A₃BBi(P₂O₇)₂ (A = Rb, Cs; B = Pb, Ba): Isovalent Cation Substitution to Sustain Large Second-Harmonic Generation Responses

Exploring novel photoelectric functional materials via chemical substitution-oriented design is an effective strategy, which can be expanded to the discovery of high-performance UV nonlinear optical (NLO) materials. Two new NLO pyrophosphates, Rb₃BaBi­(P₂O₇)₂ (I) and Cs₃BaBi­(P₂O₇)₂ (II), are rationally developed by a cation substitution technique based on A₃PbBi­(P₂O₇)₂ (A = Rb and Cs), of which I inherits the large second-harmonic generation (SHG) response (exptl 2.5 × KDP; calcd 2.9 × KDP) and moderate birefringence (0.025@1064 nm) accompanied by a broadened UV transparent region. Compounds I and II are isomeric and exhibit different Ba–P–O frameworks. Especially, I possesses a large SHG effect benefiting from the favorable pentagonal-net topological structure. Detailed theory calculations elucidate the origin of the linear and nonlinear optical properties of the compounds. The insights obtained from the atomic-level module adjustment involving lone-pair-active optical anisotropy are useful for designing more efficient UV NLO materials.