Impurity Effects on the Ionization States of F-Aggregate Color Centers in Sodium Fluoride

Abstract

A study has been made of the F‐aggregate color centers produced in NaF crystals by x irradiation at room temperature followed by various thermal and optical bleaching treatments. Crystals used were grown from Harshaw NaF. In addition to undoped samples, crystals were grown with NaOH and Na₂O₂ dopings. Important differences are observed in the doped samples in relation to the charge states of F‐aggregate centers. Electron‐deficient centers are stabilized, and the probability of their formation is enhanced in hydroxide‐doped samples. In peroxide‐doped samples, the “extra‐electron centers” are favored. Model photochemical reactions are introduced which show the generation of “electron traps” in hydroxide‐doped samples and provide excess electrons in peroxide‐doped samples. The behavior of the doped samples can be equivalently explained on the band picture in terms of the shift of the Fermi level caused by the introduction of acceptor and donor levels by the doped impurities. The postulated oxidation–reduction effects are supported by observations of impurity effects on (1) relative concentrations of charged and neutral centers after x irradiation, (2) thermal and optical bleaching rates, and (3) reactions involving conversion among various charge states.