Heterometallic 3d-4f Complexes as Air-Stable Molecular Precursors in Low Temperature Syntheses of Stoichiometric Rare-Earth Orthoferrite Powders.

Abstract

Four 3d-4f hetero-polymetallic complexes [Fe2Ln2((OCH2)3CR)2(O2CtBu)6(H2O)4] (where Ln = La (1 and 2) and Gd (3 and 4); and R = Me (1 and 3) and Et (2 and 4)) are synthesized and analyzed using elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and SQUID magnetometry. Crystal structures are obtained for both methyl derivatives and show that the complexes are isostructural and adopt a defective dicubane topology. The four heavy metals are connected with two alkoxide bridges. These four precursors are used as single-source precursors to prepare rare-earth orthoferrite pervoskites of the form LnFeO3. Thermal decomposition in a ceramic boat in a tube furnace gives orthorhombic LnFeO3 powders using optimized temperatures and decomposition times: LaFeO3 formed at 650 °C over 30 min, whereas GdFeO3 formed at 750 °C over 18 h. These materials are structurally characterized using powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray map spectroscopy, and SQUID magnetometry. EDX spectroscopy mapping reveals a homogeneous spatial distribution of elements for all four materials consistent with LnFeO3. Magnetic measurements on complexes 1-4 confirm the presence of weak antiferromagnetic coupling between the central Fe(III) ions of the clusters and negligible ferromagnetic interaction with peripheral Gd(III) ions in 3 and 4. Zero-field-cooled and field-cooled measurements of magnetization of LaFeO3 and GdFeO3 in the solid-state suggest that both materials are ferromagnetic, and both materials show open magnetic hysteresis loops at 5 and 300 K, with Msat higher than previously reported for these nanomaterials. We conclude that this is a new and facile low temperature route to these important magnetic materials that is potentially universal, limited only by what metals can be programmed into the precursor complexes.