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Theoretical studies of vacancy pair centres in the alkaline earth oxides

To, Kin Ching

Theoretical studies of vacancy pair centres in the alkaline earth oxides Thumbnail


Kin Ching To


This thesis is concerned with colour centres, particularly the FC+-centre, in the alkaline earth oxides. The FC+-centre consists of an electron trapped at an anion-cation vacancy pair and is intimately connected with the F+-centre, an electron trapped at a single anion vacancy. We emphasise in this thesis the connection between the FC+ and F+-centres. A theoretical model is developed which represents the FC+-centre as an F+-centre perturbed by the presence of a neighbouring cation vacancy. The calculated binding energy of the FC+-centre in the point ion lattice approximation is found to be too large compared with experiment. Corrections due to lattice distortion and polarisation effects taken together lower the binding energy although the polarisations of the surrounding ions actually increases the binding energy the lattice distortion decreases the binding energy by a slightly larger amount. Reasonable agreement between the experimental and theoretical binding energies is obtained for MgO only after ion size corrections to the point ion model.The axial symmetry of the FC+-centre is best revealed by the ratio of the
isotropic hyperfine constants, A00-1/A010. An attempt has been made to calculate the isotropic hyperfine constant within this model. The results show that whilst the ratio agrees reasonably well with the experiment, the absolute value of the isotronic hyperfine constants are too large compared with experiment. A slight improvement of the isotropic hyperfine constants is obtained when the ion size effect is taken into account.
A continuum calculation for the FC+-centre, in which the
electron is assumed to be trapped in the field of a finite electric dipole immersed in a dielectric medium is also attempted, and the results compare with the point ion model for the FC+-centre.


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