Synthesis, Crystal Structures, Magnetic, and Thermal Properties of Divalent Metal Formate–Formamide Layered Compounds.

A series of layered divalent metal formate compounds, [M(HCOO)2(HCONH2)2] (M = Mn (1Mn), Ni (2Ni), Cu­(3Cu), Zn­(4Zn), Mg­(5Mg)), have been prepared by solvothermal synthesis and their room temperature (RT) and low-temperature (LT) crystal structures, and thermal and magnetic properties determined. All the compounds contain octahedral metal ions connected by four anti-anti formato ligands to form (4,4) nets with the composition of M(HCOO)2. The oxygen atoms from two coordinating formamide ligands above and below the layer complete the MO6 distorted octahedral coordination. Order–disorder phase transformations involving the formamide ligands were observed in the 1Mn, 2Ni, and 4Zn compounds. Like transitions in related formate structures with perovskite like topology, the transitions correspond to the ordering of the amine groups of the terminating formamide ligands which are disordered at ambient temperature. The magnetic properties of the three magnetic members of the series 1Mn, 2Ni, and 3Cu were investigated using microcrystalline samples, over the temperature range of 2 K–300 K under different applied fields. All compounds belong to antiferromagnetic square lattices with S = 5/2, 1, and 1/2. Exchange constants for a nearest neighbor model are presented here. Specific heat measurements indicate magnetic long-range order at lower temperatures, S = 5/2 (antiferromagnetic) and S = 1 (ferrimagnetic).本研究通过溶剂热合成法制备了一系列层状二价金属甲酸酯化合物，其化学式为[M(HCOO)₂(HCONH₂)₂]（其中M = Mn（标记为1Mn）、Ni（2Ni）、Cu（3Cu）、Zn（4Zn）、Mg（5Mg）），并表征了其室温（RT）与低温（LT）晶体结构、热学及磁学性能。所有化合物中的金属离子均处于八面体配位环境，通过四个反-反构型的甲酸根配体相互连接，形成组成为M(HCOO)₂的(4,4)拓扑网格。位于该网格层上下两侧的两个配位甲酰胺配体的氧原子，共同构成了畸变八面体构型的MO₆配位环境。在1Mn、2Ni与4Zn化合物中，观测到涉及甲酰胺配体的有序-无序相变。与具有类钙钛矿拓扑结构的相关甲酸酯体系中的相变行为一致，该类相变对应于常温下呈无序状态的端基甲酰胺配体的氨基基团发生有序化的过程。针对该系列中的三个磁性成员1Mn、2Ni及3Cu，本研究采用微晶样品，在2 K至300 K的温度区间内、不同外场条件下对其磁学性能进行了系统表征。所有化合物均属于自旋量子数S分别为5/2、1与1/2的反铁磁正方形晶格体系，本文给出了近邻相互作用模型下的交换耦合常数。比热测试结果表明，在低温条件下该系列化合物存在磁长程有序行为：其中自旋量子数S=5/2的体系为反铁磁有序，S=1的体系为亚铁磁有序。