Nuclear magnetic relaxation of F-19 in fluoride glasses

Temperature and frequency dependence of the F-19 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 Gd3+ doped samples, in the temperature range of 185-1000 K, covering the region below and above th e glass transition temperature, T-g. The temperature and frequency dependen ce of the spin-lattice relaxation rate, T-1(-1), measured in the glassy sta te at temperature <300 K, is less than the observed dependence at higher te mperatures. At temperatures >T-g, the fluorine mobility increases, leading to a more efficient spins lattice relaxation process. Activation energies, for F- motion, are 0.8 eV for the fluoroindate glass and 1 eV for the fluor ozirconate glass. The addition of Gd3+ paramagnetic impurities;at 0.1-wt%, does not alter the temperature and frequency dependence of T-1(-1), but inc reases 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 de termined by the rigid-lattice nuclear dipole-dipole coupling, and it is tem perature independent within the accuracy of the measurements. Results obtai ned 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 d iffusive motion of the F- ions, with an activation energy around 0.8 eV. Fo r the doped samples, the hyperfine interaction with the paramagnetic impuri ties is most effective in the relaxation of the nuclear spin, causing an in crease in the T(2)(-1)s observed at temperatures >600 K. (C) 1999 Elsevier Science B.V. All rights reserved.