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{·}yP₂{O}₅·{xLi}₂$O glasses by means of $⁷Li$ and $³¹P$ nuclear-spin relaxation (NSR) 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 10⁴ and 10⁹ s^-1; the first one was detected by NSR as well as by conductivity. The NSR data can be linked to the conductivity data by the fluctuation-dissipation 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⁺ 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. ³¹P NSR detected both contributions resulting in two separate NSR rate maxima at about 10 and 50 K, respectively, while ⁷Li 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.