Preparation and investigation of hexagonal-tetragonal BaTiO<sub>3</sub> powders

Hexagonal-tetragonal co-existing barium titanate powders were prepared by reducing commercial barium titanate powders with their particle size of about 100 nm in a hydrogen atmosphere for 1 h at several temperatures, and the microstructures were observed. It was found that the hexagonal-tetragonal barium titanate powder had a co-existing hexagonal and tetragonal phase in a particle rather than a mixture of two phases in the powder. The hexagonal contents of about 0, 25, 50, 70, 85, and 100 wt % were obtained at temperatures of 1310, 1315, 1320, 1324, 1330, and 1333 °C, respectively. The hexagonal phase was returned to the tetragonal phase by annealing at 1200 °C in air. A model for the mechanism of a complete transformation of barium titanate polymorph from tetragonal/cubic to hexagonal phases is proposed.