rietveld method, as well as by Raman spectroscopy. Irrespective of chemical composition, this study reveals that all compositions exhibit a fluorite-like structure. Furthermore, by firing each sample at 800 and 1400 °C, we are able to analyze the transition to pyrochlore-like structures, featuring different degrees of disorder, in all but Gd1.20Y0.80Zr2O7, which retains the fluorite structure even after heating. The structural data are used to assess the existing criteria for predicting the formation and stability of the pyrochlore structure; according to this analysis, the simple radius ratio rule ( rLn/ rZr), provides a useful and sufficiently robust criterion. Because the pyrochlore structure has a strong tendency to disorder, it is not possible to define an empirical index similar to the Goldschmidt tolerance factor for perovskite.

Temperature-dependent studies of a new two-dimensional cadmium dicyanamide framework exhibiting an unusual temperature-induced irreversible phase transition into a three-dimensional perovskite-like framework.

We report the synthesis, crystal structure, and thermal, dielectric, optical and phonon properties of a new two-dimensional (2D) cadmium(ii) complex [(C3H7)4N][Cd(N(CN)2)3]. Our results show that this compound crystallizes in a two-dimensional monoclinic structure, with the space group P2/n, with ordered tetrapropylammonium cations and disorder of some dicyanamide linkers. It undergoes a structural phase transition at 245 K into another low-temperature (LT) monoclinic structure, with the space group P21/n. X-ray diffraction, dielectric, IR and Raman studies show that freezing of the dca motions stands at the origin of the phase transition. Optical studies indicate that this material has an energy band gap of 4.83 eV and exhibits intense bluish-white emission under 266 nm excitation. Upon heating, this compound undergoes an irreversible phase transition near 390 K associated with significant bond rearrangement. The high-temperature (HT) phase has a three-dimensinal (3D) perovskite-like structure. [(C3H7)4N][Cd(N(CN)2)3] is, therefore, the first example of a hybrid organic-inorganic dicyanamide exhibiting a temperature-induced reconstructive transition from a 2D (layered) structure to a 3D (perovskite-like) structure.

Electron paramagnetic resonance of a copper doped [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite framework.

We report a continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) as well as pulse electron nuclear double resonance (ENDOR) study of Cu2+ doped [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite which exhibits a structural phase transition. The multifrequency (X, Q and W-band) CW EPR measurements allow the temperature evolution of the Cu2+ ion local environment to be studied. The spectrum of the ordered (low-temperature) phase reveals an axially distorted octahedral Cu2+ site confirming the successful replacement of the Zn2+ ions and formation of the CuO6 octahedra. The CW EPR spectrum of the disordered (high-temperature) phase shows an additional broad line which gradually diminishes on cooling. The EPR linewidth of the axially symmetric Cu2+ ion site exhibits an anomaly at the phase transition point and Arrhenius-type behavior in the disordered phase. The temperature dependent Cu2+ spin Hamiltonian parameters change abruptly at the phase transition point indicating a strong first-order character of the transition. The X-band pulse ENDOR spectrum of the ordered phase reveals several protons in the vicinity of the Cu2+ center.

Polar metal-formate frameworks templated with 1,2-diaminoethane-water assemblies showing ferromagnetic and ferroelectric properties.

A set of five novel formate frameworks templated with assemblies comprising diprotonated 1,2-diaminoethane (DAE) and a water molecule of the formula: [NH3(CH2)2NH3]M2(HCOO)6·H2O, where M = Mg, Mn, Co, Ni, Zn, has been synthesized. Four compounds crystallize in the polar R3 space group and one in the chiral P6322 space group (Ni-analog) at room temperature. The polyammonium-water assemblies, mutually joined by hydrogen bonds, fill the cavities of the frameworks and are disordered in the three latter compounds. Additional disorder is found in the Ni-sample as the DAE2+-H2O couple is placed in a special position on the 63 screw axis. IR spectroscopy provides evidence of proton dynamic disorder within the assemblies, which turns into a static one at low temperatures. The crystals preserve their arrangement up to approximately 370 K as shown by differential calorimetric measurements and temperature-dependent IR spectroscopy. The ferroelectric nature of a representative of the family, DAEMgF, at room temperature has been confirmed by pyroelectric measurements. It has been found that the spontaneous polarization may be changed by an external electric field. The magnetic studies reveal a weak ferromagnetic behavior within 8.5-35 K for magnetically active ions: Mn, Co, and Ni.

The effect of K+ cations on the phase transitions, and structural, dielectric and luminescence properties of [cat][K0.5Cr0.5(HCOO)3], where cat is protonated dimethylamine or ethylamine.

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of two novel heterometallic perovskite-type metal-organic frameworks (MOFs) of the following formula: [(CH3)2NH2][K0.5Cr0.5(HCOO)3] (DMAKCr) and [C2H5NH3][K0.5Cr0.5(HCOO)3] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R3[combining macron] space group) and undergoes an order-disorder phase transition to the monoclinic system (P1[combining macron] space group) at about 190 K. The dielectric studies confirm the presence of first-order relaxor-like structural transformation. In the high-temperature phase, the dimethylammonium cations are dynamically disordered over three equal positions and upon cooling the dynamical disorder evolves into a two-fold one. This partial ordering is accompanied by a small distortion of the metal-formate framework. EtAKCr crystallizes in a monoclinic structure (P21/n space group) with ordered EtA+ cations and does not experience any phase transition. The differences in the thermal behavior caused by the substitution of Na+ ions by larger K+ ions in the [cat]MIMIII (cat = DMA+, EtA+, MI = Na+, K+ and MIII = Cr3+ and Fe3+) heterometallic MOF family are discussed taking into account the impact of the hydrogen bond (HB) pattern and other factors affecting the stability of metal-formate frameworks. The optical studies show that DMANaCr and EtAKCr exhibit Cr3+-based emission characteristics for intermediate ligand field strength.

Effect of solvent, temperature and pressure on the stability of chiral and perovskite metal formate frameworks of [NH2NH3][M(HCOO)3] (M = Mn, Fe, Zn).

We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NH2NH3][M(HCOO)3] (HyM) compounds (M = Mn, Zn, Fe). Our results show that synthesis from methanol solution leads to perovskite polymorphs while that from 1-methyl-2-pyrrolidinone or its mixture with methanol allows obtaining chiral polymorphs. Perovskite HyFe, chiral HyFe and chiral HyMn undergo phase transitions at 347, 336 and 296 K, respectively, with symmetry changes from Pnma to Pna21, P63 to P212121 and P63 to P21. X-ray diffraction and Raman studies show that the phase transitions are governed by dynamics of the hydrazinium ions. Low-temperature magnetic studies show that these compounds exhibit magnetic ordering below 9-12.5 K. Since the low-temperature structures of chiral HyMn and perovskite HyFe are polar, these compounds are possible multiferroic materials. We also report high-pressure Raman scattering studies of chiral and perovskite HyZn, which show much larger stiffness of the latter phase. These studies also show that the ambient pressure polar phases are stable up to at least 1.4 and 4.1 GPa for the chiral and perovskite phase, respectively. Between 1.4 and 2.0 GPa (for chiral HyZn) and 4.1 and 5.2 GPa (for perovskite HyZn) pressure-induced transitions are observed associated with changes in the zinc-formate framework. Strong broadening of Raman bands and the decrease in their number for the high-pressure phase of chiral HyZn suggest that this phase is disordered and has higher symmetry than the ambient pressure one.

Structural, thermal, dielectric and phonon properties of perovskite-like imidazolium magnesium formate.

We report the synthesis and characterisation of a magnesium formate framework templated by protonated imidazole. Single-crystal X-ray diffraction data showed that this compound crystallizes in the monoclinic structure in the P21/n space group with lattice parameters a = 12.1246(4) Å, b = 12.2087(5) Å, c = 12.4991(4) Å and ß = 91.39(1)°. The antiparallel arrangement of the dipole moments associated with imidazolium cations suggests the antiferroelectric character of the room-temperature phase. The studied compound undergoes a structural phase transition at 451 K associated with a halving of the c lattice parameter and the disappearance of the antiferroelectric order. The monoclinic symmetry is preserved and the new metrics are a = 12.261(7) Å, b = 12.290(4) Å, c = 6.280(4) Å, and ß = 90.62(5)°. Raman and IR data are consistent with the X-ray diffraction data. They also indicate that the disorder of imidazolium cations plays a significant role in the mechanism of the phase transition. Dielectric data show that the phase transition is associated with a relaxor nature of electric ordering. We also report high-pressure Raman scattering studies of this compound that revealed the presence of two pressure-induced phase transitions near 3 and 7 GPa. The first transition is most likely associated with a rearrangement of the imidazolium cations without any significant distortion of these cations and the magnesium formate framework, whereas the second transition leads to strong distortion of both the framework and imidazolium cations. High-pressure data also show that imidazolium magnesium formate does not show any signs of amorphization up to 11.4 GPa.

Temperature- and pressure-dependent studies of niccolite-type formate frameworks of [NH3(CH2)4NH3][M2(HCOO)6] (M = Zn, Co, Fe).

We report temperature-dependent electric, IR and Raman studies of niccolite-type formate frameworks templated by protonated 1,4-diaminobutane. Our results show that the zinc-analogue exhibits a first-order phase transition close to 240 K. Single-crystal dielectric data show a much stronger anomaly at the phase transition for ε' along the a-direction compared to the c-direction. They also reveal that dipole relaxation exists in bnZn. Pronounced temperature-dependence observed for bending and torsion modes of the NH3+ groups proves that ordering of protonated amine plays a major role in the phase transition mechanism. The ordering is associated with distortion of the zinc formate framework but the number of observed vibrational modes is much smaller than expected assuming 36-fold multiplication of the unit cell below TC. It is also much smaller than reported for the Mn-analogue, which exhibits only a two-fold increase of the unit cell below TC. We discuss the origin of this behavior. Our results also show that the Co-analogue exhibits a similar phase transition to its Zn-counterpart. However, the observed narrowing and splitting of the corresponding bands is significantly smaller, suggesting weaker distortion of the framework and the presence of some disorder for this compound even at 5 K. The Raman and IR spectra of the Fe-analogue show weak narrowing of bands upon cooling, indicative of statistical freezing of the protonated amine at low temperatures. We also report high-pressure Raman scattering studies of the zinc-analogue. This study revealed a pressure-induced reversible phase transition between 3.4 and 4.1 GPa. Large shifts and splitting of modes corresponding to the vibrations of HCOO- ions associated with weak changes of the protonated amine prove that the major contribution to the phase transition mechanism comes from distortion of the zinc formate framework.

Phase transitions and chromium(iii) luminescence in perovskite-type [C2H5NH3][Na0.5CrxAl0.5-x(HCOO)3] (x = 0, 0.025, 0.5), correlated with structural, dielectric and phonon properties.

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C2H5NH3][Na0.5Cr0.5(HCOO)3] (EtANaCr), [C2H5NH3][Na0.5Al0.5(HCOO)3] (EtANaAl) and [C2H5NH3][Na0.5Al0.475Cr0.025(HCOO)3] (EtANaAlCr). These compounds crystallize in non-centrosymmetric monoclinic polar structures (space group Pn) and undergo order-disorder phase transitions upon heating to the monoclinic centrosymmetric structure (space group P21/n) at 369 (EtANaAl) and 373 K (EtANaCr). In principle, they are ferroelectric below these temperatures. In the high-temperature phase, ethylammonium (EtA+) cations are dynamically disordered over two symmetrically independent positions while upon cooling they begin to order. The ordering is accompanied by distortion of the metal formate framework. The hydrogen bonds (HBs) between the NH3+ group and NaO6 octahedral units are more robust than between the NH3+ group and CrO6 (AlO6) octahedral units and this feature explains a much stronger distortion of the former units and a weak effect of a trivalent cation type on the phase transition temperature. The dielectric studies have confirmed the occurrence of phase transitions of dipolar character and dipole relaxation processes. The optical studies show that EtANaCr and EtANaAlCr exhibit efficient Cr(iii)-based emission characteristics for intermediate-ligand field strength.

Structure, phonon properties, and order-disorder transition in the metal formate framework of [NH4][Mg(HCOO)3].

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.

Raman and IR studies of pressure- and temperature-induced phase transitions in [(CH2)3NH2][Zn(HCOO)3].

Temperature- and pressure-dependent studies of Raman and IR spectra have been performed on azetidinium zinc formate, [(CH2)3NH2][Zn(HCOO)3]. Vibrational spectra showed distinct anomalies in mode frequencies and bandwidths near 250 and 300 K, which were attributed to structural phase transitions associated with the gradual freezing of ring-puckering motions of the azetidinium cation. Pressure-dependent studies revealed a pressure-induced transition near 0.4 GPa. Raman spectra indicate that the structure of the room-temperature intermediate phase observed near 0.4 GPa is the same as the monoclinic structure observed at ambient pressure below 250 K. The second phase transition was found near 2.4 GPa. This transition has strong first-order character and is associated with strong distortion of both the zinc formate framework and azetidinium cations. The last phase transition was found near 7.0 GPa. This transition leads to lowering of the symmetry and further distortion of the zinc formate framework, whereas the azetidinium cation structure is weakly affected.

Order-disorder transition and weak ferromagnetism in the perovskite metal formate frameworks of [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3] (M = Ni, Mn).

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.