EXAFS studies of glassy and liquid ZnCl2: A comparison with vitreous GeO2

The Zn-EXAFS (extended X-ray absorption fine structure) above its K-absorption edge has been measured for glassy ZnCl₂ at low temperature, through T_g (375 K), and into the supercooled and normal liquid state (mp = 593 K) at 673 K. By Fourier filtering and fitting the normalized glass spectra using α-ZnCl₂ as a model compound, the Zn²⁺Cl^? distance was determined to be (2.34 ± 0.01) Å and the average coordination number about the Zn²⁺ was found to be 5.1 ± 0.8. The latter value agrees with the value of 4.7 nearest neighbors obtained by the molecular dynamics computer simulation study of Woodcock et al., for liquid ZnCl₂ just above its melting point. The agreement is supportive of the notion that short-range order in the glass is reflective of that of the corresponding liquid from which it was quenched. Spectral measurement as a function of temperature indicates that there is considerable dynamic decoupling of the Zn²⁺ from its nearest Cl^? neighbor even below T_g. This is contrasted with similar data in glassy GeO₂ which show that the motion of Ge is strongly coupled with its four oxygen neighbors all the way to T_g. This difference in dynamic disorder substantiates the notion that glassy ZnCl₂, as well as vitreous BeF₂, provides a further weakened structural analog for glassy GeO₂ and SiO₂.