Acta Chimica Sinica ›› 2020, Vol. 78 ›› Issue (11): 1223-1228.DOI: 10.6023/A20080341 Previous Articles     Next Articles



赵炯鹏a,c, 王玮玮a, 韩松德b, 李泉文c, 李娜c, 刘福臣a, 卜显和b,c   

  1. a 天津理工大学 化学化工学院 天津 300384;
    b 南开大学 化学学院 元素有机化学国家重点实验室 天津 300071;
    c 南开大学 材料科学与工程学院 天津市金属与分子基材料化学重点实验室 天津 300350
  • 投稿日期:2020-08-02 发布日期:2020-09-17
  • 通讯作者: 刘福臣, 卜显和;
  • 基金资助:

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

Zhao Jiongpenga,c, Wang Weiweia, Han Songdeb, Li Quanwenc, Li Nac, Liu Fuchena, Bu Xianheb,c   

  1. a School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China;
    b State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China;
    c Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China
  • Received:2020-08-02 Published:2020-09-17
  • Supported by:
    Project supported by the National Natural Science Foundation of China (Nos. 22035003, 21531005, 21871209, 21571139).

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[FeIIIFeII(HCO2)6]n (1) was constructed by the reaction of FeCl3·6H2O, formic acid and N-methyl formamide at 140 ℃ for two days. At room temperature, 1 crystallizes in the space group P31c, in which a three-dimensional anionic niccolite topological framework is constructed by FeII, FeIII 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 130~200 K at 500 Hz~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 TN 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.

Key words: mixed-valence, phase transitions, negative magnetization, switchable magnetic dipoles, exchange bias