A neutron diffraction study of the structure and structural modification of molten zinc halides and nickel halides.
thesisposted on 19.11.2015, 09:01 by Dennis A. Allen
The structures of molten ZnCl2, ZnBr2 and ZnI2 have been determined by pulsed neutron diffraction. A tetrahedral configuration of anions around each cation was found for all three salts, which was extremely well defined for ZnCl2 and ZnBr2. Intermediate range ordering was also observed for all three salts and was seen to increase through the series from ZnCl2 to ZnI2. These observations are discussed with reference to previous studies of molten 2:1 salts, ion polarization effects and directional dependence of interionic forces. The effect of temperature upon the structure of molten ZnCl2 has also been investigated. In addition to this study of pure molten salts, the effect of structural modification of the 'network liquid' ZnCl2 by the addition of 'structure breaking' KCl has been studied. Pulsed neutron diffraction patterns were obtained from molten mixtures across the composition range. Upon addition of KCl, the local tetrahedral structure around each zinc ion was found to be very stable, even up to high KCl concentrations, and the intermediate range ordering in ZnCl2 not only persists, but was seen to increase, and to still be present even at 81% KCl concentration. Possible explanations of this behaviour are discussed and suggestions for further work are made. The structural modification of molten nickel halides by the addition of nickel metal has also been studied. A solution of NiI2 + 9 molar% Ni has been investigated by pulsed neutron diffraction from two isotopically enriched samples, and a solution of NiBr2 + 9 molar% Ni has been investigated by steady-state neutron diffraction from three isotopic samples. There was no evidence for subhalide complex ion formation involving more than one metal atom. The results suggest that nickel is ionising according to the equation Ni ? Ni2+ + 2e, with nickel ions occupying existing sites within the liquid structure and electrons acting so as to screen the interionic potentials. The maximum entropy method has been discussed and applied to the problem of structure factor data analysis. This method proved to be useful in the interpretation of some of the data presented in this thesis.