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

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. © 2011 American Chemical Society.