Nuclear spin relaxation in Li2BeF4

Abstract

Nuclear spin lattice relaxation time T1 of 19F and 7Li in the solid and liquid phases of lithium tetrafluoroberyllate (Li2BeF4) were measured in the temperature range between 250 and 630 °C by means of pulse NMR technique. Nuclear magnetic resonance absorption spectra were also measured by use of a spectrometer. T1 of 19F and 7Li in the solid phase decreases exponentially as the temperature is increased. The temperature dependence of T1 of 7Li can be attributed to the diffusion of Li nuclei with an activation energy of 1.03 eV while T1 of 19F can be explained by the dipole interaction modulated by the diffusion of F ions with an activation energy of 1.23 eV. In the liquid phase T1 of 19F goes through a minimum at about 550 °C and is expressed as T1−1∝τ/(1+ω2τ2) in terms of a correlation time, τ. We obtained τ=5×10−16 exp (1.08/kT) s. It is suggested that the relaxation mechanism of 19F in the liquid phase is due to the dissociation of F ions from BeF4 complex ions and the long distance diffusion of the F ions in the liquid.