No abstract

Tantalum pentoxide (Ta2O5) is a wide-gap semiconductor that presents good catalytic and dielectric properties, conferring to this compound promising prospective use in a variety of technological applications. However, there is a lack of understanding regarding the relations among its crystalline phases, as some of them are not even completely characterized and there is currently no agreement about which models better explain the crystallographic data. Additionally, its phase diagram is unknown. In this work we performed first-principles density functional theory calculations to study the structural properties of the different phases and models of Ta2O5, the equation of state and the zone-centered vibrational frequencies. From our results, we conclude that the phases that are built up from only distorted octahedra instead of combinations with pentagonal and/or hexagonal bipyramids are energetically more favorable and dynamically stable. More importantly, this study establishes that, given the pressure range considered, the B-phase is the most favorable structure and there is no a crystallographic phase transition to another phase at high-pressure. Additionally, for the equilibrium volume of the B-phase and the λ-model, the description of the electronic structure and optical properties were performed using semi-local and hybrid functionals.

We report a theoretical study of the electronic structure and optical properties of the B phase of niobium pentoxide (B-Nb 2 O 5 ) and tantalum pentoxide (B-Ta 2 O 5 ) by means of firstprinciple calculations. We have used density functional theory along with the revised Perdew-Burke-Ernzerhof (PBEsol) exchange-correlation functional and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. It has been found that these compounds are indirect wide-gap semiconductors, the calculated gaps for B-Nb 2 O 5 (B-Ta 2 O 5 ) are E PBEsol g ¼ 2:5ð3:1Þ and E HSE06 g ¼ 4:1ð4:7Þ eV. We have also calculated the frequencydependent and static dielectric tensor, the refraction index and the transmittance. The calculated average static dielectric constants of B-Nb 2 O 5 (B-Ta 2 O 5 ) are 33.7 (30.9), in good agreement with the available experimental data.