ENDOR measurements have been made to determine the transferred hyperfine interaction parameters for the fluorine ions forming the second-nearest anion neighbours to V2+ and Mn2+ in KMgF3. As expected, the main contribution to the interaction is due to the dipolar coupling between the fluorine nuclei and the electronic spin on the impurity ion. Contributions to the remainder of the interaction due to overlap effects with the unpaired spin on the intervening first-neighbour fluorines are calculated; the calculated parameters are found to reproduce correctly the form of the interaction, and are sufficiently large to account entirely for it in the vanadium case. The use of free-ion wave functions appears to be a good approximation.
