Stable and Metastable Equilibria in the Systems Y ₂ O ₂ ‐Al ₂ O ₃ , and Gd ₂ O ₃ ‐Fe ₂ O ₃

Abstract

The phase equilibrium relations in the systems Y 2 O 3 ‐Al 2 O 3 and Gd 2 O 3 ‐Fe 2 O 3 were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet‐type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd 2 O 3 ‐Fe 2 O 3 but only metastable in the system Y 2 O 3 ‐AI 2 O 3 . This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O 3 . A new 2:1 molar ratio cubic phase has been found in the system Y 2 O 3 ‐A1 2 O 3 . Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.