The first two new Na-containing sulfides Na2In2MS6 (M = Si (1), Ge (2)) in the Na2Q–B2Q3–CQ2 (B = Ga, In; C = Si, Ge, Sn; Q = S, Se) system were prepared for the first time through conventional high-temperature solid-state reaction. They are isostructural with space group Cc (No. 9) in monoclinic phases and feature three-dimensional frameworks built by the ∞1[In2MS6]2– (M = Si, Ge) chains through corner-sharing InS4 tetrahedra and MS4 (M = Si, Ge) tetrahedra, with Na+ cation located in the cavities. They display moderate second harmonic generation (SHG) conversion efficiencies compared with commercial AgGaS2, with phase-matching behavior at 1800 nm and laser-induced damage thresholds 6.9 and 4.0 times higher than that of AgGaS2, respectively. Therefore, the output SHG intensities of 1 and 2 will be ∼4.3 and 4.0 times larger than that of AgGaS2, when the intensity of incident laser increased to close the damage energy of 1 and 2, indicating their potential for high-power nonlinear optical application.

High-quality single crystals of RbInS2, CsInS2 were isolated from the halide flux as a major phase of the repeated fusion of CaS and In2S3 followed by slow cooling. RbInS2 and CsInS2 are practically isostructural with KInS2. RbInS2: C2/c, a = 11.071(6) Å, b = 11.068(1) Å, c = 15.610(7) Å, β = 100.36(3)°, V = 1882(2) Å3, Z = 16, dx = 3.74 g/cm3, R = 6.38%, Rw = 6.42%. CsInS2: C2/c, a = 11.197(3) Å, b = 11.158(3) Å, c = 16.358(4) Å, β = 99.92(2)°, V = 2013(2) Å3, Z = 16, dx = 4.12 g/cm3, R = 5.60%, Rw = 6.20%. The AInS2 (A = Rb, Cs) structure is characterized by double layers of vertex-sharing [In4S10] units that each consists of four [InS4] polyhedra. The charge-balancing alkali-metal cations are stuffed into the channels created by the packing of these anionic [In4S10] blocks. The optical reflectance measurements show a band gap of 3.3 eV for RbInS2 and 3.4 eV for CsInS2, suggesting that both are semiconductors.