Carsten Rosenhahn

The stannides YbPtSn and Yb2Pt3Sn5

YbPtSn and Yb 2 Pt 3 Sn 5 were prepared from the elements in scaled tantalum tubes in a high-frequency furnace in a novel water-cooled sample chamber. Both structures were refined from single-crystal X-ray data: YbPtSn (ZrNiAl type structure), space group P62m, a = 737.8(3) pm, c = 393.1(2) pm, wR2 = 0.0363, 412 F 2 values, 14 variables; Yb 2 Pt 3 Sn 5 (new structure type), space group Pnma, a = 729.5(2) pm, b 442.2(1) pm, c = 2625.2(6) pm, wR2 = 0.0373. 1378 F 2 values, 62 variables. The structure of YbPtSn contains two crystallographically different platinum positions. Both of them have tricapped trigonal prismatic coordination: [Pt 1 Sn 6 Yb 3 ] and [Pt 2 Sn 3 Yb 6 ]. Yb 2 Pt 3 Sn 5 shows a close relation to the Y 2 Rh 3 Sn 5 structure (space group Cmc2 1 ), however, with a different ordering of the platinum and tin atoms. Both ytterbium positions in Yb 2 Pt 3 Sn 5 have the high co-ordination numbers (CN) of 20 for Yb1 and 18 for Yb2. Magnetic susceptibility measurements of Yb 2 Pt 3 Sn 5 show Curie-Weiss behavior between 50 K and room temperature with an experimental magnetic moment of 2.6(1) μ B /Yb, indicating mixed valency for the ytterbium atoms. Yb 2 Pt 3 Sn 5 is a metallic conductor. 119 Sn Mossbauer spectroscopic data show one signal at δ = 1.75(1 ) mm/s for YbPtSn and δ= 2.02( 1) mm/s for Yb 2 Pt 3 Sn 5 . Both spectra are subjected to quadrupole splitting of ΔE q = 0.59(5) mm/s (YbPtSn) and ΔE q = 0.80(2) mm/s (Yb 2 Pt 3 Sn 5 ).

Structure and properties of the compounds LnAl2X2 (Ln=Eu, Yb; X=Si, Ge)

Abstract EuAl2Si2, EuAl2Ge2 and YbAl2Ge2 were synthesized by heating the elements at 1070–1270 K and characterized by single-crystal X-ray methods. They are isotypic and crystallize in the CaAl2Si2-type structure (space group P3m1) with the lattice constants (A): YbAl2Ge2: a=4.179(2), c=7.069(3). EuAl2Ge2: a=4.214(1), c=7.320(1). EuAl2Si2: a=4.181(1), c=7.259(1). Magnetic susceptibility measurements of EuAl2Si2 and EuAl2Ge2 show paramagnetic behavior above 50 K with experimental magnetic moments of 7.82(5) μB/Eu and 7.90(5) μB/Eu indicating divalent europium. Antiferromagnetic ordering is detected at TN=35.5(5) K for EuAl2Si2 and at TN=27.5(5) K for EuAl2Ge2. Both compounds undergo metamagnetic transitions at low temperatures. Previously described YbAl2Si2 shows the typical behavior of an intermediate-valent compound. Between 100 and 300 K the inverse susceptibility linearly depends on temperature with a reduced moment of 2.57(5) μB/Yb and a strongly negative paramagnetic Curie temperature of −382(5) K. Below 100 K the degree of divalent ytterbium increases. YbAl2Ge2 is a Pauli paramagnet with a room temperature susceptibility of 1.2(1)×10−9 m3 mol−1. All compounds are metallic conductors between 8 and 320 K. 151Eu Mossbauer spectroscopic measurements of EuAl2Si2 and EuAl2Ge2 show isomer shifts of −10.3(1) and −10.8(2) mm s−1, respectively, at 4.2 K in accordance with divalent europium. Full magnetic hyperfine field splitting is detected at 4.2 K. LMTO band structure calculations confirm the metallic properties for all compounds and result a fully polarized 4f7 state for EuAl2Ge2 and EuAl2Si2. For the Yb-compounds nonmagnetic 4f14 ground states were predicted, but the high 4f-contribution at the fermi level indicates the tendency to intermediate valency in YbAl2Si2.

Structure and Properties of the Stannide Eu2Au2Sn5, and its Relationship with the Family of BaAl4-Related Structures

The stannide Eu2Au2Sn5 was prepared by high-frequency melting of the elements in a sealed tantalum tube. The structure of Eu2Au2Sn5 was refined from single crystal X-ray data: P21/m, a = 928.6(2), b = 465.8(2), c = 1042.9(3) pm, ß = 92.28(2)°, wR2 = 0.0653, 1220 F2 values and 56 variables. The structure of Eu2Au2Sn5 is of a new type, it can be considered as an ordered defect variant of the BaAl4 type. Due to the ordered defects, the coordination number (CN) of the two crystallographically different europium sites is reduced from CN 16 to CN 14. The gold and tin atoms in Eu2Au2Sn5 form a complex three-dimensional [Au2Sn5] polyanion in which the europium atoms are embedded. Within the polyanion short Au-Sn and Sn-Sn distances are indicative of strongly bonding Au-Sn and Sn-Sn interactions. A detailed group-subgroup scheme for various ordered and defect variants of the BaAl4 family is presented. Eu2Au2Sn5 shows Curie-Weiss behavior above 50 K with an experimental magnetic moment of 7.90(5) μB/Eu, indicating divalent europium. Antiferromagnetic ordering is detected at 5.8(5) K at low fields and a metamagnetic transition occurs at a critical field of 1.4(2) T. Eu2Au2Sn5 is a metal with a specific resistivity of 150±20 μfΩcm at room temperature. The results of 151Eu and 119Sn Mössbauer spectroscopic experiments are compatible with divalent europium and show complex hyperfine field splitting with a transferred magnetic hyperfine field at the tin nuclei at low temperature.

Crystal Structures and Properties of Eu2GeSe4 and Eu2Ge2Se5

Europium selenogermanate(IV) and Europium selenogermanate(III) were synthesized by direct reactions of the elements at 1023 K and their crystal structures determined by single crystal methods. α-Eu2GeSe4 (P21, a = 6.964(1), b = 7.055(2), c = 8.400(2) Å , β = 108.12(2)˚, Z = 2) crystallizes as a polar variant of the monoclinic Sr2GeS4-type at room temperature. At 673 K, the structure is centrosymmetric (β-Eu2GeSe4, P21/m, a = 6.969(1), b = 7.059(2), c = 8.516(2)Å , β = 107.99(2)˚, Z = 2), and a phase transition P21→P21/m as known from Eu2GeS4 is highly probable. Eu2Ge2Se5 (P21/n, a = 8.421(4), b = 12.235(4), c = 9.127(3) Å,β = 93.67(4)˚, Z = 4) crystallizes in the Sr2Ge2Se5 type, mainly characterized by complex [Ge4Se10]8- anions with homonuclear Ge-Ge bonds. Both compounds are deep red Zintl phases according to (Eu2+)2Ge4+(Se2-)4 and (Eu2+)2(Ge3+)2(Se2-)5. Magnetic susceptibility measurements show paramagnetic behavior above 20Kwith magnetic moments of 8.00(5) μB/Eu and 8.10(5) μB/Eu, respectively, indicating Eu2+. Eu2Ge2Se5 orders antiferromagnetically at TN = 4.2(2) K and undergoes a metamagnetic transition at 2 K at a critical field of 0.5(3) T. The saturation moment at 2.2 K and 5.5 T is 7.00(2) μB/Eu. Eu2GeSe4 is a ferromagnet with TC = 5.8(2) K and a saturation moment at 2.2 K and 5.5 T of 6.92(2) μB/Eu. 151Eu Mössbauer spectroscopic measurements of Eu2GeSe4 and Eu2Ge2Se5 at 78 K show isomer shifts of -12.43(4) mm/s and -12.69(5) mm/s, respectively, in accordance with divalent europium.