Nonlinear optical crystals KPb3(3-C5H4NCOO)2Br5 and KPb3(3-C5H4NCOO)2Cl3Br2 with new Pb-centered nitrogen-halide polyhedrons obtained by the halide anionic substitution

Abstract

The rational synthesis of nonlinear optical (NLO) crystals with good performances remains a great challenge. Herein, the total/partial halide anionic substitution has been successfully expanded to discover two new NLO crystals, KPb3(3-C5H4NCOO)2Br5 and KPb3(3-C5H4NCOO)2Cl3Br2. Interestingly, KPb3(3-C5H4NCOO)2Br5 and KPb3(3-C5H4NCOO)2Cl3Br2 not only contain highly distorted [PbO2Br] and [PbO2Cl] trigonal pyramids, but also possess unique Pb-centered nitrogen-halide functional units, including [PbNBr4] square pyramid, as well as [PbNBr2Cl] four-coordinated polyhedron with mixed halogen anions. Remarkably, after the total halide anionic substitution, the net polarization of the Pb-centered nitrogen-halide functional units runs parallel to that of the π-conjugated nicotinate groups, resulting in the enhancement of macroscopic polarization. Therefore, KPb3(3-C5H4NCOO)2Br5 displays a strong phase-matching second-harmonic generation response of 2.9 × KDP. Furthermore, KPb3(3-C5H4NCOO)2Br5 has a high laser damage threshold of 149 MW/cm2, a wide transparent window of 0.39–5.5 µm, a suitable birefringence of 0.11 @ 1064 nm and good stability. Theoretical calculations have been performed to elucidate the origin of their optical properties.