Ionic conductivity enhancement in Gd 2 Zr 2 O 7 pyrochlore by Nd doping
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S.K. Deshpande University Grants Commission—Department of Atomic Energy (UGC-DAE) Consortium for Scientific Research, Bhabha Atomic Research Centre, Mumbai 400085, India
A.K. Tyagia) Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India (Received 23 May 2007; accepted 25 September 2007)
The pyrochlore compositions Gd2–yNdyZr2O7 (y ⳱ 0.0, 0.1, 0.4, 0.6, 1.0, 1.4, 1.6, and 2.0) were synthesized, and their ionic conductivity was determined (100 Hz–15 MHz, 622–696 K). The direct-current (dc) conductivity (dc) varies upon Nd substitution at the Gd site, and a peaking effect in dc was observed around y ⳱ 1.0. This indicates that a significant increase in conductivity can be obtained at moderately high temperatures by suitable doping at the Gd site with isovalent rare-earth ions like Nd. The extent of oxygen ion disorder determined from x-ray diffraction was found to decrease with increasing Nd content. The dc conductivity obeys the Arrhenius relation dcT ⳱ 0 exp(−E/kBT). The activation energy E and the preexponential factor 0, which is a measure of the concentration of the mobile species, increase while going from the ordered Nd2Zr2O7 to the least ordered Gd2Zr2O7. These two processes presumably lead to the peaking of dc at an intermediate Nd content. Our results also suggest that the cooperative motion of mobile ions does not contribute much to the increase in activation energy in this compound. I. INTRODUCTION
Pyrochlore compounds with the general formula A2B2O7, where A is a trivalent rare-earth ion and B is a tetravalent transition element, have attracted a lot of attention due to their interesting physical properties and their potential for technological applications.1–3 The electrical properties of these materials make them promising candidates for fuel-cell applications where high ionic conductivity and low activation energy are desired.4,5 Among the pyrochlores, the Gd2Zr2O7 has shown great promise due to its high oxygen-ion conductivity at relatively lower temperatures compared to the commercially used electrolytes in solid-oxide fuel cells.6 In fact, the intrinsic ionic conductivity of Gd2Zr2O7 is the highest reported among all of the reported pyrochlores.7 It has been suggested that the disorder introduced by the incorporation of Zr in the ordered pyrochlore lattice leads to enhanced oxygen-ion conduction.6 The A2B2O7 pyrochlore structure can be thought of as an ordered defect fluorite structure having a space group Fd3m.1 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0112 J. Mater. Res., Vol. 23, No. 4, Apr 2008
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The A3+ ions occupy the 16d sites (1/2,1/2,1/2), the B4+ ions are at the 16c sites (0,0,0), and the O ions occupy the 48f sites (x,1/8,1/8) and 8b sites (3/8,3/8,3/8). The 8a sites are unoccupied. Here, x is a positional parameter that indicates the extent to which the 48f oxygen ion is off-centered, and it is sensitive to the sizes and size differences between constit
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