Unveiling the enhanced structural, elastic, mechanical, and optoelectronic properties of BaWO4via oxygen vacancies and europium doping: a DFT + U insight into tailored energy applications

In this study, we examine the structural, electronic, and optical properties of Eu-doped BaWO₄ using the full potential linearized augmented plane wave (FPLAPW) method, within the framework of density functional theory (DFT). The calculations are performed using the Generalized Gradient Approximation with an optimized effective Hubbard parameter ‘U’ (GGA + U), implemented in WIEN2K software. The introduction of oxygen vacancies and Eu doping significantly impacts the elastic properties of BaWO₄, including its elastic constants, bulk modulus, shear modulus, and Poisson's ratio. These modifications result in a predictable reduction in stiffness and rigidity but enhance the material's optoelectronic functionality. By adding the Hubbard parameter term ‘U’, with a value of U = 7 eV, a more accurate description of the system is achieved, particularly in systems with a strong correlation of d- and f-electronic states. In contrast to the wide band gap (4.885 eV) of parent BaWO₄, the electronic band gap decreases to 2.80 eV for Eu-doped BaWO₄. Additionally, the creation of O-deficiency in BaWO₄ results in a reduction in the band gap value to 0.8 eV (spin-up) and 2.6 eV (spin-down). The partial density of states (PDOS) reveals that the Eu-f (Eu-d) state dominates the valence band maximum, while the conduction band minimum is attributed to the W-d state for spin-up and spin-down channels, respectively. Further analysis of the optical response, including the dielectric constant ε(ω), absorption coefficient I(ω), reflectivity R(ω), refractive index n(ω), and optical energy loss functions L(ω), with different incident photon energies, is presented. When Eu atoms are added to the BaWO₄ sample, the gap between optical bands narrows, indicating the development of intermediate energy levels. The calculated band gaps confirm that the E_g of oxygen vacancy (V_O) < oxygen vacancy shows good agreement with optoelectronic devices.