Structure and Properties of YbZnSn, YbAgSn, and Yb₂Pt₂Pb

YbZnSn, YbAgSn, and Yb2Pt2Pb were synthesized by reacting the elements in sealed tantalum tubes in a high-frequency furnace. The structures of YbAgSn and Yb2Pt2Pb were refined from single crystal X-ray data: YbAgPb type, P (6) over bar m2, a = 479.2(2) pm, c = 1087.3(3) pm, wR2 = 0.050, BASF = 0.34(8), 509 F-2 values, 18 variables for YbAgSn and Er2Au2Sn type, a = 776.0(1) pm, c = 701.8(2) pm, wR2 = 0.072, 426 F-2 values, 18 variables for Yb2Pt2Pb. The lattice constants of YbZnSn are confirmed: NdPtSb type, P6(3)mc, a = 464.7(1) pm, c = 747.7(2) pm. The stannides YbZnSn and YbAgSn crystallize with superstructures of the AlB2 type. The zinc (silver) and tin atoms form ordered Zn3Sn3 and Ag3Sn3 hexagons, respectively. The stacking sequences for the differently oriented hexagons are AB, AB for YbZnSn and ABC, ABC for YbAgSn. While exclusively Zn-Sn intralayer interactions were observed in YbZnSn, intralayer Ag-Sn and significant Sn-Sn interlayer interactions at 318 pm occur in YbAgSn. The [ZnSn] and [AgSn] polyanions in YbZnSn and YbAgSn, respectively, have a pronounced two-dimensional character. This picture of chemical bonding in YbAgSn is confirmed by TB-LMTO-ASA band structure calculations. The partial densities-of-states and the valence charge densities are discussed. The magnetic (no long-range ordering, Yb2+) and the resistivity measurements (metallic behavior) are in excellent agreement with the electronic structure calculations. Yb2Pt2Pb crystallizes with the Er2Au2Sn structure, a ternary derivative of the Zr3Al2 type. This structure is composed of distorted AlB2 and CsCl related slabs of compositions YbPt2 and YbPb. Yb2Pt2Pb shows paramagnetic behavior (4.3+/-0.4 mu(beta)/Yb) indicating trivalent ytterbium. Magnetic susceptibility measurements on YbZnSn and YbAgSn show Pauli paramagnetism with room temperature susceptibilities of 2.5(1) x 10(-9) and 4.6(1) x 10(-9) m(3)/mol. Electrical resistivity measurements indicate metallic conductivity with specific resistivities of 440 +/- 40 mu Omega cm (YbZnSn) and 490 +/- 40 mu Omega cm (YbAgSn) at 300 K. Sn-119 Mossbauer spectra of YbZnSn show a single signal at room temperature with an isomer shift of delta = 1.85(1) mm/s. YbAgSn shows two superimposed signals at 78 K: a singlet at delta = 1.94(1) mm/s and a second signal at delta = 1.99(1) mm/s subjected to quadrupole splitting of Delta E-Q = 1.35(1) mm/s, in agreement with the two crystallographically different tin sites. (C) 1999 Acamedic Press