The phase diagram for the system ZrOz-Y203 was redetermined. The extent of the fluorite-type ZrOz-Yz03 solid solution field was determined w$th a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique. Long range ordering occurred at 40 mol% Y,O, and the corresponding ordered phase was Zr3Y40L2. The compound has rhombohedra1 symmetry (space groupR3), is isostructural with UY,Ol,, and decomposes above 125OfSO"C. The results indicate that the eutectoid may occur at a temperature <400°C at a composition between 20 and 30 mol% Yz03. Determination of the liquidus line indicated a eutectic at 83 f 1 mol% Yz03 and a peritectic at 76f 1 mol% YzO3.

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