Structure of the Fluorine-Containing Trapped-Hole Center in Magnesium Oxide

A trapped-hole center (VF center) induced in MgO by ionizing radiation has previously been assigned the structure O⁺---[=]---F⁺ (only deviations from normal site charge shown). ENDOR data confirm the identification of the interacting nucleus as fluorine. The contact and dipolar splitting constants are, respectively: a=0.234±0.010 MHz and b=0.923±0.010 MHz. The value of b implies a hole-fluorine distance of 0.432 nm, compared with the normal O-O lattice distance of 0.420 nm. Calculations of the isotropic hyperfine splitting due to overlap of the hole with the fluorine $2s$ orbital suggest that the hole is primarily localized on one oxygen atom. This conclusion is supported by a comparison of g factors for trapped-hole centers.