Temperature dependence of F-centre hyperfine interactions

We report the temperature dependence of F-centre Isotropic hyperfine energies for the first shells of KCl, KBr, NaCl, LiF and Lid, and the second shell of KCl. In all cases at least 30 ENDOR data points were taken between 1.3° and 300°K, and the Fermi contact energies were found to adhere closely to a curve of the form a(T) = A + B coth (C/T).The effects of pure lattice expansion may be removed with the aid of high pressure ENDOR data. The remaining temperature dependence can be explained in terms of a configuration co-ordinate model, with the F-electron interacting with a single effective mode of lattice vibration.