Oxygen ion conductivity in doped Gd2Ti2O7 with the pyrochlore structure

Materials with the A₂B₂O₇ pyrochlore structure have interesting ionic transport properties because of their crystallographic structure, which can be described as a stable array of corner-shared BO₆ octahedra that is penetrated by a 3-dimensional tunnel configuration that is partly filled by the A₂O sublattice. The pyrochlore stochiometry means that there are built-in intrinsic oxide ion vacancies in the crystal structure in comparison to the related fluorite type structure. These are in the A₂O sublattice, so that the tunnels are only 75% occupied. The presence of these tunnels leads to the possibility of significant changes in the composition, and some ionic species in this sublattice exhibit high mobility. The cubic pyrochlore Gd₂Ti₂O₇ was doped in various ways to change its ionic and electronic transport properties. The total conductivity and partial ionic and electronic contributions were investigated by ac impedance and EMF measurement techniques. The influence of either A or B site doping with aliovalent ions that occupy sites in the A₂O and B₂O₆ sublattices was investigated. The results of these experiments are presented and discussed in relation to the crystal structure and defect chemistry of this family of oxides. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995