Flux Crystal Growth, Structure, and Optical Properties of Non-Centrosymmetric Oxysulfides Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd)

Single crystals of the non-centrosymmetric oxysulfides Ln₃Ga₃Ge₂S₃O₁₀ (Ln = La, Ce, Pr, Nd) in a hexagonal space group <i>P</i>6̄2<i>c</i> were grown by a flux crystal growth method using a eutectic BaCl₂-NaCl molten salt. Single-crystal X-ray diffraction analysis of them revealed that the Ga and Ge atoms were located on the 6<i>g</i> and 4<i>f</i> sites, which were tetrahedrally coordinated with two O and two S atoms, and four O atoms, respectively. In the structure, the GaS₂O₂ and GeO₄ tetrahedra form 1∞[Ga₃S₃O₃] triangular tubes and Ge₂O₇ tetrahedral dimers aligned along the <i>c</i> axis, which are surrounded by LaS₂O₆ square prisms in the <i>ab</i> plane. Neutron powder diffraction studies on polycrystalline samples of La₃Ga₃Ge₂S₃O₁₀ and Nd₃Ga₃Ge₂S₃O₁₀ which were prepared by high-temperature solid state reactions supported the Ga/Ge cation order determined by the single-crystal structure analysis. UV-Vis-NIR absorption spectra revealed band gaps larger than 4.60 eV for La₃Ga₃Ge₂S₃O₁₀, Pr₃Ga₃Ge₂S₃O₁₀, and Nd₃Ga₃Ge₂S₃O₁₀, while Ce₃Ga₃Ge₂S₃O₁₀ was found to have a small band gap value of 3.51 eV because of the Ce-4f¹ electronic configuration.