Low-frequency excitations ͑LFE's͒ of disorder modes were investigated in (1ϪxϪy)SiO 2 •yP 2 O 5 •xLi 2 O glasses by means of 7 Li and 31 P nuclear-spin relaxation ͑NSR͒ and ac conductivity experiments conducted at various frequencies between about 2 and 300 K. By varying the composition we were able to observe three different kinds of low-frequency excitations ranging between about 10 4 and 10 9 s Ϫ1 ; the first one was detected by NSR as well as by conductivity. The NSR data can be linked to the conductivity data by the fluctuationdissipation theorem indicating a common physical origin of the underlying relaxation process. This LFE is shown to be due to charge fluctuations related to the Li ϩ ions. The other two LFE's are observed only by NSR and not by conductivity indicating that they are mainly caused by magnetic fluctuations due to movements of the nuclear spins. 31 P NSR detected both contributions resulting in two separate NSR rate maxima at about 10 and 50 K, respectively, while 7 Li NSR shows just the 10 K maximum. Further, both contributions depend strongly on the content of phosphorous. We suppose that the corresponding LFE's are caused by fluctuations of phosphate units forming two different types of disorder configurations in the glassy network. All the data can be interpreted consistently in the framework of the asymmetric double-well-potential approach using just one set of parameters.
At low temperatures, nuclear spin relaxation (NSR) can be related to electrical conductivity in various inorganic oxide glasses by the fluctuation-dissipation theorem. The results indicate a common physical origin of the relaxation mechanism owing to fluctuating charges. In comparison, however, in heavy metal fluoride glasses, NSR is shown to detect additional fluctuations which are not observed by. conductivity experiments. The underlying relaxation process is caused by nuclear magnetic fluctuations without any accompanying charge motions. Both dielectric and NSR responses can be explained in terms of thermally activated excitations of asymmetric double-well potential (ADWP) configurations intrinsic to highly disordered solids. The ADWP configurations are characterized from the effect of alkali concentration and size on the low-temperature conductivity. The asymmetry of ADWP configurations is determined primarily by the elastic strains in the structure.
Nuclear-spin relaxation (NSR) and electrical conductivity observed in various inorganic oxide glasses at low temperatures are found to be related by the fluctuation-dissipation theorem, indicating a common physical origin of the relaxation mechanism due to fluctuating charges. However, in heavy-metal fluoride glasses NSR is shown to detect additional fluctuations which are not observed by counductivity experiments. The underlying relaxation process is caused by magnetic fluctuations without any accompanying charge motions.
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