Hydrothermal synthesis and characterisation of BaTiO3 fine powders:: precursors, polymorphism and properties

The influence of two Ti-precursors, TiO2 (anatase) and H2TiO3 (beta-titanic acid), on the purity and particle size of BaTiO3 powders prepared via hydrothermal synthesis is discussed. Amorphous H2TiO3 was found to be an excellent Ti-precursor material and offers several advantages over crystalline anatase. Phase pure powders which have small particle sizes, ca. 40-80 nm and narrow particle size distributions can be prepared at 180 degrees C after 24 h using H2TiO3 as a precursor material. Although the initial reaction is very fast, ca. 90% yield after 8-10 h, extended reaction periods at 180 degrees C are required in order to drive the reaction to completion. Lowering the reaction temperature from 180 to 85 degrees C does produce powders with even smaller particle sizes, however, very long reaction periods are required, e.g. > 72 h, to ensure complete reaction. Raman spectra of as-prepared and heat treated (1000 degrees C) powders with average particle sizes as small as ca. 29-40 nm indicate asymmetry within the TiO6 octahedra of the BaTiO3 lattice. These results contradict the widely cited 'critical' particle size theory for the stabilisation of the cubic polymorph, at least for particle sizes greater than ca. 20-40 nm. As-prepared powders contain many defects, primarily in the form of lattice OH- ions. Preliminary ac impedance spectroscopy data on samples heat treated to remove lattice hydoxyl ions demonstrate these materials to be modest proton conductors.