A systematic ab initio molecular dynamics study was carried out to identify valence electron concentration and size induced changes on structure, elastic and magnetic properties for Co(43)Fe(20)Ta(5.5)X(31.5) (X=B, Si, P, S). Short range order, charge transfer and the bonding nature are analyzed by means of density of states, Bader decomposition and pair distribution function analysis. A clear trend of a decrease in density and bulk modulus as well as a weaker cohesion was observed as the valence electron concentration is increased by replacing B with Si and further with P and S. These changes may be understood based on increased interatomic distances, variations in coordination numbers and the electronic structure changes; as the valence electron concentration of X is increased the X bonding becomes more ionic, which disrupts the overall metallic interactions, leading to lower cohesion and stiffness. The highest magnetic moments for the transition metals are identified for X=S, despite the fact that the presence of X generally reduces the magnetic moment of Co. Furthermore, this study reveals an extended diagonal relationship between B and P within these amorphous alloys. Based on quantum mechanical data we identify composition induced changes in short range order, charge transfer and bonding nature and link them to density, elasticity and magnetism. The interplay between transition metal d band filling and s-d hybridization was identified to be a key materials design criterion.
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