Impurity-associated fluorine interstitial in KMgF3

The electron-spin-resonance (ESR) spectrum of an $H_{ A}$ center has been investigated in the cubic perovskite KMg${\mathrm{F}}_3$. The $H_{ A}$ centers were produced by electron irradiation near 77 K and are the result of an interstitial fluorine atom combining with two neighboring lattice fluorine ions to form a molecular ion stabilized adjacent to an unidentified impurity. The three fluorine nuclei are magnetically inequivalent and give rise to large anisotropic hyperfine splittings in the ESR spectrum. At 10 K, the axial parameters of the three hyperfine tensors are $A_{ 1,z}=1113.8\mathrm{}$ G, $A_{ 2,z}=599.6\mathrm{}$ G, and $A_{ 3,z}=50.7\mathrm{}$ G. A partially averaged ESR spectrum is observed at 77 K and is explained in terms of a restricted motion of the interstitial about the neighboring impurity ion. The $H_{ A}$ center thermally decays at 195 K.