Concentration dependencies in the luminescence of Li4WO5: U

The uranate ion luminescence in a sequence of (Li_1?xNa_x)₄W_1?yO₅:U_y phosphors was examined at cryogenic temperatures under conditions of high resolution. It was found that as the ratio of U/W or Na/Li was increased, certain features in the sharp-line emission spectra decreased in intensity and other features increased at the same time. Careful examination of several photophysical parameters led to the conclusion that two different uranate sites yielded the observed luminescence features. One of the U(VI) ions occupies a site characterized by the presence of an inversion center, and the other ion occupies a site lacking such symmetry. The lower symmetry site may be considered a defect site, and was found to be an efficient quencher of the higher symmetry site.