Phase relationships in the yttria-rich part of the yttria-zirconia system

Phase equilibria in yttria-zirconia mixtures containing not less than 50 mol % YO_1.5 have been studied between 1200 and 2200° C. The solubility of zirconia in yttria is shown to decrease with increasing temperature: the miscibility gap between the zirconia (fluorite) and yttria (type C rare earth) solid solutions extends from 64 to 75 mol % YO_1.5 at 1400° C, from 66 to 79% at 1700° C and from 73 to 85% at 2000° C. An ordered fluorite-related phase with the ideal formula Zr₃Y₄O₁₂ and a narrow homogeneity range, which disorders at about 1370° C, is reported.