COMPOSITION DEPENDENCE OF LOW-FREQUENCY EXCITATIONS IN LITHIUM SILICOPHOSPHATE GLASSES BY NUCLEAR-MAGNETIC-RESONANCE AND ELECTRICAL-CONDUCTIVITY

Low-frequency excitations (LFE's) of disorder modes were investigated in (1-x-y)SiO2 . yP(2)O(5) . xLi(2)O glasses by means of Li-7 and P-31 nuclear-spin relaxation (NSR) and ac conductivity experiments conduct ed at various frequencies between about 3 and 300 K. By varying the co mposition we were able to observe three different kinds of low-frequen cy excitations ranging between about 10(4) and 10(9) s(-1): the first one was detected by NSR as well as by conductivity. The NSR data can b e linked to the conductivity data by the fluctuation-dissipation theor em indicating a common physical origin of the underlying relaxation pr ocess. This LFE is shown to he due to charge fluctuations related to t he Li+ ions. The other two LFE's are observed only by NSR and not by c onductivity indicating that they are mainly caused by magnetic fluctua tions due to movements of the nuclear spins. P-31 NSR detected both co ntributions resulting in two separate NSR rate maxima at about 10 and 50 K. respectively, while Li-7 NSR shows just the 10 K maximum. Furthe r, both contributions depend strongly on the content of phosphorous. W e suppose that the corresponding LFE's are caused by fluctuations of p hosphate units forming two different ty pea of disorder configurations in the glassy network. All the data can be interpreted consistently i n the framework of the asymmetric double-well-potential approach using just one set of parameters.