Comprehensive study on the electronic and optical properties of α-U₃O₈

Triuranium octoxide (U₃O₈), the most stable form of uranium oxide, is an important material that finds application in nuclear and semiconductor industry. The understanding of electronic, magnetic and optical properties of U₃O₈ is essential for these advanced applications. Therefore, in this work, we have performed the density functional theory (DFT) study to investigate the structural, electronic, magnetic and optical properties of the low-temperature orthorhombic phase of U₃O₈ (α-U₃O₈) within the generalized gradient approximation (GGA) by using WIEN2k software package. To capture the highly correlated nature of 5f electrons in uranium atoms, an on-site Coulomb repulsion term (Hubbard-U) of 4.5 eV, was considered. Further, the effect of spin orbital coupling (SOC) on the electronic structure and band gap of α-U₃O₈ is demonstrated. Work functions (ϕ) were evaluated for the planes [0 0 1], [1 0 0], [0 1 0] and [1 1 1] using the first principles code QUANTUM ESPRESSO (QE). This study verifies the importance of SOC on structural, electronic and optical properties of α-U₃O₈ and claim for indirect theoretical band gap-E_g of 2.03 eV verifying the semiconductor behaviour. The optical anisotropy is analyzed through the frequency-dependent optical properties, namely, the real and imaginary parts of the dielectric tensor (ε₁ωandε₂ω), absorption coefficient (αω), optical conductivity (σω), refractive index (nω) and Loss-function (Lω). By comparing the Fermi energy and the vacuum level energy, the work functions for the planes (1 0 0), (0 0 1), (0 1 0) and (1 1 1) are predicted as 6.31, 6.73, 7.01 and 7.03 eV respectively. Furthermore, in this article, we present our experimental investigation of E_g based on diffuse reflectance spectra (DRS) method. Three powder samples, namely 1 wt%, 2 wt% and 4 wt% of U₃O₈ diluted with KCl show that U₃O₈ exhibits semiconducting behaviour with indirect band gaps of 1.86, 1.81 and 1.72 eV respectively.