### 基于甲胺客体调制的混价态甲酸铁框架的构筑与磁电行为研究

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 E-mail:fcliu@tjut.edu.cn;buxh@nankai.edu.cn
• 基金资助:
项目受国家自然科学基金（Nos.22035003，21531005，21871209，21571139）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).

N-型亚铁磁体可以在场冷时呈现负磁化行为，但在1 T强场场冷时依然能够保持负磁化行为的化合物却很罕见.本工作以CH3NH3+为客体阳离子，成功构筑了一例红砷镍矿（49.66）（412.63）结构的混价态甲酸铁化合物[CH3NH3]n[FeIIIFeII（HCO26]n1），在1 T强场下呈现负磁化行为.1中甲酸根采用反，反模式连接FeII和FeIII形成三维阴离子框架，八面体配位构型的FeII和FeIII分别处于（49.66）和（412.63）节点，而CH3NH3+则填充在框架空隙中平衡电荷.量热、介电和单晶X-射线衍射测试结果表明，1中存在由CH3NH3+无序-有序转变诱导的结构相变并伴随介电弛豫.磁性研究表明FeII和FeIII之间存在较强的反铁磁耦合，但在冷却过程中FeII和FeIII构成的亚晶格磁有序程度存在较大差异，且具有较小自旋的FeII磁矩先于FeIII磁矩有序且平行于外加磁场，导致温度进一步降低时1呈现出负磁化行为，这表明1是一例罕见的N-型亚铁磁体.此外，1在1 T外加冷却场中依然能保持负磁化行为，表明1中存在较强的磁各向异性.值得注意的是，在100 Oe外场下冷却到17 K后，1呈现出正场调制的磁极翻转行为.进一步研究表明，1的场冷磁滞回线呈现不对称形状，并向冷却场方向发生偏转，其有效矫顽力（HCeff=（H+-H-）/2）和交换偏置场（HEB=（H++H-）/2）分别为21716 Oe和3322 Oe.总之，本研究表明客体分子尺寸和形状能够有效调控红砷镍矿结构中FeII的磁各向异性，从而实现较高的磁极翻转场、矫顽力和交换偏置场.

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.