Influence of alumina addition on crystallization and texturing behavior of LaBGeO5 glass

The structure and crystn. behavior of 25La2O3. 25B2O3. 50GeO2 glasses, melted in platinum (P glass) and corundum (A glass) crucibles, were studied by DTA, X-ray diffraction and FTIR spectroscopy. The Al2O3 dissolved from corundum crucible in the A glass was estd. to be in the range 5-7 wt%. This alumina content had almost no influence on glass transition temp. but strongly affected the structure and crystn. behavior of the A glass. In fact, the P glass showed good texture-forming ability: high quality textured glass-ceramic plates based on stillwellite-like LaBGeO5 crystals were easily obtained. On the contrary, the presence of alumina stabilized the A glass from which binary phases crystallize first, and only afterwards they are transformed in stillwellite by secondary crystn.: so in this glass texturing is hindered. Crystn. and texturing behavior of P and A glasses were well related to FTIR data. P glass contained both threefold and fourfold coordinated boron while in the A glass the presence of aluminum forced boron to assume almost exclusively threefold co-ordination. Hence the easier crystn. of stillwellite phase and the good textures obtained from the P glass in comparison to the A glass, can be well understood since all boron atoms have tetrahedral co-ordination in stillwellite LaBGeO5 crystal.