Hrtem analysis of nanocrystalline BaTiO₃ and PbTiO₃: Size effects on ferroelectric phase transition temperature

Abstract

High-Resolution Transmission Electron Microscopic studies of nanocrystalline particles of BaTiO₃ and PbTiO₃ are reported. There are characteristic differences observed for BaTiO₃ 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. The SAG preparations always showed chemical intergrowth defects whereas the SG preparations were single phase. Atomic resolution images of both varieties showed interesting surface steps and surface relaxations/reconstructions of some facets. Nanocrystalline PbTiO₃ prepared by the SG route remains tetragonal, albeit with decreasing c/a ratio, down to 25nm diameter. HRTEM observations of nanocrystalline PbTiO₃ are also presented. XRD, dielectric and Raman scattering measurements also demonstrate pronouned size effects. The relationship between the observed nanostructures and size effects on the physical properties is discussed.