Nuclear techniques such as perturbed angular correlation (PAC) sample the
hyperfine interactions of a large number of probe atoms in specific crystallographic
sites. Real crystals contain static defects producing a distribution
of electric field gradients (EFGs) that add to the ideal EFG of the crystal at
any given probe site. Also, dynamic defects like moving vacancies and interstitial
atoms can be present in the crystal and contribute to the distribution
of EFGs. The distribution of EFGs leads to line broadening and a change in
the observed asymmetry parameter η since the total EFG no longer has the
symmetry of the perfect crystal. When both defects are present in a material,
obtaining quantitative information from the analysis of PAC spectra is usually very difficult since great care has to be taken to ensure that the source
of line broadening is identified correctly. In order to relate the relationship
between the static line broadening and changes in the asymmetry parameter
η, a uniform random distribution of point charges was used to simulate the
static defect EFG. PAC spectra collected on cubic niobium metal, cubic stabilized
zirconia and Nb-doped tetragonal zirconia were fitted with this model.
Although the quality of the fits is good, more work is needed to clarify the
relationship between the new model parameters and the line broadening and
asymmetry parameter derived from conventional model fits. The PAC spectra
of Nb-doped tetragonal zirconia were fitted with a conventional static model
to establish a reliable relationship between line broadening and the asymmetry
parameter when only static defects are present in a sample. To account for effects
of dynamic defects, a four state stochastic model for vacancy motion was
adapted in order to include the line broadening and changes in the asymmetry
produced by static defects. As a result, the activation energies corresponding
to the rates at which a oxygen vacancy is trapped by, detraps from, and hops
among equivalent sites about a PAC probe atom were calculated. The values
that were found are physically reasonable, indicating that the dynamics of an
oxygen vacancy around a PAC probe atom are satisfactorily described. / Graduation date: 2003
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/30128 |
| Date | 13 March 2003 |
| Creators | Alves, Mauro A. |
| Contributors | Gardner, John A. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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