PbBi(SeO3)2F and Pb2Bi(SeO3)2Cl3: Coexistence of Three Kinds of Stereochemically Active Lone-Pair Cations Exhibiting Excellent Nonlinear Optical Properties

Stereochemically active lone-pair (SCALP) cations are one attractive type of nonlinear optical (NLO)-active units because of their large microcosmic polarizability and anisotropy. Currently, the single and/or dual lone-pair cation-based noncentrosymmetric (NCS) oxides have been extensively investigated and verified to be one class of outstanding NLO materials. From the perspective of function optimization, the integration of three kinds of SCALP cations into one crystal may synergistically improve the NLO properties, which is greatly expected but unexplored to date. Herein, by introducing flexible metal halide bonds to guarantee the stereochemical activity and overcome the energetically favorable antiparallel arrangements of lone-pair cations, the first type of three lone-pair-cation (Pb²⁺, Bi³⁺, and Se⁴⁺)-coexisting NCS oxides PbBi(SeO₃)₂F (I) and Pb₂Bi(SeO₃)₂Cl₃ (II) was obtained. As expected, both compounds show outstanding NLO properties, such as the strong second-harmonic-generation signal (10.5× and 13.5 × KDP), large birefringence (0.103 and 0.186), relatively wide energy band gaps (3.75 and 3.45 eV), and good physicochemical stability. Theoretical calculations demonstrated the effect of three lone-pair-cation-based polyhedra and the halide anion on NLO properties.