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

The title compounds were synthesized by reacting the elements in sealed tan talum tubes in a high-frequency furnace. They were investigated by X-ray di ffraction on both powders and single crystals. With the trivalent rare eart h elements they adopt the ZrNiAl-type structure, space group P (6) over bar 2m: 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 val ues for NdAgMg with 15 parameters for each refinement. Geometrical motifs i n CeAgMg are two types of silver-centered tricapped trigonal prisms: [Ag1Mg (3)Ce(6)] and [Ag2Mg(6)Ce(3)]. The silver and magnesium atoms form a three- dimensional [AgMg] network in which the cerium atoms fill distorted hexagon al channels. With divalent europium and ytterbium the orthorhombic TiNiSi-t ype 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(ytterbium) and magnes ium atoms form zigzag chains of edge- and face-sharing trigonal prisms whic h are centered by the silver atoms. Also in EuAgMg and YbAgMg a three-dimen sional [AgMg] network occurs in which the europium(ytterbium) atoms are emb edded. The magnesium positions of the cerium, neodymium, europium, and ytte rbium compound show a small mixed magnesium/silver occupancy, leading to th e refined compositions CeAg1.034(7) Mg-0.966(7), NdAg1.035(6)Mg0.965(6), Eu Ag1.032(6)Mg0.968(6), and YbAg1.053(8)Mg0.947(8) for the crystals investiga ted. The crystal chemistry and chemical bonding in these intermetallics is briefly discussed. (C) 2001 Academic Press.