Nephelauxetic effects onSm2+andEu3+in ternaryMYXcompounds

Isoelectronic ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ ions with their ground configuration 4${\mathit{f}}^6$ are used in different ternary MYX compounds to study the reduction of the Slater parameters ${\mathit{F}}_{\mathit{k}}$ and the spin-orbit coupling parameter ${\mathrm{\zeta }}_{4\mathit{f}}$, commonly referred to as the nephelauxetic effect. At first, a simulation of the 4${\mathit{f}}^6$ energy levels in the intermediate coupling scheme with the two parameters ${\mathit{F}}_2$ and ${\mathrm{\zeta }}_{4\mathit{f}}$ shows that the set of basis functions must include all the septets ${(}^7$ ${\mathit{L}}_{\mathit{J}}$) and all the quintets ${(}^5$ ${\mathit{L}}_{\mathit{J}}$), as well as some triplets ${(}^3$ ${\mathit{P}}_{\mathit{J}}$, ${}_{}{}^3{}_{}{}^{}$ ${\mathit{D}}_{\mathit{J}}$, and ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_{\mathit{J}}$) or singlets ${(}^1$ ${\mathit{S}}_{\mathit{J}}$, ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}_{\mathit{J}}$, and ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_{\mathit{J}}$) to reproduce the energy level scheme accurately, whereby a strong effect of the triplets and singlets on the ${}_{}{}^5{}_{}{}^{}$ ${\mathit{D}}_{\mathit{J}}$ multiplets is most prominent. With structural parameters of different host materials at ambient and elevated pressures, different distance dependences of the parameters ${\mathit{F}}_2$ and ${\mathrm{\zeta }}_{4\mathit{f}}$ for ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ 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 ${\mathrm{Eu}}^{3+}$ ion, whereas the SRC is more dominant for the larger ${\mathrm{Sm}}^{2+}$ ion. Finally, ab initio calculations of Coulomb integrals are used to evaluate the Slater parameters ${\mathit{F}}_{\mathit{k}}$ of ${\mathrm{Sm}}^{2+}$ within the SRC model and close agreement of these results with the experimental data is then obtained.