Amorphous Alloy Thin Films from Molecular Precursors. Evidence of Structure and Stoichiometry from Crystallization and Effects of Precursor Ligand Structure on Stoichiometry

The crystallization at 400 °C of amorphous thin films formed by the thermal decomposition of HFe₃- (CO)₉BH₄ at low low pressure has been studied by using the techniques of X-ray diffraction and Mössbauer spectroscopy. The results unambiguously show that the films are iron rich in terms of the ideal stoichiometry of Fe/B = 3, signifying some loss of boron during film formation. The iron-main group atom phase that crystallizes from the amorphous film is orthorhombic Fe₃B_1-x.C_x, with x for the specific film examined lying between 0.3 and 0.4. The cleavage of CO is postulated to account for the overall Fe/B ratio of 3, formation of the mixed boride/carbide phase and the presence of B₂O₃ in the film. That is, the boron sequesters the oxygen atom as B₂O₃, whereas the carbon atom replaces boron so lost in Fe₃B to form the Fe₃B_1-xC_x phase. Deposition of films from a closely related precursor, HFe₃(CO)₁₀BH₂, have also been examined. Most of the boron is lost during deposition even at the lowest substrate temperature. Crystallization of these amorphous films requires higher temperatures and yields α-Fe and a phase indistinguishable from orthorhombic Fe₃C. Decomposition during sublimation with the production of Fe(CO)₅ accounts for the qualitatively different behavior of HFe₃(CO)₁₀BH₂.