Magnetic Resonance in Solids
Abstract
Available from UMI in association with The British Library. Requires signed TDF. In this thesis electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments are reported on paramagnetic defects in two host materials: (a) Cr^{3+} in the ferroelectric lecontite and (b) colour centres and the Fe^ {3+} centre in lithium oxide. A. Lecontite. Lecontite, NaNH_4 SO_4{cdot}2H _2O, undergoes a phase transition to a ferroelectric phase below T_{c } = 101 K. Reported here are EPR measurements on doped Cr^{3+} ions at two distinct sites, made at a variety of temperatures in an attempt to ascertain the details of the structural changes occurring at T_{c}. The room temperature data are compared to various models for the two defects using the Newman superposition model. The results show that the Cr^{3+ } ions are at Na^+ sites, highly distorted due to charge compensation of the trivalent dopant and therefore that the EPR of these defects is not representative of the bulk material. B. Lithium oxide. Investigations of three defects in lithium oxide, Li_2O are reported here. (i) The electronic F^+ -centre has been investigated using EPR and ^7Li ENDOR to determine the transferred hyperfine structure (THFS) due to hyperfine interactions of the defect wavefunction with the ligand nuclei. Comparison with F-centres in other simple ionic hosts shows that the centre is highly localised and this observation is explained by the charge states of the ligands and the defect site. (ii) EPR and ^7Li ENDOR of the interstitial neutral hydrogen centre H^0 show a novel tunnelling effect. At low temperatures the defect has tetragonal symmetry but at higher temperatures there is evidence of tunnelling between six equivalent sites, leading to effective cubic symmetry. (iii) THFS of Fe^{3+ } centres, measured using ^7 Li ENDOR, interpreted using a molecular orbital model and EPR measurements, interpreted using the Newman superposition model, are used to determine the distortions of the neighbouring ligands.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1992
- Bibcode:
- 1992PhDT........31O
- Keywords:
-
- LECONTITE;
- LITHIUM OXIDE;
- Physics: Condensed Matter