A Polyselenide with a Novel Se 7 8– Unit: the Structure of Sr 19– x Pb x Ge 11 Se 44 with x = 5.0 and 6.4

Mixed Anion Semiconductor In8S2.82Te6.18(Te2)3

The new heteroanionic compound In₈S_2.82Te_6.18(Te₂)₃ crystallizes in the monoclinic space group C2/c with lattice parameters a = 14.2940(6) Å, b = 14.3092(4) Å, c = 14.1552(6) Å, and β = 90.845(3)°. The three-dimensional (3D) framework of In₈S_2.82Te_6.18(Te₂)₃ is composed of a complex 3D network of corner-connected InQ₄ tetrahedra with three Te₂^2- dumbbell dimers per formula unit. The optical bandgap is 1.12(2) eV and the work function is 5.15(5) eV. First-principles electronic structure calculations using density functional theory (DFT) indicate that this material has potential as a p-type thermoelectric material as it is a narrow bandgap semiconductor, incorporates several heavy elements, and has multiple overlapping bands near the valence band maximum.

Effect of Pb Substitution in Sr2-xPbxGeSe4 on Crystal Structures and Nonlinear Optical Properties Predicted by DFT Calculations

We investigated the Sr_2-xPb_xGeSe₄ series from 0 ≤ x ≤ 2 to study the impact of Pb on structure and properties. While the noncentrosymmetric (NCS) compounds γ-Sr₂GeSe₄ and α-Pb₂GeSe₄ have already been reported previously, the substitution variants Sr_1.31Pb_0.69GeSe₄ (space group Ama2, a = 10.31220(1) Å, b = 10.39320(1) Å, c = 7.42140(1) Å) and Sr_0.19Pb_1.81GeSe₄ (I4̅3d, a = 14.6177(3) Å) are introduced here for the first time. The experimentally determined optical band gaps decrease as predicted with increasing Pb content from γ-Sr₂GeSe₄ to Sr_1.31Pb_0.69GeSe₄, Sr_0.25Pb_1.75GeSe₄, and α-Pb₂GeSe₄ from 2.00, to 1.65, 1.45 and 1.42 eV, respectively. The nonlinear optical (NLO) properties of the orthorhombic compounds γ-Sr₂GeSe₄ and Sr_1.3Pb_0.7GeSe₄ (approximated with the supercell "Sr₃PbGe₂Se₈") were studied both theoretically, using first-principle calculations, and experimentally. The calculations found the effective nonlinear susceptibility, d_eff, of γ-Sr₂GeSe₄ and "Sr₃PbGe₂Se₈" at the static limit to be 10.8 and 8.8 pm V^-1, respectively. The experimental d_eff values of γ-Sr₂GeSe₄, Sr_1.31Pb_0.69GeSe₄, Sr_0.25Pb_1.75GeSe₄, and α-Pb₂GeSe₄ were 2.6, 2.3, 0.68, and 0.79 pm V^-1, respectively.

BaCuSiTe3: A Noncentrosymmetric Semiconductor with CuTe4 Tetrahedra and Ethane-like Si2Te6 Units

BaCuSiTe₃ was prepared from the elements in a solid-state reaction at 973 K, followed by slow cooling to room temperature. This telluride adopts a new, hitherto unknown structure type, crystallizing in the noncentrosymmetric space group Pc with a = 7.5824(1) Å, b = 8.8440(1) Å, c = 13.1289(2) Å, β = 122.022(1)°, and V = 746.45(2) ų (Z = 4). The structure consists of a complex network of two-dimensionally connected CuTe₄ tetrahedra and ethane-like Si₂Te₆ units with a Si-Si bond. This semiconducting material has an optical band gap of 1.65 eV and a low thermal conductivity of 0.50 W m^-1 K^-1 at 300 K. Calculations of its optical properties revealed a moderate birefringence of 0.23 and a second-order harmonic generation response of d_eff = 3.4 pm V^-1 in the static limit.