Alternative configuration interaction expansions for transition metal ions with intermediate oxidation states in crystals:: The structure and absorption spectrum of Cs2GeF6:Mn4+

The vertical absorption spectrum of the MnF62- cluster embedded in the Cs2GeF6 host crystal was recently calculated using the averaged coupled pair functional method leading to very large discrepancies with accurate one- and two-photon spectra. The same multiconfigurational expansions had previously been successful in similar systems which involved transition metal impurities in lower oxidation states. In this paper we show that the ligand-to-metal charge transfer configurations become so important in this intermediate oxidation state impurity (and, possibly, non-negligible ligand-ligand weak bonding interactions) that none of the 18 molecular orbitals of F 2p character should be left inactive in the correlation treatment. This requirement can be satisfied in MnF62- because of the higher oxidation state of manganese, which enhances the ligand field splittings in the Mn 3d(3) configuration manifold so much that one dominant Mn 3d(3) configuration rather than the full Mn 3d(3) active space can be used as a single reference for single and double excitations from all occupied ligand 2p orbitals. The results of this work, together with those of previous studies, outline two different alternative truncation schemes of the valence electron correlation which produce the same, necessary, high accuracy in structural and spectroscopic properties of transition metal ions doped in ionic crystals. Whether one or the other should be used depends, basically, on the formal oxidation state of the transition metal impurity. (C) 2001 American Institute of Physics.