Nuclear magnetic relaxation of 19F in fluoride glasses

Abstract

Temperature and frequency dependence of the 19F nuclear spin relaxation of the fluoroindate glass, 40InF 3–20ZnF 2–20SrF 2–2GaF 3–2NaF–16BaF 2 and the fluorozirconate glass, 50ZrF 4–20BaF 2–21LiF–5LaF 3–4AlF 3, are reported. Measurements were undertaken on pure and Gd 3+ doped samples, in the temperature range of 185–1000 K, covering the region below and above the glass transition temperature, T g. The temperature and frequency dependence of the spin-lattice relaxation rate, T 1 −1, measured in the glassy state at temperature <300 K, is less than the observed dependence at higher temperatures. At temperatures > T g, the fluorine mobility increases, leading to a more efficient spin-lattice relaxation process. Activation energies, for F − motion, are 0.8 eV for the fluoroindate glass and 1 eV for the fluorozirconate glass. The addition of Gd 3+ paramagnetic impurities, at 0.1 wt%, does not alter the temperature and frequency dependence of T 1 −1, but increases its magnitude more than one order of magnitude. At temperatures <400 K, the spin–spin relaxation time, T 2 −1, measured for all samples, is determined by the rigid-lattice nuclear dipole–dipole coupling, and it is temperature independent within the accuracy of the measurements. Results obtained for the pure glass, at temperatures >400 K, show that T 2 −1 decreases monotonically as the temperature increases. This decrease is explained as a consequence of the motional narrowing effect caused by the onset of the diffusive motion of the F − ions, with an activation energy around 0.8 eV. For the doped samples, the hyperfine interaction with the paramagnetic impurities is most effective in the relaxation of the nuclear spin, causing an increase in the T 2 −1s observed at temperatures >600 K.