Phase Equilibria in the System Vanadium Oxide-Niobium Oxide

The phase diagram for the pseudo-binary system V2O5-Nb2O5 was constructed from fusion characteristics and x-ray diffraction data. Three compounds were postulated for the system. The compound VNb9O25 and V2Nb23O62 were found to melt incongruently at about 129O °C and 1322 °C respectively. The third compound occurring at about 93 to 94 mole percent Nb2O5 melted incongruently at approximately 1332 °C. Apparently,Nb2O5 accepted up to 5 mole percent V2O5 in solid solution, however, V2O5 did not accept any appreciable Nb2O5 in solid solution. In addition, the Nb2O5 rich portions of the following binary systems were reinvestigated: ZnO - Nb2O5, NiO - Nb2O5, A12O3 - Nb2O5, TiO2 - Nb2O5, and ZrO2 - Nb2O5. In each case an Nb2O5 solid solution had been reported previously in excess of 9O mole percent Nb2O5.However, subsequent data indicated that these Nb2O5 solid solutions contained discrete compounds which are related to those found in the pseudo-binary V2O5 - Nb2O5.

Phase Equilibrium Relations in the Binary System Barium Oxide-Niobium Pentoxide

A large portion of the phase equilibrium diagram for the binary system barium oxide-niobium pentoxide has been constructed from observations of fusion characteristics and X-ray diffraction data. In the system five binary compounds were observed with BaO: Nb₂O₅ ratios of 5:2, 1:1, 6:7, 3:5, and 1:3 and a 6:1 compound was postulated. The 1:1 compound was found to melt congruently at 1,455 °C and have only one stable polymorph, although a second metastable polymorph can also be prepared. The 5:2 compound melts congruently at 1,542 °C; the 6:7, 3:5, and 1:3 phases melt incongruently at 1,330, 1,290, and 1,315 °C, respectively. The phase relations of the 6:1 compound could not be determined due to the reaction between this phase and platinum metal. No 2:1 compound was observed in this system.

Solid State Reactions Involving Oxides of Trivalent Cations

Selected mixtures in 69 binary systems involving Al2O3, Ga2O3, Cr2O3, Fe2O3, Sc2O3, In2O3, Y2O3, and the rare earth oxides were studied by X-ray diffraction techniques after heat treatment at various temperatures. A plot of the radii of the A+3 cations versus the radii of B+3 cations shows the regions of stability for the different structure types found for the double oxides of the trivalent cations. The following structure types were encountered: A, B, and C-type rare earth oxide; corundum, beta gallia; kappa alumina; garnet; perovskite; and several types which could not be definitely related to known structures. The majority of A+3B+3O3 compounds have the perovskite structure. Several phases, including (1-x)Fe2O3·xAl2O3 ss and (1-x)Fe2O3·Ga2O3 ss, appear to have structures similar to kappa alumina. Solid solution definitely occurs in many of the garnet type compounds which contain gallia. Based on the data collected in this survey, the subsolidus phase equilibria relationships of 79 binary systems were drawn.

Phase Equilibria in Systems Involving the Rare-Earth Oxides. Part I. Polymorphism of the Oxides of the Trivalent Rare-Earth Ions

The polymorphic relationships of the pure rare-earth oxides have been reinvestigated using X-ray diffraction methods for identification of phases. The oxides of the trivalent rare earth ions crystallize in three different types: A, B, and C. Each oxide has only one truly stable polymorph: La2O3, Ce2O3, Pr2O3, and Nd2O3 belong to the A type; Sm2O3, Eu2O3, and Gd2O3 to the B type; Tb2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3 to the C type. In addition Nd2O3, Sm2O3, Eu2O3, and Gd2O3 have low-temperature, apparently metastable, C-type polymorphs. The low-temperature form inverts irreversibly to the stable form at increasingly higher temperatures for decreasing cation radius.