UNCOMMON NUCLEAR-SPIN RELAXATION IN FLUOROZIRCONATE GLASSES AT LOW-TEMPERATURES

The temperature and frequency dependence of Li-7, F-19, and Na-23 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 Li-7, F-19, and Na-23 suggests that the mode, which is responsible for the observed peak, is due to localized motions of a fraction of the fluorine atoms.