Spectroscopy and theoretical investigations of high symmetry lanthanide complexes


Student thesis: Doctoral Thesis

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  • Wenyu LI


Awarding Institution
Award date3 Oct 2012


Hexanitritolanthanates provide an unique opportunity to investigate the energy level structures and crystal field of the lanthanide ion (Ln3+) in Th site symmetry, with 12-coordination to oxygen. However the electronic spectra are mainly vibronic and particularly extensive and complex. Advantage may be taken of the high energy of NO2-vibrations, using 15N substitution, in assigning the electronic emission spectra. These features form a fingerprint in less-congested spectral regions. The technique is demonstrated for the case of Ln = Eu, from the study of the 5D0 emission spectra. The versatile NO2- ligand is a chelating, bidentate ligand in the high symmetry M2M'Eu(NO2)6 (M = Cs, Rb; M' = Na, Li) systems. The investigation of the vibronic structure and energy levels of these systems is of interest for comparison with the abundant data from Cs2NaEuX6 (X = F, Cl) crystals. The vibrational structure in the electronic emission spectra of the hexanitritolanthanates has been investigated from the analysis of the 10 K and 100 K spectra. Most of the vibronic intensity of the 5D0 → 7F0, 7F2 transitions is due to low energy skeletal modes which correlate with τu (Th) infrared vibrations. The energy levels derived from the vibronic analysis indicate a weak crystal field in these systems. The luminescence spectra of Eu3+ at a Th point group site in the hexanitritolanthanate systems Cs2NaEu(14NO2)6, Cs2NaEu(15NO2)6, Rb2NaEu(14NO2)6, Cs2LiEu(14NO2)6 and Cs2NaY(14NO2)6:Eu3+ have been recorded between 19500-10500 cm-1 at temperatures down to 3 K. The spectra comprise magnetic dipole allowed zero phonon lines, odd-parity metal-ligand vibrations, internal anion vibrations and lattice modes, with some weak vibrational progressions based upon vibronic origins. With the aid of density functional calculations, the vibrational modes in the vibronic sidebands of transitions have been assigned. The two-centre transitions involving NO2 - stretch and scissor modes are most prominent for the 5D0 → 7F2 hypersensitive transition. The onset of NO2 - triplet absorption above 20000 cm-1 restricts the derived Eu3+ energy level dataset to the 7FJ (J = 0 - 6) and 5D0,1 multiplets. Twenty two levels have been included in crystal field energy level calculations of Eu3+ in Cs2NaEu(NO2)6, using 7 adjustable parameters, resulting in a mean deviation of ~20 cm-1. The comparison of our results is made with Eu3+ in the double nitrate salt. In both cases, the fourth-rank crystal field is comparatively weaker than in the europium hexahaloelpasolites. The Pr3+ ion in Cs2NaPr(NO2)6 is situated at a site of Th symmetry with 12- coordination to O atoms of bidentate nitrito groups. First-principles calculations of the vibrational modes of the complex were carried out using the density functional theory with the generalized gradient approximation Perdew-Burke-Ernzerhof exchange-correlation functional. The calculations that treated the Pr3+ 4f electrons as valence electrons showed better agreement with the experimental vibrational assignments compared with those treating the 4f electrons a part of the inner core. The 1D2→3H4 emission spectra of Cs2NaPr(NO2)6 at 7 K enabled assignments to be made for the crystal-field (CF) levels of the ground-state multiplet. The emission of the dilute system Cs2NaY(NO2)6:Pr3+ was dominated by NO2 - triplet emission, which was quenched at elevated temperatures by energy transfer to trace Eu3+ impurity. From magnetic dipole calculations and the vibronic fingerprint, detailed assignments are given for the complex 10 K electronic absorption spectrum of Cs2NaPr(NO2)6 between 3940 and 18800 cm-1, and the derived Pr3+ 4f2 energylevel data set has been fitted by calculation. By comparison with Cs2NaPrCl6, the fourthorder CF parameter in Cs2NaPr(NO2)6 is relatively small so that interaction with a 4fnp configuration is not important. From the NO2 - absorption bands above 20 000 cm-1, the N O bond length change upon excitation is small, whereas the angle O-N-O opens by more than 10K in the triplet state. By contrast to the NO2 - internal vibration frequencies, which except for the wagging mode show only minor changes with the environment, the tripletstate energy shows a linear decrease with an increase of the lanthanide (Ln3+) ionic radius in Cs2NaLn(NO2)6. Using the eigenvectors from the energy-level fit, the variation of the inverse magnetic susceptibility with temperature has been calculated between 1 and 100 K and the values are somewhat lower than those from experiment. The assignment of the low temperature 5D4 electronic emission spectra of Cs2NaTb(14NO2)6 and Cs2NaTb(15NO2)6 enabled the locations of 7FJ (J = 0-6) and 5D4 crystal field levels to be identified for Tb3+ at the Th site symmetry in these hexanitrito complexes. The complex vibronic spectra comprised metal-ligand, lattice mode and two centre NO2 - transitions, and the 14,15N vibrational energy shifts of the latter were most helpful in spectral assignments. Variable temperature studies enabled symmetry representations to be assigned to some crystal field levels since thermally-populated upper 5D4 levels also produced emission. The crystal field analysis of the energy level dataset by 5 adjustable parameters yielded a mean deviation of 16.5 cm-1. All of the three independent crystal field parameters were negative, with B62 dominant. The comparison of energy parameters with those from previous studies of Cs2NaLn(NO2)6 (Ln = Pr, Eu) systems showed (i) a reasonably linear increase with lanthanide atomic number Z (Z2) for the Slater (spin orbit coupling) parameter; (ii) irregular trends for the crystal field parameters for the three systems, attributed to the small number of crystal field levels in the datasets, which are thereby biased by the inclusion of only a few multiplet terms. The luminescence of Eu3+ doped in a series of double perovskite lattices Ba2LnMO6 (Ln = Y, Gd; M = Nd, Ta) has been recorded at room temperature and 10 K. Together with FT-IR and FT-Raman spectra, and aided by DFT vibrational energy calculations, assignments have been made for the crystal field levels of the 5DJ (J = 0,1) and 7FJ (J = 0-2) multiplets. The luminescence spectra are consistent with monoclinic symmetry of these systems. The crystal field parameters from the fitting of the energy level dataset of Ba2YNbO6:Eu3+ enable the crystal field strength to be calculated and the order of magnitude: free ion <Cl-< O2-< F- is found for EuX6 systems. By contrast, the nephelauxetic seriesCl- ~ O2-> F-> free ion is found from the comparison of Slater and spinorbit coupling constants of these systems.

    Research areas

  • Spectra, Rare earth metals