Synthesis of magnetite nanoparticles by thermal decomposition of ferrous oxalate dihydrate

Two different polymorphs of ferrous oxalate dihydrate were synthesized by precipitation of ferrous ions with oxalic acid: alpha-Fe(C2O4) center dot 2H(2)O with a monoclinic unit cell is obtained after precipitation and ageing at 90 degrees C, whereas the orthorhombic beta-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be tailored from prismatic crystals of the alpha-polymorph over star-like aggregates of alpha/beta-mixtures to non-agglomerated crystallites of beta-oxalate. Thermal decomposition in air gives hematite at T >= 250 degrees C; if the thermolysis reaction is performed at low oxygen partial pressures (e.g., T = 500 C and p(O2) = 10(-25) atm) magnetite is obtained. The synthesized magnetite is stoichiometric as signaled by lattice parameters of a(0) = 8.39 angstrom. The thermal decomposition of ferrous oxalate is monitored by thermal analysis, XRD, and IR-spectroscopy. The morphology of the oxalate crystals is preserved during thermal decomposition; the oxalates are transformed into spinel particle aggregates of similar size and shape. The crystallite size of the magnetite particles increases with temperature and is 40 or 55 nm, if synthesized from beta-oxalate at 500 degrees C or 700 degrees C, respectively. The saturation magnetization of the magnetite particles decreases with decreasing particle size. Since the particles are larger than the critical diameter for superparamagnetic behavior they display hysteresis behavior at room temperature.