Crystal Structures and Nonlinear Optical Properties of Semiconducting Main Group Chalcogenides

Abstract

I have investigated ten compounds where the experimental crystal structures have not been reported. Of these compounds, five are noncentrosymmetric, two are oxychalcogenides, and four are new structure types. The noncentrosymmetric compounds discovered are part of two distinct families: A2BQ4 and A6M4T4Q16 (A = Sr–Ba; M = Cu–Ag; B = Si–Sn; Q = O–Te). The A2BQ4 family contains Sr2SiSe4 (P21, a = 6.8866(19) Å, b = 7.0062(19) Å, c = 8.520(2) Å, β = 108.200(6)°) and Sr2SnSe4 (Ama2, a = 10.4802(10) Å, b = 10.6965(10) Å, c = 7.5411(7) Å). DFT calculations revealed Sr2SiSe4 and Sr2SnSe4 to have direct band gaps of 2.31 eV and 2.05 eV, respectively. Sr2SiSe4 was determined to have a second harmonic generation intensity of 0.5 × AGS, a laser-induced damage threshold of 3.1 × AGS, and a thermal conductivity of 0.46 W m-1 K-1 at 523 K. The A6M4T4Q16 family contains Sr6Ag3.91Si4Se16 (I4¯3d, a = 14.621(2) Å), Sr6Ag3.63Sn4Se16 (I4¯3d, a = 14.9295(5) Å), and Sr6Cu2.33Ag1.55Si4Se16 (I4¯3d, a = 14.4904(5) Å). DFT calculations revealed Sr6Ag3.91Si4Se16, Sr6Ag3.63Sn4Se16, and Sr6Cu2.33Ag1.55Si4Se16 to have direct band gaps of 1.14 eV, 1.09 eV, and 1.28 eV, respectively. Sr6Ag3.63Sn4Se16 was determined to have a second harmonic generation intensity of 0.5 × AGS, and a laser-induced damage threshold of 3.3 × AGS. The centrosymmetric compounds discovered are part of three distinct families: ABQ3, A3B2Q7, and oxychalcogenides. The ABQ3 family contains SrSiSe3 (P21/c, a = 6.6948(2) Å, b = 17.4606(4) Å, c = 15.8647(3) Å, β = 117.0100(10)°) and BaGeSe3 (P21/c, a = 15.6603(7) Å, b = 6.0379(3) Å, c = 14.0072(6) Å, β = 116.3830(10)°). DFT calculations revealed SrSiSe3 and BaGeSe3 to have band gaps of 2.15 eV and 1.65 eV, respectively. The A3B2Q7 family contains Sr3Sn2S7 (C2/m (no. 12), a = 23.207(2) Å, b = 7.9768(8) Å, c = 17.216(3) Å, β = 132.1738(17)°), which possesses rare SnS5 polyhedra in addition to SnS4 tetrahedra. DFT calculations revealed Sr3Sn2S7 to have a direct band gap of 1.10 eV. The oxychalcogenide family contains Sr3Ge3OS8 (Pnma, a = 15.6628(6) Å, b = 13.9919(5) Å, c = 6.6687(3) Å) and Ba4Ge2O3Se5 (Pnma, a = 11.536(14) Å, b = 10.539(11) Å, c = 11.836(13) Å). DFT calculations revealed Sr3Ge3OS8 and Ba4Ge2O3Se5 to have band gaps of 2.55 eV and 2.25 eV, respectively.