Visualizing the alignment of lone pair electrons in La3AsS5Br2 and La5As2S9Cl3 to form an acentric or centrosymmetric structure
Literature Information
Andrea Cicirello, Andrew Swindle, Jian Wang
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.
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Source Journal
CrystEngComm

CrystEngComm is the forum for the design and understanding of crystalline materials. We welcome studies on the investigation of molecular behaviour within crystals, control of nucleation and crystal growth, engineering of crystal structures, and construction of crystalline materials with tuneable properties and functions. We publish hypothesis-driven research into… how crystal design affects thermodynamics, phase transitional behaviours, polymorphism, morphology control, solid state reactivity (crystal-crystal solution-crystal, and gas-crystal reactions), optoelectronics, ferroelectric materials, non-linear optics, molecular and bulk magnetism, conductivity and quantum computing, catalysis, absorption and desorption, and mechanical properties. Using Techniques and methods including… Single crystal and powder X-ray, electron, and neutron diffraction, solid-state spectroscopy, spectrometry, and microscopy, modelling and data mining, and empirical, semi-empirical and ab-initio theoretical evaluations. On crystalline and solid-state materials. We particularly welcome work on MOFs, coordination polymers, nanocrystals, host-guest and multi-component molecular materials. We also accept work on peptides and liquid crystals. All papers should involve the use or development of a design or optimisation strategy. Routine structural reports or crystal morphology descriptions, even when combined with an analysis of properties or potential applications, are generally considered to be outside the scope of the journal and are unlikely to be accepted.