B11spin-lattice relaxation and disorder modes in ionic glassy conductors (AgI)x(Ag2O⋅nB2O3)1−x

The temperature and frequency dependence of the $^{11}\mathrm{B}$ nuclear spin-lattice-relaxation rate has been investigated in a number of ionic glassy conductors of general composition (AgI${)}_{\mathit{x}}$(${\mathrm{Ag}}_{2}$O\ensuremath{\cdot}n${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, with n=1, 2 and variable x. At Tg100 K a BPP-type maximum in ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ is found that is consistent with a thermally activated reorientational motion of ${\mathrm{BO}}_{4}$ groups. [BPP indicates Bloembergen, Percell, and Pound, Phys. Rev. 73, 679 (1948).] It is found that the sum of the activation energy for ${\mathrm{BO}}_{4}$ motion with the activation energy recently measured for ${\mathrm{Ag}}^{+}$ local motion is equal to the activation energy derived from conductivity measurements. At low temperature one observes the usual quasilinear temperature dependence of ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ 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 ${\mathrm{BO}}_{4}$ units, responsible for the frequency dependence of the experimental relaxation rate.