Modulating optical performance by phase transition in a nonlinear optical material β-Li₂RbBi(PO₄)₂

As nonlinear optical (NLO) materials, phosphates often suffer from weak second harmonic generation (SHG) response and low birefringence. Here, we report the successful synthesis of a new UV NLO orthophosphate, β-Li₂RbBi(PO₄)₂, by the high temperature solution method, and demonstrate that it crystallizes in a polar space group of P2₁ and is composed of 1D ¹[Bi₄(PO₄)₈]_∞ infinite chains that are constructed from ¹[Bi₂O₁₁]_∞ chains and [PO₄] clusters. The optical functional motifs ¹[Bi₄(PO₄)₈]_∞ are arranged in a parallel mode, which greatly improves the polarizability of the phosphate. As a result, β-Li₂RbBi(PO₄)₂ presents the largest SHG response, 5.2 × KH₂PO₄ (KDP), at 1064 nm incident radiation among all the Bi-based phosphates. Furthermore, first-principles calculations reveal that the synergistic effect of P-O and Bi-O groups contributes significantly to the optical properties of the title compound.