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

Three new low-dimensional quaternary sulfides, namely, Tl₂Cu₂GeS₄(<b>1</b>), Tl₂Ag₂GeS₄(<b>2</b>), and Tl₂Ag₂SnS₄(<b>3</b>) were synthesized by solid-state reactions and characterized by single-crystal X-ray diffraction, spectroscopic, thermal, and photocatalytic studies. The compounds <b>1</b>, <b>2</b>, and <b>3</b>, 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 <b>1</b>, <b>2</b>, and <b>3</b> are semiconductors with the respective energy band gap values of 1.68, 2.02, and 1.90 eV. Compared to compounds <b>1</b> and <b>2</b>, compound <b>3</b> 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.