Chemical bonding and local symmetry in cobalt- and iron-metalloid alloys

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Abstract
A model is proposed that quantitatively accounts for the moment variation in transition metal–metalloid (T–M) crystals and glasses. The model, formulated from the valence bond theory, assumes that each Co atom surrounding an M atom contributes a d orbital to participate in p–d hybrid bonding. The hybridized d orbital is then considered to be nonmagnetic. Hence the moment variation in Co–M alloys is determined by the local symmetry of the M atom and not by the valence of M. The moment reduction is caused by orbital transfer rather than charge transfer. Excellent quantitative agreement is found when the model is compared with experimental data for crystalline and amorphous Co–M alloys. It is found that amorphous alloys retain the same local environment around the metalloid atom as in the crystalline cases and that the bonding is equivalent. The bond model predicts zero moment change for dilute bcc Fe alloys because the bonding levels in the Fe band have no uncompensated spin. Reasonable agreement with experiment is obtained, but the size of the metalloid affects the Fe–Fe atom exchange which in turn affects the magnetic moment. The bond model for Ni–M alloys is presented in another paper.