Model Of Glass Structure Research Articles

Extended defect structures in bulk As 2S 3 glass II. low-frequency Raman spectra and the glass transition

A new Extended Defect model for As 2S 3 glass that is a superset of paracrystalline or cluster models of glass structure is proposed. It focuses attention on internal surfaces in the bulk structure that govern intermediate range order and contribute to observed variations in macroscopic properties as a function of the cooling conditions under which the glass forms. Variations in the breadth of the glass transition and changes in the low frequency Raman bands in the acoustic region provide experimental support for the new model.

Bond-orientational order in liquids and glasses

Bond-orientational order in molecular-dynamics simulations of supercooled liquids and in models of metallic glasses is studied. Quadratic and third-order invariants formed from bond spherical harmonics allow quantitative measures of cluster symmetries in these systems. A state with short-range translational order, but extended correlations in the orientations of particle clusters, starts to develop about 10% below the equilibrium melting temperature in a supercooled Lennard-Jones liquid. The order is predominantly icosahedral, although there is also a cubic component which we attribute to the periodic boundary conditions. Results are obtained for liquids cooled in an icosahedral pair potential as well. Only a modest amount of orientational order appears in a relaxed Finney dense-random-packing model. In contrast, we find essentially perfect icosahedral bond correlations in alternative "amorphon" cluster models of glass structure.

Infrared spectra, glass transition temperature, and conductivity of superionic conducting glasses in the systems Ag XAg 2OB 2O 3 ( X = I, Br)

Comparative studies on glass formation, IR spectra, glass transition temperature, and conductivity were made for the systems Ag XAg 2OB 2O 3 ( X = I, Br). IR spectra showed that (i) each glass contained BO 3 and BO 4 groups, (ii) non-bridging oxygens were present in BO 3 groups only, and (iii) Ag + ions were bound to the non-bridging oxygens with strong partial covalency. The glass transition temperature, ranging from 100 to 400°C, increased with increasing B 2O 3 content in the composition region of low B 2O 3 content, and showed a “boron anomaly” in the region of low Ag X and high B 2O 3 contents. The transport number of Ag + ions was unity, and the electronic conductivity was found to be negligibly small, about 5–6 orders of magnitude smaller than the total conductivity. Ionic conductivities, ranging from 10 −2 to 10 −6 ω −1 cm −1 at room temperature, were a little smaller in AgBr containing glasses than in AgI containing ones of the same Ag X, Ag 2O, and B 2O 3 contents. The conductivities increased with increasing Ag X content, while at a given Ag X content, especially in the composition region of high Ag X content, the values decreased with increasing Ag 2O content. These results suggest that not all of the Ag + ions in the glass contribute to the conduction. A model of glass structure is proposed on the basis of the results shown above.

Viscous liquids and the glass transition. VI. Relaxations in simple molecule glasses in the 4–77 K range

Previous dielectric studies from our laboratory have shown that a secondary relaxation β, usually attributed to molecular flexibility, occurs as often, with the same strength, and in the same place (0.75 Tg for 1 kHz measuring frequency) in glasses made of simple rigid molecules. We concluded that the β relaxation is a universal feature of amorphous packing. We have now extended our previous studies on molecular and fused salt glasses to the 4–77 K temperature range, and found relaxations at about 0.2–0.5 Tg in seven of 16 substances examined. These low temperature relaxations appear in some rigid molecule and in some flexible molecule glasses, but not all. We conclude flexibility is not a necessary condition for a low temperature relaxation, but feel the evidence that such relaxations are also characteristic of amorphous packing is weak. Four alcohols studied gave evidence of an additional loss peak below 4 K. We suggest this peak may be related to the well-known very low temperature anomalies common to most glasses. A general explanation of secondary relaxations in glasses in terms of the amorphous cluster model of glass structure is proposed.