Tube Elements Research Articles

Fluorite‐Related Phases in the System Li‐Pt‐Sn: Synthesis and Structures of Li2PtSn, Li3Pt2Sn3, Li2.27Pt2Sn3.73 and Li2.43Pt2Sn3.57

AbstractThe intermetallic compounds Li2PtSn, Li3Pt2Sn3, Li2.27Pt2Sn3.73 and Li2.43Pt2Sn3.57 were synthesised by reaction of the elements in sealed tantalum tubes. They were investigated by X‐ray diffraction on powders and single crystals: F$\bar 4$ 3m, a = 626.0(1) pm, wR2 = 0.0834, 70 F2 values, 7 variables for Li2PtSn; Ia$\bar 3$ , a = 1264.3(1) pm, wR2 = 0.0340, 416 F2 values, 23 variables for Li3Pt2Sn3; Ia$\bar 3$ , a = 1269.7(1) pm, wR2 = 0.1130, 521 F2 values, 24 variables for Li2.27(1)Pt2Sn3.73(1); Ia$\bar 3$ , a = 1266.6(1) pm, wR2 = 0.1094, 620 F2 values, 25 variables for Li2.43(1)Pt2Sn3.57(1). The latter refinement was based on a crystal twinned by merohedry. Li2PtSn crystallises with an ordered variant of the BiF3 type. The other stannides adopt a superstructure of the fluorite type where all three subcell axes are doubled. In Li3Pt2Sn3 the platinum atoms are located on the calcium sites while the tin and 1/3 of the lithium atoms are ordered on the fluorine site. The remaining lithium atoms fill octahedral voids of the fcc arrangement of platinum. The superstructure is discussed on the basis of a group‐subgroup scheme and experimental evidence is given for a solid solution Li3−xPt2Sn3+x. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

γ1-Pt 5Zn 21—a reappraisal of a γ-brass type complex alloy phase

The constitution of the γ-phase field in the PtZn system was reinvestigated. The samples were prepared from the elements in sealed silica tubes at 1320 K followed by slow cooling to 970 K. Three structurally distinct γ-phases were identified by means of single crystal X-ray diffraction. The γ 1- and γ 1′-phase represent two structurally intimately related 2 a×2 a×2 a superstructures of the ordinary γ-Pt 2Zn 10.75. Three crystal structures were refined from single crystal intensity data: γ 1-Pt 5.12Zn 20.47 ( F 4 ̄ 3m , a=1809.1(1) pm, cF416-6.6, R F=0.043), γ 1-Pt 4.72Zn 20.92 ( F 4 ̄ 3m , a=1812.8(2) pm, cF416-5.8, R F=0.034), and γ 1′-Pt 4.25Zn 20.34 ( F 4 ̄ 3m , a=1814.0(1) pm, cF416-22.7, R F=0.054). The structures are discussed in terms of four symmetrically inequivalent 26 atom clusters each comprising inner tetrahedral (IT), outer tetrahedral (OT), octahedral (OH) and cuboctahedral (CO) positions. The homogeneity range of the γ 1-phase extends between 18.4 and 20.0 at.% Pt, a range which encompasses the composition Pt 5Zn 21. Two symmetrically independent, ordered Pt 4Zn 22 clusters form the compositionally invariant part of the structure. Two complementary, partly disordered Pt 6± δ Zn 20− δ′ clusters account for the homogeneity range of the phase. They vary in composition due to mutual Pt/Zn substitution, vacancies at Zn sites, and partial positional disorder. The closely related phase γ 1′-Pt 4.25Zn 20.34 is separated from the γ 1-phase by a small miscibility gap which is indicated by isopiestic measurements and a discontinuous jump of the vacancy concentration. The melting point of the phases increases with rise of the Pt content: 1136(2) K ( γ), 1238(5) K ( γ 1′) and 1251(3) K ( γ 1). The magnetic susceptibility is dominated by the core diamagnetism of the constituents. χ mol was found to vary between −4.7×10 −10 and −3.1×10 −10 m 3 mol −1.

Stem anatomy ofCipocereus(Cactaceae)

The genus Cipocereus belongs to the subfamily Cactoideae, tribe Cereeae, and includes columnar or semi-erect species, endemic of the Serra do Espinhaço of Minas Gerais, Brazil. It is composed of five species: C. bradei, C. crassisepalus, C. laniflorus, C. minensis and C. pusilliflorus. It is considered to be one of the most primitive of the tribe, due to a vascular cylinder composed of secondary xylem with heavy lignification, among other morphological characters. The aim of the present work is to verify the occurrence of diagnostic and taxonomic features. Stem and branch samples of each species were cut at apical, medium and basal portions. Sections were stained with safranin and astrablue and mounted in synthetic resin. The species share the following characters: a uniseriate epidermis, with thick cuticle; an external cortex portion differentiated to have a hypodermis, with many layers of cells with very thick walls; a well developed palisade chlorenchyma containing mucilage cells. During the secondary vascular formation, sclereid precursors are present outside the secondary phloem, that give rise to sclereids in later stages. Primary rays are formed by the interfascicular cambium and secondary rays by the fascicular cambium, among the axial elements of xylem and phloem. The secondary phloem is composed of sieve tube elements with transverse simple sieve plate, companion cells, axial and radial parenchyma. The xylem is composed of vessel elements with simple perforation plates, septate libriform fibres, large and high rays, axial scanty paratracheal parenchyma. Species differences have been found in the hypodermis structure, the presence of crystals and mucilage cells.

The Indium‐Rich Intermetallics YbCoIn5, YbRhIn5, and YbPtIn4

AbstractThe indium‐rich intermetallic compounds YbCoIn5 and YbRhIn5 were prepared from the elements in sealed tantalum tubes by the indium flux technique with a starting composition 1:1:7. The tubes were heated at 1320 K (YbCoIn5) and 1300 K (YbRhIn5) for six hours followed by slow cooling (5 K/h) to 670 K. YbPtIn4 was prepared in a sealed molybdenum crucible from an arc‐melted precursor alloy YbPtIn2 with additional indium (6 h at 1370 K followed by slow cooling at a rate of 5 K/h). The structures of the three indides were investigated by single crystal X‐ray data: P4/mmm, HoCoGa5 type, a = 455.90(7), c = 743.3(1) pm, wR2 = 0.0487, 173 F2 values for YbCoIn5, a = 459.52(6), c = 744.2(2) pm, wR2 = 0.0803, 238 F2 values for YbRhIn5 with 12 variables per refinement, and Cmcm, YNiAl4 type, a = 444.5(1), b = 1655.0(1), c = 751.6(1) pm, wR2 = 0.1036, 482 F2 values, 24 variables for YbPtIn4. The transition metal atoms in YbCoIn5 and YbRhIn5 have eight indium neighbors in a square prismatic coordination by indium atoms. These prisms are condensed via common edges forming layers. The ytterbium and further indium atoms are located between these layers. The ytterbium atoms are cuboctahedrally coordinated by twelve indium atoms. The cell volumes indicate intermediate‐valent or divalent ytterbium for YbCoIn5 and YbRhIn5. The YbPtIn4 structure consists of a three‐dimensional [PtIn4] network in which the ytterbium atoms are located in distorted hexagonal channels. Common structural motifs of the three structures are distorted bcc‐like indium cubes which resemble the structure of elemental indium.

Ternary Stannides LiTSn4 (T = Ru, Rh, Ir)Chemical Bonding and Physical Properties

Pure samples of the ternary stannides LiTSn4 (T = Ru, Rh, Ir) have been synthesized from the elements in sealed tantalum tubes. They crystallize with an ordered version of the PdGa5 type. LiTSn4 (T = Ru, Rh, Ir) are all metallic conductors and Pauli paramagnets. Spectroscopic measurements indicate significant differences in the chemical bonding properties: As revealed by both 119Sn Mossbauer spectroscopy and solid-state NMR data, the local electron distribution at the tin site is more anisotropic in LiRuSn4 compared to the other two materials. In addition, 7Li Knight shift and spin-lattice relaxation times indicate that LiRuSn4 has a much higher electron density at the lithium atom arising from conduction electrons at the Fermi level. These findings are in good agreement with the results of DFT band structure calculations.

Clean Diesel technology approved for use in mines

The stannides YPdSn and YPd2Sn were synthesized by high-frequency melting of the elements in sealed tantalum tubes. Both structures were refined on the basis of single crystal X-ray diffractometer data: TiNiSi type, Pnma, a=715.4(1), b=458.8(1), c=789.1(1) pm, wR2=0.0461, 510 F2 values, 20 variables for YPdSn and MnCu2Al type, Fm3¯m, a=671.44(8), wR2=0.0740, 55 F2 values, 5 parameters for YPd2Sn. The yttrium atoms in the new stannide YPdSn are coordinated by two tilted Pd3Sn3 hexagons (ordered AlB2 superstructure). In the Heusler phase YPd2Sn each yttrium atom has octahedral tin coordination and additionally eight palladium neighbors. The cubic site symmetry of yttrium is reflected in the 119Sn Mössbauer spectrum which shows no quadrupole splitting. In contrast, YPdSn shows a single signal at δ=1.82(1) mm/s subjected to quadrupole splitting of ΔEQ=0.93(1) mm/s. Both compounds have been characterized by high-resolution 89Y solid state NMR spectroscopy, which indicates the presence of strong Knight shifts. The spectrum of YPd2Sn is characterized by an unusually large linewidth, suggesting the presence of a Knight shift distribution reflecting local disordering effects. The range of 89Y Knight shifts of several binary and ternary intermetallic yttrium compounds is briefly discussed.

Xyloglucan endotransglycosylases have a function during the formation of secondary cell walls of vascular tissues.

Xyloglucan transglycosylases (XETs) have been implicated in many aspects of cell wall biosynthesis, but their function in vascular tissues, in general, and in the formation of secondary walls, in particular, is less well understood. Using an in situ XET activity assay in poplar stems, we have demonstrated XET activity in xylem and phloem fibers at the stage of secondary wall formation. Immunolocalization of fucosylated xylogucan with CCRC-M1 antibodies showed that levels of this species increased at the border between the primary and secondary wall layers at the time of secondary wall deposition. Furthermore, one of the most abundant XET isoforms in secondary vascular tissues (PttXET16A) was cloned and immunolocalized to fibers at the stage of secondary wall formation. Together, these data strongly suggest that XET has a previously unreported role in restructuring primary walls at the time when secondary wall layers are deposited, probably creating and reinforcing the connections between the primary and secondary wall layers. We also observed that xylogucan is incorporated at a high level in the inner layer of nacreous walls of mature sieve tube elements.

Ternary Antimonides YbTSb (T = Ni, Pd, Pt, Cu, Ag, Au) – Synthesis, Structure, Homogeneity Ranges, and 121Sb Mössbauer Spectroscopy

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.

Structure and Properties of Gd TMg ( T=Pd, Ag, Pt)

The title compounds were prepared by reacting the elements in sealed tantalum tubes in a water-cooled sample chamber in a high-frequency furnace. X-ray powder and single-crystal investigations showed isotypism with the ZrNiAl type, space group P&6macr;2 m: a=750.1(1) pm, c=404.10(4) pm, wR2=0.0703, 250 F 2 values, 14 parameters for GdPdMg, a=768.0(2) pm, c=419.92(9) pm, w R2=0.0579, 261 F 2 values, 16 parameters for GdAgMg, and a=738.0(1) pm, c=409.02(5) pm, w R2=0.0742, 244 F 2 values, 14 parameters for GdPtMg. The structures contain two crystallographically different transition metal ( T) sites which both have a tricapped trigonal prismatic coordination: [ T(1)Gd 6Mg 3] and [ T(2)Mg 6Gd 3]. Together the transition metal and magnesium atoms build three-dimensional networks in which the gadolinium atoms fill distorted hexagonal channels. The magnesium position of the silver compound shows a small degree of magnesium/silver mixing resulting in the composition GdAg 1.06(1)Mg 0.94(1) for the crystal investigated. The magnetic properties of all compounds were investigated using AC and DC susceptibility as well as 155Gd Mössbauer spectroscopy measurements. All investigated materials show irreversibilities between field cooled and zero-field-cooled DC magnetizations and magnetic hysteresis behavior as is typical for ferromagnets. The remanent magnetizations and coercive fields are relatively small. The Curie temperatures were determined from inflection points of the experimental susceptibilities. Additional anomalies below the ferromagnetic transitions suggest spin-reorientation processes.

Phloem-and xylem-restricted plant pathogenic bacteria

1. Introduction: an overview This review concerns plant pathogenic bacteria, which are strictly restricted either to the sieve tubes in the phloem or to the vessels in the xylem. These bacteria are endogenous as opposed to exogenous bacteria such as species of Erwinia , Pseudomonas , Ralstonia or Xanthomonas , which colonize the apoplast (intercellular spaces) of plant tissues, even though some of the exogenous bacteria, such as Xylophilus ampelina , can also induce vascular infections. Due to their vascular habitat, the endogenous bacteria have a systemic distribution throughout the plant, they are transmitted from plant to plant by graft inoculation, and most of them are vectored by insects which feed in the phloem (leafhoppers, psyllids), or the xylem (sharpshooters). Because of these virus-like properties, the diseases caused by endogenous bacteria have long been taken for virus diseases. The principal conducting cells of the phloem are the sieve tube elements [37]. These elements are joined end to end into sieve tubes, and are associated with parenchymatic, nucleated cells, the companion cells, an important role of which is to load sucrose into the sieve tubes. The sieve tube elements are living cells, which become enucleated at maturity. The sieve plates are lateral wall areas between two adjacent sieve elements. The sieve plates are clustered with pores, resembling giant plasmodesmata and interconnecting two adjacent sieve elements through their cytoplasms. The diameter of a pore ranges from a fraction of a micron (m )t o 15m and more, and is large enough to allow passage of sieve tube-restricted bacteria. The principal conducting elements of the xylem are tracheids and vessel members [37]. Both are dead cells, contain no cytoplasm, and have lignified secondary walls. The vessel elements are joined end to end into vessels, and the adjoining ends have open perforation plates. These openings allow relatively unimpeded longitudinal spread of the xylem-restricted bacteria within

Enhancement of mechanical and superconducting properties of MgB2

We demonstrate the higher mechanical strength and density and superconducting properties of MgB2 compounds by controlling the initial mixing ratio of Mg and B in a Ta tube. It is found that the extra Mg as a starting element in a sealed Ta tube establishes a good environment for the formation of MgB2 with a large grain size of 9–16 μm diam, rather than existing as an impurity. As a result, MgB2, which was synthesized from mixing a ratio of Mg:B=1.3:2, shows much higher mechanical strength and density. In addition, electrical and superconducting properties are also enhanced for application, which indicates a more effective interconnection between grains. The critical current density is increased almost by two times compared with the compound, synthesized from the stoichiometric starting elements, which is important for the application of MgB2.

Fabrication and characterization of vertically aligned carbon nanotubes on silicon substrates using porous alumina nanotemplates.

An ethylene-air laminar diffusion flame successfully provided silicon substrates of anodic aluminum oxide (AAO) template with vertically oriented well-aligned carbon nanotubes. Field emission scanning electron microscopy (SEM) showed that open-tipped carbon nanotubes consisting of tube elements with the same length and diameter uniformly coated the template. High-resolution transmission electron microscopy (TEM) analyses revealed these nanotubes to be multiwalled carbon nanotubes, some well graphitized. It was found that cobalt catalyst particles, but not the porous aluminum templates, helped the growth of carbon nanotubes through graphitization and bonding of carbon nanotubes to the silicon substrates.

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

Abstract Ternary intermetallic compoundsREPdCd (RE=La, Ce, Pr, Nd, Sm, Eu, Tb)were synthesized by reaction of the elements in sealed tantalum tubes in a high-frequency furnace. All compounds were investigated by X-ray diffraction on powders. Those with RE = La, Ce, Pr, Nd, Sm, and Tb contain trivalent rare earth metals and they crystallize with the hexagonal ZrNiAl type structure. NdPdCd [a = 764.8(3), c = 401.3(2) pm] and TbPdCd [a = 756.5(2), c = 389.0(1) pm] are reported here for the first time as well as EuPdCd. The compounds with La, Ce, Pr, and Sm [J. Alloys Compd. 182, 87 (1992)] are confirmed. The crystal structure of EuPdCd was refined from X-ray single crystal diffractometer data: TiNiSi type, Pnma, a = 745.9(3), b = 439.8(2), c = 863.7(3) pm, wR2 = 0.0405, 683 F2 values, and 20 variable parameters. The three-dimensional [PdCd] substructure consists of strongly puckered, orthorhombically distorted Pd3Cd3 hexagons with Pd-Cd distances ranging from 283 to 285 pm. The europium atoms fill the cavities within the [PdCd] network. Chemical bonding in the series REPdCd and REPdIn is compared.

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

Single-phase samples of REAgMg (RE=La, Ce, Eu, Yb) and EuAuMg were prepared by reacting the elements in sealed tantalum tubes in a high-frequency furnace. LaAgMg and CeAgMg adopt the hexagonal ZrNiAl-type structure, while EuAgMg, YbAgMg, and EuAuMg crystallize with the orthorhombic TiNiSi type. Chemical bonding was exemplarily investigatedfor EuAgMg and EuAuMg on the basis of TB-LMTO-ASAcalculations. Magnetic susceptibility measurements indicatePauli paramagnetism for LaAgMg and YbAgMg with room-temperature susceptibilities of 2.4(1)×10−9 and 1.5(1)×10−9 m3/mol, respectively. CeAgMg remains paramagnetic down to 2 K. The experimental magnetic moment of 2.52(2) μB/Ce above 50 K is compatible with trivalent cerium. EuAgMg and EuAuMg are paramagnetic above 50 K with experimentalmagnetic moments of 7.99(5) μB/Eu for the silver and 7.80(5) μB/Eu for the gold compound, indicating divalent europium. Ferromagnetic ordering is detected at TC=22.0(3) K (EuAgMg) and TC=36.5(5) K (EuAuMg). At 4.2 K and 5 T the saturation magnetizations are 7.1(1) and 7.3(1) μB/Eu for EuAgMg and EuAuMg, respectively. According to the very small hysteresis, EuAgMg and EuAuMg may be classified as soft ferromagnets. All compounds are metallic conductors. For EuAgMg and EuAuMg freezing of spin-disorder scattering is observed below TC. At 78 K 151Eu Mössbauer spectra show isomer shifts of −9.00(4) and −8.72(8) mm/s for EuAgMg and EuAuMg, respectively. Full magnetic hyperfine field splitting is detected at 4.2 K with hyperfine fields of 17.4(1) and 18.3(2) T at the europium nuclei of EuAgMg and EuAuMg.

The Association of Phytoplasma with Stunting, Leaf Necrosis and Witches’ Broom Symptoms in Magnolia Plants

In 1999–2000 a severe disease was observed on plants of four Magnolia spp. cultivated in a commercial nursery in Poland. Affected plants showed a progressive loss of vigour, were stunted, and had severely malformed leaves, leaf necrosis and witches’ broom. Phytoplasma was detected in magnolias with severe symptoms and in dodder‐inoculated Catharanthus roseus seedlings by nested polymerase chain reaction (PCR) assay with primer pair R16F1/R0 followed by universal (rA/fA) and group specific (R16(I)F1/R1) primer pairs which amplified a fragment of phytoplasma 16S rDNA. The PCR products (560 bp or 1.1 kb) of all samples used for restriction fragment length polymorphism analysis after digestion with endonuclease enzymes AluI and MseI produced the same profile which corresponded to that of an aster yellows phytoplasma reference strain. Phytoplasma DNA was detected throughout the growing season in roots, stems and young but not mature leaves. Electron microscope examination of the ultra‐thin sections of the leaf and stem of diseased magnolias showed collapsed and degenerated sieve tube elements with wall thickening. The reduced lumen of these sieve elements contained numerous vesicles and membrane‐bound structures, but no typical phytoplasma cells. This is the first report of aster yellows phytoplasma in magnolia identified by molecular assays.

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.

Secondary Au−Au and Sn−Sn Interactions in the Superstructure of YbAuSnMissing Link in the Series of KHg2 Superstructures

The stannide YbAuSn has been synthesized in quantitative yield by reacting the elements in sealed tantalum tubes in a high-frequency furnace. The structure was determined from single-crystal X-ray data: Imm2, a = 469.7(1) pm, b = 2191.2(4) pm, c = 812.7(1) pm, wR2 = 0.111, 2061 F2 values, and 58 variables. It crystallizes with a pronounced KHg 2 type subcell (a, 1/3b, c, Imma). In the superstructure the tripled b axis allows an ordered stacking of Au 3 Sn 3 hexagons with weak gold-tin, gold-gold, and tin-tin interlayer bonding interactions. The determined stacking sequence- + - - + -,- + - - + - in YbAuSn realizes a so far missing sequence within a general ordering scheme of KHg 2 type superstructures. The group-subgroup relation in going from the KHg 2 subcell and chemical bonding in YbAuSn are discussed. Each ytterbium site has an ordered near-neighbor environment of six gold and six tin atoms in the form of two tilted hexagons. Magnetic susceptibility measurements show a nonmagnetic ground state of the ytterbium atoms. YbAuSn is a metallic conductor with a specific resistivity of 50 μΩ cm at room temperature. 119 Sn Mossbauer spectroscopic data show one signal at an isomer shift of 1.922(5) mm/s subjected to quadrupole splitting of 0.929(9) mm/s.

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 tantalum tubes in a high-frequency furnace. They were investigated by X-ray diffraction on both powders and single crystals. With the trivalent rare earth elements they adopt the ZrNiAl-type structure, space group P62m:a=782.5(3) pm, c=432.8(1) pm, wR2=0.0420, and 277 F2 values for CeAgMg and a=775.7(3) pm, c=426.1(1) pm, wR2=0.0387, and 271 F2 values for NdAgMg with 15 parameters for each refinement. Geometrical motifs in CeAgMg are two types of silver-centered tricapped trigonal prisms: [Ag1Mg3Ce6] and [Ag2Mg6Ce3]. The silver and magnesium atoms form a three-dimensional [AgMg] network in which the cerium atoms fill distorted hexagonal channels. With divalent europium and ytterbium the orthorhombic TiNiSi-type occurs, space group Pnma:a=777.4(2) pm, b=463.0(1) pm, c=898.8(2) pm, wR2=0.0430, and 401 F2 values for EuAgMg and a=753.66(7) pm, b=446.49(7) pm, c=887.2(1) pm, wR2=0.0742, and 485 F2 values for YbAgMg with 21 parameters for each refinement. The europium(ytterbium) and magnesium atoms form zigzag chains of edge- and face-sharing trigonal prisms which are centered by the silver atoms. Also in EuAgMg and YbAgMg a three-dimensional [AgMg] network occurs in which the europium(ytterbium) atoms are embedded. The magnesium positions of the cerium, neodymium, europium, and ytterbium compound show a small mixed magnesium/silver occupancy, leading to the refined compositions CeAg1.034(7)Mg0.966(7), NdAg1.035(6)Mg0.965(6), EuAg1.032(6)Mg0.968(6), and YbAg1.053(8)Mg0.947(8) for the crystals investigated. The crystal chemistry and chemical bonding in these intermetallics is briefly discussed.

The Stannides RERhSn (RE = Ho - Yb) and ScTSn (T = Pd, Pt) - Structure Refinements and 119Sn Mössbauer Spectroscopy

Abstract The stannides RERhSn (RE = Ho -Yb) and ScTSn (T = Pd, Pt) were prepared by reaction of the elements in sealed tantalum tubes in a high-frequency furnace, by arc-melting, or by a tin-flux technique in quartz tubes. The rhodium based stannides crystallize with the ZrNiAl type structure, space group P6̄2m. The four structures were refined from single crystal X-ray data: a = 754.5(3), c = 377.1(1) pm, wR2 = 0.0357, 233 F2 values for HoRhSn, a = 753.3(1), c = 372.16(8) pm, wR2 = 0.0721, 233 F2 values for ErRhSn, a = 753.7(3), c = 369.0(2) pm, wR2 = 0.0671,233 F2 values for TmRhSn, and a = 753.17(5), c = 366.53(4) pm, wR2 = 0.0566, 180 F2 values for YbRhSn with 14 parameters for each refinement. ScPdSn and ScPtSn adopt the HfRhSn type, a superstructure of ZrNiAl, space group P6̄2c: a = 747.5(1), c = 710.2(1) pm, for ScPdSn, and a = 738.37(9), c = 729.47(9) pm, wR2 = 0.0452,369 F2 values, 18 variables for ScPtSn. Structural motifs in these stannides are transition metal centered trigonal prisms formed by the rare earth and tin atoms. While these prisms are regular in the rhodium based stannides, significant distortions occur in ScPdSn and ScPtSn. The formation of the superstructure can be ascribed to packing reasons. The shortest interatomic distances occur between the transition metal (T) and tin atoms. These atoms form three-dimensional [FSn] networks in which the rare earth atoms fill distorted hexagonal channels. The series RERhSn displays a somewhat unique behavior. The a lattice parameter is more or less independent of the rare earth element, while the c lattice parameter shows the expected lanthanoid contraction. 119Sn Mössbauer spectroscopic data of the rhodium stannides show signals at isomer shifts varying from 1.77 to 1.82 mm/s subject to quadrupole splitting between 0.75 to 0.82 mm/s.

Synthesis, Structure, and Magnetic Properties of EuAgCd and YbAgCd

Abstract New intermetallic compounds EuAgCd and YbAgCd were synthesized in quantitative yield by reaction of the elements in sealed tantalum tubes in a high-frequency furnace. Both com­ pounds were investigated by X-ray diffraction on powders and single crystals: KHg2 type, Imma, a = 490.41(8), b = 771.0(1), c = 834.4(2) pm, wR2 = 0.0624, 255 F2 values, 12 variables for EuAgCd, and MgZn2 type, Pb3/mmc, a = 584.66(5), c = 946.83(9) pm, wR2 = 0.0502, 187 F2 values, 11 variables for YbAgCd. Owing to the very small difference in scattering power, no long range ordering of the silver and cadmium atoms is evident from the X-ray data, although Ag-Cd ordering is expected. The silver and cadmium atoms randomly occupy the mercury and zinc positions of the KHg2 and MgZn2 type structures, respectively. In EuAgCd the [AgCd] substructure consists of strongly puckered, orthorhombically distorted Ag3 Cd3 hexagons, while a three-dimensional network of face-and comer-sharing tetrahedra is observed in YbAgCd. The rare earth atoms fill the space between the Ag3 Cd3 hexagons (EuAgCd) or within the three-dimensional tetrahedral network (YbAgCd). Magnetic susceptibility measurements in­ dicate Pauli paramagnetism for YbAgCd and Curie-Weiss behavior above 60 K for EuAgCd with an experimental magnetic moment of 7.82(3) μB/Eu indicating divalent ytterbium and europium. Ferromagnetic ordering at Tc = 28.0(5) K is observed for EuAgCd. At 2 K and 5 T the saturation magnetization is 5.85(5) μB/Eu.