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 (lMn), 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 M06 distorted octahedral coordination. Order--disorder phase transformations involving the formamide ligands were observed in the lMn, 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 lMn, 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). [ABSTRACT FROM AUTHOR]