NEPHELAUXETIC EFFECTS IN EUROPIUM-DOPED LNOX (LN = LA, GD, OR Y, X = F, CL, OR BR) CRYSTALS - HIGH-PRESSURE EFFECTS

The free-ion parameters for Eu3+ in LaOF, LaOBr, GdOCl, and YOCl crystals and in LaOCl crystals at high pressure (up to 13 GPa) are calculated and fitted to the experimental 'free-ion' energy levels. The Slater parameters F(k) and the spin-orbit coupling parameter zeta decrease with increasing pressure and with decreasing Eu3+-ligand distances along the two host series LaOCl-GdOCl-YOCl and LaOBr-LaOCl-LaOF. The shift of the free-ion parameters along the latter series exhibits an 'anti-nephelauxetic series' and therefore has a positive correlation with the overlap integrals. On the basis of the free-ion calculations and the calculations of the overlap integrals between Eu3+ and the ligands, a combined cloud-expanding model is proposed to explain the behaviour of the free-ion parameters.