Hexagonal-phase retention in pressureless-sintered barium titanate

Undoped and MgO-doped TiO2- and BaO-excess non-stoichiometric barium titana te (BaTiO3) compositions were pressureless sintered using a conventional fu rnace and CO2 laser. High-temperature hexagonal BaTiO3 was metastably retai ned to room temperature as revealed by both X-ray diffractometry and transm ission electron microscopy. The sintered microstructure of the TiO2 excess composition is characterized by a bimodal grain size distribution containin g large plate-like grains. The microstructure of BaO-excess BaTiO3 with MgO doping is characterized by large spherulitic grains with petals 1-2 mm lon g. Hexagonal phase retention in the undoped material is only obtained when quenching the laser sintered BaO-excess powder. However, it occurs in both non-stoichiometric BaTiO3 compositions with MgO doping, regardless of the s intering techniques. A possible mechanism for the hexagonal phase stabiliza tion associated with oxygen vacancies is discussed.