Size effect on crystal structure and chemical bonding nature in BaTiO3 nanopowder

Summary The size effect on the crystal structure including the chemical bonding nature has been investigated for several kinds of BaTiO₃ nanopowder with the particle sizes down to 50 nm in diameter, by means of powder diffraction using high-energy synchrotron radiation. The Rietveld refinement reveals that the BaTiO₃ nanopowder consists of tetragonal and cubic structure components at 300 K. The feature of coexistence can be illustrated by the core/shell model for the particle, in which the shell with a cubic structure covers the core with a tetragonal structure. The thickness of the cubic shell is almost constant irrespective of the particle sizes, and is estimated as approximately 8 nm. Hence, the critical particle-size, where the entire particle is covered with the cubic shell, is suggested as 16 nm. The charge density distributions of the BaTiO₃ nanopowder in the cubic phase at 410 K are revealed by the maximum entropy method. Changes in the bonding electron density and the ionic valence expected are not observed clearly even in the 50 nm crystal compared with the bulk crystal.