SUB-T-G PHASE-SEPARATION AND DEVITRIFICATION OF A LITHIUM TRIBORATE SOL-GEL GLASS

Sol-gel synthesis of glassy lithium triborate, Li2O-3B(2)O(3), its str uctural characterization and the changes induced by thermal annealing are described. The as prepared glass consists of chains of six-membere d B-O rings having two trigonal and one tetrahedral boron. A relativel y small fraction of the rings is hydrolyzed and two borons per formula unit are bonded to methoxy groups. Thermal annealing at less than or similar to 270 degrees C causes release of most of the -OH as water, t he formation of highly mobile ring-based units, and partial structural reordering. Annealing at higher temperatures leads to loss of methoxy groups and a sub-T-g phase separation, that leads to the formation of crystalline lithium diborate and glassy lithium tetraborate. The samp le annealed for 6 h at 500 degrees C is almost fully polymerized and d isplays a crystalline phase and medium-range structure very similar to melt-quenched lithium triborate.