FRAMEWORK COMPOSED OF INTERCONNECTED QUADRUPLE-OCTAHEDRA INFINITE CHAINS - SYNTHESIS AND STRUCTURE OF CALCIUM YTTERBIUM SULFIDE, CAYB2S4, WITH THE YB3S4-TYPE STRUCTURE

Single crystals of the ternary calcium ytterbium compound, CaYb2S4, have been synthesized using an eutectic, halide flux and have been structurally characterized by the X-ray single-crystal diffraction method. The compound crystallizes in an orthorhombic unit cell, Pnma (No. 62), with cell dimensions of a = 12.807 (3) Å, b = 3.836 (2) Å, c = 12.964 (3) Å, V = 636.9 (7) Å3, and Z = 4. The least-squares refinement of 658 observed reflections (with F20 > 3σ (F20)) gives a final structure solution of R = 0.026, Rw = 0.030, and GOF = 1.06 for 45 variables. This compound has adopted a Yb3S4-type structure in which the 7-coordinated divalent Yb2+ sites are occupied by Ca2+ cations. The refined occupancy factor on the calcium site suggests some nonstoichiometry, which is attributed to the solid solution of a ytterbium-rich phase. The crystal formula found is Ca1-xYb2+xS4 (x = 0.04). The CaYb2S4 structure is a three-dimensional lattice which consists of interconnected quadruple-octahedra infinite chains. Each quadruple chain is built up from four edge-sharing octahedra that are composed of two pairs of asymmetric Yb(1)S6 and Yb(2)S6 groups. Structure comparison with another ternary rare-earth sulfide bearing a larger alkaline earth metal cation, BaSm2S4 (CaFe2O4-type), gives rise to some important insight for structure formation of rare-earth chalcogenide compounds. © 1992.