Ž . Chemical Physics 237 1998 59–65 Theoretical study of the crystal field excitations in CoO C. de Graaf, W.A. de Jong, R. Broer ) , W.C. Nieuwpoort Department of Chemical Physics and Materials Science Centre, UniÕersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received 19 March 1998 Abstract A theoretical investigation of the crystal field excitations in CoO is presented. Special attention is given to the excitation energy of the 4 A state. In recent experimental and theoretical studies an excitation energy around 3.1 eV was reported. 2g This is in disagreement with the 2.1 eV deduced from optical spectroscopy data. After analyzing electron correlation effects, spin–orbit interactions and the material model to represent the CoO crystal, we can confirm the interpretation of the optical data, not only for the 4 A state but also for all other low-lying crystal field excitations. Electron correlation effects are 2g found to have a significant differential effect on the excitation energies, ranging from q0.3 to y0.6 eV. Spin–orbit interactions are less important, affecting the excitation energies by at most 0.05 eV. Finally, we discuss the effect of the Pauli repulsion between the cluster ions and the first shell of ions around the cluster. This affects the excitation energies by a small, but significant, amount. q 1998 Elsevier Science B.V. All rights reserved. 1. Introduction A very characteristic property of the insulating late transition metal oxides is the appearance of sharp peaks in the band gap. These peaks can be ascribed to local transitions within the open 3d-shell on the transition metal ion. These so-called crystal field transitions are all dipole forbidden, but they can be observed in optical experiments due to phonon coupling, which lifts the inversion site symmetry. In 1959 Pratt and Coelho measured the optical absorp- wx tion spectrum of CoO 1 . The spectrum was inter- preted based on the ligand field theory developed by w x Tanabe and Sugano 2,3 and the lowest eight crystal field transitions were assigned. Until recently these were the only experimental data available for CoO. ) Corresponding author. However, the development of new techniques for measuring electronic transitions, in particular elec- Ž . tron energy loss spectroscopy EELS , led to a re- newed interest in the crystal field transitions. This technique is not only surface sensitive but there is also the advantage that certain transitions which are optically forbidden can gain intensities in off-specu- lar geometries. In 1989 EELS measurements were performed on various polycrystalline transition metal Ž . monoxides TMO; TM s Mn, Co, Ni, Cu by Kemp wx et al. 4 . The optical absorption data for some of the bulk crystal field excitations were confirmed by these measurements. Gorschluter and Merz reported EELS ¨ Ž . Ž . wx data for the NiO 100 and CoO 100 surface 5 . For NiO a surface-specific peak was observed at 0.6 eV in addition to some peaks assigned to bulk states. For CoO no such surface-specific transition was ob- served, but some bulk peaks appear consistent with the spectra of Pratt and Coelho. Haßel et al. found 0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0301-0104 98 00241-9