THE EFFECT OF THE PARTICLE SIZES AND THE CORRELATIONAL SIZES OF DIPOLES INTRODUCED BY THE LATTICE-DEFECTS ON THE CRYSTAL-STRUCTURE OF BARIUM-TITANATE FINE PARTICLES

Barium titanate single-crystal particles with various average particle sizes (20-100 nm) were prepared by a hydrothermal method. In the particles as prepared, there were a lot of hydroxyl group and barium vacancies as the lattice detects, and the concentration of the lattice defects increased with decreasing particle sizes. But most of the hydroxyl group in the particles desorbed by the thermal treatment at 600 degrees C. The crystal structure of the barium titanate particles as prepared was an expanded cubic. (a similar to 4.02 Angstrom), but after a thermal treatment above 600 degrees C to remove the hydroxyl group, the cubic phase changed to tetragonal phase (c/a much less than 1.011). This phenomenon was observed in the particles with average particle sizes from 20 to 100 nm and the erg-stal structure was independent of the particle sizes. This result could be explained using the following model, that as the physical quantity, with a liner dimension than the particle sizes, there are the correlational sizes of dipoles introduced ba the lattice defects, and the correlational size can determine the stability of the lattice vibration and then the stability of the crystal structure.