Electrical conductivity study of defects in zirconium doped yttrium aluminium garnet ceramics

High temperature measurements of conductivity as f(T) of Y3Al5O12 (YAG) doped with 50 ppm Zr in CO/CO2 and H2/H2O gas mixtures show a significant difference in the activation energy (3.54 resp. 1.91 eV) at constant oxygen partial pressure. The sign of the Seebeck coefficient indicates that a H2/H2O atmosphere introduces protons as conducting species into the garnet lattice. Impedance spectroscopy revealed that this is a bulk effect, while the grain boundaries are nearly unaffected by ambient atmosphere. Ionic and electronic conductivity as f(pO2) show that ionic conductivity dominates in the high pO2 regime, while at low pO2 electronic conductivityincreases with pO2-1/5 indicating that either singly or doubly ionised oxygen vacancies or defect clusters appear as prevailing ionic defects