Trends in the band structures of the group-I and -II oxides.

Abstract

Measured and calculated band structures for the six lightest group-I and -II oxides are reported. Band structures have been measured using electron momentum spectroscopy, a technique that maps the ground-state occupied orbitals resolved both in energy and momentum. Measurements are compared with first-principles calculations carried out within the linear combination of atomic orbitals approximation using both Hartree-Fock (HF) and density functional (DFT) methods. Three DFT functionals are used representative of the local density approximation, the generalized gradient approximation, and a hybrid method incorporating exact exchange. The calculated O 2p bandwidths and O 2p-2s band gaps generally scale linearly with the inverse of the oxygen-oxygen separation squared, but consistently show an anomaly at Li(2)O. These trends, including the anomaly, are also observed in the experimental data. HF calculations consistently overestimate the oxygen 2p-2s band gap by almost a factor of two. Measured band gaps lie within the range of the three DFT functionals employed, with evidence that the description of exchange becomes more important as the cation size increases. Both HF and DFT calculations overestimate the oxygen valence bandwidths, with DFT giving more accurate predictions. Both observed and calculated bandwidths converge as the cation size increases, indicating that exchange-correlation effects become less important as the metallic ion becomes larger.