OXYGEN DEFECTS IN BAFBR SAND BAFCL

Oxygen in oxide form is a common contaminant of BaFBr and BaFCl crysta ls. Upon x-irradiation of oxide-containing crystals of both materials, O-F(-) centers are formed, as has been shown previously by electron p aramagnetic resonance (EPR) methods. It is shown here by EPR that upon x-irradiation O- centers are also formed on the Br- and Cl- sites, re spectively. With electron nuclear double resonance spectroscopy, hyper fine interactions with the nearest F-19 neighbors have been determined for O-Br(-) in BaFBr. Their interpretation leads to the conclusion th at the O- ion is displaced along the c axis from the Br- site by about 1 Angstrom, in qualitative agreement with previous theoretical calcul ations. The EPR hyperfine data for O-Cl(-) centers in BaFCl indicate t hat a similar displacement occurs. Also by EPR, (O-2)(Cl)(-) centers c ould be identified in x-irradiated BaFCl crystals containing O2- enric hed with the magnetic O-17 isotope.