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.