“…The ZrO 2 exhibits thermal stability, electrical conductivity and oxygen vacancies, and its properties can be improved by the introduction of lanthanide and alkaline-earth cations such as Y 3+ , La 3+ , Ca 2+ or Mg 2+ into the ZrO 2 lattice [19,20]. The replacement of the Zr 4+ cation with one of these cations with lower positive charge leads to a negative overall charge that is compensated by an increased number of oxygen vacancies, to maintain electrical neutrality.…”