Determination of the Electronic Structure and UV-Vis Absorption Properties of (Na2-xCux)Ta4O11 from First-Principle Calculations

Density functional theory (DFT) and density functional perturbation theory (DEPT) were applied to study the structural, electronic, and optical properties of a (Na2-xCux)Ta4O11 solid solution to accurately calculate the band gap and to predict the optical transitions in these materials using the screened coulomb hybrid (HSE06) exchange-correlation formalism. The calculated density of states showed excellent agreement with UV-vis diffuse reflectance spectra predicting a significant red-shift of the band gap from 4.58 eV (calculated 4.94 eV) to 2.76 eV (calculated 2.60 eV) as copper content increased from 0 to 83.3%. The band gap narrowing in these materials, compared to Na2Ta4O11, results from the incorporation of new occupied electronic states, which are strongly localized on the Cu 3d orbitals, and is located within 2.16-2.34 eV just above the valence band of Na2Ta4O11. These new occupied states, however, possess an electronic character localized on Cu, which makes hole mobility limited in the semiconductor.