High temperature-pressure synthesis of Ti-doped tohdite and corundum from hydrous Al_2O_3-TiO_2 gel

摘要

Hydrous Al_2O_3-TiO_2 (78:22 in molar ratio) gel was fired at various P-T conditions using a piston-cylinder apparatus and identified by XRD. FTIR, optical microscopy and electron microscopy. Below 675 deg C, the sample remained amorphous at ambient pressure, yet transformed at 1.5 kbar to Ti-doped tohdite, which is elongated along the crystallographic c-axis, with well-developed (0001) base and {1010} ledges. Tohdite has a significant water/hydroxy content and is therefore susceptible to pore coalescence parallel to the basal layer upon electron dosage. Tohdite also contains Ti~(4+) up to 3 at.percent, which replaces Al~(3+) in tetrahedral and/or octahedral sites to form superstructures and defect microstructures. In contrast, a higher T-P condition (above 675 deg C and 8 kbar) caused the formation of more stable Ti-doped corundum, which is hexagonal-rhombohedral in shape and in epitaxial association with rutile nuclei. Ti-doped tohdite and corundum shed light on a sol-gel route for their occurrence in peraluminous metamorphic rock. The nanoporous and nanodelaminated tohdite may have potential catalytic applications.