Phase Transitions and Coexistence of Magnetic and Electric Orders in the Methylhydrazinium Metal Formate Frameworks

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.