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Electron spin resonance studies of reaction intermediates in metallic halides Catton, Richard Carl


The object of this work was to produce and identify reaction intermediates in systems containing group I and group II halides treated with fluorine. The systems which are studied in detail are the NaCl and KCl/F₂ and SrCl₂/F₂ systems. In accordance with predictions from kinetic studies, ESR has shown that treatment of vacuum-sublimed NaCl with F₂ at room temperature produces a defect containing an unpaired electron, and that the decay of this defect is retarded by Cl₂. The ESR spectrum indicates an anisotropic g-factor and unresolved hyperfine structure. In KCl, a spectrum is obtained which is closely similar except that the hyperfine structure is partly resolved. The spectra appear consistent with interactions of the unpaired electron with Cl atoms, rather than with F or such impurities as 0 or Br, and can be accounted for adequately by a model of the defect as linear Cl[formula omitted], i.e., an H center or something very similar. For NaCl, the spectra indicate that the vacuum-sublimed material is sometimes partly oriented. It is suggested that centers with one-electron deficiency are observable at such high temperatures in vacuum-sublimed material because this consists of essentially perfect crystals devoid of sites which could accept a second electron from the centers. The reaction of SrCL₂ powder or single crystal with fluorine at room temperature produces a defect, stable after removal of fluorine but located close to the reaction interface only, which has an electron spin resonance absorption. The ESR spectrum is consistent with a model of the defect as a chlorine atom displaced from an anion site towards a neighbouring anion vacancy. The principal directions of the hyperfine tensor and g-tensor are two two-fold axes and a four-fold axis of the SrCl₂ lattice. The tensor components indicate that the unpaired electron is localized on the Cl atom, and suggests that the atom is subject to a strong crystal field determind chiefly by two nearest-neighbour cations which define a two-fold axis of the crystal. The unpaired electron is in an orbital mainly of p-character and aligned along a two-fold axis which is probably the one perpendicular to the line of the cations. The single-crystal spectrum, although haying orientation-dependent line positions, has line shapes and intensities resembling those of a powder spectrum. This suggests a range of relative displacement of Cl atoms and neighbouring cations along a "reaction co-ordinate" which is probably a four-fold axis of the crystal.

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