Far-infrared spectra of inorganic nitrate and chloride glasses, liquids, and crystals: Complex ions or optical phonons?

Polyethylene-sandwiched films of glassy and viscous liquid metal nitrate, nitrate-chloride, and chloride mixtures, sufficiently thin for their absorption spectra in the far ir to be determined in direct transmission studies, have been prepared. Comparisons of the spectra of these amorphous phases with spectra of the same thin-film samples after crystallization, have been made. Intense absorption bands arising from the vibrations of the quasilattices are observed in all instances, and in the case of the Cd(NO₃) ₂+TlNO₃ glasses and liquids, the separate modes are almost as well defined as for the crystalline double salt TlNO₃· Cd(NO₃)₂. The dependence of lattice vibration frequencies on cation charge and mass, and on cation mole fraction within a given binary system, have been studied. In the mixed systems frequency shifts to both high and lower values than those found in the individual components are observed. No reflections of the abrupt changes in thermodynamic and flow properties, which occur at the glass transition temperature, are found in the vibrational spectra. The nature of collective vibrational modes of motion in condensed phases and the relation of the common "complex-ion" interpretation of vibrational spectra in molten salts to the "quasilattice dynamics" view of the same spectra, are considered in some detail. Information on the internal modes of the NO₃^- ion, and evidence for a distribution of NO₃^- ion environments, in the glass, are also presented.