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UBC Theses and Dissertations

Paramagentic impurity centres in alkali halides and strontium compounds Ng, Hok Nam


Paramagnetic impurities containing oxygen were produced in the reactions of KBr, KCl, NaCl and SrCl(2) with fluorine. Molecular oxygen, FOO・ radical and C10(2) were identified by ESR as reaction by-products. The spectra assigned to CIO(2) have predominant powder character even in single crystals of halides. The results are correlated with previous work in this laboratory. The origin of oxygen impurity is suggested to be surface hydroxide ions in KBr and KC1, and entrapped water in NaCl and SrCl(2). Nucleation processes and other anomalous features observed in these reactions by previous workers are explained by the presence of impurities. Oxygen was found to be incorporated into the SrCl(2) crystals by recrystallisation from the melt in the presence of oxygen. It exists in the form of superoxide ion O(2)¯ which occupies an interstitial position between two lattice anions and is associated with two anion vacancies. The molecular axis lies in a [00l] direction of the crystal and the degeneracy of the 2pπ(g) -molecular orbital is lifted by the crystal field. The bonding between the O(2)¯ ion and its neighbouring cations and anions is discussed in terms of a Cl(6)Sr・O(2)¯・SrCl(6) "complex". The orbital angular momentum reduction factor for O(2)¯ in SrCl(2) has been calculated from the experimental g-factors and found to be anomalously large. A survey was made on impurities incorporated into strontium compounds through processes of recrystallisation from melts or from aqueous solutions. Strontium carbonate was identified in melt-recrystallised Sr(NO3)(2) by Infrared Spectroscopy and X-ray powder method. It was produced by partial decomposition of the nitrate in the presence of atmospheric CO(2). NO(3)²¯ ion was identified in the recrystallised material by ESR. ESR and infrared studies suggest that the NO(3)²¯ ion substitutes a carbonate ion in the SrCO(3) lattice. Results from a similar study on Ba(NO3)(2) also support this conclusion.

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