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

Abstract

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 Pb₆(HPO₃)(H₂PO₃)Cl₉ and noncentrosymmetric Pb₆(HPO₃)₂Br₈(H₂O)·H₂O, were obtained via a hydrothermal method. Pb₆(HPO₃)(H₂PO₃)Cl₉ is the first reported lone-pair metal phosphite with two kinds of phosphite groups (HPO₃^2- and H₂PO₃^-) and Pb₆(HPO₃)₂Br₈(H₂O)·H₂O is the first inorganic NLO phosphite halide with a phase-matchable SHG effect of 1.02 × KDP. In addition, the Pb-centered polyhedral units of PbOCl₄, PbOCl₆, PbO₂Cl₅, PbO₂Br₅, PbOBr₆, and PbO₃(H₂O)Br₃ 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.