High-resolution valence-band XPS spectra of the nonconductors quartz and olivine

High-resolution core-level and valence-band x-ray photoelectron spectra (XPS) of the nonconductor silicates, quartz, and two olivines ([Mg,Fe](2)SiO4, Mg rich and Fe rich) have been obtained with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. Observed total linewidths are about 1.3 eV for several of the peaks in these spectra, much narrower than previously obtained. The quartz and olivine valence-band spectra are very different, even though the dominant contributions to both come from molecular orbitals of SiO4 groups. High-quality calculations of the band structure using pseudopotential density functional theory with generalized gradient approximation (GGA) for quartz yield a theoretical XPS valence-band spectrum in the Gelius approximation that is in excellent agreement with the experimental spectrum. GGA+U (where U is the on-site Coulomb energy) calculations on Mg-rich and Fe-rich olivines yield theoretical XPS spectra in good qualitative agreement with experiment. The valence-band spectral contributions are readily assigned using the calculated partial density of states. For example, the Fe 3d t(2g) and e(g) orbitals in M1 and M2 sites of the olivine are located at the top of the olivine valence band and are readily resolved in both observed spectra and theoretical calculations. The valence-band spectrum of quartz is about 3 eV more dispersed than the valence bands of the olivines and considerably greater than the 1.4 eV difference calculated previously. The difference in dispersion arises mainly from two effects. The dispersion of the quartz valence band is primarily a response to the nature of metal atoms in the second coordination sphere of the central Si atom of SiO4 tetrahedra. For example, the Si-O moiety is more negatively charged in olivine than in quartz because of the transfer of electrons from Mg to O-Si. The calculations indicate very small Mg 2s contributions to the forsterite (Mg2SiO4) valence band. Hence all Mg 2s electrons effectively reside on O atoms (O 2p orbitals) and the Mg-O bond is ionic. Polymerization (network formation) also contributes to dispersion in that it results in large splittings in the Si 3s and Si 3p partial density of states. Fe 3d electrons of olivines are of both bonding and nonbonding according to the calculations. In the Mg-rich olivine, the great majority of Fe 3d electrons are nonbonding and give rise to separate Fe 3d t(2g) and e(g) peaks at the top of the valence band. The same no-bonding contributions are observed in Fe-rich olivine, but O 2p contributions near the top of the valence band indicate appreciable Fe-O bonding.