Phase transformations in the high-temperature form of pure and TiO ₂-stabilized Ta₂O₅

The high-temperature forms of undoped tantalum pentoxide (H-100Ta ₂O₅) and TiO₂-modified Ta₂O ₅ (H-92Ta₂O₅-8TiO₂) were investigated by in situ synchrotron X-ray diffraction and Raman scattering measurements. Two unquenchable and reversible phase transformations were observed in pure H-Ta₂O₅, while only one was detected for TiO₂-stabilized H-Ta₂O₅. Diffraction studies were consistent with displasive transformations, but hot-stage Raman spectroscopy indicated the existence of transient intermediate forms during the transformations. Use of complementary techniques enabled the reinterpretation of phase transformations in light of a newly proposed crystal structure model for H-Ta₂O₅,¹ and emphasized the structural contributions of the oxygen sublattice.