Exploring Polymorphism in High-valent Tantalum Trioxides*

Tantalum oxides (TaO, TaO₂, and Ta₂O₅) are key oxide materials for modern electronic devices, such as dynamic random-access memory and sensors, possessing the usual +2, +4, and +5 valence charge state of Ta. Interestingly, new forms of tantalum oxides have been proposed with an unusual stoichiometry of TaO₃ and are predicted to have a high-valent charge state of Ta⁶⁺. However, not much is known about these highvalent TaO₃ nor is the possibility of structural polymorphism (as found in other more well-known heavy metal oxides like WO₃ and MoO₃) explored. In this work, we have collected structural motifs from previous studies about TaO₃, especially synthesis process. Using first-principles density-functional theory calculations, we study the thermodynamics and lattice dynamics of polymorphs of TaO₃ (namely, the cubic ReO₃, new stacked and sheet structures) and explore the electronic and optical properties of these new TaO₃ polymorphs for potential device applications.