Phase Equilibria as Related to Crystal Structure in the System Niobium Pentoxide-Tungsten Trioxide

The phase equilibrium diagram for the binary system niobium pentoxide-tungsten trioxide has been constructed from results of x-ray diffraction studies on both single crystals and powders and from fusion characteristics. Twelve stable compounds have been found in the system. The exact composition of eight of these compounds has been established by single crystal analyses at the Nb₂O₅:WO₃ ratios of 6:1, 13:4, 7:3, 8:5, 9:8, 1:1, 4:9, and 2:7. The approximate compositions of the remaining four other phases are 30:1, 6:11, 1:11, and 1:15. The 6:1, "6:11", 4:9, and 2:7 phases melt eongruently at 1476, 1378, 1380, and 1357 °C, respectively. The "30:1", 7:3, 8:5, 9:8, "1:11", and "1:15" phases melt incongruently at 1470, 1440, 1385, 1375, 1356, and 1358 °C, respectively; and the 13:4 and 1:1 phases decompose before melting at 1435 and 1115 °C, respectively. The 8:5, 9:8, "6:11", 2:7, "1:11", and "1:15" compounds are shown on the phase diagram as having minimum temperatures of stability. One metastable phase having a narrow range of composition near the 3:8 ratio was also encountered. Although Nb₂O₅ apparently exhibits no solid solution, WO₃ was found to accept a maximum of three mole percent niobia in solid solution enabling all the reported polymorphs of WO₃ to be obtained at room temperature.