Application of an Ion‐Packing Model Based on Defect Clusters to Zirconia Solid Solutions: II, Applicability of Vegard's Law

Lattice parameter data of cubic phases and cube roots of unit cell volumes of tetragonal phases in homogeneous ZrO₂‐containing solid solutions were compiled to examine the validity of Vegard's law. Except for ZrO₂–CeO₂ and ZrO₂–UO₂ systems, the data for cubic phases were expressed by the equation d=a_sX+b, where d, a_s, X, and b denote the lattice parameter, a constant depending on dopant species, the dopant content, and a constant independent of dopant species, respectively. For tetragonal phases, the cube roots of unit‐cell volumes could be fitted by a similar equation except for the data in the ZrO₂–MO₂ systems (M = Ge and U). The constant a_s was calculated using an ion‐packing model and was independent of the defect cluster models. The calculated a_s is close to the experimentally observed one, although the former is slightly smaller than the latter in the ZrO₂–MO_u systems (u= 1 and 1.5). This difference was ascribed to the lack of consideration of the ionic distortions from the ideal sites of the fluorite‐type structure.