Vibrational spectra of molecular ions isolated in glassy and crystalline alkali metal oxyanion salt matrices

Molecular anions (NO−3 and ClO−3) have been isolated in glassy alkali metal oxyanion matrices (MClO3 and MClO4) at 80°K so that the splittings of the impurity molecular ion degenerate modes become a more direct measure of the anion site asymmetry in amorphous salt media. In general it is found that the individual components of the impurity ion ν3 asymmetric stretching band are much sharper and the splitting of the ν3 components significantly reduced over values for pure glasses or molten salts. Because of the band narrowing, distinct maxima are observed for both ν3 components for KNO3 and NaClO3, for which a doublet structure has been inferred but not previously observed. Of principal interest is the observation that the isolated NO−3 ν3 splitting, for a LiClO4 or KClO4 glass matrix, is unchanged by crystallization of the salt. In other words, the vibrational data show that the distortion experienced by the NO−3 anion in an amorphous salt is not appreciably different than for an asymmetric site in a perchlorate crystal. Consequently, the vibrational data are no more indicative of ion-pair configurations in the glass or molten salt phase than for the crystal phase.