Glassy Materials: Fundamental Aspects and Ionic Conduction

Doctoral thesis, 1999

Materials of very different categories, ranging from polymers to metals, can form glasses. The disordered structure of glassy materials often provide unique properties to the system and glasses are commonly used in various technological applications. However, even though glassy materials have been in use for a long time their structures and physical properties are not well understood. This thesis concerns experimental investigations of structure and dynamics of glasses. One part of the work concerns fundamental properties, such as glass structure and collective vibartional excitations, and the other deals with ion conduction in glassy systems. Using neutron and x-ray diffraction, in combination with reverse Monte Carlo (RMC) simulations, and Raman spectroscopy, the structure of meta-phosphate glasses has been investigated, with the focus on the influence on the structure of different cations. Collective vibrational excitations, in a strong (B2O3) and a fragile glass (Ca0.4K0.6(NO3)1.4) former have been studied with high resolution (meV) inelastic x-ray scattering, in the meso-scopic regime. The behaviour of the acoustic modes is shown to be different for strong and fragile glasses. It is suggested that this is related to differences in intermediate range connectivity of the structure and fluctuation of force constants. A high ionic conductivity at room temperature is a property unique to glasses (with the exception of some crystalline compounds). In this work, structural aspects of the ionic conductivity has been investigated in several different systems, including the new ion-conducting glass system the silver-thio-boro-silicates, using diffraction techniques and RMC simulations. Furthermore, a structural approach has been taken to explain the mixed-alkali effect in ion conducting glasses. Three series of mixed-alkali meta-phosphate glasses have been investigated. From the results it is evident that the local environment of the alkali ions remain the same in the mixed glasses as in the single alkali glasses. Hence, there is a large energy mismatch between different alkali sites in the structure resulting in a considerably lowered mobility of the ions.