F19NMR studies of CaF2crystals doped with NdF3, EuF3, DyF3, HoF3, or TmF3

${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ NMR at 46.6 MHz has been studied for single crystals of Ca${\mathrm{F}}_2$ doped with Nd${\mathrm{F}}_3$, Dy${\mathrm{F}}_3$, Ho${\mathrm{F}}_3$, and Tm${\mathrm{F}}_3$ for doping concentrations of 0.5, 1.0, and 2.0 mol%. ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ NMR at 84.67 MHz has been done for crystals doped with Nd${\mathrm{F}}_3$ and Eu${\mathrm{F}}_3$. Data on the main satellite lines, due to lattice fluorides having one nearest-neighbor rare-earth ion, show that the anisotropic portion of the NMR shift is entirely due to the direct dipolar interaction for all ions except ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Eu}}^{3+}$. Analysis of the isotropic portion of the shift and the nondipolar portion of the anisotropic shift reveals that the spin-transfer mechanism is primarily the polarization mechanism in the first half of the rare-earth series but that the covalent mechanism becomes predominant in the second half of the series. Evidence for extensive clustering is found in all systems studied in the form of resonance lines that must arise from lattice fluorides with two nearest-neighbor rareearth ions. A possible form for the cluster is presented that will explain all the NMR results. $T_1$ measurements on the main ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ line were done as a function of orientation for crystals doped with Nd${\mathrm{F}}_3$ and Eu${\mathrm{F}}_3$. In both cases, large variations with orientation were found. It is shown that present theories will account for this variation.