Thomas Fickenscher

Intermetallic rare earth (RE) magnesium compounds REPdMg and RE2Pd2Mg

Abstract The intermetallic rare earth metal compounds REPdMg (RE=Nd, Eu, Tb, Ho, Tm, Yb) and RE2Pd2Mg (RE=Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho) were synthesized from the elements by reactions in sealed tantalum tubes in a high-frequency furnace. All compounds were investigated by X-ray powder diffraction. With a trivalent rare earth element, the REPdMg intermetallics adopt the hexagonal ZrNiAl structure, while those with a divalent rare earth metal crystallize with the orthorhombic TiNiSi type. The structures of EuPdMg and YbPdMg have been refined on the basis of single-crystal diffractometer data: Pnma, a=753.85(9), b=440.27(4), c=866.27(9) pm, wR2=0.0770, 366 Fo2 values, 20 variables for EuPdMg and a=729.4(2), b=424.3(2), c=850.5(3) pm, wR2=0.0622, 546 Fo2 values, 20 variables for YbPdMg. In both structure types, the palladium and magnesium atoms build a three-dimensional [PdMg] network in which the rare earth metal atoms fill distorted hexagonal channels. With a lower magnesium content the compounds RE2Pd2Mg with the tetragonal Mo2FeB2 structure, space group P4/mbm were obtained. Single crystal data yielded a=774.4(2), c=393.37(8) pm, wR2=0.0556, 216 Fo2 values, 13 variables for Nd2Pd2Mg and a=762.56(8), c=380.55(7) pm, wR2=0.0476, 298 Fo2 values, 12 variables for Tb2Pd2Mg. The magnesium position of the neodymium compound shows a small magnesium/palladium mixed occupancy leading to the refined composition Nd2Pd2.11Mg0.89. The compounds may be considered as intergrowths of CsCl and AlB2 related slabs of compositions REMg and REPd2. The crystal chemistry and chemical bonding in these intermetallics is briefly discussed. The magnetic properties of YbPdMg, Pr2Pd2Mg and Sm2Pd2Mg were investigated with a SQUID magnetometer. YbPdMg is Pauli paramagnetic with a room temperature susceptibility of 246(5)×10−6 cm3/mol. Pr2Pd2Mg is Curie–Weiss paramagnetic above 100 K with an experimental magnetic moment of 3.86(5) μB/Pr atom. Antiferromagnetic ordering occurs at 15(1) K. Pr2Pd2Mg is a metamagnet with a critical field of 0.8(1) T. The saturation magnetization at 2 K and 5 T is 2.65(5) μB/Pr atom. Sm2Pd2Mg shows Van Vleck paramagnetism.

Syntheses and Crystal Structures of LaRhMg, CeRhMg, PrRhMg, and NdRhMg

New intermetallic rare earth compounds LaRhMg, CeRhMg, PrRhMg, and NdRhMg were prepared by reaction of the elements in sealed tantalum tubes in a high-frequency furnace. The compounds were investigated by X-ray diffraction both on powders and single crystals. LaRhMg crystallizes with the LaNiAl type structure, space group Pnma, Z = 8, a = 760.1(2), b = 419.92(8), c = 1702.6(2) pm, wR2 = 0.0482, 740 F2 values and 38 variable parameters. The cerium compound adopts the ZrNiAl structure: P6¯2m, Z = 3, a = 752.3(1), c = 417.6(1) pm, wR2 = 0.0497, 250 F22 values and 17 variable parameters. PrRhMg and NdRhMg crystallize with the TiNiSi type: Pnma, Z = 4, a = 721.62(7), b = 415.98(4), c = 869.47(8) pm, wR2 = 0.1864, 440 F2 values, 20 variables for PrRhMg and a = 720.6(1), b = 417.6(1), c = 868.8(1) pm, wR2 = 0.0779, 425 F2 values, 22 variables for NdRhMg. Refinements of the occupancy parameters revealed mixed Mg/Rh occupancy for the magnesium sites of the cerium and the neodymium compound leading to the compositions CeRh1.262(8)Mg0.738(8) and NdRh1.114(9)Mg0.886(9) for the investigated single crystals. From a geometrical point of view, the four crystal structures are built up from different rhodium centered trigonal prisms. The rhodium and magnesium atoms form three-dimensional [RhMg] networks in which the rare earth metal atoms are located in different types of channels. The networks show Rh—Mg and Mg—Mg bonding. Synthese und Kristallstrukturen von LaRhMg, CeRhMg, PrRhMg und NdRhMg Die neuen intermetallischen Seltenerdverbindun-gen LaRhMg, CeRhMg, PrRhMg und NdRhMg wurden durch Reaktion der Elemente in geschlossenen Tantalampullen in einem Hochfrequenzofen synthetisiert. Die Verbindungen wurden uber Rontgen-Pulver- und Einkristalluntersuchungen charakterisiert. LaRhMg kristallisiert im LaNiAl-Typ, Raumgruppe Pnma, Z = 8, a = 760, 1(2); b = 419, 92(8); c = 1702, 6(2) pm; wR2 = 0, 0482; 740 F2-Werte und 38 variable Parameter. Die Cerverbindung ist isotyp zu ZrNiAl: P6¯2m, Z = 3, a = 752, 3(1); c = 417, 6(1) pm; wR2 = 0, 0497; 250 F2-Werte und 17 variable Parameter. PrRhMg und NdRhMg kristallisieren im TiNiSi-Typ: Pnma, Z = 4, a = 721, 62(7); b = 415, 98(4); c = 869, 47(8) pm; wR2 = 0, 1864; 440 F2-Introduction Werte, 20 Variable fur PrRhMg und a = 720, 6(1); b = 417, 6(1); c = 868, 8(1) pm; wR2 = 0, 0779; 425 F2-Werte; 22 Variable fur NdRhMg. Verfeinerungen der Besetzungsparameter ergaben eine gemischte Mg/Rh-Besetzung fur die Magnesium-Positionen der Cer- und Neodymverbindung, was zu den Zusammensetzungen CeRh1.262(8)Mg0.738(8) und NdRh1.114(9)Mg0.886(9) fur die untersuchten Einkristalle fuhrte. Rein geometrisch betrachtet sind die vier Strukturen aus unterschiedlichen, rhodium-zentrierten trigonalen Prismen aufgebaut. Die Rhodium- und Magnesiumatome bilden dreidimensionale [RhMg] Netzwerke aus, in denen die Seltenerdmetallatome zwei unterschiedliche Arten von Kanalen besetzen. Innerhalb der Netzwerke findet man Rh—Mg- und Mg—Mg-Bindungen.

Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds

We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic RETMg intermetallics with RE = La, Eu, Gd, Yb and T = Ag, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic (RE = La, Yb) or they order either ferro- or antiferromagnetically with transition temperatures ranging from about 13 to 81 K. All of them are metallic, but the resistivity varies over three orders of magnitude. The magnetic order causes a strong decrease of the resistivity and around the ordering temperature we find pronounced magnetoresistance effects. The magnetic ordering also leads to well defined anomalies in the specific heat. An analysis of the entropy change leads to the conclusion that generally the magnetic transition can be described by an ordering of localized S = 7/2 moments arising from the half-filled 4f7 shells of Eu2+ or Gd3+. However, for GdAgMg we find clear evidence for two phase transitions, indicating that the magnetic ordering sets in partially below about 125 K and is completed via an almost first-order transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets and rather pronounced for the ferromagnets for low magnetic fields around the zero-field Curie temperature.

Synthesis and Structure of RE2Rh2Cd (RE = La, Ce, Pr, Nd, Sm)

New intermetallic cadmium compounds RE2Rh2Cd (RE= La, Ce, Pr, Nd, Sm) were synthesized from the elements in sealed tantalum tubes in a high-frequency furnace. They were characterized through X-ray powder data: Mo2FeB2 type, space group P4/mbm, Z = 2. Single crystal data of the cerium compound (a = 762.8(1), c = 377.8(1) pm, wR2 = 0.0662, 199 F2 values, and 13 variable parameters) revealed small defects on the rhodium position leading to the composition Ce2Rh186(3)Cd for the investigated crystal. According to the course of the cell volumes Ce2Rh2Cd may be classified as a mixedvalent compound. The Ce2Rh2Cd structure is an intergrowth of slightly distorted AlB2 and CsCl related slabs of compositions CeRh2 and CeCd. Within the CeRh2 slab short Ce-Rh contacts (284-300 pm) are indicative of strong Ce-Rh bonding. The Rh-Rh distance within the AlB2 related slab is 289 pm.

Ternary stannides RE2Au5Sn2 (RE = Gd-Er) – i3 superstructures of the Fe2P type

Abstract The stannides RE2Au5Sn2 (RE = Gd-Er) were synthesized by arc-melting of the elements and subsequent annealing in tantalum crucibles in sealed evacuated silica tubes. The samples were characterized by powder X-ray diffraction and two structures were refined on the basis of single-crystal X-ray diffractometer data: ordered Au7Ga2 variant, P6¯2m,

Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)

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The stannides RE3Au6Sn5 (RE = La, Ce, Pr, Nd, Sm) – synthesis, structure, magnetic properties and 119Sn Mössbauer spectroscopy

a = 820.14(4), c = 942.72(5) pm, wR2 = 0.0798, 789 F2 values, 34 variables for Tb2Au4.70Sn2.30 and a = 816.1(1), c = 941.3(2) pm, wR2 = 0.0600, 723 F2 values, 35 variables for Er2Au5.03Sn1.91. All stannides exhibit small homogeneity ranges resulting from Au/Sn mixing and/or defects within the tin substructure. The RE2Au5Sn2 structures derive from the aristotype ZrNiAl (ordered version of Fe2P) by a group-subgroup relation (isomorphic transition of index 3). The crystal chemical details are discussed in comparison with the ZrNiAl type stannide CeRhSn which crystallizes with the subcell structure.

Ternary antimonides YbTSb (T = Ni, Pd, Pt, Cu, Ag, Au): Synthesis, structure, homogeneity ranges, and 121Sb Mössbauer spectroscopy

We report measurements of the thermal expansion for two Eu2+- and two Gd3+-based intermetallics which exhibit ferro- or antiferromagnetic phase transitions. These materials show sharp positive (EuAgMg and GdAuMg) and negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal expansion coefficient α which become smeared and/or displaced in an external magnetic field. Together with specific heat data, we determine the initial pressure dependences of the transition temperatures at ambient pressure using the Ehrenfest or Clausius–Clapeyron relation. We find large pressure dependences, indicating strong spin–phonon coupling, in particular for GdAgMg and EuAuMg where a quantum phase transition might be reached at moderate pressures of a few GPa.

Palladium-Cadmium Ordering In Repdcd (Re = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb) And Comparison With Isotypic Indium Compounds

Abstract The Ce3Pd6Sb5-type rare earth stannides RE3Au6Sn5 (RE = La, Ce, Pr, Nd, Sm) were synthesized by arc-melting of the elements and subsequent annealing in open tantalum crucibles within sealed evacuated silica ampoules. The polycrystalline samples were studied by powder X-ray diffraction. The structures of three crystals were refined from single crystal X-ray diffractometer data: Pmmn, a = 1360.3(9), b = 455.9(2), c = 1023.6(4) pm, wR2 = 0.0275, 1069 F2 values, 48 variables for Ce3Au6Sn5, a = 1352.4(4), b = 455.1(1), c = 1023.7(3) pm, wR2 = 0.0367, 1160 F2 values, 48 variables for Nd3Au6Sn5, and a = 1339.8(2), b = 452.80(7), c = 1012.4(2) pm, wR2 = 0.1204, 1040 F2 values, 49 variables for Sm3Au5.59(2)Sn5.41(2). One of the gold sites of the samarium compound shows a significant degree of Au/Sn mixing. The RE3Au6Sn5 structures are composed of three-dimensional [Au6Sn5] polyanionic networks with the two crystallographically independent rare earth atoms in larger cages, i.e., RE1@Au10Sn6 and RE2@Au8Sn8. The [Au6Sn5] network is stabilized by Au–Sn (266–320 pm), Au–Au (284–301 pm) as well as Sn–Sn (320 pm; distances given for the cerium compound) interactions. Temperature-dependent magnetic susceptibility measurements reveal an antiferromagnetic ordering only for Sm3Au6Sn5, while the other compounds exhibit Curie–Weiss paramagnetism. 119Sn Mössbauer spectroscopy shows resonances in the typical range for intermetallic tin compounds where tin takes part in the polyanionic network [isomer shifts between 1.73(1) and 2.28(1) mm·s−1]. With the help of theoretical electric field gradient calculations using the WIEN2k code it was possible to resolve the spectroscopic contributions of all three crystallographically independent atomic tin sites in the 119Sn spectra of RE3Au6Sn5 (RE = La, Ce, Pr, Nd, Sm).

Magnetic and Electrical Properties, 151Eu Mössbauer Spectroscopy, and Chemical Bonding of REAgMg (RE=La, Ce, Eu, Yb) and EuAuMg

The ternary antimonides YbTSb (T = Ni, Pd, Pt, Cu, Ag, Au) were synthesized by reaction of the elements in sealed tantalum tubes in a high-frequency furnace. The structures of YbCuSb (NdPtSb type), YbAgSb (TiNiSi type), and YbAuSb (NdPtSb type) were confirmed on the basis of X-ray powder diffraction data. Those of the nickel, palladium, and platinum based antimonides (cubic MgAgAs type) were refined from single crystal X-ray data. The nickel based antimonide has a pronounced homogeneity range YbNixSb. The structures of five crystals have been investigated. The cubic lattice parameter increases with increasing nickel content from 613.13(6) pm(x = 0.17) to 621.25(5) pm (x = 0.63). Full occupancy of the palladium and antimony sites was observed for YbPdSb while the platinum compound shows some platinum vacancies leading to the composition YbPt0.969(7)Sb for the investigated crystal. A new, hightemperature modification of YbPdSb was obtained by rapidly quenching an arc-melted sample: TiNiSi type, Pnma, a = 725.6(2), b = 458.3(1), c = 785.4(2) pm, wR2 = 0.1255, 421 F2 values, 20 variables. The antimonides YbTSb (T = Ni, Pd, Pt, Cu, Ag, Au) show single 121Sb Mössbauer signals at isomer shifts ranging from -7.34 to -7.82 mm/s. The crystal chemistry and chemical bonding of these antimonides is discussed.