Heterometallic 3d-4f complexes as air-stable molecular pre-cursors in low temperature syntheses of stoichiometric rare-earth orthoferrite powders

Four 3d-4f hetero-polymetallic complexes [Fe₂Ln₂((OCH₂)₃CR)₂(O₂C^tBu)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 synthesised and analysed using elemental analysis, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) 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 optimised temperatures and decomposition times: LnFeO₃ formed at 650 ℃ over 30 min, whereas GdFeO3 formed at 750 ℃ over 18 h. These materials are structurally characterised using powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray map spectros-copy, and SQUID magnetometry. EDX spectroscopy mapping reveals a homogenous spatial distribution of elements for all four materials consistent with LnFeO₃. Magnetic measurements on complexes 1-4 confirms the presence of weak antiferro-magnetic 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 (ZFC) and field-cooled (FC) measurements of magnetization of LnFeO₃ and GdFeO₃ in the solid state suggests that both materials are ferromagnetic, and both materials show open magnetic hysteresis loops at 5 K and 300 K, with M_sat higher than previously reported for these as 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.