Optical absorption measurements in combination with an EPR investigation of colour centres produced by X‐rays in synthetic chlorapatite single crystals are presented. Models for both electron‐excess and hole‐excess centres are proposed and theoretical calculations of some of the hole‐excess centres have been carried out. It turns out that the colour centres produced at temperatures higher than 77°K can be associated with oxygen impurities. Subtractive coloration at 5°K gives rise to the formation of F‐centres.
Flux‐grown synthetic calciumchlorapatite (Ca10 (PO4)6Cl2) single crystals are known to show significant off‐stoichiometry; the CaCl2 deficiency can be as large as 3 to 5%. This has severe implications for many physical properties of these materials; especially the defects as produced by means of X‐rays will be strongly modified by the presence of large concentrations of vacancies. It is found that at 300 K predominantly complex colour centres are created. Below 50 K rather simple electron‐excess centres are formed during X‐irradiation. A systematic study of the photo‐chemical bleaching and annealing properties of the colour centres enables one to develop models for the electron‐excess centres observed at different temperatures.
The position of fluoride ions occurring as minority in hydroxyapatite has been determined by means of nuclear magnetic resonance. The fluoride ions form magnetic resonance part of the linear chains of hydroxyl ions in one of the configurations OH-F-vacancy or OH-F-HO. A fluoride ion in this position will block diffusion along these chains. This may help to understand how very small amounts of fluoride are able to reduce the development of caries.
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