Visualizing the alignment of lone pair electrons in La3AsS5Br2 and La5As2S9Cl3 to form an acentric or centrosymmetric structure

Abstract

Acentric structures host numerous important applications such as second harmonic generation, nonreciprocal responses, etc. In this work, a heteroanionic system of La₃AsS₅Br₂ and La₅As₂S₉Cl₃ exhibits a good example of how the alignment of lone pair electrons affects crystal structure. Noncentrosymmetric (NCS) chalcohalide La₃AsS₅Br₂, isostructural to Pr₃AsS₅Cl₂, and centrosymmetric chalcohalide La₅As₂S₉Cl₃ were successfully synthesized by a salt flux growth method. Crystal structures were determined by single crystal X-ray diffraction. Both compounds contain trigonal pyramidal [AsS₃] units with stereochemically active lone pairs in As³⁺, aligning in the same direction in La₃AsS₅Br₂ and in opposite directions in La₅As₂S₉Cl₃, which account for their acentric crystal structure and centrosymmetric structure, respectively. Electron localization function (ELF) calculations confirmed that the alignment of the [AsS₃] motifs contributes to the acentric nature of La₃AsS₅Br₂. La₃AsS₅Br₂ is predicated to be an indirect bandgap semiconductor by theory calculations with a bandgap of 2.27 eV, which is verified by UV-Vis spectroscopy measurements to be 2.8(1) eV. The acentric structural nature of La₃AsS₅Br₂ was demonstrated by a moderate second harmonic generation (SHG) response of 0.23× AgGaS₂, where La₅As₂S₉Cl₃ exhibited no response under the same conditions.