Noncentrosymmetric chalcogenides containing stereochemically active lone pair elements, such as As, Sb, and Bi, offer a useful way to tune the band gap and enhance the second harmonic generation (SHG) response. Ba2As2–x Sb x Q5, Ba2As2–x Bi x Q5 (Q = S and Se), and Ba2As2S4.8Se0.2 are new chalcoarsenates that crystallize in the noncentrosymmetric monoclinic space group P21. These five new compounds crystallized with a three-dimensional (3D) Ba2As2Se5-type structure, containing AsQ3 units and [As2Q4]2– dimeric units. The [As2Q4]2– dimeric unit can be tuned by replacing As with Sb or Bi and Se with S. In contrast, Ba2As2S5 crystallizes in the orthorhombic space group Pca21. Differential thermal analysis suggested that Ba2As2S5, Ba2As2Se5, Ba2AsSbSe5, and Ba2As1.25Bi0.75Se5 melt congruently. The [M2Q4]2– unit plays a significant role in tuning the band gap of these compounds, decreasing from 2.02 eV in Ba2As2S5 to 1.37 eV in Ba2As1.25Bi0.75Se5. Powder SHG measurements showed all the compounds are essentially phase-matchable at 3300 nm. Ba2AsSbSe5 exhibits the highest χ(2) of 36 pm/V and a laser-induced damage threshold of 0.10 GW/cm2, comparable to that of AgGaQ2. These materials show significantly improved SHG behavior response to BaGa4Q7 compounds, making them attractive for commercial applications.

Cu2TSiS4 (T = Mn and Fe) polycrystalline and single-crystal materials were prepared with high-temperature solid-state and chemical vapor transport methods, respectively. The polar crystal structure (space group Pmn21) consists of chains of corner-sharing and distorted CuS4, Mn/FeS4, and SiS4 tetrahedra, which is confirmed by Rietveld refinement using neutron powder diffraction data, X-ray single-crystal refinement, electron diffraction, energy-dispersive X-ray spectroscopy, and second harmonic generation (SHG) techniques. Magnetic measurements indicate that both compounds order antiferromagnetically at 8 and 14 K, respectively, which is supported by the temperature-dependent (100–2 K) neutron powder diffraction data. Additional magnetic reflections observed at 2 K can be modeled by magnetic propagation vectors k = (1/2,0,1/2) and k = (1/2,1/2,1/2) for Cu2MnSiS4 and Cu2FeSiS4, respectively. The refined antiferromagnetic structure reveals that the Mn/Fe spins are canted away from the ac plane by about 14°, with the total magnetic moments of Mn and Fe being 4.1(1) and 2.9(1) μB, respectively. Both compounds exhibit an SHG response with relatively modest second-order nonlinear susceptibilities. Density functional theory calculations are used to describe the electronic band structures.