The electronic band structure of Li2O:: testing theoretical predictions using electron momentum spectroscopy

Using the technique of electron momentum spectroscopy (EMS) we have measured the oxygen 2p- and 2s-derived valence bands and lithium 1s-derived core level in lithium oxide. All three sets of bands have been measured in a single experiment allowing the energy gap between the bands to be determined. At the F point the O(2p)-O(2s) band gap is measured to be 16.1 +/- 0.2 eV, and the O(2s)-Li(1s) band gap is 34.3 +/- 0.2 eV. We can also determine bandwidths since EMS measures the full band structure directly, resolved both in energy and momentum. As expected, the O(2s) and Li(1s) bands are essentially non-dispersing, while the O(2p) has an observed width of 1.6 +/- 0.2 eV. The experiment is compared with calculations using the linear combination of atomic orbitals approach. At the Hartree-Fock (HF) level these calculations overestimate the gap between the valence bands and the width of the O(2p) band. The three density functional methods used give a reduced intervalence band gap and bandwidth. The hybrid gradient corrected method, PBE0 (where PBE stands for 'Perdew-Burke-Ernzerhof'), gives the closest agreement for the band gap at 16.7 eV, while the gradient corrected method, PBE, gives the best value for the bandwidth at 2.0 eV. At all levels the O(2s)-Li(1s) gap is underestimated; HF gives the closest agreement at 31.8 eV.