Construction, Magnetic and Dielectric Properties of Mixed-Valence Iron Formate with Methylammonium Guest

Although N-type ferrimagnets exhibit negative magnetization under positive magnetic fields, compounds that could maintain negative magnetization behavior under strong magnetic field (e.g. 1 T) are still rare. In this work, a mixed-valence metal-formate [CH3NH3](n)[(FeFeII)-Fe-III(HCO2)(6)](n) (1) was constructed by the reaction of FeCl3 center dot 6H(2)O, formic acid and N-methyl formamide at 140 degrees C for two days. At room temperature, 1 crystallizes in the space group P (3) over bar 1c, in which a three-dimensional anionic niccolite topological framework is constructed by Fe-II, Fe-III ions, and anti,anti formate. The guest CH3NH3+ cations fill in the cavities of the framework as the charge balancer with the N atoms displaying threefold disorder and the atoms having a twofold disorder. The heat capacity measurement shows two different peaks, being the signatures of phase transitions. The change in heat capacity at 136 K corresponds to the phase transition triggered by the order-disorder phenomena of the CH3NH3+ cations. At low temperature phase, 1 has a symmetry of 2/m in space group C2/c, in which the threefold disorder of the N atoms of the CH3NH3+ was freezed. The order-disorder phase transition also results in dielectric relaxation in the temperature range 130 similar to 200 K at 500 Hz similar to 1 MHz. The change of heat capacity at 40 K is associated with the ferromagnetic order of the antiferromagnetically coupled FeIII and FeII sublattices. 1 is a N-type ferrimagnet with negative magnetization below T-N on cooling under the applied field, and thermo-driven magnetic poles reversal could be found in 1 with large applied field of 1 T. Furthermore, positive field regulated switchable magnetic dipoles switching of the magnetization, together with obvious huge positive exchange bias is also observed in 1. These results reveal the significant magnetic anisotropy in 1, and the guest in the framework not only can tune the structural phase transitions but also modulate the anisotropy of the host framework leading to different magnetism.