Ionic conductivity enhancement in Gd2Zr2O7 pyrochlore by Nd doping

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 (sigma(dc)) varies upon Nd substitution at the Gd site, and a peaking effect in sigma(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 sigma T-dc = sigma(o) exp(-E/k(B)7). The activation energy E and the preexponential factor sigma(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 sigma(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.