Oxide-Ion Transport in Gadolinium Zirconate - Titanates under High Pressure
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Oxide-Ion Transport in Gadolinium Zirconate - Titanates under High Pressure Hitoshi Takamura1, 3, Hirofumi Kakuta2, Atsunori Kamegawa1, 3, Masuo Okada1, 3 and Harry L. Tuller4 1 Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-02 Aoba-yama, Sendai 980-8579, Japan 2 Institute for Materials Research, Tohoku University, 2-2-1 Katahira, Sendai 980-8577, Japan 3 CREST, Japan Science and Technology Agency 4 Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA ABSTRACT The oxide-ion conduction of gadolinium zirconate - titanates, Gd2(Ti1-xZrx)2O7 (GTZ), has been investigated under high pressure in the range of 2 to 6 GPa with the aid of a cubic-anvil-type apparatus. For these pyrochlore compounds, the oxide-ion conductivity at elevated temperatures in the range of 750 to 960 °C decreased with increasing pressure, indicating the presence of a positive activation volume, ∆V. The activation volumes ∆V of GZ (x = 1.0) and GTZ (x = 0.95) were 2.0 and 1.3 cm3/mol, respectively, at 750 °C and decreased with increasing temperature. INTRODUCTION Pyrochlore compounds, A2B2O7, are of interest both as electrodes and as electrolytes in solid oxide fuel cells [1-3]. From a scientific viewpoint, the high oxide-ion conductivity originating from intrinsic structural disorder is also of great interest. In the ordered pyrochlore structure such as (Gd2Ti2O7; GT), eight built-in vacancies reside on 8b crystallographic sites as a result of the large difference in the ionic radius of Gd3+(1.053 nm) and Ti4+ (0.605 nm); however, this ordered oxygen sublattice can be disordered by means of the incorporation of lager cations such as Zr4+(0.72 nm) on B sites. As a result of the formation of Frenkel-type defects, high oxide-ion conductivity appears in the pyrochlore structure without need for acceptor dopants. It should be also noted that the presence of antisite cation defects plays an important role in the enhancement of oxide-ion conductivity. Gadolinium zirconate (Gd2Zr2O7; GZ), for example, exhibits a high oxide-ion conductivity of 10-2 S/cm at 1000 °C [1]. To further understand the nature of Frenkel-type defect formation and oxide-ion transport in gadolinium zirconate - titanates, Gd2(Ti1-xZrx)2O7 (GTZs), we performed conductivity measurements under high pressure in the range of 2 to 6 GPa. The effect of pressure on the oxide-ion conductivity is characterized by the “activation volume,” which can be derived from the following equation [4]: (1)
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where the pressure dependence of the pre-exponential factor in the temperature dependence of oxide-ion conductivity is neglected. As for the activation enthalpy for oxide-ion conductivity, the activation volume consists of two terms; i.e. a defect formation term, ∆Vf, and a migration term, ∆Vm. These quantities are considered to be sensitive to the type of defects and migration pathways. The theoretical background and results on high-pressure effects on ionic cond
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