Anion-vacancy couples

Surface-trapped electrons can be formed at the surface of alkali-earth oxides by different methods, among these methods the UV irradiation of the solid in the presence of hydrogen has been found the most reliable and reproducible and leads to the formation of a particular type of surface colour centre named Fj (H) centre . The mechanism leading to the formation of these centres implies the heterolytic chemisorption of hydrogen at the surface of activated MgO and the formation of and H ions stabilised onto a couple of low-coordinated O -Mg ions. Upon UV irradiation a fraction of the hydride H ions are ionised and the released electrons stabilised into suitable surface anion vacancies close to the OH formed by reaction of H with surface O ions. At the end of this process the sample develops a blue colour and exhibits an EPR signal with g values slightly lower than the free spin value... 

Typical lattice defects include cation vacancies substitutional or interstitial ions are other types of more complicated structural defects. A cation vacancy behaves like a negative charge. If the temperature is high, ions are sufficiently mobile that an anion could be expelled from the lattice by the Coulomb potential of the cation vacancy. Cation and anion vacancies could form a dipole oriented along one of the six crystallographic axes. This vacancy coupling is then able to induce a crystalline dipole. Similar dipoles can also appear when an ion is substituted for the host ion. 

Even given the excess of F in the structure the theory of the distorted tetrahedron is not universally accepted. Regnier et al. (1994) used FTIR spectroscopy, NMR, and quantum mechanical calculations on synthetic and natural samples to demonstrate that the repulsion between the F anion and the carbonate anionic complex is too great to form the distorted tetrahedron. They do not dismiss the possibility that F couples with COs to balance the charge, but that if they do both enter the structure, the F enters independently and fills a vacancy in the structure. Nathan (1996) however points out that all of the samples used in these experiments are actually fluorine-deficient apatites and thus unlikely to have the COsF pseudo-tetrahedron, so clearly more research is needed before final judgments can be made. 

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