Nuclear magnetic relaxation in molten salts. I. Spin-lattice relaxation time of 23Na in molten sodium nitrate

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Abstract
The spin‐lattice relaxation time (T1) of 23Na in molten sodium nitrate has been measured at 8.0 and 22.3 MHz from the melting point (309°C) to 600°C using a pulse spectrometer. T1 was found to be independent of frequency over this range and equal to about 50 × 10−3 sec. 1/T1 was a linear function of 1/D (where D is the rate of diffusion of Na+); but 1/T1 did not go to zero when 1/D was extrapolated to zero, as predicted by the Bloembergen‐Pound‐Purcell model. All of these observations can be explained assuming the quasilattice random flight (QLRF) model and a quadrupolar relaxation mechanism. Relaxation by dipole‐dipole interactions and by paramagnetic impurities are both shown to be completely negligible. The correlation time τc is obtained from the temperature dependence of 1/T1 and found to agree with the value obtained from compressibility and thermal expansitivity data. The electric field gradient at the Na+ nucleus is also calculated from the relaxation time and shown to be in excellent agreement with the value estimated from the radial distribution function of the melt. Upper and lower limits are given for the lifetime of the diffusing species, NaNO3.