PHYSICAL REVIEW B VOLUME 43, NUMBER 10 1 APRIL 1991 Uncommon nuclear-spin relaxation in Auorozirconate glasses at low temperatures S. Estalji, O. Kanert, and J. Steinert Institute of Physics, University of Dortmund, Dortmund, Germany H. Jain Department of Materials Science & Engineering, Lehigh Uniuersity, Bethlehem, Pennsylvania 18015 K. L. Ngai naval Research Laboratory, 8'ashington, D. C. 20375-5000 (Received 6 September 1990; revised manuscript received 26 November 1990) The temperature and frequency dependence of Li, ' F, and 'Na nuclear-spin relaxation (NSR) has been studied in various Auorozirconate (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 di6'erent types of disor- dered modes. The relative magnitude of NSR rate peaks of Li, ' F, and Na suggests that the mode, which is responsible for the observed peak, is due to localized motions of a fraction of the fluorine atoms. I. INTRODUCTION It has long been known that at low temperatures fun- damental differences exist between the physical properties of vitreous materials and that of crystalline solids. ' The anomalies are commonly explained by low-frequency ex- citations of disordered modes with a broad distribution of their characteristic parameters which are intrinsic to the glassy state of matter. The physical origin of the modes, however, is still lacking and therefore a subject of current interest. Microscopic experimental methods such as neu- tron scattering or nuclear magnetic resonance (NMR) have proven to be successful tools for obtaining more de- tailed insights into the nature of these properties. In par- ticular, nuclear-spin relaxation (NSR) techniques are able to provide detailed information about the mechanisms of atomic fluctuations in solids. As shown in Table I, fluorozirconate glasses are mul- ticomponent systems. Since the Arst discovery of a fluorozirconate glass in 1974 by M. Poulain, the ZrF4- based fluoride glasses have gained considerable attention. This is mainly associated with their superior infrared transparency compared to oxide-based glasses. A minimum loss of only 0.01 dB/km occurs at about 2.5 pm, which is an order of magnitude lower than that for high silica glasses. Hence, fluoride glass fibers seem to be ideal candidates for long-length Aber communication links. Until now, little is known about the structure of ZrF4- based glasses. According to Phifer et al. the glasses can be thought of as a complex ion structure consisting of a network of ZrF7 and ZrF8 coordination polyhedra with charge compensating ions (Ba +, alkali-metal ions, etc. ) inserted among the polyhedra. Computer simulation studies suggest that probably three kinds of fluorine exist: (i) internal bridging fluorine pairs, (ii) bridging fluorines connecting the basis units into an extended network, (iii) nonbridging fluorines which interact ionically with the cations (Ba +, alkali-metals ions, etc. ). The presence of three discrete fluorine sites was confirmed recently by ' F NMR line-shape studies. The positions of the alkali ions are mostly unknown. At higher temperatures part of the fluorine ions be- comes mobile. Hence, alkali-metal-free Zr-F glasses are fluorine conductors, whereas alkali-metal ions exhibit substantial mobilities in Zr-F glasses only above an amount of about 20 mol%. TABLE I. Composition of the Auorozirconate glasses (in mol%). Glass ZBLA(I) ZBLA(II) ZBLALi ZBLAN ZrF4 58 59.45 48.22 27.43 HfF4 27.43 BaFq 33 30.87 22.41 19.76 LaF3 5 5.69 4.64 3.06 AlF3 4 3.99 3.54 3. 19 LiF 21.19 NaF 19.19 Ts [K] 584 595 528 553 43 7481 1991 The American Physical Society