VIBRONIC INTENSITIES IN THE OPTICAL-ABSORPTION SPECTRA OF PR3-XCL6 - CONCENTRATION AND TEMPERATURE-DEPENDENCE( IN CS2NAPRXY1)

The (3)H4-->P-3(0) transition in the absorption spectra of Pr3+ in, th e hexachloroelpasolites Cs2NaPrxY1-xCl6 (x=0.1 to x=1) have been measu red from 300 to 10 K. The electronic spectra are dominated by electric -dipole allowed vibronic origins enabled by Herzberg-Teller coupling. The two odd parity vibrations of the [PrCl6](3-)- ion (upsilon(3) and upsilon(4)) appear strongly in the vibronic sidebands and are accompan ied by weaker lattice vibrations. The small change in the oscillator s trength of the vibronic transitions is readily accounted for by the ch ange in lattice parameter with x. There is no evidence for cooperative effects.