EXAFS studies of structural changes in fragile glasses of zinc nitrate and nickel nitrate hydrates.

The ionic structures of aqueous solutions of two sets of transition metal nitrates have been studied in the liquid and glass states by EXAFS spectroscopy. Experiments were carried out on Zn(NO3)2.xH2O, with x=2, 6, 12 and NiNO.9HO over the temperature range 30<T(K)<250. The glass transition regime was monitored by means of an in-situ DSC probe. The EXAFS data were analysed by recently developed Monte Carlo procedures, enabling a discussion of the glass structure in terms of pairwise and higher order correlations. Results for the zinc nitrate hydrates show complex behaviour depending on the concentration. This behaviour is explained in terms of first hydration shell stability and NO3- penetration. This result contrasts with that for the equivalent correlation in nickel nitrate, and is taken as evidence for a more extensive free energy landscape of zinc nitrate hydrates. The results are also consistent with the known hydration properties of Zn2+ and Ni2+, and help explain why Zn2+ is biologically active in solution.