Condensed Phase Relations in the Systems ZrO ₂ ‐WO ₂ ‐WO ₃ and HfO ₂ ‐WO ₂ ‐WO ₃

Abstract

Phase relations for the systems ZrO 2 –WO 2 –WO 3 and HfO 2 –WO 2 –WO 3 from 1000° to 1700° C were determined by the quenching technique using sealed sample containers. In the system ZrO 2 –WO 3 , 1:2 compound, ZrW 2 O 8 forms, having a cubic structure with a = 9.159 A. The ZrW 2 O 8 melts incongruently at 1257°± 3°C to ZrO 2 and liquid and has a lower limit of stability at 1105°C, below which ZrO 2 and WO 3 coexist in equilibrium. One eutectic and one peritectic were established: at 1231°± 3°C and 74 mole % WO 3 , and at 1257°± 3°C and 71 mole % WO 3 , respectively. Along the join ZrO 2 –WO 2 , no compound formed. Two invariant points were determined: ZrO 2 , WO 2 , W, and liquid are in equilibrium at 1430°± 5°C and 76 mole % WO 2 , whereas WO 2 , W 18 O 49 , W, and liquid coexist at 1530°± 5°C and 89 mole % WO 2 ‐ Equilibrium relations in the system ZrO 2 –WO 2 –WO 3 were investigated at four temperatures. At 1200°C, a cubic phase with composition near W 20 O 58 was found; it exists in equilibrium with ZrO 2 , W 18 O 49 , W 20 O 58 , and WO 3 . As the temperature increases, the liquid formed along the ZrO 2 –WO 3 join extends into the ternary system, crosses the join ZrO 2 –W 20 O 58 at 1300°C, and crosses the join ZrO 2 –W 18 O 49 at 1400°C. The cubic phase can take more zirconium into its solid solution at 1300° than at 1200°C. At 1500°C, the system can no longer be treated as a simple ternary oxide system because of the presence of metallic tungsten, and equilibrium relations are presented on the basis of the system ZrO 2 –W–WO 3 . Phase equilibrium relations in the systems HfO 2 –WO 3 , HfO 2 –WO 2 , and HfO 2 –WO 2 –WO 3 in the temperature ranges studied are much like those in the corresponding zirconium system.