The valence band structures of BeO, MgO, and CaO

Abstract

We have performed direct measurements of the valence band structures of the light alkaline earth oxides BeO, MgO, and CaO using electron momentum spectroscopy (EMS). From these measurements, we have determined the band dispersions, valence bandwidths, and O(2s)–O(2p) intervalence bandgaps at the Γ point. For comparison we have also performed Hartree–Fock (HF) and density-functional (DFT) calculations in the linear combination of atomic orbitals (LCAO) approximation. Intervalence bandgaps compare reasonably well with the DFT calculations and previous experimental and theoretical studies. Our measured bandwidths, however, are significantly smaller. In particular, we find that contrary to conventional wisdom, the local density approximation of DFT overestimates the valence bandwidths of these ionic solids.

Electronic band structure of calcium oxide

“…The majority of available theoretical calculations predict a p-bandwidth value within 2-3 eV [9,11,12,13,14,15,16,17,5,18,52]. Our previous calculations [52] give a range from 2.61 to 2.88 eV, which is close to the present, raw LMTO result.…”

“…They give values of 0.6 eV and 0.7 eV, respectively, which are about two times narrower than the full LMTO bandwidth and would probably be at odds with our experimental result after accounting for the experimental conditions. Our previous work [52], where we calculate the bandwidth using the linear combination of atomic orbital (LCAO) approximation in the framework of density-functional theory (DFT) with three different local density approximation Hamiltonians, predicts a range from 1.01 to 1.37 eV, a result, which is close to the present calculation. Unfortunately, there is no experimental data on the bandwidth of the s-band against which we can compare.…”

“…Our previous calculations [52] give a range from 2.61 to 2.88 eV, which is close to the present, raw LMTO result.…”

“…The theory changes from 0.2 to 1.6 over the same momentum range. We observed a similar disagreement in the relative p-to -s-band EMD ratio for MgO [31] and BeO [51,52].…”

“…In this case, our attempt to account for multiple scattering leads to worse agreement between theory and experiment. Comparing our experimental value with the DFT-LCAO result from our previous work [52], we find that the full valence bandwidth of 16.7 eV obtained in the B3-PW scheme is the closest.…”