Peculiar size effect in nanocrystalline BaTiO3

Abstract

Dense BaTiO3 ceramics with grain sizes of 35nm to 5.6μm were prepared, and the electrical properties investigated in the temperature range 500–700°C by means of impedance spectroscopy. Charge carriers (oxygen vacancies and holes) are depleted in the space charge regions at the BaTiO3 grain boundaries. When the grain size is ⩾250nm, the width of the space charge region was determined to be ∼40nm. Therefore, the depletion regions were expected to overlap when the grain size decreases to 35nm; in such a situation, charge carriers would be depleted over the entire grain, resulting in depressed conductivity. However, the conductivity of the 35nm grain size sample was measured to be one to two orders of magnitude higher than those of the microcrystalline samples, and the activation energy markedly lower. Moreover, we determined a width of ∼7nm for the space charge regions in the 35nm grain size sample; therefore, the space charge regions do not overlap. The enhanced conductivity is ascribed to a reduced oxidation enthalpy in nanocrystalline BaTiO3, and the distorted grain boundaries in nanocrystalline BaTiO3 are believed to be the atomic level origin of the reduced oxidation enthalpy.