Features of structure and its relaxations in alkali borate glasses

The structure of alkali borate glasses in R(2)O-B2O3 systems (where R = Na, K, Rb, and Cs) is investigated by the small-angle X-ray scattering (SAXS) technique. The angular dependence of SAXS intensity exhibits a portion that satisfies the relationship I(s) similar to s(-2). This indicates that inhomogeneity regions arising from the chemical inhomogeneity of alkali borate glasses have a fractal nature. The inhomogeneity regions in low-alkali borate glasses are chain-branched clusters. At low concentrations of alkali ions, an increase in concentration causes these clusters to increase in number and, possibly, in size. The formation of a continuous cluster occurs at a certain concentration that depends on the type of alkali ion. Further increase in the concentration of alkali metal oxide leads to an increase in the degree of branching of the continuous cluster. A certain regularity in the distribution of chemical inhomogeneity regions is revealed at low temperatures. The degree of regularity increases with a decrease in temperature and becomes maximum at temperatures T-s close to a temperature of the transition to a disordered solid. After abrupt changes in temperature, the development of structural inhomogeneity brings about the change in the distribution of chemical inhomogeneities. The redistribution of inhomogeneity regions throughout the bulk of glass proceeds further even if the structure of a matrix attains equilibrium states; i.e., the level of thermal density fluctuations and structural inhomogeneity. Therefore, the structural relaxation in alkali borate glasses involves the processes of changes in the short-range and medium-range order structures due to thermal density fluctuations and development of structural inhomogeneity and also the redistribution processes of chemical inhomogeneity regions.