B-11 SPIN-LATTICE RELAXATION AND DISORDER MODES IN IONIC GLASSY CONDUCTORS (AGI)X(AG2O.NB2O3)1-X

The temperature and frequency dependence of the B11 nuclear spin-lattice-relaxation rate has been investigated in a number of ionic glassy conductors of general composition (AgI)x(Ag2OnB2O3)1-x, with n=1, 2 and variable x. At T>100 K a BPP-type maximum in T1-1 is found that is consistent with a thermally activated reorientational motion of BO4 groups. [BPP indicates Bloembergen, Percell, and Pound, Phys. Rev. 73, 679 (1948).] It is found that the sum of the activation energy for BO4 motion with the activation energy recently measured for Ag+ local motion is equal to the activation energy derived from conductivity measurements. At low temperature one observes the usual quasilinear temperature dependence of T1-1 and an unusual frequency dependence not described by a power law. An interpretative model of nuclear relaxation due to disorder modes is developed to explain the frequency dependence. The analysis of the data indicates the presence of a relatively large number of slowly fluctuating two-level systems, formed by coupled BO4 units, responsible for the frequency dependence of the experimental relaxation rate. © 1990 The American Physical Society.