Effect of Nb substitution for Ti on the electrical properties of Yb2Ti2O7-based oxygen ion conductors

We have studied the effect of niobium doping on the electrical conductivity of Yb2Ti2O7-based oxygen ion conductors. Yb2[Ti1 − xNbx]2O7 (x = 0.01, 0.04, 0.1) and (Yb0.8Tb0.1Ca0.1)2[Ti1 − xNbx]2O6.9 (x = 0; 0.05; 0.1) pyrochlore solid solutions were synthesized through coprecipitation followed by firing at 1550 °C for 4 h. The materials were examined by XPS, XRD, scanning electron microscopy and impedance spectroscopy. Yb2(Ti0.99Nb0.01)2O7 was shown to have the highest oxygen ion conductivity in air (2.3 × 10− 3 S/cm at 750 °C), which is however markedly lower than that of undoped Yb2Ti2O7. In the (Yb0.8Tb0.1Ca0.1)2[Ti1 − xNbx]2O6.9 (x = 0; 0.05; 0.1) system, the highest conductivity is offered by (Yb0.8Tb0.1Ca0.1)2[Ti0.95Nb0.05]2O6.9 (σ = 4.44 × 10− 3 S/cm at 650 °C). Additional oxygen vacancies created by Ca doping in pyrochlore structure reduce the detrimental effect of Nb4 + doping on the oxide ion transport up to 5% Nb. The conductivity of the Yb2(Ti0.99Nb0.01)2O7 and (Yb0.8Tb0.1Ca0.1)2[Ti0.95Nb0.05]2O6.9 solid solutions was measured both in air and under reducing conditions (5% H2 in N2 and CO2 atmospheres). A comparative study of both these compositions under 5% H2 in N2 atmosphere showed that the transport mechanism was not affected by complex doping of the lanthanide and titanium sublattices in the Yb2Ti2O7-based materials and was related to oxygen vacancies. Conductivity measurements in CO2 were done to ensure correct evaluation of the ionic conductivity of (Yb0.8Tb0.1Ca0.1)2[Ti0.95Nb0.05]2O6.9, because in air it seems to be a mixed p-type and ionic conductor.