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 Fe3B1-xCx, 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 Fe3B1-xCx 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. © 1991 American Chemical Society