Syntheses, structure, magnetism, and optical properties of the partially ordered quaternary interlanthanide sulfides PrLnYb2S6 (Ln=Tb, Dy)

Dark red single crystals of PrLnYb₂S₆ (Ln=Pr/Yb, Tb, Dy) have been synthesized through the reactions of elemental rare earth metals and S using a Sb₂S₃ flux at 1000 °C. These isotypic compounds adopt the F-Ln₂S₃ three-dimensional open-channel structure type. Eight-coordinate Pr³⁺ ions sit in the channels that are constructed from three different edge-shared double chains running down the b-axis that contain Yb(1)S₆ octahedra, Yb(2)S₆ octahedra, and LnS₇ monocapped trigonal prisms. Each double chain connects to four other neighbors by sharing vertices and edges. Considerable disordering in Ln positions was observed in single X-ray diffraction experiments only in the case of Pr/Yb. Least-squares refinements gave rise to the formulas of Pr_1.34Yb_2.66S₆, of PrTbYb₂S₆, and PrDyYb₂S₆, which are confirmed by the elemental analysis and magnetic susceptibility measurements. Pr_1.34Yb_2.66S₆, PrTbYb₂S₆, and PrDyYb₂S₆ are paramagnetic down to 2 K, without any indications of long-range magnetic ordering. The optical transitions for Pr_1.34Yb_2.66S₆, PrTbYb₂S₆, and PrDyYb₂S₆ are at approximately 1.6 eV. Crystallographic data are listed as an example for PrTbYb₂S₆: monoclinic, space group P2₁/m, a=10.9496(10) Å, b=3.9429(4) Å, c=11.2206(10) Å, β=108.525(2)°, V=459.33(7) Å³, Z=2.