Alkali-Metal Thiogermanates: Sodium Channels and Variations on the La₃CuSiS₇ Structure Type

Five new isotypic quaternary chalcogenides containing rare-earth metal atoms crystallizing in the hexagonal noncentrosymmetric space group P6₃ (No. 173) with the La₃CuSiS₇ structure type have been synthesized by reacting the appropriate anhydrous rare-earth trichloride with sodium thiogermanate, Na₂GeS₃. The reaction between LnCl₃ and Na₂GeS₃ in an evacuated fused-silica ampule produced high yields of good-quality crystals of NaLn₃GeS₇ [Ln = Ce (I), Nd (II), Sm (III), Gd (IV), and Yb (V)], while a similar reaction between EuCl₃ and Na₂GeS₃ yielded a quinary chloride thiogermanate, Na_1.2Eu_3.4Cl₂Ge₃S₉ (VI), incorporating a cyclic trimeric Ge₃S₉ building unit and adopting a structure related to La₃CuSiS₇. The crystal structure of the compounds comprises a complex network of bicapped trigonal-prismatic LnS₈ and GeS₄ tetrahedra, which creates channels along the [001] direction. The Na⁺ cations reside in these channels within trigonally distorted octahedral coordination environments, surrounded by six S atoms. For compounds III–V, the temperature dependence of the magnetic susceptibility indicates that these compounds are paramagnetic with μ_eff. = 1.86, 8.01, and 3.87 μ_B, for III–V, respectively. The experimental μ_eff for IV is close to the theoretical value of 7.94 for free Gd³⁺ ions, while μ_eff values for III and V deviate from their theoretical values of 0.86 and 4.54 μ_B for Sm³⁺ and Yb³⁺ ions, respectively. These compounds are semiconductors with optical band gaps of around 1.3 eV for III and V. Extended Hückel calculations suggest that the valence band comprises primarily S 3p and the bottom of the conduction band is dominated by empty rare-earth 5d orbitals. Compound VI exhibits a sharp optical absorption of around 2.18 eV, which is attributed to the f → d transition of Eu^II. The effective magnetic moment of 7.94 μ_B/Eu is in excellent agreement with the theoretical value of 7.94 μ_B for the free Eu²⁺ ion.