Electrical conductivity anomaly around fluorite–pyrochlore phase boundary

Abstract

The relationship between electrical conductivity and crystal structure was investigated for Ln 2Zr 2O 7 ( Ln=La, Nd, Sm, Eu, Gd, Y, or Yb) and ( Ln 1− x Ln x ′) 2Zr 2O 7 ( 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( Ln 3+)/ r(Zr 4+) or r( Ln av. 3+)/ r(Zr 4+), were larger than 1.26, where r( Ln av. 3+) is estimated from the ionic radius of the component ions and the composition using the following equation: r( Ln av. 3+)=(1− x) r( Ln 3+)+ xr( Ln′ 3+). The lattice parameter increased linearly with increasing ionic radius ratios. The electrical conductivity at 800 °C in air for Ln 2Zr 2O 7 systems showed the sharp maximum at the vicinity of the phase boundary between fluorite- and pyrochlore-type phases. The electrical conductivity of ( Ln 1− x Ln x ′) 2Zr 2O 7 system also showed the maximum at the phase boundary for some combinations of Ln 3+ and Ln′ 3+. The pyrochlore-type Eu 2Zr 2O 7, 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.