Crystal-field energy level analysis for Nd³⁺ions at the low symmetry C₁site in [Nd(hfa)₄(H₂O)](N(C₂H₅)₄) single crystals

Abstract

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.

Alternative crystal field parameters for rare-earth ions obtained from various techniques: IV. Comparative analysis of crystal field parameters obtained from inelastic neutron scattering and related studies of RE ions (RE=Er3+, Ho3+, Nd3+, Pr3+) in REBa2Cu3O7−δ high-Tc superconductors

“…These alternative CFP sets may be utilized in the multiple correlated fitting technique [11,52] to improve reliability of the final fitted CFPs. Applications of the multiple correlated fitting technique in optical spectroscopy [44,[71][72][73] and EMR [74] convincingly confirm its usefulness.…”

Alternative crystal-field parameters for rare-earth ions obtained from various techniques: III. Low symmetry aspects inherent in monoclinic parameters obtained by Mössbauer spectroscopy for Tm3+ ions in Tm2BaXO5 (X=Co, Cu, Ni)

“…The present considerations provide deeper meaning to the fitted monoclinic CFP sets [4,5] and enable application of the multiple correlated fitting techniques [7,9] to improve the reliability of the final fitted results as successfully shown in [3,8,34]. Clarifications of low symmetry aspects inherent in monoclinic CF parameters may be helpful in future spectroscopic studies of Tm 2 BaXO 5 and related systems.…”

“…These examples amply illustrate the fact that for monoclinic and triclinic cases the rhombicity ratio B 2 2 /B 0 2 [9,10] is meaningful only for orthorhombic-like CFP sets expressed in the principal axis system of the rank-2 CF terms [33]. The low symmetry aspects inherent in such sets may be easily misinterpreted if proper care is not taken in their analysis [30,34].…”

Kramers doublets, phonons, crystal-field excitations, and their coupling in Nd2ZnIrO6

“…1), which appear in lowtemperature regime below ~ 150-200 K. We have assigned these modes to the crystal-field excitations within the ground state multiplets of Nd 3+ ( 4 9/2 I , ground state has five Kramer's doublets, see inset of Fig. 5(c) for their energy levels [37][38]). As the frequencies of Raman active modes are independent of the incident photon energy, and as result appear at the same position in Raman-shift even with different energy of the excitation source; also, Raman active modes appear at the same position in both stokes and anti-stokes side of the spectrum.…”