It was recalled that O in the form of oxides was a common contaminant of these materials. Upon irradiating oxide-containing crystals of these materials, OF- centers were formed; as had been shown previously by using electron paramagnetic resonance methods. It was shown here, again by means of electron paramagnetic resonance, that - during X-irradiation - O- centers were also formed at the Br- or Cl- sites. By using electron nuclear double resonance spectroscopy, hyperfine interactions with the nearest 19F neighbors were detected in the case of OBr- in BaFBr. The interpretation of these results led to the conclusion that the O- ion was displaced along the c-axis, from the Br- site, by about 0.1nm. This was in qualitative agreement with theoretical calculations. The electron paramagnetic resonance hyperfine data for OCl- centers in BaFCl indicated that a similar displacement occurred there. Again by using electron paramagnetic resonance methods, (O2)Cl- centers could be identified in X-irradiated BaFCl crystals which contained O2- and were enriched with the magnetic 17O isotope.

R.S.Eachus, R.H.D.Nuttall, M.T.Olm, W.G.McDugle, F.K.Koschnick, T.Hangleiter, J.M.Spaeth: Physical Review B, 1995, 52[6], 3941-50