Phase equilibria as related to crystal structure in the system niobium pentoxide-tungsten trioxide

The phase equil ibrium 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 s table compounds have been found in the system_ The exac t compos ition of eight of these compounds has been established by single crystal analyses at th e Nb,O" : WO" ratios of 6 : J, 13: 4, 7: 3, 8: 5,9: 8, 1 : 1,4 :9, and 2: 7. The approximate compos ition s of the remaining four other phases are 30: 1,6: ll, 1: 11, and I : 15. The 6: 1, "6: 11 ",4: 9, and 2 : 7 phases melt congruently at 1476, 1378,1380, and l357 C, respectively. The "30: I", 7 : 3, 8 : 5, 9 : 8, "1 : ll" , and "1: 15" phases melt incongruently at 1470, 1440, 1385,1375,1356, and 1358 C, respectively; and the 13: 4 and I : I phases decompose before melting at 1435 and l1l5 C, respectively. The 8: 5, 9: 8, "6: 11 ", 2: 7, "} : 11 ", and "l : 15" compounds are shown on t he phase diagram as having minimum temperatures of stability. One metastable phase having a narrow range of composition near th e 3 : 8 ratio was also encountered_ Although Nb,O" apparently exhibits no solid solut ion , WO" was found to accept a maximum of three mole percent niobia in solid solution enab lin g a ll the reported polymorphs of WO" to bt' obtained at room temperature.