Kazuto Yokota

Coloration caused by mechanical stress on rare earth oxysulfides

Abstract Oxysulfide powders of yttrium, gadolinium, terbium and lanthanum display distinctive colors when they are either heavily ball-milled or pressed in a steel die. These colors are due to optical absorption bands in the visible region, peak positions of which obey an empirical rule like the Mollowo-Ivey relation of color centers in alkali halides. The c-axis shrinks in the colored samples. The colors are bleached and the cell constants are restored to the original values by heat treatment up to 450°C. Some distortions, however, still remain in the annealed samples.

Color centers in rare earth oxysulfide phosphors

Abstract Color centers are found to be induced by mechanical stress on Ln 2 O 2 S powders. They have a significant influence on the practical use of the phosphors. Optical coloration and the Mollwo-Ivey relation suggest that atomically dispersed electron excess centers in the lattice may be responsible for the coloration. From ESR spectra observed in the stressed Y 2 O 2 S, F + centers are considered to be the most probable model, though the signal is absent in La 2 O 2 S.