First-principles studies of Ta₂O₅ polymorphs

Using density functional theory (DFT) with generalized gradient approximations (GGA) and the projector-augmented wave method, we have investigated the structure, energetics, elastic tensors, and mechanical properties of four crystalline forms of Ta₂O₅ with exact stoichiometry as well as a model amorphous structure. We have also constructed a virtual crystal potential to address partial oxygen occupancy and compared it to models of explicit oxygen vacancies and of the oxygen-rich system. Our calculations show that mechanical properties of these polymorphs are highly anisotropic. By comparison with experimental data, we find that all crystalline phases and the simulated amorphous phase have a Young's modulus higher than the amorphous thin film that is probed experimentally, but the variation among crystalline structures is as high as a factor of 2. Electronic properties of three Ta₂O₅ polymorphs have been calculated using a hybrid DFT and explicit exchange functional method that improves the gap size compared to that obtained by GGA. We suggest that further experimental measurements on tantala crystals are needed to understand the physical properties of this important material.