Diffusion Narrowing of Nuclear Magnetic Resonance Linewidth of ${\mathrm{F}}^{19}$ in Ca${\mathrm{F}}_2$: ${\mathrm{Sm}}^{2+}$

Nuclear magnetic resonance linewidths of ${\mathrm{F}}^{19}$ in a synthetic Ca${\mathrm{F}}_2$: ${\mathrm{Sm}}^{2+}$ single crystal and natural Ca${\mathrm{F}}_2$ single crystals have been measured over a temperature range from 88 to 668°K and over a range of $H_1$ from 2.0 to 29 mG. Up to temperatures of about 400°K results are in agreement with existing theories of saturational narrowing. At higher temperatures the line narrowed in the samarium-doped crystal but not in the natural crystals. This narrowing was attributed to a decrease of the fluorine dipole interaction due to fluorine interstitial diffusion. An activation energy $E=0.26\mathrm{}$ eV was calculated.