New Metal Chalcogenides Ba₄CuGa₅Q₁₂ (Q = S, Se) Displaying Strong Infrared Nonlinear Optical Response

A series of new metal chalcogenides Ba₄CuGa₅Q₁₂ (Q = S, S_0.75Se_0.25, Se) were synthesized using KBr flux at 750 °C. The three compounds are isostructural and adopt the noncentrosymmetric space group P4̅2₁c. Crystal data are as follows: Ba₄CuGa₅S₁₂, 1, a = 13.040(1) Å, c = 6.304(1) Å, and Z = 2; Ba₄CuGa₅S_9.00(1)Se_2.92(1), 2, a = 13.1585(2) Å, c = 6.3520(2) Å, and Z = 2; Ba₄CuGa₅Se₁₂, 3, a = 13.598(1) Å, c = 6.527(1) Å, and Z = 2. The three-dimensional framework in 1 is constructed by infinite columns _∞¹[CuGa₄S₁₀]^7– that surround the discrete GaS₄ tetrahedra situated on a 4̅ axis. The discrete GaS₄ tetrahedra on the stacking (112) planes and canted oriented edge-sharing CuS₄ tetrahedra within the columns may account for the occurrence of strong second-harmonic generation (SHG) responses. Compounds 1–3 are transparent in the mid-infrared range and have the absorption edges at 2.82, 2.05, and 1.45 eV, respectively. The new nonlinear optical (NLO) materials are type-I nonphase matching at 693 nm and display strong SHG intensities that are ∼2.7, ∼2.6, and ∼1.1 times that of AgGaSe₂ at 808 nm. Raman spectroscopic characterization of the compounds is reported.