Physical properties of alkaline-earth and alkali borate glasses prepared over an extended range of compositions

By using rapid cooling we have greatly extended the reported glass-forming ranges in the binary magnesium, calcium, strontium, and barium borate systems. We observed phase separation for low alkaline-earth oxide contents, typically below about 15 mol% alkaline-earth oxide, but we have been able to increase the alkaline-earth oxide limit of glass formation to approximately 60-65 mol%. We have determined the density and the glass transition temperature for a large number of glasses within each of these systems. We compare these data with atomic arrangement studies based on spectroscopy. Using a model derived from the NMR data of Greenblatt and Bray [Phys. Chem. Glasses 8 (5) (1967) 190-193] we have determined the associated volumes of the four basic borate structural units thought to be present in each system. These groups include trigonal borons with three (f(1)), two (f(3)), and one bridging oxygens (f(4)) as well as the tetrahedral boron unit (f(2)). We compare these volumes with those obtained from the binary alkali borates. In addition, we have calculated the volumes of the borate groups using Shannon and Prewitt radii. This allowed us to determine and compare the packing fractions of each borate arrangement in the alkaline-earth and alkali borate systems. The T, data display two distinct regions, a high temperature region for the alkaline-earth borates and one for the alkali borates. These regions are separated by approximately 150 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.