Stable and Metastable Phase Relations in the System ZrO₂‐CaO

The chemical equilibrium for the system ZrO₂‐CaO was rede termined for temperatures above ∼1000°C by heating reactive powders for extended periods of time. The eutectoid decom position of the cubic solid solution was found to occur at 1140°±40°C and 17.0±0.5 mol% CaO. Decomposition reac tions in sintered cubic solid solutions were studied using elec tron diffraction and transmission techniques. Decomposition follows metastable extensions of the boundary lines which sena rate two‐phase regions from the adjacent cubic solid solution region. The appearance of the diffuse diffraction features in the electron diffraction patterns of cubic solid solutions Seems to be associated with short‐range ordering of oxygen vacancies.‘Duo ordered phases, CaZr₄O₉ and Ca₆Zr₁₉O₄₄, were syn thesized and their upper limits of stability were determined to be 1235°±15° and 1355°±15°C, respectively.