Electrical conductivity anomaly around fluorite–pyrochlore phase boundary

The relationship between electrical conductivity and crystal structure was investigated for Ln2Zr2O7 (Ln=La, Nd, Sm, Eu, Gd, Y, or Yb) and (Ln1−xLnx′)2Zr2O7 (Ln=Gd, Sm, or Nd; Ln′=Y, Yb, or Gd) systems. The crystal structure of both systems changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ln3+)/r(Zr4+) or r(Lnav.3+)/r(Zr4+), were larger than 1.26, where r(Lnav.3+) is estimated from the ionic radius of the component ions and the composition using the following equation: r(Lnav.3+)=(1−x)r(Ln3+)+xr(Ln′3+). The lattice parameter increased linearly with increasing ionic radius ratios. The electrical conductivity at 800 °C in air for Ln2Zr2O7 systems showed the sharp maximum at the vicinity of the phase boundary between fluorite- and pyrochlore-type phases. The electrical conductivity of (Ln1−xLnx′)2Zr2O7 system also showed the maximum at the phase boundary for some combinations of Ln3+ and Ln′3+. The pyrochlore-type Eu2Zr2O7, which is located at the nearest position to the phase boundary, showed the highest conductivity of 8.3×10−3 S cm−1 at 800 °C. On the other hand, the activation energy for the conduction remarkably decreased with the increasing ionic radius ratios in the fluorite-type phase range and showed the minimum at the given compositions, at which the maximum electrical conductivities were observed and then increased.