Electronic and optical properties of pyrochlore Re2Ti2O7 (Re  = Sm and Eu) from first-principles.

Rare-earth titanate oxides are believed to be prospective functional materials for photocatalytic and photoluminescent applications because of their excellent optical properties and thermal stability of their physical properties. However, the relationships between optical properties and photoelectron trapping mechanisms are unclear. Herein, the structure, electronic, and optical properties of pyrochlore-structure Re₂Ti₂O₇ (Re = Sm and Eu) were investigated using the first-principles approach with the Hubbard parameter U (GGA + U). The calculated bandgap is 2.5 eV for Sm₂Ti₂O₇ and 2.4 eV for Eu₂Ti₂O₇, which is in good agreement with the experimental observation. The results indicate that the strongly localized f states at the top of valence band are charge-trapping sites for photoexcitation of Re₂Ti₂O₇, where electrons can absorb photon energy and transfer from the valence band to the conduction band, resulting in the photocatalytic and/or fluorescent effects in the visible and early UV regions. The important optical parameters, dielectric function ε (ω) , refractive index n (ω) , extinction coefficient k (ω) , reflectivity R (ω) , absorption coefficient I (ω) , optical conductivity σ (ω) , and electron energy-loss L (ω) were studied in detail, indicating that these optical parameters of Sm₂Ti₂O₇ and Eu₂Ti₂O₇ are insensitive to the ultra-violet (UV) radiation, but both Sm₂Ti₂O₇ and Eu₂Ti₂O₇ exhibit excellent optical properties in the visible and early UV regions. This work provides a clear understanding on the photoelectron trapping mechanism of pyrochlore-structure Re₂Ti₂O₇, which will help to improve the photocatalytic and photoluminescent performance of Re₂Ti₂O₇ and broaden their applications. [ABSTRACT FROM AUTHOR]