Prediction of crystalline Ta₄O₉ phase using first principles-based cluster expansion calculations

Tantalum is the only element of Group 5 in the periodic table that lacks any experimental reports on the existence of reduced crystalline oxide between the pentoxide (Ta2O5) and the dioxide (TaO2). We computationally predict the existence of a novel tantalum oxide with Ta4O9 stoichiometry, which lies at the midpoint between Ta2O5 and TaO2. The ground-state Ta4O9 structure was found through simulated annealing based on a cluster expansion model, which is trained using 186 density functional theory calculations. The newfound Ta4O9 material has space group number 10 (P2/m), and it can be viewed as an oxygen-deficient λ-Ta2O5 structure in which oxygen vacancies aggregate pair-wise in nearest-neighbor sites. Tad–Tad bonds fill the spatial void of the oxygen vacancies, keeping the system non-magnetic and non-metallic. The synthesis of the new Ta4O9 crystal is deemed feasible through a controlled reduction of λ-Ta2O5. The reported Ta4O9 has the potential to open new avenues in catalysis and resistive switching device applications where the reduced tantalum oxides are broadly employed.