Experimental and Theoretical Studies of the Vibrational and Electronic Spectra of a Lanthanide Ion at a Site of Th Symmetry: Pr3+ in Cs2NaPr(NO2)6

The Pr3+ ion in Cs2NaPr(NO2)(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 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 D-1(2) -> H-3(4) 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+ 4f(2) energy-level data set has been fitted by calculation. By comparison with Cs2NaPrCl6, the fourth-order 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 10 degrees 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 triplet-state energy shows a linear decrease with an increase of the lanthanide (Ln(3+)) ionic radius in Cs(2)NaLn(NO2)(6). Using the eigenvectors from the energy-level fit, the variation of the inverse magnetic susceptibility with temperature has been calculated between I and 100 K and the values are somewhat lower than those from experiment.