From Pb6(HPO3)(H2PO3)Cl9 to Pb6(HPO3)2Br8(H2O)•H2O: Halogen Regulation to Achieve Inorganic Metal Phosphite Halide Nonlinear Optical Material with Unprecedented Pb-Centered Polyhedral Units

Nonlinear optical (NLO) crystals are widely used in various fields. The introduction of lone-pair cations is regarded as an effective strategy to explore NLO crystals. In this work, two novel lead phosphite halides, centrosymmetric Pb-6(HPO3)(H2PO3)Cl-9 and noncentrosymmetric Pb-6(HPO3)(2)Br-8(H2O)center dot H2O, were obtained via a hydrothermal method. Pb-6(HPO3)(H2PO3)Cl-9 is the first reported lone-pair metal phosphite with two kinds of phosphite groups (HPO32- and H2PO3-) and Pb-6(HPO3)(2)Br-8(H2O)center dot H2O is the first inorganic NLO phosphite halide with a phase-matchable SHG effect of 1.02 x KDP. In addition, the Pb-centered polyhedral units of PbOCl4, PbOCl6, PbO2Cl5, PbO2Br5, PbOBr6, and PbO3(H2O)Br-3 in these two structures have never been reported before. An in-depth study on the structure-property relationship of the two compounds with halogen substitution is also performed.