Electron nuclear double resonance in calcium fluoride containing Yb3+ and Ce3+ in tetragonal sites

This paper describes endor of 19F nuclei in crystals of calcium fluoride containing the impurities Ce3+ and Yb3+ at low concentrations. The presence of a charge compensating interstitial F- ion (which reduces the symmetry of the rare-earth site to tetragonal) has been confirmed by the observation of endor lines from its nucleus. Each rare-earth ion has eight nearest neighbour (n. n.) F- ions which are separated into two inequivalent sets of four by the presence of the interstitial F- ion. The hyperfine parameters for each of these sets and for the interstitial have been measured. Formulae have been deduced for the contributions to these parameters from covalent bonding with the 4f electrons, but for n. n. the formulae are too complicated for precise fitting with the experimental parameters, and for the interstitial the simple covalent bonding theory gives the wrong predictions. The most likely cause of the discrepancy is core polarization of the rare-earth ion which allows core electrons to take part in the covalent bonding. The hyperfine interactions with next nearest neighbours (n. n. n.) have also been measured. These have almost pure dipolar form so that it is possible to use them to map out the distortion of the lattice due to the interstitial F- ion.