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.

本工作报道了四种钙钛矿型金属甲酸酯骨架（perovskite-type metal formate frameworks）材料[CH3NH2NH2][M(HCOO)3]（记为MHyM，其中M分别为Mn、Mg、Fe和Zn）的合成。该系列化合物均存在两次结构相变。第一相变的转变温度受二价金属离子种类的影响较弱，升温过程中其转变温度区间为310~327 K。通过X射线衍射（X-ray diffraction）、差示扫描量热法（DSC）以及振动光谱研究发现，该相变具有二级相变特征。其与甲基肼阳离子（methylhydrazinium，MHy+）的部分有序化以及晶体对称性从非极性空间群R-3c到极性空间群R3c的转变相关。热释电测量结果表明，室温相具有铁电性。第二次低温相变则具有一级相变特征，其源于MHy+阳离子的进一步有序化以及金属甲酸酯骨架的畸变。磁化率测试数据显示，MHyMn和MHyFe分别在9 K和21 K时发生类铁磁相变。由于低温相为极性相，该系列化合物有望成为潜在的多铁性材料。MHyFe在磁有序状态下还表现出额外的磁异常现象，该现象大概率源于磁矩的阻塞效应。本工作同时报道了MHyMn的高压拉曼散射（high-pressure Raman scattering）研究结果，该研究发现其在4.8~5.5 GPa区间内发生压力诱导的可逆相变。数据分析表明，该相变会同时导致锰甲酸酯骨架以及MHy+阳离子结构发生显著变化。