The luminescence of V2+(d3) and V3+(d2) ions in ZnS and an advanced interpretation of their excitation levels

At low temperatures, ZnS:V crystals display three structured photoluminescence bands. Their individual fine structures, their behaviour under various conditions of preparation and excitation and their different excitation spectra demonstrate the presence of the charge states V3+(d2), represented by the 3T2(F) to 3A2(F) emission band centred around 5600 cm-1; V2+(d3), represented by the 4T2(F) to 4T1(F) emission band around 4800 cm-1; and possibly V+(d4), tentatively connected with an emission near 3800 cm-1. A model of one-electron states is presented in which donor-type as well as acceptor-type changes of the vanadium oxidation states become comprehensible, commencing from V2+, the state with neutral effective charge. The excitation spectra for the V3+ and V2+ emission bands are interpreted on the basis of both a conventional and a modified Tanabe-Sugano scheme. Modifications considered concern mainly the inclusion of (i) a fit to the free-ion terms and (ii) different radial extensions of t2-and e-type wavefunctions. Values are derived for the crystal-field splitting parameter Dq and the Racah parameters B and C. Evidence is presented for a Jahn-Teller splitting of the 3T1(P) V3+ level and the 4T1(P) V2+ level.