First-principles calculations of optoelectronic properties of CaO: Eu+2 (SrO: Eu+2) for energy applications

We have performed the first-principles density functional theory (DFT) and DFT+U calculations on the electronic and optical properties of CaO: Eu+2 (SrO: Eu+2) phosphors compounds. Herein, we have focused on the polarization of the electronic structures, i.e., the energy bandgap and the density of states. All electrons were treated within the most common exchange and correlation functional called generalized gradient approximation plus optimized effective Hubbard parameter U as GGA+U. GGA+U is a very effective tool for describing the electronic band energy upto considerable accuracy. Hence, we have opted for the arbitrary values of U as 3.0, 4.0, 5.0 and 7.0 eV to treat the strongly correlated electrons for obtaining the matching result with the experimental one. However, GGA+U is highly expensive in terms of computation due to interaction of d or f electrons. The result shows that the appearance of Eu-4f states at the valance band maximum of the spin-up causes a substantial impact on the electronic properties of the studied compounds. The value of energy bandgap is smaller in case of spin up as compared to spin down case. In case of majority spin, the energy gap of 2.224 (2.14) eV belongs to the Eu-4f orbitals and governs the CBM. The partial densities of states (PDOS) structure displays a strong hybridization that may be pointed to the formation of covalent bonds. The calculated and the measured values are in good agreement with each other. In the study of optical properties of the compound, the optical spectral structure shows a lossless region and uniaxial anisotropy. The value of uniaxial anisotropy is positive at static limit and its value is negative above this value.