Neutron studies on a mixed valence metal(II)-iron(III) formate framework with metal(II) = Mn(II), Fe(II), and Co(II)

Abstract

Considerable attention is currently focused on metal-organic polymers combining two or more functionalities, e.g., conductivity and magnetism, magnetic and optical properties or porosity and magnetism. The inclusion of two different metallic atoms into a niccolife-like structure framework has led to the formation of isostructural compounds with very different magnetic behaviours, due to the compensation or not of the different spins involved in each lattice. A previous nuclear characterization has been done using single crystal X-ray and neutron diffraction, this last one only in the [NH2(CH3)2]n[Fe(lI)Fe(III)(HC00)6]n isostructural compound. The neutron studies show a sequence of phase transitions involving nuclear transformation as well as a long range magnetic order. The nuclear phase transition involves the ordering of the dimethylammonium countercation within the cavities framework which is related with a change in the electric behaviour from paraelectric to antiferroelectric. Moreover, below Tn, the magnetic order in these compounds vary from an antiferromagnetic behaviour with a weak spin canting for Mn(lI)Fe(III) compound to a ferrimagnetic behaviour for Co(II)Fe(lII) and Fe(lI)Fe(III) compounds. This is due to the competition of the different magnetic moments present in each sublattices (the different spin states intervening in those complexes are S = 5/2, for Fe(III) and Mn(II) while it is 3/2 and 2 for Fe(II) and Co(II), respectively). Neutron studies using single crystal and powder diffraction techniques have heen carried out at Intitut Laue Langevin (Grenoble, France) to elucidate the different magnetic behaviour present in these compounds.