Hexabarium 17-titanate in pressureless-sintered TiO2-excess barium titanate Original Research Article

The microstructure of TiO2-excess BaTiO3 ceramics sintered by using a CO2-laser and conventional furnace has been analysed via scanning and transmission electron microscopy. Second phases located at the triple-grain junctions and along the grain boundaries of laser-sintered samples contain monoclinic-Ba6Ti17O40. They exhibit a characteristic lamellar structure arising from the solidification of the BaTiO3–Ba6Ti17O40 eutectic generated at ≈1332 °C upon fast cooling. This microstructure can be taken as the diagnostic feature to distinguish between Ba6Ti17O40 formed by solid-state reaction and that by eutectic solidification. A self-consistent crystallographic orientation relationship of (001)t//(101)m, (11̄1)t/(001)m, (11̄0)t//(1̄03)m, (11̄1̄)t//(1̄01)m and [010]m//[1̄1̄0]t between tetragonal-BaTiO3 and monoclinic-Ba6Ti17O40 has been found. The monoclinic Ba6Ti17O40 is stable below the eutectic temperature of 1332 °C after post-sintering annealing at 1000 °C for 100 h.