An electron paramagnetic resonance and electron nuclear double resonance study was made of the so-called Fe3+(I) centers which were produced by the X-irradiation, at below 200K, of Fe-doped crystals that had been grown in a Cl atmosphere. It was shown that the Fe3+(I) centers comprised a substitutional Fe3+ ion, with a defect in the nearest-neighbor Cl site. This defect, which was responsible for a very large axial zero-field splitting of the Fe3+(I) centers, was thought to be an interstitial Cli- ion which formed a weakly-bound (Cli-)2 split interstitial with the nearby substitutional Cl- ion, or a divacancy that consisted of a nearest-neighbor Cl- vacancy with a Na+ vacancy next to it on the same axis. At room temperature, the Fe3+(I) centers were transformed into cubic Fe3+ centers, where the Fe3+ ion was situated in an interstitial site.

Electron-Nuclear Double-Resonance Study of a Substitutional Fe3+ Defect Complex in X-Irradiated NaCl Crystals. T.Pawlik, S.V.Nistor, J.M.Spaeth: Journal of Physics - Condensed Matter, 1997, 9[36], 7631-42