Electrical transport and defect properties of Ca‐ and Mg‐doped yttrium aluminum garnet ceramics

Electrical transport properties of a sintered yttrium aluminum garnet (YAG) doped with Ca or Mg have been investigated as a function of temperature and partial pressure of oxygen. Conductivity and ionic transport measurements show Ca- and Mg-doped YAG to be mixed ionic-electronic ¿-type conductors exhibiting an activation energy of about 2 eV. Thermally stable inclusions of alumina, which were found to be present in sintered YAG powders, act as a source of Al vacancies. Al vacancies are dominating the defect chemistry at high oxygen partial pressure. The electronic contribution to total conductivity becomes more pronounced when the temperature increases. At low ¿O2 a hole-release from O·o centers (trapped holes) influences the transport behavior. We developed a possible defect model which explains the experimental data qualitatively. This model considers the influence of dispersed particles of constant activity over a ¿O2 regime ranging from 105 to 10¯¯¹² Pa and their influence on the defect chemistry.