Optical Spectra and Magnetic Behavior of a Wide Range of Europium(III) Oxo-Compounds: Analysis of the Ligand-Field Effects

Abstract

The europium-oxygen interaction in nine different europium(III) oxo-compounds (including C-type EuO) was investigated on the basis of powder reflectance spectra (near-IR/vis/UV) and temperature-dependent magnetic measurements. Computation of the transition energies and of the effective Bohr magneton numbers for Eu in the different ligand fields were performed within the framework of the angular overlap model (AOM) using the computer program BonnMag. These calculations show that all electronic transition energies in the optical spectra, the magnetic susceptibilities as well as their temperature dependence, are very well-accounted for by AOM. BonnMag provides a facile way to perform these calculations. Analysis of the obtained "best fit" AOM parameters e(Eu-O) shows that these are significantly influenced by the further bonding partners of oxygen ("second-sphere ligand-field effect"). An increase of e(Eu-O) from 404 cm (EuPO) to 687 cm (EuSbO), both normalized to d(Eu-O) = 2.38 Å, is found. Correlation of this variation to oxide polarizability and optical basicity of the oxo-compounds is discussed.

Analysis of Ligand Field Effects in Europium(III) Phosphates

“…Based on various considerations and observations (see Experimental Section) we estimate the accuracy of μ exp / μ B to be better than ± 0.03. This estimation is supported by comparison of our data for several europium(III) oxo‐compounds reported in reference. For none of the phosphates reported here hints on magnetic ordering were found.…”

“…The radial part of the overlap integral is well accounted for by the dependence of e σ (Eu 3+ –O 2– ) on d (Eu 3+ –O 2– ) –7 [Equation (1)]. As already discussed in reference to get an even better fit of calculated to observed excited state energies a more elaborate description of the free ion electronic levels can be introduced.…”

“…Only recently, with the introduction of the newly developed computer program BonnMag ligand field analysis for all f n systems (lanthanoides and actinoides; 1 ≤ n ≤ 13) has become feasible within the AOM framework . In a recent paper we have shown the applicability of this semi‐empirical approach using BonnMag by analyzing the ligand field effects experienced by Eu 3+ in a series of oxo‐compounds . This study showed, that the Slater‐Condon‐Shortley parameters F 2 , F 4 , and F 6 and the spin‐orbit coupling constant ζ of the free Eu 3+ ion are well suited for modeling of the excited states energies and magnetic susceptibilities of several europium(III) oxo‐compounds.…”

“…In literature the “best fit” AOM parameters e σ,max (EuPO 4 ) = 404 cm –1 and the corresponding e π,iso (Eu–O) = 101 cm –1 were already derived. The splitting of the 5 D 2 level of Eu 3+ in EuPO 4 as well as the observed temperature dependence of the Bohr magneton numbers, which depend primarily on the ligand field splitting of the 7 F 1 level, are well accounted for by these values.…”

“…We could also show, that e σ,max (Eu–O) depends to a first approximation linearly on the optical basicity and increases from acidic EuPO 4 to basic Eu 2 O 3 . According to these findings we have calculated the e σ,max values for all chromophores [Eu III O n ] of the whole series of europium(III) phosphates using e σ,max (EuPO 4 ) = 404 cm –1 as reference number and allowing for the different d min (Eu–O) and optical basicities Λ as it was reported in reference. Table provides a summary of the AOM parameterization used in these calculations including the optical basicity Λ for each compound.…”