Ba6(Bi1−xEux)9B79O138 (0 ≤ x ≤ 1): synergetic changing of the wavelength of Bi3+ absorption and the red-to-orange emission ratio of Eu3+

Abstract

Very recently, a novel barium bismuth borate Ba₆Bi₉B₇₉O₁₃₈ (BBBO) was shown to be a new analogue of REBaB₉O₁₆ (RE = rare earth), which is a family of excellent phosphor hosts, because it has two types of cationic positions suitable for bi- and tri-valent activators (or sensitizers). In this study, we prepared Eu³⁺-to-Bi³⁺ substituted solid solutions Ba₆(Bi_1−xEu_x)₉B₇₉O₁₃₈ (0 ≤ x ≤ 1) and performed a systematic UV-photoluminescence investigation. As anticipated, efficient energy transfer occurs in the whole series of compounds and the majority of the energy absorbed by Bi³⁺ was transmitted to Eu³⁺, causing a great enhancement of Eu³⁺ emission. More interestingly, the Eu³⁺-to-Bi³⁺ substitution leads to a centrosymmetric-to-noncentrosymmetric transition of the local coordination symmetry of the trivalent cations due to the size difference between Eu³⁺ and Bi³⁺. This is observed in the synergetic changing of the wavelength of Bi^{3+ 1}S₀ → ³P₁ absorption (excitation spectra) and the red-to-orange emission ratio (emission spectra) when changing x. BBBO is a new polyborate host with a freshly resolved structure whose interesting preliminary results suggest the further development of BBBO-based phosphors.