THE INFLUENCE OF THE HOST LATTICE ON THE LUMINESCENCE OF DIVALENT EUROPIUM

The luminescence of Eu2+ in several silicate host lattices has been st udied. In these structures there are chains of alkaline earth ions. In some cases the Eu2+ ion in these chains shows emission at long wavele ngths. This effect is due to preferential orientation of a d-orbital. In (Ca,Sr)(2)MgSi2O7:Eu2+ it is shown that if the cell parameters are increased by replacing (part of) the calcium by strontium ions, the ef fect of preferential orientation of a d-orbital in the chain direction decreases and the Eu2+ emission shifts to shorter wavelengths. Howeve r, in SrSiO3:Eu2+ and CaSiO3:Eu2+ the chains of alkaline earth ions ar e close to each other and a blue Eu2+ emission is observed. This indic ates that preferential orientation of only one d-orbital is required t o obtain a Eu2+ emission at low energy. Finally, in BaSiO3:Eu2+, BaSi2 O5:Eu2+ and Sr2LiSiO4F:Eu2+, where there is a network of alkaline eart h ions through the lattice, the conduction band is lowered and an impu rity-trapped exciton emission is observed. This situation is similar t o that in BaF2:Eu2+.