Effect of Oxide Additions on the Polymorphism of Tantalum Pentoxide (System Ta₂O₅-TiO₂)

The phase equilibrium relationships of the TiO₂-Ta₂O₅ system were determined in air. An equimolar compound, TiTa₂O₇, was found to melt congruently at about 1662 °C and have a monoclinic unit cell a = 20.291 Å, b = 3.804 Å, c = 11.831 Å, β = 120°14', apparently isostructural with TiNb₂O₇. In addition, two other compounds are postulated to occur at about TiO₂:49Ta₂O₅ and TiO₂:7Ta₂O₅ and to dissociate at about 1230 and 1190 °C, respectively. TiO₂ apparently accepts a maximum of 9 mole percent Ta₂O₅ in solid solution at 1630 °C. Two eutectics occur in the system at 54 mole percent Ta₂O₅ and 1650 °C and 31 mole percent Ta₂O₅ and 1630 °C. The high temperature form of Ta₂O₅ is apparently triclinic at room temperature with a = 3.801 Å, b = 3.785 Å, c = 35.74 Å, α = 90°54.4', β = 90°11.5', and γ = 89°59.9'. A metastable transition occurs at 320 °C to a monoclinic form which inverts to tetragonal at 920 °C with a = 3.81 Å, c = 36.09 Å. Several intermediate morphotropic phases are found with up to 9 mole percent TiO₂ in solid solution.