Experimental and theoretical studies of the vibrational and electronic spectra of a lanthanide ion at a Site of T _h symmetry: Pr ~(3+) in Cs_2NaPr(NO_2)_6

摘要

The Pr~(3+) ion in Cs_2NaPr(NO_2)_6 is situated at a site of T_h 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 Pr ~(3+) 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 ~1D_2 → ~3H_4 emission spectra of Cs_2NaPr(NO _2)_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 Cs_2NaY(NO_2)_6:Pr~(3+) was dominated by NO_2~- triplet emission, which was quenched at elevated temperatures by energy transfer to trace Eu~(3+) impurity. From magnetic dipole calculations and the vibronic fingerprint, detailed assignments are given for the complex 10 K electronic absorption spectrum of Cs _2NaPr(NO_2)_6 between 3940 and 18800 cm ~(-1), and the derived Pr~(3+) 4f~2 energy-level data set has been fitted by calculation. By comparison with Cs_2NaPrCl _6, the fourth-order CF parameter in Cs_2NaPr(NO _2)_6 is relatively small so that interaction with a 4fnp configuration is not important. From the NO_2~- 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 10° in the triplet state. By contrast to the NO_2~- internal vibration frequencies, which except for the wagging mode show only minor changes with the environment, the triplet-state energy shows a linear decrease with an increase of the lanthanide (Ln~(3+)) ionic radius in Cs_2NaLn(NO _2)_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.