Structure and glass transition thermodynamics of liquid zinc chloride from far-infrared, raman, and probe ion electronic and vibrational spectra

With a view to elucidating the structure of liquid ZnCl₂ and deciding the extent of the analogy to SiO₂ and BeF₂ (liquid and vitreous) provided by ZnCl₂ (liquid and vitreous), a variety of measurements on ZnCl2 have been performed. Density and expansivity measurements and probe ion electronic spectra are used to demonstrate the existence of an open tetrahedrally coordinated structure. Details supplied by x-ray patterns, Raman spectra, and particularly, by far-infrared spectral studies, are then used to establish a close similarity between the glass quasilattice and the network lattice of the a-Znd2 crystal polymorph. Finally a new technique, impurity ion (CN^-) vibrational spectroscopy, is introduced to show that the defects characteristic of the glass structure involve small numbers of singly and doubly broken anion bridges, no evidence for intermediate states being found. On the basis of these observations a split-level cell model in which cells have discrete energies depending on the presence or absence of such Zn-Cl bonds, is constructed. The model proves capable of accounting for the volume data and for the characteristic changes in equilibrium thermodynamic properties which, it is known, must occur near (usually somewhat below) the glass transition temperature. The thermodynamic relations obtained for the simplest case are the same as those from the Macedo-Capps-Litovitz quasichemical two-state model. The liquid is best regarded as a weakened version of this network in which the progressive disruption of the anion bridges is associated with a greatly reduced structural relaxation time for the quasilattice.