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

Abstract

High-resolution core-level and valence-band x-ray photoelectron spectra (XPS) of the nonconductor silicates, quartz, and two olivines (${[\mathrm{Mg},\mathrm{Fe}]}_{2}\mathrm{Si}{\mathrm{O}}_{4}$, 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 $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ 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 $\mathrm{Si}{\mathrm{O}}_{4}$ 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. $\mathrm{GGA}+U$ (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 $3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ more dispersed than the valence bands of the olivines and considerably greater than the $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ 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 $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra. For example, the $\mathrm{Si}\ensuremath{-}\mathrm{O}$ moiety is more negatively charged in olivine than in quartz because of the transfer of electrons from Mg to $\mathrm{O}\ensuremath{-}\mathrm{Si}$. The calculations indicate very small Mg $2s$ contributions to the forsterite $({\mathrm{Mg}}_{2}\mathrm{Si}{\mathrm{O}}_{4})$ valence band. Hence all Mg $2s$ electrons effectively reside on O atoms (O $2p$ orbitals) and the $\mathrm{Mg}\ensuremath{-}\mathrm{O}$ 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 $\mathrm{Fe}\ensuremath{-}\mathrm{O}$ bonding.