Studies of disorder in fastionics and of a nuclear quadrupole interaction in ordered markets

Thesis



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 t_c, and spin wave theory at low temperatures; and the itinerant model at absolute zero and t_c.

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.

Authors: Walker, A

• Publication date: 1980
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Nuclear quadrupole resonance; Order-disorder models