Bernd D. Mosel

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 Stannides CeAuSn, Ce3Rh4Sn13, and Ce3Ir4Sn13

Abstract CeAuSn, Ce3Rh4Sn13, and Ce3lr4Sn13 were prepared by reaction of the elements in an arc-melting furnace and subsequent annealing at 970 K for two weeks. The three stannides were investigated by X-ray powder and single crystal techniques. CeAuSn crystallizes with the NdPtSb type, space group P63mc: a = 472.7(2), c = 771.6(3) pm, wR2 = 0.0230,208 F2 values, 11 variable parameters, and BASF = 0.40(2). The gold and tin atoms form a pronounced two-dimensional [AuSn] polyanion which consists of slightly puckered Au3Sn3 hexagons. ,19Sn Mössbauer data at 78 K show one signal at an isomer shift of δ = 1.90(7) mm/s subjected to unresolved quadrupole splitting of ΔEQ = 0.55(2) mm/s. Ce3Rh4Sn13 and Ce3lr4Sn13 adopt the cubic Yb3Rh4Sn13 type structure, space group Pm3n: a = 970.51(3) pm, wR2 = 0.0721, 267 F2 values (Ce3Rh4Sn13) and a = 972.29(6) pm, wR2 = 0.0850, 267 F2 values (Ce3lr4Sn13) with 14 variable parameters for each refinement. Striking structural motifs in Ce3Rh4Sn13 are condensed distorted trigonal [RhSn6] prisms with Rh-Sn distances of 266 pm. The polyhedral network leaves two different cages which are occupied by cerium (6c position) and tin (2a position) atoms. The Sn2 atoms show occupancy parameters of only 92% (Ce3Rh4Sn13) and 76% (Ce3Ir4Sn13) and an extremely large displacement parameter indicating a rattling of these atoms within the icosahedral Sn12 cages. Magnetic susceptibility measurements of Ce3Rh4Sn13 show paramagnetic behavior down to 2 K with an experimental magnetic moment of 2.45(2) μB/Ce. No magnetic ordering is observed. Magnetization measurements show a moment of 0.78(2) μB/Ce at 2 K and 5.5 T. Resistivity data reveal only a very weak temperature dependence. The two crystallographically different tin sites are resolved in the 119Sn Mössbauer spectrum which shows a signal at δ = 2.12(1) mm/s subject to quadrupole splitting of 1.54(1) mm/s, superimposed by a singlet at δ = 2.47(1) mm/s. The Seebeck coefficient of Ce3Rh4Sn13 is within a few μ V/K of zero over the temperature range of 10 - 300 K.

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 refinement, magnetic susceptibility, electrical conductivity and europium-151 Mössbauer spectroscopy of EuNiIn4

The title compound was prepared by a reaction of the elemental components at 970 K in a tantalum tube. EuNiIn4 adopts the orthorhombic YNiAl4-type structure. It was refined from single-crystal X-ray data: space group Cmcm, a= 447.31 (4) pm, 6= 1695.88(15) pm, c= 722.32(6) pm, V= 0.5479(1) nm3, Z = 4, wR2 = 0.046l, 637 F2 values and 24 variable parameters. Magnetic susceptibility measurements show Curie–Weiss behaviour above 50 K. At 16(1) K, a phase transition to the antiferromagnetic state is observed in the temperature dependence of the inverse magnetic susceptibility. The experimental magnetic moment µexp= 7.86(5)µB/Eu is close to that of the free Eu+2 ion µeff=7.947 µB. EuNiIn4 is a good metallic conductor with a specific resistivity of 14 µΩcm at room temperature. 151Eu Mossbauer measurements can be fit by a single Eu site with an isomer shift δ=– 10.9 mm s–1 against EuF3 which is typical for divalent Eu. At TN, = 32(1) K magnetic order begins, which can be detected by Mossbauer spectroscopy when the fluctuation rate is smaller than the inverse half-life of the excited state of the 151Eu nuclide, which occurs in parts of the present sample only at temperatures significantly lower than TN.

Electronic structure, physical properties and ionic mobility of LiAg2Sn

The stannide LiAg2Sn was synthesized from the elements by reaction in a sealed tantalum tube in a resistance furnace. LiAg2Sn crystallizes with a ternary ordered version of the cubic BiF3 structure, space group Fmm: a = 659.2(2) pm, wR2 = 0.0450, 69 F2 values, 5 variables. The silver and tin atoms form an antifluorite structure of composition Ag2Sn (285 pm Ag–Sn) in which the lithium atoms fill octahedral voids. Electronic structure calculations reveal weak Ag–Ag and strong Ag–Sn bonding within the Ag2Sn substructure. LiAg2Sn is weakly Pauli paramagnetic and a good metallic conductor. Nevertheless, the modestly small 7Li Knight shift is consistent with a nearly complete state of lithium ionization. The high local symmetry at the tin site is reflected by the absence of a nuclear electric quadrupolar splitting in the 119Sn Mossbauer spectra and a small chemical shift anisotropy evident from 119Sn solid state NMR. Static 7Li solid state NMR spectra reveals motional narrowing effects above 300 K, consistent with lithium atomic mobility on the kHz timescale.

151Eu Mössbauer spectroscopic, electric, and magnetic measurements of EuAgGe and EuAuGe

The physical properties of EuAgGe and EuAuGe, the structures of which are derived from the CeCu2 type, have been investigated in detail by means of magnetic susceptibility, electrical conductivity and 151Eu Mössbauer measurements. Above 50 K both germanides show Curie--Weiss behavior with experimental magnetic moments of \mu exp=7.70(5) \mu B (EuAgGe) and \mu exp=7.40(5) \mu B (EuAuGe) and Weiss constants of -2(1) K (EuAgGe) and 33(1) K (EuAuGe). For EuAgGe, a magnetic phase transition is observed below 18(1) K. Zero-field cooling and field cooling measurements indicate cluster glass behavior (weak ferromagnetism, mictomagnetism). Magnetization measurements at 5 K show a saturation magnetic moment of 3.3(2) \mu B/Eu at 5.5 T. 151Eu Mössbauer measurements show a Eu(II) valence state (\delta =-10.4 mm/s). While magnetic hyperfine splitting appears in the spectra at temperatures as high as 15 K, complete magnetic ordering is not reached at temperatures down to 4.2 K. EuAuGe orders ferromagnetically at 32.9(2) K. Magnetization measurements at 2 K show a saturation magnetic moment of 6.2(1) \mu B/Eu at 5.5 T, respectively, indicating that all spins are ordered ferromagnetically at low temperatures. 151Eu Mössbauer measurements show a Eu(II) valence state (\delta =-10.6 mm/s) and two spectral components in an approximate 1:1 ratio, subjected to magnetic hyperfine splitting effects at T1=32(2) and T2=18(4) K, respectively. Thus, the transition temperature of 32.9 K observed in the susceptibility measurements appears to be associated with ordering of only one of the two crystallographically distinct europium sites in this compound. Electrical conductivity measurements indicate metallic behavior for both germanides.

Magnetic hyperfine interactions in the Zintl phase EuZnSn

Magnetic susceptibility, 119 Sn and 151 Eu Mossbauer effect measurements are reported for the Zintl phase EuZnSn which crystallizes with the TiNiSi-type structure. The stannide shows Curie–Weiss behaviour above 40 K with an experimental magnetic moment of µ exp =7.88(5) µ B /Eu, close to that of the free Eu 2+ ion of µ eff =7.94 µ B . At 20.5(2) K a magnetic phase transition to the antiferromagnetic state is observed in the temperature dependence of the inverse susceptibility at a magnetic flux density of 0.01 T. Magnetization measurements at 5 K indicate metamagnetism with a saturation magnetic moment of 6.80(5) µ B /Eu and a critical field of 0.7(1) T. 151 Eu Mossbauer measurements show an isomer shift δ=-10.9 mm s -1 against EuF 3 , as is typically observed for divalent europium. While onset of magnetic ordering is evident at T 0 =26.5(5) K, full magnetic order is detected near 15 K. 119 Sn Mossbauer spectra show a large transferred magnetic field of 12.8 T at 4.2 K. The temperature dependence of this field is closely correlated with the internal magnetic field measured at the 151 Eu site (23.4 T at 4.2 K).

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.

Magnetic, electrical and 151Eu Mössbauer properties of EuPtGe

The title compound was prepared from the elements in a tantalum tube at 1170 K. EuPtGe [single-crystal X-ray data: space group P213, Z= 4, a= 654.63(9) pm, R1= 0.0208, 378 F2 values and 11 parameters] crystallizes with the LaIrSi type structure, an ordered ternary version of the SrSi2 type. Magnetic susceptibility and 151Eu Mossbauer measurements show no magnetic order down to 4.2 K. The experimentally determined magnetic moment, µexpc= 7.80(5)µB/Eu compares well with the free ion value, µeff= 7.94 µ/Eu for Eu2+, in accordance with the isomer shift δ=–10.4 mm s–1 which is typical for divalent Eu. EuPtGe is a metallic conductor with a specific resistivity at room temperature of 145 µΩ cm.

The Zintl Phase Eu2Si

The Zintl phase Eu2Si was synthesized from elemental europium and silicon in a sealed tantalum tube in a high-frequency furnace at 1270 K and subsequent annealing at 970 K. Investigation of the sample by X-ray powder and single crystal techniques revealed: Co2Si (anti-PbCl2) type, space group Pnma, a = 783.0(1), b = 504.71(9), c = 937.8(1) pm, wR2 = 0.1193, 459 F2 values and 20 variables. The structure contains two europium and one silicon site. 151Eu Mossbauer spectroscopic data show a single signal at an isomer shift of −9.63(3) mm/s, compatible with divalent europium. Within the Zintl concept electron counting can be written as (2Eu2+)4+Si4−, in agreement with the absence of Si-Si bonding. Each silicon atom has nine europium neighbors in the form of a tri-capped trigonal prism. The silicon coordinations of the Zintl phases Eu2Si, Eu5Si3, EuSi, and EuSi2 are compared. Die Zintl-Phase Eu2Si Die Zintl-Phase Eu2Si wurde aus elementarem Europium und Silicium in einer verschweisten Tantalampulle im Hochfrequenzofen bei 1270 K synthetisiert und anschliesend bei 970 K getempert. Untersuchungen der Probe mit Rontgen-Pulver- und Einkristalldaten ergab: Co2Si (anti-PbCl2)-Typ, Raumgruppe Pnma, a = 783, 0(1); b = 504, 71(9); c = 937, 8(1) pm; wR2 = 0, 1193; 459 F2-Werte und 20 variable Parameter. Die Struktur enthalt zwei kristallographisch unterschiedliche Europiumplatze. 151Eu Mossbauer-spektroskopische Daten zeigen nur ein Signal bei einer Isomerieverschiebung von −9.63(3) mm/s, was mit zweiwertigem Europium kompatibel ist. Im Rahmen des Zintl-Konzepts kann die Formel als (2Eu2+)4+Si4− geschrieben werden, in Ubereinstimmung mit der Abwesenheit von Si—Si-Bindungen. Jedes Siliciumatom hat neun Europiumnachbarn in Form eines dreifach uberkappten trigonalen Prismas. Die Siliciumkoordination in den Zintl-Phasen Eu2Si, Eu5Si3, EuSi, und EuSi2 wird verglichen.