Crystal-field energy level analysis forNd3+ ions at thelow symmetry C1 site in [Nd(hfa)4(H2O)](N(C2H5)4) single crystals

Abstract

Optical absorption measurements ofNd3+ ions in singlecrystals of [Nd(hfa)4(H2O)](N(C2H5)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 groupP 21/n). Each Nd ion is coordinated to eight oxygen atoms that originate from thehexafluoroacetylacetonate ligands and one oxygen atom from the water molecule.A total of 85 experimental crystal-field (CF) energy levels arising from theNd3+ (4f3) electronic configuration were identified in the optical spectra and assigned. A three-stepCF analysis was carried out in terms of a parametric Hamiltonian for the actualC1 symmetryat the Nd3+ ion sites. In the first step, a total of 27 CF parameters (CFPs) in the Wybourne notationBkq, admissible by group theory, were determined in a preliminary fitting constrained by theangular overlap model predictions. The resulting CFP set was reduced to 24 specificindependent CFPs using appropriate standardization transformations. Optimizations of thesecond-rank CFPs and extended scanning of the parameter space were employed in the secondstep to improve reliability of the CFP sets, which is rather a difficult task in the case of nosite symmetry. Finally, seven free-ion parameters and 24 CFPs were freely varied, yieldingan rms deviation between the calculated energy levels and the 85 observed ones of11.1 cm−1. Our approach also allows prediction of the energy levels ofNd3+ ions that are hidden in the spectral range overlapping with strong ligand absorption, whichis essential for understanding the inter-ionic energy transfer. The orientation of the axissystem associated with the fitted CF parameters w.r.t. the crystallographic axes isestablished. The procedure adopted in our calculations may be considered as a generalframework for analysis of CF levels of lanthanide ions at low (triclinic) symmetry sites.