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_kappa 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.