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).