High‐Temperature Phase Relations in the System Y₂O₃‐Ta₂O₅

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid‐state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite‐type orthorhombic phases (W2 phase, space group C222₁), fluorite‐type cubic phases (F phase, space group Fm3m)and another orthorhombic phase (O phase, space group Cmmm)are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.