R. Küchler

Effect of devitrification on the ionic diffusion of Li-disilicate

Abstract A simple theoretical model has been developed for describing the variation of diffusion of ions in an initially homogeneous glass as it transforms into a two-phase structure. The model is applicable to those microstructures in which the second phase grows as a discontinuous dispersion without affecting the diffusion properties of the glassy matrix. The validity of the model is demonstrated by the measurement of ionic conductivity and of diffusion-induced 7Li nuclear spin relaxation during the devitrification of Li-disilicate, containing 0 and 1 mol% P2O5, respectively. Evaluation of the data yields the basic parameters of devitrification.

Study of low-frequency excitations in disordered solids by nuclear magnetic resonance and electrical conductivity☆

Abstract 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 which belong to structural low-energy configurations intrinsic to the glassy state. 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 magnetic fluctuations without any accompanying charge motions.

A comprehensive view of the local structure around Rb in rubidium germanate glasses

Abstract A description of the local structure around Rb atoms in rubidium germanate glasses is obtained using: extended X-ray absorption fine structure analysis for interatomic bond distance, coordination number, and topological disorder; X-ray photoelectron spectroscopy for the distribution of electron density; infrared spectroscopy for the strength of various bonds; and 87 Rb nuclear magnetic resonance spectroscopy for charge distribution around Rb nucleus. The results describe the nature and origin of compositional, spatial and bond strength distribution and homogeneity in the structure.

Recent progress in the area of NMR characterization of ionic transport and relaxation in glasses

Results of temperature and frequency dependence of nuclear spin relaxation (NSR) are presented for different inorganic glasses between about 1 K and the glass transition temperature. Below about 200 K the data can be interpreted consistently in the framework of thermally activated low-frequency excitations (LFEs) of disorder modes intrinsic to the glassy state. The modes are modelled by asymmetric double-well potential (ADWP) configurations with a suitable density of states of the barrier height and asymmetry. Varying the nuclear probe allows the detection of different disorder modes. Above 200 K, NSR is dominated by diffusive ionic jumps resulting in an asymmetric NSR rate maximum. Evaluation of the data is shown to agree best with results of Monte Carlo (MC) simulations of the hopping process as performed by the Bunde and Dieterich group.

Ionic motion in lithium silicophosphate glasses by nuclear spin relaxation and electrical conductivity

Abstract The diffusive motion of Li + ions has been investigated in lithium silicophosphate glasses between 300 K and the glass transition temperature by means of 7 Li and 31 P nuclear spin relaxation and electrical conductivity experiments. The data are analyzed in terms of the coupling model. The effects of the mobile cation concentration and of the nature of the network atoms on the ionic transport properties have been determined. The results are interpreted in view of various theoretical approaches. Moreover, we are able to observe an additional fast relaxation process which is related to local rearrangements of the network atoms (β-process). As this process is only detected by nuclear spin relaxation and not by conductivity, we conclude that the underlying mechanism is caused by local rearrangements of uncharged [PO 4 ] 0 tetrahedra belonging to the glassy network.

Correspondence between nuclear spin relaxation and ionic conduction in lithium germanate glasses

Abstract AC electrical conductivity, σ, and 7Li nuclear spin relaxation in rotating frame have been studied in the glass series xLi2O:(1−x)GeO2, where x = 0.002−0.25. The temperature dependence of spin lattice relaxation rate (1/T1ϱ) scales linearly with σT when compared at the same frequency (20 kHz). This demonstrates a direct correspondence between nuclear spin relaxation and electrical conductivity due to lithium diffusion in these glasses above room temperature.

Ionic-to-electronic conductivity transition in an oxide glass doped with gold

Gold doped ruby glasses are classical examples of metal-glass nanocomposites that have been investigated for their striking optical properties. For the example of weakly Au doped lithium-borate glasses these materials are shown to exhibit a transition from ionic to electronic conduction within the same sample. It is achieved via appropriate heat treatment, presumably by introducing polarons formed from the trapping of electrons at partially ionized gold atoms. This unique electrical response is expected to introduce new functionality to this class of nanocomposites.

Nearly constant loss behavior of lithium disilicate during devitrification

Abstract The temperature and frequency dependence of the electrical conductivity has been measured in Li-disilicate glass as a function of its devitrification from the glassy to the fully crystallized state. We found that below ∼50 K the conductivity is dominated by the nearly constant loss (NCL) behavior at all stages of devitrification. The findings are confirmed by 7Li nuclear spin relaxation measurements. The magnitude of the conductivity is shown to decrease remarkably with increasing devitrification, indicating significantly different jellyfish configurations in the crystalline and glassy phases. The evolution of conductivity with devitrification time can be explained quantitatively by a simple two-phase, Johnson–Mehl–Avrami type model based on a linear superposition of the NCL contributions of the glassy and crystalline phases.

Chemical structure of lithium silicophosphate glasses

Abstract We have investigated the chemical structure of a series of lithium silicophosphate glasses by means of X-ray photoelectron spectroscopy (XPS). The fine structure of the oxygen 1s spectrum was analyzed in terms of three contributions due to bridging (BO), non-bridging (NBO) oxygen, and oxygen (BO6) linking an uncharged PO4 tetrahedron with a double charged SiO62− octahedron. The relative fractions and the mean binding energies of BO, NBO and BO6 have been determined as a function of the glass composition, especially the modifying cation concentration. The results are discussed in view of the nuclear magnetic resonance spectroscopy of the same lithium silicophosphate glasses.

Structural basis of low-frequency excitations in silicophosphate glasses

Abstract Relations between the structure and low-frequency excitations (LFEs) which dominate the dynamics at low temperatures were investigated in lithiumsilicophosphate glasses by means of nuclear magnetic resonance (NMR), electrical conductivity and X-ray photoelectron spectroscopy (XPS). By varying the glass composition we observed three different kinds of LFEs which could be analyzed consistently by an asymmetric double well potential (ADWP) approach. We were able to link the different LFEs with structural units of the glasses detected by magic angle spinning NMR and XPS. In particular, [SiO6]2− units are shown to be connected to LFEs caused by charge fluctuations of Li+ ions, whereas fluctuations of neutral [PO4]0 units are found to be the source of the two other LFEs.