Novel Delta-Ta2O5 Structure Obtained from DFT Calculations

We present a novel structure (called delta'-Ta2O5) of hexagonal delta'-Ta2O5 (tantala) based on first-principles calculations. Previous calculations have predicted a hexagonal structure and determined lattice constants in agreement with X-ray diffraction. Starting from this structure and determining an O-defective crystal, we have observed a strong reconstruction of the crystal structure incompatible with the pristine one. We show that this reconstruction must already be present in stoichiometric tantalum oxide, and we provide its geometrical and electronic structure. The energy stabilization is explained by a decreased O-O repulsion, by a more reasonable coordination of the tantalum atoms, and by an increase in the electronic gap.