Spectroscopy of hexanitritoelpasolite crystals: the effect of the rare-earth ion on the progressions in the nitrite vibration

The low temperature absorption spectra of the cubic hexanitritoelpasolite crystals Cs(2)NaLn(NO2) (Ln, rare-earth ion) in the blue spectral region are dominated by progressions in the bending mode of the nitrite ion. The vibrational structure on which these progressions are based is strongly dependent on the rare-earth ion which is surrounded by twelve oxygen ions from six nitrite groups. The lanthanide-oxygen distances show a remarkable change with temperature. The f-f transitions of the rare-earth ion are extremely weak in comparison to the corresponding hexachloroelpasolite crystals Cs(2)NaLnCl(6) which have a similar structure except for the high frequency vibrations. We compare the 12 K absorption spectra of Cs2NaY(NO2)(6), with hexanitritoelpasolite crystals containing Ln = La, Sm, Gd, and Er. The change in the relative intensities of the origins of the nitrite progression lines is related to the change in the ionic radii of the rare-earth ions, which in turn affects the metal oxygen distance and the O-N-O bond angle. (C) 1998 Elsevier Science B.V. All rights reserved.