In search of the ground-state crystal structure of Ta2O5 from ab initio and Monte Carlo simulations

Abstract

Tantalum oxides (Ta2O5) are characterized by attractive physical and chemical properties, such as high dielectric constants and anti-reflection behaviour. In principle, their electronic properties can be accurately investigated from first-principles simulations. However, the existence of several stable polymorphs of these oxides represents a major difficulty to calculate and disentangle their respective spectral features. To determine the ground-state structure of the thermally grown oxide, we use linear-response time-dependent density functional theory calculations for investigating the energy loss function in the optical limit of various polymorphs. We show that the experimental energy loss signal, extracted from the reflection energy loss spectra (REELS) recorded on a thermally grown thin film, can be rationalized and interpreted by assuming that the γ-phase of Ta2O5 represents the underlying structural model of the oxide. We find that the inclusion of both local field effects and spin–orbit coupling is crucial to compute the energy loss function of these materials. Finally, to further validate the γ-Ta2O5 polymorph as a model of the experimental tantalum oxide, we compute the REELS spectra using a Monte Carlo approach, finding an excellent agreement with the as-acquired experimental data.