Halogen regulation triggers structural transformation from centrosymmetric to noncentrosymmetric switches in tin phosphate halides Sn2PO4X (X = F, Cl)

Abstract

Generally, phosphates are employed to design ultraviolet nonlinear optical crystals. However, phosphates always suffer from too small birefringence to effectively achieve phase matching. Herein, the Sn²⁺ cation with a stereochemically active lone pair was introduced into a phosphate as a birefringence-active unit, and two tin phosphate halides Sn₂PO₄X (X = F, Cl) were successfully synthesized. Interestingly, the different coordination environments of the Sn-polyhedron caused by the partial substitution of halide ions (F⁻/Cl⁻) with different ionic radii for the oxygen ligands lead to structural transformation from centrosymmetric (CS) Sn₂PO₄F to noncentrosymmetric (NCS) Sn₂PO₄Cl. Thus, Sn₂PO₄X (X = F, Cl) were characterized by enlarged birefringence, and Sn₂PO₄Cl is type I phase-matchable with a moderate SHG response, and is a potential ultraviolet nonlinear optical material. Furthermore, both phosphates show wide band gaps. Theoretical analysis confirmed that their excellent optical properties result from the synergistic effect of the [SnOX] (X = F, Cl) polyhedra with stereochemically active lone pairs and the [PO₄]³⁻ tetrahedron.