Synthesis and Crystal Structures of REAgMg (RE = La, Ce, Nd, Eu, Gd, Tb, Ho, Tm, and Yb)

摘要

The title compounds were synthesized by reacting the elements in sealed tantalum tubes in a high-frequency furnace. They were investigated by X-ray diffraction on both powders and single crystals. With the trivalentt rare earth elements they adopt the ZrNiAl-type structure, space group P62m:a = 782.5(3) pm, c = 432.8(1) pm, wR2 = 0.0420, and 277 F~2 values for CeAgMg and a = 775.7(3) pm, c = 426.1(1) pm, wR2 = 0.0387, and 271 F~2 values for NdAgMg with 15 parameters for each refinement. Geometrical motifs in CeAgMg are two types of each refinement. Geometrical motifs in CeAgMg are two types of silver-centered tricapped trigonal prisms: [Ag1Mg_3Ce_6Ce_6] and [Ag2Mg_6Ce_3]. The silver and magnesium atoms form a three-dimensional [AgMg] network in which the cerium atoms fill distorted hexagonal channels. With divalent europium and ytterbium the orthorhombic TiNiSi-type occurs, space group Pnma:a = 777.4(2) pm, b = 463.0(1) pm, c = 898.8(2) pm, wR2 = 0.0430, and 401 F~2 values for EuAgMg and a = 753.66(7) pm, b = 446.49(7) pm, c = 887.2(1) pm, wR2 = 0.0742, and 485 F~2 values for YbAgMg with 21 parameters for each refinement. The europium(ytter-bium) and magnesium atoms form zigzag chains of edge- and face-sharing trigonal prisms which are centered by the silver atoms. Also in EuAgMg and YbAgMg a three-dimensional [AgMg] network occurs in which the europium(ytterbium) atoms are embedded. The magnesium positions of the cerium, neodymium, europium, and ytterbium compound show a small mixed magnesium/silver occupancy, leading to the refined compositions CeAg_1.034(7)Mg_0.947(8) for the crystals investigated. The crystal chemistry and chemical bonding in these intermetallics is briefly discussed.