New quaternary selenogermanates of Rb2Ag2GeSe4 and Cs2Cu2GeSe4

Synthesis and Characterization of Low-Dimensional Quaternary Sulfides of Thallium, Tl2Cu2GeS4, Tl2Ag2GeS4, and Tl2Ag2SnS4

Three new low-dimensional quaternary sulfides, namely, Tl2Cu2GeS4(1), Tl2Ag2GeS4(2), and Tl2Ag2SnS4(3) were synthesized by solid-state reactions and characterized by single-crystal X-ray diffraction, spectroscopic, thermal, and photocatalytic studies. The compounds 1, 2, and 3, respectively, have centrosymmetric layered, noncentrosymmetric layered, and noncentrosymmetric one-dimensional structures, in which Cu+, Ge4+, and Sn4+ ions have tetrahedral coordination and Ag+ ion has linear, trigonal planar and tetrahedral coordinations. The monovalent thallium cations have TlS8 dodecahedral coordination and TlS7 and TlS6 coordination polyhedra of irregular shape. The compounds 1, 2, and 3 are semiconductors with the respective energy band gap values of 1.68, 2.02, and 1.90 eV. Compared to compounds 1 and 2, compound 3 has a higher efficiency value of 22.9% for the photodegradation of aqueous solution of methylene blue. Band structures and projected density of states of all three compounds were computed using density functional theory, and these results are in good agreement with experiments.

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.