Lone pair electron-induced attrition of lanthanide ions from a sillenite-structured bismuth gallate host†

Abstract

The crucial role of lone pair of electrons in the functionality of materials has been unprecedented. The present study reports a prolific correlation between the local structure and the stereochemical activity of the lone pairs of Bi³⁺ ions in the sillenite structure. The structural changes induced by variations in annealing temperatures greatly influenced the stereochemical activity of Bi³⁺ ion lone pairs, which, in turn, dictated the capacity for the incorporation of probing lanthanide ions in its proximity. Increased annealing temperature induces a transformation of Bi₁₂GaO_20±δ samples from a nanoscale to a bulk length scale. During the nano-to-bulk conversion of sillenite-structured Bi₁₂GaO_20±δ, the emergence of long-range ordering synchronized with the enhanced stereochemical activity of the lone pair, and this amelioration in activity led to the attrition of luminescent lanthanide ions (Eu³⁺, Tb³⁺, Er³⁺, and Yb³⁺) from the sillenite phase. Based on detailed experimental and theoretical investigations, it was envisioned that while finite size impeded the long-range translational repeatability of lone pairs in the nano-crystalline Bi₁₂GaO_20±δ sample, the concurrent existence of random geometrical distortion induces averaging of interactions at the focal Bi-site, suppressing the stereochemical activity. In high-temperature annealed bulk Bi₁₂GaO_20±δ sillenite samples, the long-range symmetrical arrangement of Bi³⁺ lone pairs balances the repulsive interactions. To maintain this balance, guest (doped) lanthanide ions were eliminated from the host lattice, resulting in phase separation. It is believed that the observations and rationalizations established between the local structure and the stereochemical activity of Bi³⁺ ion lone pair will aid in designing future bismuth-based functional materials.