Synthesis, structure and properties of oxalato and thio-oxalato molecule based magnets

A number of techniques have been used to investigate the structure and magnetic properties of a range of materials based on transition metal complexes of the oxalate (C2O42-) and dithio-oxalate (C2S2O22-) ions. Structural studies were carried out using X ray and neutron diffraction. Magnetic properties were studied using DC magnetisation, neutron diffraction, muon spin relaxation and Mossbauer spectroscopy. Supporting evidence for structural studies was also gained from extended X ray absorption fine structure (EXAFS). The materials studied are all mixed valence transition metal compounds with general formula AMIIMIII(L)3 (A = NR4+, PPh4+), MII = Cr,Mn,Fe,Ni,Co, MIII = Cr,Fe and L C2O42-, C2S2O22-, in which the metals are linked by either the oxalate or dithio-oxalate ligand to give 2 dimensional honeycomb lattices. These sheets are separated by layers of diamagnetic, univalent organic cations which provide charge balance to the solid. This results in a 2 dimensional infra-layer magnetic interaction. The project involves the study of the effect of sublattice doping of oxalate systems with paramagnetic ions on the magnetic properties of previously studied compounds and the synthesis and characterisation of previously unreported compounds based upon the dithio-oxalate ligand. Unexpected changes in magnetic properties were observed upon variation of the organic cation in the case of the ferric dithio- oxalates. The structure of these compounds was studied as function of temperature using high resolution powder diffraction and the local environment of the metal ions studied using EXAFS spectroscopy. The nature of the local magnetic fields in selected compounds were studied using a combination of Mossbauer spectroscopy and muon spin relaxation. It was discovered during the course of the project that the dithio-oxalate materials undergo solid state reactions resulting in a change of magnetic behaviour with time. The nature of this reaction was studied using Mossbauer spectroscopy. (author)