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The Nitrogen Vacancy (NV) centre is a defect in diamond consisting of a substitutional nitrogen atom accompanied by a vacant nearest-neighbour lattice site. Substitutional nitrogen impurities are common in diamond, and vacancies can be introduced with radiation damage (there are other methods). The NV centre is formed by annealing the diamond: during this process the vacancy becomes mobile and is eventually 'trapped' by the substitutional nitrogen.

Why does the vacancy become mobile at high temperature, and why is it trapped by the nitrogen?

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This was shown by Konstantin Iakoubovskii and Guy J Adriaenssens 2001 J. Phys.: Condens. Matter 13 6015 doi:10.1088/0953-8984/13/26/316. Their optical absorption experiments show that single substitutional nitrogen centers trap vacancies about eight times more efficiently than the substitutional nitrogen pairs.

In my own reasoning, I would say that have to look at the temperature effect on the binding energy. Annealing decreases the binding energy and relaxes the lattice so that the vacancies can move around and find an energetically favorable state. At higher temperature, more movement is possible. Because the nitrogen atoms produces strain in the diamond lattice and are also mobile, it is reasonable to say that once they occupy a vacancy site, this reduces the overall energy of the lattice, in effect causing the vacancies to be filled with a nitrogen impurity.

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