Hexagonal-phase retention in pressureless-sintered barium titanate

Abstract

Undoped and MgO-doped TiO₂- and BaO-excess non-stoichiometric barium titanate (BaTiO₃) compositions were pressureless sintered using a conventional furnace and CO₂ laser. High-temperature hexagonal BaTiO₃ was metastably retained to room temperature as revealed by both X-ray diffractometry and transmission electron microscopy. The sintered microstructure of the TiO₂-excess composition is characterized by a bimodal grain size distribution containing large plate-like grains. The microstructure of BaO-excess BaTiO₃ with MgO doping is characterized by large spherulitic grains with petals 1–2 mm long. 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 sintering techniques. A possible mechanism for the hexagonal phase stabilization associated with oxygen vacancies is discussed.