Significant differences between nuclear-spin relaxation and conductivity relaxation in low-conductivity glasses

Abstract

A unified model of ionic motion in glasses is presented which relates consistently the different quantities observed in nuclear-spin relaxation and electrical-conductivity relaxation due to thermally activated ionic diffusion. Corresponding experimental data obtained in a low-ionic-conductivity fluorozirconate glass are shown to be in reasonable agreement with the present approach. Limitations of the model are pointed out and are discussed in view of observed deviations between the different experimental results.

Glass structure and ion dynamics of lead–cadmium fluorgermanate glasses

“…Although the exact nature of the relaxation mechanism is not well established at the moment, there is sufficient experimental evidence to support the idea that it corresponds to timedependent fluctuations of the nuclear spin coupling between mutually interacting ion pairs, caused by thermally activated hopping motions of the charged particles in a disordered lattice. 23,36,37 The kind of approach used assumes that the Coulomb interaction among the ions and their interaction with the glassy network will cause the decay of the correlation function to slow down for times tϾ1/ c , where c denotes the characteristic frequency of the slowing-down process and is in the range c Ϸ10…”

“…33,41 However, this value is more than one order of magnitude higher than the conductivity measured in fluoride glasses at the same temperature. 36,42 It is interesting to note that in fluoride glasses the 19 F NMR relaxation rates increase very little between room temperature and T g ͑e.g., from Ϸ2 s Ϫ1 at 300 K to 7 s Ϫ1 at T g in fluorozirconate glass 24 ͒. In contrast, the T 1 Ϫ1 values, measured in our fluorogermanate glasses, increase almost three orders of magnitude between room temperature and T g ͑Fig.…”

“…Although, the relation ͑7͒ has been verified for various inorganic oxide glasses at low temperatures, the theoretical bases of a general correspondence between EC and NSLR experiments are still under discussion. 36,43 The motion of ions in the glasses produces a non-Debye frequency dependence of the ac conductivity, which exhibits a power-law dependence at high frequencies but is constant at low frequencies, and can be described by 36,43,44 …”

“…22 Alternatively, it is also possible that the shape of the correlation function decay be inherently nonexponential, due to intrinsic cooperative interactions ͑mainly Coulomb interactions͒ among the mobile charged ions and their interaction with the glassy network. 23,36,37 In the case of heavy metal fluoride glasses, the temperature dependence of the 19 F T 1 Ϫ1 has been interpreted in terms of three main mechanisms: ͑a͒ spin relaxation processes attributed to low-frequency excitation of disorder modes, usually noticeable at temperatures below 300 K, ͑b͒ diffusive ionic motions which become significant in the region 300 KϽTϽT g , and ͑c͒ fast diffusive ionic motions, which are responsible for a 19 F spin-lattice relaxation rate maximum, generally localized at temperatures above T g .…”

New Developments in Solid Electrolytes for Thin-Film Lithium Batteries

“…Because of their more open disordered structure, amorphous materials typically have higher ionic conductivities than the corresponding crystalline material (Angell, 1983;Martin, 1991). In addition, single ion conduction can be realized because glassy materials belong to decoupled systems in which the mode of ion conduction relaxation is decoupled from the mode of structural relaxation (Kanert et al, 1994;Patel and Martin, 1992). For these reasons, amorphous or glassy materials are thus among the more promising candidates of solid electrolytes because of their properties of single ion conduction and high ionic conductivities.…”