Self-Activated and Bismuth-Related Photoluminescence in Rare-Earth Vanadate, Niobate, and Tantalate Series-A First-Principles Study.

Abstract

Rare-earth vanadates, niobates, and tantalates have shown self-activated and Bi3+-activated emissions. Their intrinsic emission has been attributed to self-trapped excitons (STEs), but the detailed information concerning the geometric and electronic structures of the excited states has remained unknown. Regarding the Bi3+ dopants in these hosts, the luminescence has been attributed to two different mechanisms, i.e., Bi3+↔ (V/Nb/Ta)5+ metal-to-metal charge transfer and interconfigurational (3P0,1 → 1S0) transition. Here, first-principles calculations using hybrid functionals are employed to resolve these issues. The STEs are shown to be composed of an electron localized on an individual vanadium, niobium, or tantalum ion and a hole localized on a single nearest-neighbor oxygen ion that is not shared by covalent complexes, and the bond length of the (V/Nb/Ta)-O bond with oxygen accommodating the hole is significantly elongated. The Bi3+-related emission is identified as the recombination of an exciton with a hole and an electron localized correspondingly at Bi3+ and (V/Nb/Ta)5+ ions, while the excitation is dominated by the 6s → 6p transition of Bi3+. Furthermore, Bi3+ has a hole trap level in all of the hosts considered with the trap levels in the vacuum-referred binding energy diagram being nearly flat but has an electron trap level only in rare-earth tantalates. Furthermore, the long-wavelength emission observed in niobates and tantalates is interpreted based on our calculations to be excitons bound to intrinsic defects. The insights gained in this work deepen our understanding of the STEs and form the basis for interpreting similar luminescence phenomena in other ternary closed-shell d0 transition-metal oxides. The clarification of Bi3+-related transitions and the analyses with the vacuum-referred binding energy diagram may find applications for the design and optimization of Bi3+-activated phosphors.