BaCu2MIVQ4 (MIV = Si, Ge, and Sn; Q = S, Se): synthesis, crystal structures, optical performances and theoretical calculations

Abstract

Five non-centrosymmetric (NCS) quaternary metal chalcogenides, BaCu₂SiSe₄, BaCu₂GeS₄, BaCu₂GeSe₄, BaCu₂SnS₄, and BaCu₂SnSe₄, were successfully synthesized by solid-state reaction in vacuum-sealed silica tubes. They crystallized in the three different space groups: Ama2 for BaCu₂SnSe₄, P3₁21 for BaCu₂GeS₄ and BaCu₂GeSe₄, and P3₂21 for BaCu₂SiSe₄ and BaCu₂SnS₄. Note that BaCu₂GeS₄ and BaCu₂GeSe₄ show the mirror symmetrical structures to those of BaCu₂SiSe₄ and BaCu₂SnS₄. In comparison with their structures, it can be found that the [CuSe₄] units are connected together to form a two-dimensional (2D) layer structure in BaCu₂SnSe₄, which is different from the 3D framework structure formed by the interlinked [CuQ₄] (Q = S and Se) units in other four title compounds. In addition, BaCu₂SnSe₄ exhibits only one type of tunnel structure with the isolated [BaSe₈] units existing in each tunnel, which is also different from the other title compounds (two types of tunnels with the isolated [BaSe₈] units and [BaSe₆]_n chains located). The interesting structural changes also indicate that slight change of cation size would result in different structure features, and future structure prediction should devote considerable attention to the different chalcogen atoms. Moreover, important optical properties (optical bandgap, infrared (IR) absorption edge, second harmonic generation (SHG) response) of the title compounds were systematically investigated. Among them, IR and Raman spectra indicate that all of them exhibit the wide IR absorption edges (∼22 μm). Powder SHG measurement shows that BaCu₂SnS₄ possesses good SHG response about 1.6 times that of benchmark AgGaS₂ (AGS) at the particle size 55–88 μm. All results indicate that BaCu₂SnS₄ can be expected as a potential IR nonlinear optical (NLO) candidate. Theoretical calculation was also used to analyze the structure–property relationship and their electronic structures and origin of NLO effect were studied in detail.