Uncommon nuclear-spin relaxation in fluorozirconate glasses at low temperatures

The temperature and frequency dependence of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$, and ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ nuclear-spin relaxation (NSR) has been studied in various fluorozirconate (Zr-F) glasses between about 0.4 and 600 K. Below about 200 K the NSR is caused by low-frequency excitations of disordered modes intrinsic to the glassy state, which induce transitions between the nuclear-spin levels. In strong contrast to oxide glasses, however, the NSR rates in Zr-F glasses exhibit a significant peak around 15 K. The findings can be explained quantitatively by thermally activated excitations of two different types of disordered modes. The relative magnitude of NSR rate peaks of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$, and ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ suggests that the mode, which is responsible for the observed peak, is due to localized motions of a fraction of the fluorine atoms.