Hyperfine interactions of F centres in alkaline earth fluorides

Abstract

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 CaF 2 , SrF 2 and BaF 2 . 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 CaF 2 to BaF 2 ; 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 CaF 2 , SrF 2 and BaF 2 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.