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

The influence of TiO2 (anatase) and H2TiO3 (beta-titanic acid) precursors 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 (about 40-80 nm) and narrow size distributions can be prepared at 180 C after 24 h using H2TiO3 as a precursor material. Although the initial reaction is very fast, about 90% yield after 8-10 h, extended reaction periods at 180 C are required in order to drive the reaction to completion. Lowering the temperature from 180 to 85 C produces powders with even smaller particle sizes; however, very long reaction periods are required, e.g. over 72 h, to ensure complete reaction. Raman spectra of as-prepared and heat treated (1000 C) powders with average particle sizes as small as about 20-40 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 sizes greater than about 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 OH- ions show these materials to be modest proton conductors. 54 refs.