D. Reinen, W. Rauw, U. Kesper, M. Atanasov, H U. Güdel, M. Hazenkamp and U. Oetliker
Colour, luminescence and bonding properties of tetrahedrally coordinated chromium(IV), manganese(V) and iron(VI) in various oxide ceramics
J. Alloys Comp. 246, 193-208 (1997)                 

Abstract: The spectroscopic properties (absorption, luminescence, EPR) of d²-configurated cations (Fe(VI), Mn(V), Cr(IV)) doped into the tetrahedral sites of a great variety of oxidic solids are reviewed and supplemented by new results. Angular overlap model calculations on the basis of the host site geometry, utilising a modified ligand field theory with different Racah parameters for e and t₂ orbitals, have been performed, which nicely fit the experimental data. In particular, they give evidence, that - in the case of Cr(IV) - the E¹ State is usually positioned well above the luminescing triplet state, in agreement with the experiment. Further, the calculations allow a characterization of the transition-metal-oxygen bond. It is finally shown, that the doping cation does not necessarily adopt the host site geometry; geometric criteria are given.