Syntheses and Crystal Structures of New Ternary Selenides:  Rb₃Yb₇Se₁₂ and CsEr₃Se₅

The new compounds Rb₃Yb₇Se₁₂ and CsEr₃Se₅ have been synthesized through the reaction of elemental rare-earth metal and Se with halide flux at 680 °C. The structures of the compounds were determined by the single-crystal X-ray diffraction method. The structures of CsEr₃Se₅ and Rb₃Yb₇Se₁₂ are considerably similar to each other. Rb₃Yb₇Se₁₂ crystallizes in space group Pnnm of the orthorhombic system with cell dimensions a = 12.385(4) Å, b = 25.688(7) Å, c = 4.063(1) Å, and Z = 2. CsEr₃Se₅ crystallizes in space group Pnma of the orthorhombic system with cell dimensions a = 22.059(9) Å, b = 4.095(2) Å, c = 12.155(5) Å, and Z = 4. The structures of these compounds have infinite one-dimensional channels. The channels consist of edge-sharing RESe₆ (RE = rare-earth metal) octahedral layers and alkali-metal ions reside in the channels. The RESe₆ octahedra in both compounds are distorted with bond lengths varing from 2.752(3) Å to 2.939(2) Å. The result of magnetic susceptibility and resistivity measurement of Rb₃Yb₇Se₁₂ showed that it is paramegnetic and is an insulator. Structural comparison between Rb₃Yb₇Se₁₂ and CsEr₃Se₅ is discussed. A series of additional isostructural compounds Cs₃Y₇Se₁₂ and Rb₃Er₇Se₁₂ have been prepared, and structural diagrams of ternary selenides with respect to cationic radius ratios were drawn.