Four new quaternary chalcogenides A2Ba7Sn4Q16 (A = Li, Na; Q = S, Se): syntheses, crystal structures determination, nonlinear optical performances investigation

Abstract

Four new quaternary chalcogenides A₂Ba₇Sn₄Q₁₆ (A = Li, Na; Q = S, Se) were successfully synthesized in vacuum-sealed silica tubes for the first time. All of them crystallize in the noncentrosymmetric (NCS) cubic I3d space group with the cell parameters ranging from 14.807 to 15.4163 Å and Z = 4, respectively. Interestingly, their isostructures are different from the structural transformations (from tetragonal to trigonal) appearing in previously reported Na₂BaSnQ₄ compounds. As observed in their structures, (Na/Ba)Q₄ and SnQ₄ units alternately connect with each other by sharing corners to build a three-dimensional (3D) framework structure with charge-balanced Ba cations located within tunnels, which is different from the observation that the Na atoms located inside the tunnels formed by the interconnection of the BaS₈ and SnS₄ units in Na₂BaSnS₄. Optical properties (diffuse-reflection, IR, and Raman spectra) were also systematically studied and the results indicate that their optical bandgaps are in the range from 1.75 to 2.5 eV. Moreover, the second harmonic generation (SHG) responses of the title compounds were also measured to be about 0.1–0.5 times higher than that of benchmark AgGaS₂ under 2.09 μm radiation.