LITHIUM SODIUM METABORATE GLASSES - STRUCTURAL ASPECTS AND VITRIFICATION CHEMISTRY

Vibrational and devitrification techniques are employed to investigate the structure and glass-forming ability of x NaBO2 . (1 - x)LiBO2 gla sses with 0 less-than-or-equal-to x less-than-or-equal-to 0.75. Alkali substitution is found to result in non-additive network changes due t o the appearance of structures encountered in the newly discovered com pound Li3NaB4O8 (x = 0.25). A decrease of T(g) with x is observed and attributed mainly to the partial suppression of the formation of tetra hedra-containing, beta'- and gamma-LiBO2-type, arrangements in the gla ss network. The dependence of the crystallization products on temperat ure is very pronounced for x = 0 and progressively diminishes for x -- > 0.25. This diminishment is taken to indicate that, upon increasing x (0 less-than-or-equal-to x less-than-or-equal-to 0.25), the structura l rearrangements associated with the liquid-to-glass transition become less pronounced. In turn, alkali mixing in the system under investiga tion is correlated with decreasing melt 'fragility' and a less pronoun ced pathway glass morphology. Such remarks may be important in assessi ng the mixed alkali effect from a structural viewpoint, especially sin ce they are compatible with earlier findings on lower modification bor ate analogs.