Hyperfine interactions of F centres in alkaline earth fluorides

The hyperfine interactions of the F centre electron (an electron trapped at an anion vacancy) with the four nearest fluorine shells have been measured by the endor method in CaF2, SrF2 and BaF2. The third shell fluorines are divided into two separate groups, labelled 3 a and 3 b, which are characterized by the presence and absence of a cation between the F centre and the fluorine. The hyperfine interaction with 3 a fluorines shows the unusual feature of increasing form CaF2 to BaF2; all other shells show a decrease. In the case of the F centre with the magnetic Ba nuclei in the nearest Ba shell has also been measured by the endor method. The hyperfine constants of the F centre in CaF2, SrF2 and BaF2 have been calculated and are compared with experiment. The theoretical wavefunctions used are the sum of a spherically symmetric part and anisotropic parts. The wavefunctions are orthogonalized to the other occupied orbitals in the crystal. The isotropic parts are taken from Bennett & Lidiard's point-ion calculation and give reasonable agreement with experiment. The anisotropic terms are needed because the F centre has only tetrahedral symmetry. The admixture of these terms is calculated by perturbation theory using the point ion model and anisotropic function from a continuum calculation. It is possible to get reasonable agreement with the anisotropy in the hyperfine constants found experimentally. The discrepancies are analysed as a preliminary to more sophisticated colour centre calculations.