Optical absorption measurements of Nd(3+) ions in single crystals of [Nd(hfa)(4)(H(2)O)](N(C(2)H(5))(4)) (hfa = hexafluoroacetyloacetonate), denoted Nd(hfa) for short, have been carried out at 4.2 and 298 K. This compound crystallizes in the monoclinic system (space group P 2(1)/n). Each Nd ion is coordinated to eight oxygen atoms that originate from the hexafluoroacetylacetonate ligands and one oxygen atom from the water molecule. A total of 85 experimental crystal-field (CF) energy levels arising from the Nd(3+) (4f(3)) electronic configuration were identified in the optical spectra and assigned. A three-step CF analysis was carried out in terms of a parametric Hamiltonian for the actual C(1) symmetry at the Nd(3+) ion sites. In the first step, a total of 27 CF parameters (CFPs) in the Wybourne notation B(kq), admissible by group theory, were determined in a preliminary fitting constrained by the angular overlap model predictions. The resulting CFP set was reduced to 24 specific independent CFPs using appropriate standardization transformations. Optimizations of the second-rank CFPs and extended scanning of the parameter space were employed in the second step to improve reliability of the CFP sets, which is rather a difficult task in the case of no site symmetry. Finally, seven free-ion parameters and 24 CFPs were freely varied, yielding an rms deviation between the calculated energy levels and the 85 observed ones of 11.1 cm(-1). Our approach also allows prediction of the energy levels of Nd(3+) ions that are hidden in the spectral range overlapping with strong ligand absorption, which is essential for understanding the inter-ionic energy transfer. The orientation of the axis system associated with the fitted CF parameters w.r.t. the crystallographic axes is established. The procedure adopted in our calculations may be considered as a general framework for analysis of CF levels of lanthanide ions at low (triclinic) symmetry sites.
Both the fundamental assumptions of the angular overlap model (AOM) and its common simplifications are shown to have a sound basis in the ab initio calculations of the crystal-field effect in uranium (III), (IV), (V), neptunium (IV) and plutonium (IV) ions in various crystals. The traditional sigma - pi approach is confirmed as a well-aimed initial step towards an interpretation of the effect. The specific role of the delta -contribution as an important lattice-dependent correction indicates the necessity of its inclusion in the model. The practical two-step interpretation method that naturally emerges from the ab initio calculations is recommended. The effectiveness of the AOM for actinide ionic systems is illustrated using available phenomenological results. The universal character of the AGM parametrization and simple rules concerning the relationships between the parameters-their mutual hierarchy, spectrochemical ordering and the definite dependence on the metal ionization degree-are pointed out and discussed in the light of other phenomenological models including the Newman's twin superposition model.
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