Ln polyoxocations: yttrium oxide solution speciation & solution deposited thin films

Abstract

Rare earth oxide materials, including thin film coatings, are critically important in magnetic, luminescent and microelectric devices, and few substitutes have been discovered with comparable performance. Thin film coatings from solution are almost unknown for rare earth oxides, likely due to their high activity towards hydrolysis which yields poor quality thin films. The hexamer [Ln₆(O)(OH)₈(H₂O)₁₂(NO₃)₆]²⁺ is a rare example of a metal-oxo cluster isolated and stabilized without additional supporting organic ligands. Herein we report a new method for both the preparation and stabilization in non-aqueous media, which makes these clusters valuable precursors for solution-processed thin films. Solution characterization (NMR, small-angle X-ray scattering and Raman spectroscopy) in wet organic solvents indicated that the clusters evolve via a fragmentation and reaggregation process. This is especially true for hexamers of the smaller Ln³⁺-ions: the higher charge density yields higher hydration rates. This process produced an entirely new hexadecameric cluster formulated Y₁₆O₃(OH)₂₄(NO₃)₁₈(OSMe₂)₁₆(OCMe₂)₂(H₂O)₄. The new structure represents an intermediate hydrolysis product on the pathway from hexanuclear clusters to metal oxyhydroxide bulk solid. DMSO solvent ligands displace aqua ligands on the cluster and likely explain the additional stability observed for these clusters in organic solvents. The enhanced cluster stability in DMF and DMSO also enables solution-processing methods to create high quality thin films.