Stability of doped zirconia under extreme conditions: towards long-term and secure storage of radioactive waste

Extreme temperatures and pressures were applied to systems based on stabilized zirconia, ZrO2, doped with Ce4+ ions as surrogate for tetravalent Actinides in order to conclude on their long term stability in deep geological underground. Both tetragonal and cubic Yttrium-stabilized ZrO2 (YSZ) exhibit excellent phase and structural stabilities up to 1150 K. In addition, incorporated guest Ce4+ did not show any increase in their mobility at elevated temperatures. Application of external pressure did not induce any structural or phase changes in cubic YSZ doped with 5 at.% Ce as well. However, a corresponding tetragonal analogue with lower Yttrium content exhibits a 2nd order phase transition towards higher cubic symmetry around 9 GPa. Remarkably, no discharge of the guest Ce4+ ions was observed throughout the transition and further upon increase in pressure. This together with T-dependent data indicates excellent affinity of guest Ce atoms with the host YSZ matrices. The parent YSZ phases are, therefore, promising candidates as host materials for long term underground immobilization for radiotoxic tetravalent elements like U, Th or Pu.