Z. Phys. B Condensed Matter 79, 57-60 (1990) Condensed Zeitschrift Matter for Physik B 9 Springer-Verlag 1990 Calculated optical curve for spin polarized MnO J. Hugel and M. Belkhir Cloes, Universit6de Metz, France Received November 28, 1989 A spin polarized band structure has been used to calcu- late the dielectric function of MnO. It appears that d to d band transitions are at the origin of the absorption edge owing to the p d mixing. Like NiO, the onset excita- tions are of p--+ d charge transfer character. Only one metal ion is implied in the transfer process. Introduction Among the transition metal monoxides MnO sets the least problems when the electronic structure is described within the spin polarized band scheme. This is essentially the result of the atomic exchange which opens a conse- quent gap (2.2 eV, Belkhir and Hugel [1]) between the full majority and the empty minority d bands. Since the antiferromagnetic unit cell of D~ a symmetry contains two metal ions, "majority" labels an electron in a metal ion with the spin parallel to the atomic moment, whereas "minority" labels an electron in a metal ion with the spin and atomic moment antiparallel. The presence of the forbidden band confirms the pre-eminence of the spin polarized approach with respect to the NaC1 crystal structure where no d band separation can be induced by the crystal field (Mattheiss [2]). Concerning the opti- cal properties, the absorption edge has been found around 3.8 eV (Iskenderov et al. [3], Hufmann et al. [4]) and the first maximum in the reflectance curves arises at 4.7 eV (Messick et al. [5], Ksendzov et al. [6]). Below the absorption edge the structures have been assigned to excitonie crystal field transitions between 3d states [3, 4]. Just above, Ksendzov et al. [6] concluded that transitions from the 3 d bands to the conduction bands give rise to the singularities observed in the first part of the reflectance curve. In contrast, the examination of the band structure conducts, on energy grounds, to consider transitions out of the majority d bands to the minority d bands as the most likely to generate the absorption edge. Pure d metal intersite dipolar transitions cannot be envisaged because they are precluded by the selection rules. The remaining possibility is to ask if the mixing between p and d orbitals in the one electron crystal function is sufficient to give an observable manifestation in the dielectric function. The study of the dielectric function will serve to provide assignments for the structures which can be put in corre- spondence with the experimental spectrum. Compared to NiO, MnO has received less experimen- tal and theoretical attention. Despite the large amount of efforts devoted to NiO, the controversy about the interpretation of the onset energy in terms of one or two ions process is not settled (see for instance Merlin [7], Williams et al. [8]). It is only admitted that the onset mechanism corresponds to a p -~ d charge transfer excita- tion. The purpose of our work is to take advantage of the information of the band results in order to furnish an independent interpretation of the optical spectra and to specify the nature of the absorption edge. Results Figure 1 presents the spin polarized valence and conduc- tion bands of MnO obtained by extending a previous work [1] to the 4s and 4p metal functions. As mention- ned in the introduction, the significant feature is the pre- sence of the 2.2 eV d gap containing the Fermi level. The p and d band organisation is in qualitative agree- ment with the antiferromagnetic structure calculations of Wilson [9], Andersen et al. [10], and Terakura et al. [11]. The difference between the various spin polarized bands lies in the width of the d gap and in the additional splitting of the majority and minority d bands into eg and t2g subbands. While the calculated structures exhibit comparable band profiles, there is less in common with the experimental electronic structure of [6]. The major points of divergence are the very broad forbidden band- width (4.7 eV) and the fact that the 3d minority band