Electronic states, bonding, and x-ray absorption spectra ofPd2Si

The electronic properties of ${\mathrm{Pd}}_2$ Si are analyzed in light of a self-consistent calculation of the electronic states. By using the linear muffin-tin orbitals method in the atomic-sphere approximation, energy bands, densities of states, x-ray photoabsorption spectra, and the total energy have been determined. The basic bonding interaction between Pd and Si is found to be a bonding-antibonding coupling between Pd 4d and Si 3p orbitals. This leaves the Si 3s states in an atomiclike configuration. Many features of the experimental spectroscopic data can be interpreted using this overall picture, but there are details which depend on the complexities of the crystal structure. We found that the Si 3p states just below the Fermi level, often attributed to the filled portion of the Pd d–Si p antibonding states, are indeed due to a Pd p–Si p coupling. The antibonding states of this coupling are entirely unoccupied and are responsible for both the low-energy structure determined by bremsstrahlung isochromat spectroscopy (BIS) and the occurrence of the white line in the Pd $L_2$,3 x-ray absorption spectra. Furthermore, a Pd p–Si d structure provides an explanation of the higher-energy BIS features and of the main peak in the Pd $L_1$ absorption data. These results show that many-body effects are hardly necessary in order to interpret the existing x-ray absorption spectra. The zero-temperature heat of formation is calculated to be -11.3 kcal/g-at. and the room-temperature experimental value is -6.9 kcal/g-at.