The structure of some trapped hole centres in CaF2. - I

Electron spin resonance studies have been made of trapped hole centres with an F 2 - molecular ion structure in CaF 2 . x-irradiation of CaF 2 at 77 °K produces self-trapped hole centres. These become mobile at higher temperatures and may be trapped at special sites in the lattice to form other, relatively stable, F 2 - molecular ions with axes near <100>, <111> and <110> directions. The <100> molecular ion which is stable at room temperature has a symmetry which is lower than that of the self-trapped hole and this is shown to be consistent with the hole being trapped adjacent to the site of a substitutional Na + ion. The <111> case is treated in detail and is found to give an axially symmetric F 2 - molecular ion in which the probabilities of locating the hole near each of the two nuclei are unequal. This is shown to be consistent with the hole being trapped at the site of an interstitial anion. Activation energies for thermal annealing of these centres and for the self-trapped holes have been found by pulse annealing to be, respectively, 0·55 ± 0·10 and 0·13 ± 0·02 ev.