Nuclear Magnetic Resonance Studies of Pure and Samarium-Doped CaF2 Crystals

Abstract

The spin—lattice relaxation time T₁ of ¹⁹F was measured over a temperature range of 77° to 1300°K in natural and synthetic CaF₂ crystals and in CaF₂ crystals doped with 0.05% Sm, 0.16% Sm, and 1.20% Sm. Most of the data was taken with the magnetic field along the [100] and [111] directions using the magnetic recovery method. For annealed samples T₁ at room temperature and below is found to be inversely proportional to the concentration of samarium. Measurements show that in the high‐temperature regions T₁ is primarily governed by translational diffusion. Activation energies in the extrinsic and intrinsic regions were found to be 0.36 and 2.3 eV, respectively, for the pure sample and 0.53 and 2.4 eV for the doped samples. These are in good agreement with the activation energies obtained from conductivity measurements. The knee separating the extrinsic and intrinsic regions occurred at higher temperatures as the amount of dopant was increased. The T₁‐vs−1/T curves had a minimum at 1120°K for all the samples. Below 400°K T₁ was found to be directly proportional to temperature for the doped samples. The low‐temperature data indicate that the correlation time of samarium at room temperature is of the order of 10⁻¹⁴ sec.