Nuclear quadrupolar relaxation in molten salts

Abstract

From the straightforward extension of a general theory for nuclear quadrupolar spin–lattice relaxation in monatomic liquids the rate R_Q,α for an α ion in a simple uni-univalent ionic melt is expressed in terms of the local electric field gradients (e.f.g.) which originate from unlike or like charges external to the ion of interest, the van Hove time-dependent pair distribution function, and the static pair distribution function between unlike or like ions. R_Q,α may be approximated in terms of the partial elastic limit S_γδ(k, 0) of the dynamic liquid structure factor and the partial weighting factor I_{αγ, αδ}(k) involving the e.f.g. contribution. For a sodium cation in molten NaCl the terms of I_Naγ,Naδ(k)(γ, δ; Na or Cl) are estimated under an assumption of an electrostatic quadrupolar interaction. These numerical values are 10–100 times greater than those of I(k) for liquid Ne calculated from exchange–van der Waals e.f.g. The experimental results of the partial static-structure factor S_γδ(k) and the static pair radial distribution function _gNaγ(r)(γ; Na or Cl) for molten NaCl are of great consequence in calculations of both S_γδ(k, 0) and I_αγ,αδ(k). Numerical calculation of R_O,Na for the sodium cation in the molten NaCl is in rough agreement with experiment. The relaxation process of ²³Na in molten NaNO₃ is caused by both translation and reorientational motions. The rotation-like motion gives rise to the time dependent e.f.g. at the Na nucleus and contributes mainly to the observed relaxation rate. Numerical calculations of the temperature dependence of R_Q,Na in molten NaNO₃ are in no good agreement with experiment.