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 hydr othermal synthesis is discussed. Amorphous H2TiO3 was found to be an excell ent Ti-precursor material and offers several advantages over crystalline an atase. 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 de grees C are required in order to drive the reaction to completion. Lowering the reaction temperature from 180 to 85 degrees C does produce powders wit h even smaller particle sizes, however, very long reaction periods are requ ired, e.g. > 72 h, to ensure complete reaction. Raman spectra of as-prepare d 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 B aTiO3 lattice. These results contradict the widely cited 'critical' particl e size theory for the stabilisation of the cubic polymorph, at least for pa rticle sizes greater than ca. 20-40 nm. As-prepared powders contain many de fects, primarily in the form of lattice OH- ions. Preliminary ac impedance spectroscopy data on samples heat treated to remove lattice hydoxyl ions de monstrate these materials to be modest proton conductors.