It is shown that the ions Cr − , Br − , and I − play an essential part in the blue centers of , , and phosphors. The centers are assumed to be , and , respectively, (or the corresponding bromides and iodides), and to occupy normal zinc and sulfur sites in the zinc sulfide lattice. The spectral distribution of the fluorescence is the same for the centers containing chlorine or bromine ions; Ag, Zn, and Cu cause bands at slightly different wave lengths. A difference in peak positions for wurtzite and sphalerite is explained by the difference in separation between the upper occupied and the lower empty energy band of the base lattice.
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KROEGER et al. (1948) studied this question.