Linear augmented Slater-type orbital study of Pt–5d-transition-metal alloying

We have used a self-consistent density-functional theory with an augmented Slater-type-orbital basis to calculate the heats of formation, charge components, and densities of states for ordered 5d-transition-metal–platinum alloys. Crystal structures considered were CsCl, CuAu-I, AuCd, CrB, ${\mathrm{Cu}}_3$Au, ${\mathrm{Al}}_3$Ti, ${\mathrm{MoSi}}_2$, and ${\mathrm{MoPt}}_2$, though not all structures were calculated for each alloy system. Calculated heats of formation are in accord with the limited experimental data for close-packed systems; however, the ill-packed CrB structure cannot be described sufficiently well with our muffin-tin potentials. Correct phase-diagram behavior, i.e., the correct structure at a given composition and the competition between phases at different composition, is predicted. Charge transfer was inspected with use of the orbital population and Wigner-Seitz sphere counts as were two observables sensitive to charge behavior. These were the electron contact density at the nucleus and the initial-state shift of core-electron energy levels. Neither of these are simply related to the charge-transfer terms. In addition, the charge transfer does not correlate with traditional notions concerning differences in electronegativity. One complication, encountered previously in calculations for Au alloys, is that a significant component of the change in charge at an atomic site upon alloying has nothing to do with charge transfer, hybridization, or screening in the traditional chemical sense but is, instead, simply associated with the overlap of tails of charge best understood as being associated with orbitals centered on neighboring sites. Thus while it has been long recognized that orbital population analyses are not unique because they are strongly affected by the details of how overlap is dealt with, it turns out that analyses of charge transfer, based on integrating charge over a Wigner-Seitz cell or sphere, are also blurred by the overlapping.