Intergrowth defects and dielectric response of BaTiO₃ nano-particles

Abstract

Characteristic differences are observed for the dielectric response and microsturctures of BaTiO₃ nanoscale fine powders prepared using sol gel (SG) and steric acid gel (SAG) methods. The former exhibit a critical size below which there is no paraelectric/ferroelectric phase transition whereas BaTiO₃ prepared via the SAG route remained cubic for all conditions of preparation. The SAG preparations always showed chemical intergrowth defects and (111) multiple twinning. The density of such defects tended to increase as the particle size decreased. The SG preparations were single phase and became tetragonal above some critical size (> 130 nm). Some SG BaTiO₃ showed (111) twinning.

The electron optical image analysis of nanocrystalline BaTiO₃ particles by computer simulation and image-matching techniques has identified the intergrowth phase as polytypic variants of the perovskite structure of BaTiO₃; i.e., mixtures of cubic and hexagonal BaTiO₃. These defects are stoichiometric and were found for SAG BaTiO₃ particles of all sizes. Image analysis was used to determine that the nanoscale multiple twins involve a pure mirror symmetry operation with BaO atoms forming the twin composition plane.

The relationship between the observed nanostructures and defects and the dielectric properties is discussed.

Key Words:

• Dielectric response
• BaTiO₃
• DRAM