Enhancement of p-type conductivity in nanocrystalline BaTiO3 ceramics

Abstract

Undoped BaTiO 3 ceramic samples with an average grain size of ϳ35 nm were prepared and the electrical properties investigated. The defect structure is dominated by acceptor impurities; therefore, the conductivity of nanocrystalline BaTiO 3 is of p-type. Comparing with microcrystalline BaTiO 3 , the conductivity of nanocrystalline BaTiO 3 is about 1 to 2 orders of magnitude higher and the activation energy remarkably lower, which is ascribed to a greatly reduced oxidation enthalpy in nanocrystalline BaTiO 3 ͑ϳ0.3 versus ϳ0.92 eV for microcrystalline BaTiO 3 ͒. respectively, either in bulk form or thin-film form, have been prepared and the electrical properties investigated. Among these nanocrystalline oxides, the most dramatic property changes were observed for nanocrystalline CeO 2 . Comparing with microcrystalline counterparts, nanocrystalline CeO 2 is characterized by an orders-of-magnitude enhanced n-type conductivity and a significantly reduced activation energy. This phenomenon was explained by a greatly reduced reduction enthalpy of nanocrystalline CeO 2 , and the grain-boundary sites of lower vacancy formation enthalpy were proposed to be the atomic-level origin of this behavior.1,2 However, Kim and Maier 5 proposed that the higher conductivity of nanocrystalline CeO 2 is due to the electron accumulation at the grain boundaries. With its comparatively high electronic bulk contribution and high density of grain boundaries, the grain boundaries in nanocrystalline CeO 2 can become electronically conducting and dominate the overall behavior.Nanocrystalline BaTiO 3 materials of high density have also been prepared in thin-film form with an average grain size of ϳ25 nm, 10 and in bulk form with an average grain size of ϳ70 nm, 11 respectively, and the ferroelectric properties characterized. It is found that the ferroelectricity is weakened and the dielectric constant is remarkably lower for nanocrystalline BaTiO 3 ceramics.11 The ferroelectric response is even absent when the grain size is 25 nm.10 In neutral to oxidizing atmospheres, acceptor-doped BaTiO 3 shows a p-type conductivity. 12,13 In this letter, the defect and transport properties of nanocrystalline BaTiO 3 ceramics, nominally undoped but actually doped with acceptor impurities, are characterized, featuring an enhanced p-type conductivity.BaTiO 3 powder with an average particle diameter of ϳ10 nm was synthesized through the hydrolytic decomposition of a barium-titanium-isopropoxide solution in a waterin-oil microemulsion, consisting of 10.47 wt. % of Tergitol NP35, 80.70 wt. % of cyclohexane, 6.04 wt. % of 1-octanol and 2.79 wt. % of ultrapure and degassed water. Since the reaction is confined to the ultrasmall space of individual aqueous micelles, this approach allows the formation of nanopowders.14 The Ti/ Ba ratio of the powder was determined to be 1.0035± 0.0004 by x-ray fluorescence spectroscopy. According to inductively coupled plasma mass spectroscopy analyses, the nominally undoped BaTiO 3 powder was actually doped with acceptor impurities ͑...

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Evidence of Eu2+ 4f electrons in the valence band spectra of EuTiO3 and EuZrO3

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