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.

We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NHNH][M(HCOO)] (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 Pna2, P6 to P222 and P6 to P2. 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.

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.