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Title: Dynamics and structure of molten salt mixtures
Author: Durell, Martin
Awarding Body: University of Leicester
Current Institution: University of Leicester
Date of Award: 1998
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The self-scattering SNi( Q , o) Partial Dynamic Structure Factor (PDSF) for liquid NiI 2 and NiI2+Ni has been measured using Time of Flight (TOF) inelastic neutron scattering. The narrowing of the quasi-elastic region of the SNi( Q , o) PDSF shows the mobility of the cations to be curtailed for NiI2+Ni. Measurements taken in the high temperature solid phase show that both the pure salt and the metal-salt mixture melt from a fast ion conductive state. The presence of two distinct 'soft' modes in the quasi-elastic and inelastic regions of the power spectra for the autocorrelation functions have been ascribed to ionic diffusion in molten NiI2. Strong mass-charge coupling in the melt is indicated by a prominent CLmq ( Q , o) mode at E ~ 20meV. The structure factors have been measured for CsCl-MgCl2, CsCl-NiCl 2 and CsCl-ZnCl2 binary melts using TOF and steady state neutron diffraction techniques. The results exhibit similarities with those obtained from KCl-MCl2 studies. Badyal and Howe's model of ordering between the alkali cation and [MCl4]2- units is supported by the First Sharp Difraction Peak (FSDP) for the CsCl mixtures residing at lower Q values than those for KCl melts. The similar Full Width Half Maximum (FWHM) values for the two sets of results is believed to be due to increased stability of units [MCl4]2- in the presence of Cs+ ions, thereby compensating for the low charge density of the larger alkali cation. The structure factors of AlCl3-ZnCl2, AlCl 3-CSCl and equimolar AlCl3-ZCl (Z=Li, Na, K, Cs) binary melts have been measured using TOF neutron diffraction. The constant displacement of the FSDP to higher Q values across the AlCl3-ZnCl2 composition range indicates a reduction in ordering repetition length. This has been associated with a reduction in voids in the melt and a change from comer to edge-sharing polyhedra with increasing ZnCl2 concentration. The formation of a charge ordered structure comprised of Cs+ ions and [AlCl4]- units has been associated with a narrowing of the FSDP and a reversal in migration of the feature. The positions of the FSDPs for the equimolar AlCl3-ZCl melts indicate an increase in the repetition length with increasing ion size. A model where complete dissociation of AlCl3 structure occurs near the equimolar concentrations for all the AlCl3-ZCl mixtures with dative covalent bonding of the smaller alkali species to [AlCl4]- units has been proposed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available