Structural, Spectroscopic, Magnetic and Thermal Properties in the [SrM(C3H2O4)2(H2O)5] 2 H2O (M = Mn, Fe, Co, Ni) System: Precursors of SrMO3x Mixed Oxides

Abstract

The [SrM(C3H2O4)2(H2O)7] (M = Mn, Fe, Co, Ni) compounds have been prepared and characterised by chemical analysis and X-ray diffraction. The crystal structure of [SrCo(C3H2O4)2(H2O)5] · 2 H2O has been solved [monoclinic, P21/c, Z = 4, a = 6.880(1), b = 14.360(2), c = 15.800(2) Å. β = 101.69(1)°]. The compound exhibits a three-dimensional chain/snake structure consisting of –CoO6–[SrO7]∞–CoO6– repeating units parallel to the [0 1 1] and [0 –1 1] directions. UV/Vis data are consistent with the cations being in a high-spin octahedral symmetry. Magnetic measurements show antiferromagnetic interactions for all compounds with approximate θ values of –0.5 K, except for the cobalt compound (–24.5 K) where the decrease of the χmT curve can also be attributed to the spin-orbit coupling. Thermal analyses performed in air and under nitrogen show three consecutive steps: dehydration, ligand pyrolysis, and inorganic residue evolution. Thermal treatments in tubular furnaces were performed in order to obtain pure SrMO3–x phases. In all cases the oxides are nonstoichiometric and oxygen-deficient with structures related to that of perovskite. Lower temperatures and reaction times than those given in the literature have been used when using the ceramic method. These “soft” treatments gave rise to homogeneous particles of small grain size.