Ambient pressure synthesis of Eu³⁺ doped δ-BiB₃O₆ by seed-assisted high temperature solid state reactions

δ-BiB₃O₆ shows a comparable nonlinear optical performance with the famous α-BiB₃O₆. It can be prepared either by a high temperature-high pressure method or by a sol-gel method associated with RE³⁺-doping (RE = La, Ce, Pr, and Nd). A direct solid state reaction at 700 °C stabilized 20 atom% La³⁺ doped δ-BiB₃O₆ compared to the previously reported record (15 atom%) in the literature. La³⁺ doping would not only expand the thermodynamically stable region, which is critical for the synthesis, but also hasten the crystallization process. Moreover, Eu³⁺, a smaller RE cation, was also doped for the first time at a high doping level (7 atom%) simply by a seed-assisted solid state reaction, which apparently provides additional functionality and will prompt further efforts on single crystal growth of RE³⁺-doped δ-BiB₃O₆ under ambient pressure. The combined powder X-ray diffraction and transmission electron microscopy analysis underpinned the phase purity and doping homogeneity of cation doped bismuth borates. UV-excited red luminescence was observed for δ-Bi_0.93Eu_0.07B₃O₆.