No abstract

This paper elucidates some subtle properties of the orthorhombic and monoclinic symmetry crystal-field ͑CF͒ Hamiltonians ͑H CF ͒, which are not fully understood by some authors. These properties bear on interpretation of experimental data obtained in, e.g., optical spectroscopy, inelastic neutron scattering, and magnetic susceptibility measurements. Our reanalysis of CF parameters ͑CFPs͒ is based on application of the standardization idea as well as the idea of dataset closeness. The closeness of any two CFP sets may be represented by the closeness factor C and the norm ratio R = N A / N B for the respective H CF terms: k =2,4,6, and the global ones C gl . The specific aims of this paper are twofold. The first aim is to clarify the controversy arising when comparing the standard CF parameters with the nonstandard CFPs. This controversy, originating from the lack of awareness of the standardization applicable to the orthorhombic ͑as well as monoclinic and triclinic͒ CFP datasets, is evident, e.g., in the recent study of Nd 3+ ion in Nd 2 BaCuO 5 by R. S. Puche et al. ͓Phys. Rev. B 71, 024403 ͑2005͔͒. This leads to an unjustified criticism of the data of other authors. Consideration of standardization and dataset closeness also helps in identifying other inconsistencies concerning R. S. Puche et al.'s results. The second aim is to determine the alternative physically equivalent CFP datasets generated by the standardization transformation. These CFP datasets are utilized in the multiple correlated fitting technique ͑MCFT͒ to improve the reliability of the final fitted CFPs. Additonal refittings of the original energy level data are carried out starting from distinct regions in the parameter space. The independently fitted nonstandard CFPs obtained in this way are then transformed to the standard region and intercorrelated. Our considerations enable us to solve the controversy in question and improve the understanding of the intricacies inherent in the low-symmetry CFP datasets. A comparative analysis of the CFPs for various rare earth R 3+ ions in R 2 BaXO 5 ͑X = Cu, Zn, Ni, and Co͒, and related crystals will be carried out elsewhere.

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.

Uranium (3+) doped single crystals of Cs2NaYCl6 and Cs2LiYCl6 with a 2.0% and 0.1% U3+ concentration have been obtained by the Bridgman-Stockbarger method. Luminescence spectra of the crystals were recorded at 160, 70, and 15 K. The emission bands observed in the visible and near infrared regions have been assigned to transitions from the lowest components of the I11/24, F3/24, and G7/24 multiplets to the crystal-field components of the I9/24 ground level. Absorption spectra were recorded from 4 000 to 25 000 cm−1 at 4.2 K. The zero phonon transitions were identified from an analysis of the vibronic side bands as well as in least-squares fits by applying a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions for an ion of the 5f3 configuration and Oh symmetry. Eight energy level parameters were varied simultaneously in the least-squares adjustments yielding a mean error of 57 and 61 cm−1 for U3+:Cs2LiYCl6 and U3+:Cs2NaYCl6, respectively. The analysis of the spectra enabled the assignment of 25 crystal field levels for U3+:Cs2NaYCl6 and 27 for U3+:Cs2LiYCl6. The received total splitting of the ground level is equal to 625 and 595 cm−1, respectively.

No abstract