Detection of dimers by ¹⁹F NMR in CdF₂ doped with ErF₃ and YbF₃

The ¹⁹F NMR of single crystals of CdF₂ containing large concentrations of ErF₃ or YbF₃ has been studied at 30 and 46.5 MHz from 190° to 373°K. In addition to the main resonance, two types of weaker resonances were detected: one type displaying symmetry about the [111] axis and the other displaying symmetry about the [100] axis. The 111 lines are attributed to the lattice fluorides adjacent to one rare earth ion while the 100 lines are attributed to the interstitial fluoride. It is shown that the anisotropy of the interstitial fluoride resonance can only be explained by the presence of two rare earth ions at 90° angles and therefore the bulk of rare earth ions and interstitial fluorides must be present as (RE ³⁺-F_i^-)₂ dimers. The anisotropic portion of the NMR shifts is shown to be entirely due to the dipolar contribution of spin residing on the rare earth ion. Further it is shown that the isotropic shift is of opposite sign to that predicted by the equation currently used to explain contact shift of lanthanide ions.