Ab initio study of local d-d excitations in bulk CoO, at the CoO(100) surface, and in octahedral Co2+ complexes.

Abstract

Quantum chemical ab initio calculations are presented for the lowest d-d excitation energies of ${\mathrm{Co}}^{2+}$ ions in bulk CoO, at the CoO(100) surface and, for comparison, in ${\mathrm{CoF}}_{2}$, in [Co(${\mathrm{NH}}_{3}$${)}_{6}$${]}^{2+}$ complexes, and for ${\mathrm{Co}}^{2+}$ impurity ions in LiF. Different cluster models have been used for describing the octahedral or distorted octahedral surrounding of the ${\mathrm{Co}}^{2+}$ ions: A pure point-charge model, a ${\mathrm{CoO}}_{6}^{10\mathrm{\ensuremath{-}}}$ cluster (${\mathrm{CoO}}_{5}^{8\mathrm{\ensuremath{-}}}$ at the surface) embedded in a point-charge (Madelung) field, and a ${\mathrm{CoO}}_{6}^{10\mathrm{\ensuremath{-}}}$ cluster surrounded by 18 effective core potentials in the next positive coordination shell and embedded in the point-charge field. The calculations for the ground state and the lowest excited states have been performed at the complete active space self-consistent field, valence configuration interaction, and multiconfiguration coupled-electron pair approach levels. The best results for the lowest excitation energies for bulk CoO are 0.80 eV ${(}^{4}$${\mathit{T}}_{2\mathit{g}}$) and 1.71 eV ${(}^{4}$${\mathit{A}}_{2\mathit{g}}$) which is slightly lower than experimental data derived from optical and electron-energy-loss (EEL) spectra. At the CoO(100) surface, the threefold spatial degenerate $^{4}$${\mathit{T}}_{1\mathit{g}}$ ground state of bulk CoO is split by about 60 meV into a lower $^{4}$${\mathit{A}}_{2}$ and a higher $^{4}$E component. The same small splitting has been observed experimentally in high-resolution EEL spectra as a low-energy shoulder of the first Fuchs-Kliewer surface phonon. The splitting of the first excited $^{4}$${\mathit{T}}_{2\mathit{g}}$ state of CoO at the CoO(100) surface is \ensuremath{\sim}0.4 eV and gives rise to a surface state at 0.35 eV (experimentally at 0.45 eV).