NEPHELAUXETIC EFFECTS ON SM2+ AND EU3+ IN TERNARY MYX COMPOUNDS

Isoelectronic Sm2+ and EU(3+) ions with their ground configuration 4f(6) are used in different ternary MYX compounds to study the reduction of the Slater parameters F-kappa and the spin-orbit coupling parameter zeta(4f), commonly referred to as the nephelauxetic effect. At first, a simulation of the 4f(6) energy levels in the intermediate coupling scheme with the two parameters F-2 and zeta(4f) shows that the set of basis functions must include all the septets ((7)L(J)) and all the quintets ((5)L(J)), as well as some triplets (P-3(J), D-3(J), and F-3(J)) Or singlets (S-1(J), P-1(J), and D-1(J)) to reproduce the energy level scheme accurately, whereby a strong effect of the triplets and singlets on the D-5(J) multiplets is most prominent. With structural parameters of different host materials at ambient and elevated pressures, different distance dependences of the parameters F-2 and zeta(4f) for Sm2+ and EU(3+) are derived from the ambient-pressure and high-pressure energy-level schemes, demonstrating a breakdown of the nephelauxetic series. Only a combination of at least two microscopic covalency mechanisms, central-field covalency (CFC) and symmetry-restricted covalency (SRC), allows for a description without contradictions. Thereby the CFC contribution is dominant for the small EU(3+) ion, whereas the SRC is more dominant for the larger Sm2+ ion. Finally, ab initio calculations of Coulomb integrals are used to evaluate the Slater parameters F-kappa of Sm2+ within the SRC model and close agreement of these results with the experimental data is then obtained.