High-Pressure-High-Temperature Study of Spin-Lattice Relaxation in Pure and Doped LiBr Single Crystals

Abstract

Measurements of the spin-lattice relaxation time of Li in pure and doped single crystals of LiBr, as a function of temperature, clearly indicate that the spin relaxation is due predominantly to the diffusive motion above room temperature. The magnitude and temperature dependence of the relaxation time and the magnitude of temperature at which $T_{1min}$ occurs favor Torrey's theory of dipolar relaxation due to translational diffusion of ${\mathrm{Li}}^{+}$ ions over Reif's quadrupolar mechanism due to vacancy diffusion. The study of temperature dependence facilitated a determination of activation energies for creation and motion of vacancies in this material. The low-temperature slopes of the normalized frequency $y(=\frac{{\omega }_0}{2\nu })$ versus $\frac{1}{\mathrm{kT}}$ curves yielded a value of 0.35 eV corresponding to the activation energy for motion of a vacancy. The high-temperature slope fixed the value of the activation energy of formation of a Schottky pair at 1.70 eV. These values are in essential agreement with those obtained from ionic conductivity measurements. The effect of pressure on longitudinal relaxation time at temperatures ranging from room temperature to about 400°C was investigated up to about 5 kbar. The activation volume for the motion of a vacancy was found to be 6.4 ${\mathrm{cm}}^3$/mole. The study of pressure dependence at high temperatures gave a value of 29.5 ${\mathrm{cm}}^3$/mole for the formation volume of a Schottky-pair defect. These values of the activation volumes are comparable to the previous determinations in other alkali halides by ionic conductivity studies and are also in reasonable agreement with those predicted by Keyes's strain-energy model.