Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.618359
Title: Studies of disorder in fastionics and of a nuclear quadrupole interaction in ordered markets
Author: Walker, Alison Bridget
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1980
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Abstract:
This thesis consists of two completely separate parts. In Part I, we look at disorder in fastionic conductors, and in Part II we examine the pseudoquadrupole effect in ordered magnets. Part I looks at two aspects of the disordered state in fastionics, where the disorder is due to ions moving off their regular sites to positions close to other regular sites, a feature especially characteristic of the fluorites. The first aspect is how Coulomb interactions could be responsible for the co-operative behaviour of defects which causes the transition to fastionic behaviour. We look at this with a model of charged defects on a lattice, applying techniques involving classical diagrammatic perturbation theory to find the free energy of our model system. Using elementary thermodynamics, we show how this model can predict co-operative behaviour. The second aspect is the nature of the disorder above the fastionic transition. We look at disorder in lead fluoride using a molecular dynamics simulation with an interionic potential that we obtained. We use the simulation to examine the distribution of anions in both real space and k space. Simulations have been made on the other fluorites CaF2 and SrCl2 and it is possible that the high dielectric constant of lead fluoride might lead to qualitatively different behaviour. Our results show that this is not the case and we find defect concentrations similar to those obtained from CaF2 and SrCl2. Our Is space analysis however gives defect concentrations an order of magnitude larger, in approximate agreement with experiment. In Part II we set up a theory for the pseudoquadrupole effect in cubic ferromagnets and show that it is related to the difference in longitudinal and transverse magnetic susceptibilities. Model calculations are performed for a Heisenberg ferromagnet using molecular field theory near the critical temperature tc, and spin wave theory at low temperatures; and the itinerant model at absolute zero and tc. We find that the pseudoquadrupole effect in iron and nickel and at impurities in these metals appears to be very much less than measured quadrupole effects. We also look at the effect in GdAl2 and show that it can not explain a temperature dependent quadrupole interaction seen experimentally We therefore conclude that the observed quadrupole effects are due to real electric field gradients.
Supervisor: Gehring, Gillian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.618359  DOI: Not available
Keywords: Order-disorder models ; Nuclear quadrupole resonance
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