低温磁制冷功能导向的Fe(III)/Mn(II)分子基材料的构筑与性能研究

Molecular magnetic refrigerants have attracted considerable interest for their synthetic tunability and functional tailorability. Theoretically, the free Mn(II) and Fe(III) ions with d5 electronic configurations has a larger single-ion magnetic entropy change than GdIII ions (271.1 J kg-1 K-1 for Mn(II); 266.7J kg-1 K-1 for Fe(III); 109.9 J kg-1 K-1 for Gd(III)). Therefore, better magnetic refrigerants could be expected in some Fe(III)/Mn(II)-based complexes. However, because of the strong magnetic interactions between Fe(III)-Fe(III)/Mn(II)-Mn(II), it is difficult to realize large magnetocaloric effect, which further restricts the use of Fe(III)/Mn(II)-based complexes as magnetic refrigerants. In this project, we use the method of magnetic isolation, based on the topology and magnetic engineering, to avoid the strong magnetic couplings between adjacent Fe(III)/Mn(II) ions. Based on these considerations, excellent Fe(III)/ Mn(II)-based magnetic refrigerants with particular topology could be anticipated. This project will not only enrich the kind and synthetic method of molecular magnetic refrigerants, but also provide guidance for the design and application of such materials.

分子基磁制冷材料因其结构的可调控性和功能的可裁剪性引起研究者的极大兴趣。理论上，高自旋的Mn(II)和Fe(III)的单离子磁熵变(Mn(II): 271.1 J kg-1 K-1; Fe(III): 266.7J kg-1 K-1)远高于Gd(III) (109.9 J kg-1 K-1)，因此，基于Fe(III)/Mn(II)分子基材料有望展示出更好的磁制冷效果。然而Fe(III)-Fe(III)/Mn(II)-Mn(II)间强的磁相互作用，使得很难获得较高的磁熵变，进而限制其在磁制冷方面的应用。本项目结合拓扑学以及磁工程原理，采用磁隔离的研究思路，有望避免邻近Fe(III)/Mn(II)金属间的强磁相互作用，获得具有特定拓扑结构的优异Fe(III)/Mn(II)分子基磁制冷材料。本项目将拓展分子基磁制冷材料的种类及合成方法，为该类材料的设计合成及应用研究提供指导。

在当今全球变暖、能源紧缺的形势下，新型磁制冷材料的研发显得愈发必要，而且这一领域已成为各国科学家的研究热点。分子基低温磁制冷材料因其结构的可调控性和功能的可裁剪性受到研究者的广泛关注。研究报告主要从钆基分子基磁制冷材料和过渡金属分子基磁制冷材料两个方面开展工作，具体如下：.一、基于晶体工程策略，以低温磁制冷功能为导向，制备出四例钆基配合物，[Gd(oxa)(H2PO2)(H2O)2] (1), (NH4)[Gd(C2O4)(SO4)(H2O)] (2), [Gd(C2O4)0.5(CO3)(H2O)] (3), Gd(OH)SO4 (4)。磁性测试表明1-4的最大磁熵变(−ΔSmmax)分别为46.6 J kg−1 K−1，42.4 J kg−1 K−1，50.7 J kg−1 K−1，53.5 J kg−1 K−1。需要说明的是，1-4的−ΔSmmax与商用钆镓石榴石Gd3Ga5O12 (GGG)的性能相当(−ΔSmmax = 38.4 J kg−1 K−1, ΔH = 7T）。这些特点使1-4有望成为潜在低温磁制冷剂。.二、基于八面体场轨道正交作用原理，制备系列异金属甲酸化合物[AI][CrIIIMII(HCO2)6] (5: AI = NH4H2OI, M = Mn; 6: AI = CH3NH3I, M = Fe; 7: AI = CH3NH2CH3I, M = Co; 8: AI = CH3NH3I, M = Ni)。每个MII通过六个甲酸连接邻近的六个CrIII，同时每个CrIII通过六个甲酸连接邻近的六个MII，整个结构可以看做双节点6-连接的砷镍矿(niccolite)框架。由于MnII-CrIII之间的弱磁相互作用以及较高的磁密度，5的最大磁熵变为43.9 J kg-1 K-1。与5相比，由于CrIII和MII离子间铁磁相互作用强度的增加，可以观测到化合物6-8中磁有序温度的升高。该研究为如何实现过渡金属的弱相互作用及调控过渡金属间相互作用强度提供一种方法。