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

Abstract

This study investigates novel hetero-polymetallic 3d-4f complexes with the goal of using them as single-source precursors (SSPs) for technologically important nanoscale perovskite materials, in particular the rare-earth orthoferrites of prototypical formula LnFeO3. Recently, these types of heterometallic complexes have been identified as promising candidates as molecular nanomagnets. They possess pre-formed interatomic bonds corresponding to the solid-state materials, to improve the control of the stoichiometry of the materials and of the phase present. Molecular synthesis used Fe(III) and easily accessible 4f starting metal sources, pivalate and tripodal alkoxy ligands, in order to produce a range of new tetrametallic 3d-4f heterometallic clusters of general formula [Fe2Ln2((OCH2)3CMe)2(O2CtBu)6(H2O)4], where Ln = La (1), Gd (2), Eu (3), Y (4), Tb (5), Er (6), Pr (7), Sm (8), and Dy (9). A slightly different formula was found for compound (10) when preparing it, namely [Fe2Yb2((OCH2)3CMe)2(O2CtBu)4(NO3)2(H2O)2] (10b), as two of the pivalates were exchanged for nitrate ions. These ten 3d-4f heterometallic compounds are analysed using elemental analysis, fourier transform infrared spectroscopy (FT-IR), X-ray crystallography, SQUID magnetometry and thermogravimetric analysis (TGA). The single crystal structural characterisation shows that the compounds are isostructural and share the same core. Magnetic measurement for all the compounds reveals an antiferromagnetic behaviour between the neighbouring Fe(III) ions. Compound 6 (Er) shows an SMM behaviour with a relatively low energy barrier of 9 K. Thermal decomposition technique is used to prepare rare-earth orthoferrites, LnFeO3, at different temperatures and decomposition times. These materials are structurally characterised using powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray map spectroscopy, UV-Vis spectroscopy and SQUID magnetometry. The calculated energy band gap using a Tauc plot for all phases range between 3.71 eV to 1.46 eV. The field-dependent magnetisation demonstrates ferromagnetic behaviour. Ultimately, this is a new and facile low temperature route to these important perovskite materials that is potentially universal, limited only by which metal cations can be incorporated into the precursor complexes.