Complex dielectric spectra up to 20 GHz are measured at 25 "C 'for n-butanol tetrachloromethane mixtures.Within this frequency range the spectra can be described by two Debye-processes. Their relaxation times TI') and r12) are compared with the hypothetical single-particle relaxation time of the dipoles Z; estimated from deuteron magnetic relaxation rates in these mixtures. The results indicate a collective nature of the primary relaxation time rj", due to cooperative motion of several molecules. Although not accurate. in detail, some theoretical models can reproduce major features of the concentration dependence of TI'). In these models, r!') is coupled to r; via the Kirkwood correlation factor gl, which can be determined independently from the static permittivity. There is no obvious interrelation between 4 and the secondary relaxation time I\*', the nature of which remains puzzling.

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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.

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