Nuclear Magnetic Relaxation in LiF at High Temperatures

Measurements of T₁ and T₂, the spin-lattice and spin-spin relaxation times, are reported for Li⁷ and F¹⁹ nuclei in a single crystal of LiF. T₁(Li) has been measured from room temperature to the melting point (1120^∘{}K) and the other quantities from 830^∘{}K to the melting point. Above 620^∘{}K, relaxation is caused exclusively by the diffusion of Li and F ions. T₁(Li), T₂(Li), and T₁(F) are mainly determined by νLi, the jump frequency of Li ions, and T₂(F) mainly by νF. Using a theory of relaxation developed elsewhere, νLi and νF are obtained for the region of intrinsic diffusion as νLi=1.7×10¹⁶exp(-1.81 eV/kT) and νF=4.5×10¹⁷exp(-2.2 eV/kT). For the extrinsic region, the motional activation energy of Li ions is obtained as 0.71 eV. The values of νLi are in good agreement with those obtained from conductivity measurements; to our knowledge, νF has never been measured by other methods. Below 620^∘{}K quadrupolar relaxation due to lattice vibrations and relaxation by paramagnetic impurities become important, and rough values are obtained for these contributions to T₁(Li). Large angular variations of T₁ and T₂ are observed in the region of relaxation due to atomic diffusion. T₂(Li) and T₁(Li) vary by a factor of 2 and 1.5, respectively, as a function of crystal orientation, in good agreement with theoretical prediction. A crude measurement of T₁(Li) in molten LiF is discussed.