Energy isosbestic points in third-row transition metal alloys

The total electronic energies of the six electrons per atom (e per atom) al loys W, TaRe, HfOs, and YIr and the seven electrons per atom alloys Re, WOs , TaIr, HfPt, and YAu have been calculated in the local density approximati on Of density functional approximation oi theory. When one considers common alloy structures such as atomically ordered variants: of the body-centered cubic, face-centered cubic, or hexagonally closest packed structures and p lots the total electronic energy as a function of the unit cell parameter, one finds fur both the six and seven electrons per atom series energetic is osbestic points. An energetic isosbestic point corresponds to a critical va lue of the size parameter for which all members of the 6 or 7 e(-) per atom series of compounds have nearly identical total electronic energy. Just as in spectroscopy, where the existence of such isosbestic points is the hall mark of two compounds present in the mixture, an energy isosbestic points i s implies there are just two separate energy curves. For both series it is found that the total electronic energy can be viewed as the weighted sum of a purely covalent term and a purely ionic term. Two semi-quantitative mode ls are proposed to account for these two separate energies. In the first mo del the total energy is viewed as the sum of the elemental structural energ y plus an ionic energy based on the Born-Mayer ionic model. In the second m odel one considers within the confines of mu (2)-Huckel theory the evolutio n of the total electronic energy as the Coulombic H-ii integrals change in value.