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

A trapped-hole center ($V_{\mathrm{F}}$ center) induced in MgO by ionizing radiation has previously been assigned the structure ${\mathrm{O}}^{+}$—[=]—${\mathrm{F}}^{+}$ (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: $a=0.234\mathrm{}\pm 0.010$ MHz and $b=0.923\mathrm{}\pm 0.010$ 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.