Structure in nitrate glasses: Multiple anion sites or random continuum?

The Raman spectra have been studied of tertiary glasses formed by the progressive replacement of Ca(II) by Mg(II) in an equimolar Ca(NO3)2:KNO3 mixture. The Raman bands of the three internal modes of the nitrate ion show progressive growth of a new set of components as the Mg(II) concentration increases. The changes are interpreted in the light of previous experimental and theoretical studies. A case is made for the existence of discrete anion sites in which the field perturbing the anion is dominated by one species of cation. Even in the case of the (Ca/K)NO3 glass, there is evidence for discrete sites rather than a continuum of anion environments.