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Now showing items 1 - 16 of 79

  • Two superstructures of Ce3Rh4Ge4

    Vosswinkel, Daniel   Hoffmann, Rolf-Dieter   Svitlyk, Volodymyr   Hermes, Wilfried   Greiwe, Magnus   Niehaus, Oliver   Chevalier, Bernard   Matar, Samir F.   Al Alam, Adel F.   Nakhl, Michel   Ouaini, Naim   Poettgen, Rainer  

    Two different samples of Ce3Rh4Ge4 were synthesized from different starting compositions by melting of the elements in an arc-melting furnace followed by annealing sequences in a sealed tantalum ampoule in a muffle furnace. The structures of two different stacking variants were refined on the basis of temperature dependent single-crystal X-ray diffractometer data. At high temperature Ce3Rh4Ge4 adopts the U3Ni4Si4 type structure with strongly enhanced anisotropic displacement parameters for the Rh1 atoms. For the two different crystals, additional reflections start to appear at different temperatures. The first crystal showed additional reflections already at room temperature (stacking variant I) and the second one showed additional reflections emerging below 270 K (stacking variant II). Stacking variant I could be described with the (3 + 1)D superspace group I2/m(alpha 0 gamma)00; alpha =3D 1/2a*, gamma =3D 1/2c*; (Z =3D 2), 1252 F-2 values, 48 variables, wR =3D 0.0306 for the main and wR =3D 0.0527 for 440 1st order satellite reflections, similar to Pr3Rh4Ge4. For stacking variant II the (3 + 1) D superspace group is Immm(alpha 00)00s; alpha =3D 1/2a*; (Z =3D 2). The structure could be refined with 1261 F-2 values, 53 variables and residuals of wR =3D 0.0331 for the main reflections and wR =3D 0.1755 (R1(obs) =3D 0.0788) for the 1st order satellite reflections, [a =3D 406.2(1), b =3D 423.7(1) and c =3D 2497.1(1) pm]. The commensurate description could be transformed to a three-dimensional (3D)supercell with space group Pnma and Z =3D 4: a =3D 812.5(1), b =3D 423.7(1), c =3D 2497.1(2) pm, 1261 F-2 values, 69 variables and wR =3D 0.0525. The relation of the U3Ni4Si4 type structure, the (3 + 1) D modulated and the 3D supercells are discussed on the basis of group-subgroup schemes. Ab initio electronic structure calculations are in line with the diffraction experiments, revealing the lowest total energy for the Pnma phase.
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  • Na3GaF6 - A crystal chemical and solid state NMR spectroscopic study

    Boehnisch, David   Seidel, Stefan   Benndorf, Christopher   Jansen, Thomas   Funke, Lena   Hoffmann, Rolf-Dieter   Heletta, Lukas   Stahl, Juliane   Johrendt, Dirk   Eckert, Hellmut   Juestel, Thomas   Poetgen, Rainer  

    Na3GaF6 and Na3GaF6:Mn4+ samples were obtained from NaNO3 and Ga(NO3)(3)center dot 9H(2)O in hydrofluoric acid using K2MnF6 or NaMnO4 as manganese sources. The structure of Na3GaF6 was studied by single crystal X-ray diffraction at 90, 293, 440 and 500 K, confirming the monoclinic cryolite type structure, space group P2(1)/c. The gallium atoms show slightly distorted octahedral coordination by fluorine atoms, similar to the Nal atoms. Coordination number 8 is observed for Na2. Both sodium sites are clearly distinguished by Na-23 MAS-NMR spectroscopy. Above 400 K the spectra reveal distinct chemical exchange effects, signifying sodium ion hopping between these two sites. At the same time static F-19 NMR spectra indicate pronounced motional narrowing effects in this temperature region. The nearly invariant Ga-69 MAS-NMR spectra suggest that any reorientational motion involving the GaF63- ions (if present) occurs with preservation of the center of mass of these octahedra.
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  • EuAuGe Type Indides RAgIn (R =3D Ca, Sr, La, Eu)

    Stein, Sebastian   Schmolke, Kai Heinz   Block, Theresa   Heletta, Lukas   Hoffmann, Rolf-Dieter   Poettgen, Rainer  

    The equiatomic intermetallic phases CaAgIn [a =3D 482.75(7), b =3D 750.0(1), c =3D 835.5(1) pm], SrAgIn [a =3D 495.86(5), b =3D 794.71(9), c =3D 851.89(9) pm], LaAgIn [a =3D 489.99(5), b =3D 767.93(9), c =3D 837.53(9) pm], and EuAgIn [a =3D 493.02(7), b =3D 781.6(1), c =3D 844.2(1) pm] were synthesized from the elements in sealed niobum containers. They crystallize with the EuAuGe type structure, space group Imm2. The four structures were refined from single-crystal Xray data. The silver and indium atoms build up orthorhombically distorted, puckered Ag3In3 hexagons, which are stacked in AA' sequence, leading to direct Ag-Ag and In-In interlayer bonding (e.g. 303 and 304 pm in CaAgIn). Temperature dependent magnetic susceptibility measurements show a magnetic moment of 7.40(1) mu(B) per europium atom. EuAgIn orders antiferromagnetically at 5.7(5) K. The divalent nature of europium is also evident from Eu-151 Mosbauer spectra: delta =3D -10.50(1) mm.s(-1) at 78 K.
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  • Structural phase transitions in YPtGe2 and GdPtGe2

    Janka, Oliver   Hoffmann, Rolf-Dieter   Heying, Birgit   Poettgen, Rainer  

    The germanides YPtGe2 and GdPtGe2 were synthesized from REGe2 precursor compounds and platinum by arc-melting and their structures were studied on the basis of temperature-dependent single crystal X-ray diffraction data. At room temperature both germanides adopt the orthorhombic YIrGe2 type structure, space group Immm, with enhanced U-11 displacement parameters for the Ge1 atoms. Below 174 and 145 K, respectively, satellite reflections emerge in the diffraction patterns, giving rise to modulations. The low-temperature structures were refined in the superspace group Pnnn(1/2,1/2,gamma)qq0 (48.1.11.3). The structural phase transition is also evident in the magnetic susceptibility, specific heat and resistivity data. The high-and low-temperature modifications are discussed on the basis of a group-subgroup scheme.
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  • SrPt3In2 - an orthorhombically distorted coloring variant of SrIn5 dagger

    Muts, Ihor R.   Hlukhyy, Viktor   Galadzhun, Yaroslav V.   Solokha, Pavlo   Seidel, Stefan   Hoffmann, Rolf-Dieter   Poettgen, Rainer   Zaremba, Vasyl' I.  

    The new intermetallic phase SrPt3In2 was synthesized by induction-melting of the elements in a sealed tantalum ampoule followed by long-term annealing for crystal growth. The SrPt3In2 structure was refined from single crystal X-ray diffraction data: Imma, a =3D 1674.7(6), b =3D 921.2(4), c =3D 971.2(4) pm, wR(2) =3D 0.0551, 1192 F-2 values and 55 variables. Electronic structure calculations indicate strong covalent Pt-In bonding and a substantial charge transfer from the strontium atoms to the three-dimensional [Pt3In2](delta-) polyanionic network. The strontium atoms fill larger cavities within the network and the bonding of strontium to the polyanion is of the electrostatic type. The Bader charge calculations classify SrPt3In2 as a ternary platinide. The close relationship between the SrPt3In2 structure and the aristotype CaCu5 is discussed on the basis of a group-subgroup scheme in the Barnighausen formalism along with other CaCu5 coloring variants and superstructures.
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  • The modulated structure of intermediate-valent CeCoGa

    Niehaus, Oliver   Hoffmann, Rolf-Dieter   Chevalier, Bernard   Poettgen, Rainer  

    CeCoGa was synthesized by melting of the elements in an arc-melting furnace as well as in a sealed niobium tube in an induction furnace. A further annealing step improves the purity and crystallinity of the samples significantly. Its structure was refined on the basis of single-crystal X-ray diffractometer data at different temperatures. Already at room temperature CeCoGa crystallizes in a superstructure of the HT-CeCoAl type. This superstructure can be described in the (3 + 1)D superspace group C2/m(alpha 0 gamma) 00; alpha = 2/3, gamma = 1/3 with a temperature independent q-vector (Z = 4). For the 300 K data (also for 90 K) the commensurate modulated structure could be refined with 1336 F-2 values, 56 variables and residuals of wR = 0.0348 for the main and wR = 0.0605 for the satellites of 1st order [a = 1101.7(1), b = 436.0(1) and c = 482.4(1) pm, beta = 103.2(1)degrees]. Furthermore a description in a transformed 3D supercell with the space group C2/m and Z = 12 is possible. For the 90 K data this 3D supercell was refined with 1289 F-2 values, 56 variables and a residual of wR = 0.0409 [a = 1618.8(1), b = 435.3(1) and c = 1094.1(1) pm, beta = 119.3(1)degrees]. The relation of the HT-CeCoAl type structure, the (3 + 1)D modulated and the 3D supercell structure are discussed on the basis of a group-subgroup relation. By thermal analysis and magnetic measurements the phase transition temperature to the HT-CeCoAl type structure is stated slightly above 475 K. Furthermore intermediate cerium valence was identified by the magnetic susceptibility.
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  • Lead flux crystal growth of Ce2Ru12P7

    Niehaus, Oliver   Hoffmann, Rolf-Dieter   Poettgen, Rainer  

    Well-shaped needles of the metal-rich phosphide Ce2Ru12P7 were grown in a lead flux, while a polycrystalline sample became available through a ceramic route followed by arc-melting. Ce2Ru12P7 crystallizes with the hexagonal Zr2Fe12P7 type: P (6) over bar, a =3D 940.7(2), c =3D 389.8(1) pm, wR2 =3D 0.0342, 1001 F-2 values and 45 variables. Striking structural motifs in the Ce2Ru12P7 structure are tri-capped trigonal prisms that are centered by the phosphorus atoms: P1@Ru7Ce 2, P2@Ru7Ce2 and P3@Ru-9. Adjacent prisms are condensed via common edges to propeller-like building units.
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  • Monoclinic superstructure of Pr3Rh4Ge4

    Vosswinkel, Daniel   Hoffmann, Rolf-Dieter   Greiwe, Magnus   Eul, Matthias   Poettgen, Rainer  

    Pr3Rh4Ge4 was synthesized by melting of the elements in an arc-melting furnace followed by annealing in a sealed tantalum ampoule in a muffle furnace. Structure refinement was based on temperature dependent single-crystal X-ray diffractometer data. At ambient temperature Pr3Rh4Ge4 adopts the U3Ni4Si4 type structure with strongly enhanced anisotropic displacement parameters for Rh1. Below 230 K additional reflections start to appear and at 110 K the structure could be described with the (3+1)D superspace group I2/m(alpha 0 gamma)00; alpha = 1/2, gamma = 1/2; (Z = 2). This commensurately modulated structure could be refined with 1448 F-2 values, 39 variables and residuals of wR = 0.0417 for the main reflections and wR = 0.1520 for the satellites of 1st order, [a = 408.36(2), b = 421.12(3) and c = 2504.4(2) pm]. The commensurate description could be transformed to a 3D supercell with space group A2/m and Z = 4: a = 816.72(2), b = 421.12(3), c = 2537.5(1) pm, beta = 99.26(1)degrees, 1448 F-2 values, 69 variables and wR = 0.0499. The relation of the U3Ni4Si4 type structure, the (3+ 1)D modulated and the 3D supercell is discussed on the basis of a group-subgroup scheme. Temperature dependent magnetic susceptibility data reveal Curie-Weiss paramagnetism with an experimental moment of 3.72(2) mu(B) /Pr atom and a Weiss constant of -12.6(5) K. No magnetic ordering is evident down to 3 K.
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  • The quaternary arsenide oxides Ce9Au5-xAs8O6 and Pr9Au5-xAs8O6

    Bartsch, Timo   Hoffmann, Rolf-Dieter   Pottgen, Rainer  

    The quaternary gold arsenide oxides Ce9Au5-xAs8O6 and Pr9Au5-xAs8O6 were synthesized from the rare earth elements (RE), rare earth oxides, arsenic and gold powder at maximum annealing temperatures of 1173 K. The structures were refined from single crystal X-ray diffractometer data: Pnnm, a =3D 1321.64(6) pm, b =3D 4073.0(3), c =3D 423.96(2), wR2 =3D 0.0842, 3106 F-2 values, 160 variables for Ce9Au4.91(4)As8O6 and Pnnm, a =3D 1315.01(4), b =3D 4052.87(8), c =3D 420.68(1) pm, wR2 =3D 0.0865, 5313 F-2 values, 160 variables for Pr9Au4.75(1)As8O6. They represent a new structure type and show a further extension of pnictide oxide crystal chemistry. A complex polyanionic gold arsenide network [Au5As8](15-) (with some disorder in the gold substructure) is charge compensated with polycationic strands of condensed edge-sharing O@RE4/4 and O@RE4/3 tetrahedra ([RE4O3](2)(12+)) as well as RE3+ cations in cavities.
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  • The ternary germanides UMnGe and U2Mn3Ge

    Hoffmann, Rolf-Dieter   P?ttgen, Rainer   Chevalier, Bernard   Gaudin, Etienne   Matar, Samir F.  

    The title compounds were prepared by induction levitation melting of the elemental components and subsequent annealing. UMnGe (Pnma, a = 686.12(9), b = 425.49(6) and c = 736.5(1) pm) adopts the orthorhombic structure of TiNiSi and U2Mn3Ge (P6(3)/mmc, a = 524.3(2) and c = 799.2(3) pm) possesses the hexagonal Mg2Cu3Si-type structure (ordered variant of the hexagonal Laves phase MgZn2). Both structures were refined from X-ray powder data to residuals of R-1 = 0.021 and 0.014 for UMnGe and U2Mn3Ge, respectively. The manganese and germanium atoms in UMnGe build up a three-dimensional [MnGe] network of ordered Mn3Ge3 hexagons with Mn-Ge distances ranging from 248 to 259 pm. The uranium atoms are coordinated by two tilted Mn3Ge3 hexagons. The manganese atoms in U2Mn3Ge build up Kagome networks with 252 and 272 pm Mn-Mn distances which are connected via the germanium atoms (254 pm Mn-Ge) to a three-dimensional network. A remarkable feature of the U2Mn3Ge structure is a short U-U distance of 278 pm between adjacent cavities of the [Mn3Ge] network. From OFF based electronic structure calculations both germanides are found more cohesive than the Laves phase UMn2, thus underpinning the substantial role of Mn-Ge bonding. Calculations for both germanides show ferrimagnetic ground states with antiparallel spin alignments between U and Mn. The valence bands show bonding characteristics for interactions of atoms of different chemical natures and significant Mn-Mn bonding in U2Mn3Ge. Preliminary investigation of UMnGe by magnetization measurements confirms an antiferromagnetic arrangement below T-N = 240 K. (C) 2013 Elsevier Masson SAS. All rights reserved.
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  • Structure and lithium mobility of Li(4)Pt(3)Si

    Dinges, Tim   Hoffmann, Rolf-Dieter   van Wuellen, Leo   Henry, Paul   Eckert, Hellmut   Poettgen, Rainer  

    The lithium-rich silicide Li(4)Pt(3)Si was synthesised from the elements by high-temperature synthesis in a sealed niobium ampoule. Its structure was refined on the basis of single-crystal X-ray diffraction data: R32, a = 693.7(2), c = 1627.1(4) pm, wR2 = 0.0762, 525 F(2) values and 21 variables. The striking structural motifs of the Li(4)Pt(3)Si structure are silicon atoms with a slightly distorted trigonal prismatic platinum coordination with short Si-Pt distances (238-246 pm). Always two trigonal prisms are condensed via a common Pt(3) triangle, and these double units built up a three-dimensional network by condensation via common corners. The channels left by this prismatic network are filled by two crystallographically independent lithium sites in a 3:1 ratio. The single crystal X-ray data were fully confirmed by neutron powder diffraction and (7)Li magic-angle spinning (MAS)-nuclear magnetic resonance (NMR) results. The two distinct lithium sites are well differentiated by their (7)Li isotropic chemical shift and nuclear electric quadrupolar interaction parameters. MAS-NMR spectra reveal signal coalescence effects above 300 K, indicating chemical exchange between the lithium sites on the millisecond timescale. The spectra can be simulated with a simple two-site exchange model. From the resulting temperature-dependent correlation times, an activation energy of 50 kJ/mol is extracted.
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  • The new high-pressure borate hydrate Cu3B6O12 center dot H2O

    Neumair, Stephanie C.   Kaindl, Reinhard   Hoffmann, Rolf-Dieter   Huppertz, Hubert  

    The new compound Cu3B6O12 center dot H2O, synthesized under high-pressure/high-temperature conditions of 7.5 GPa and 700 degrees C in a Walker-type multianvil apparatus, possesses a structure similar to CuB2O4 (3 x CuB2O4 = Cu3B6O12). Via the high-pressure technique, it was possible to incorporate water molecules into the structure, leading to a symmetry reduction compared to the water free compound (I (4) over bar 2d -> Fdd2). Cu3B6O12 center dot H2O crystallizes in the orthorhombic space group Fdd2 (Z = 8) with the lattice parameters a = 1620.4(3), b = 1657.6(3), c = 564.2(2) pm, V = 1.5154(5) nm(3), R-1 = 0.0375, wR(2) = 0.0640 (all data). The crystal structure of Cu3B6O12 center dot H2O is built up from corner-sharing BO4 tetrahedra forming a three-dimensional network with channels along the c-axis. Inside these channels, the water molecules and a part of the copper ions are arranged. (C) 2011 Elsevier Masson SAS. All rights reserved.
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  • SrPt2Al2 - A(3+2)D-incommensurately modulated variant of the CaBe2Ge2 type structure

    Hoffmann, Rolf-Dieter   Stegemann, Frank   Janka, Oliver  

    The first ternary compound in the Sr-Pt-Al system, SrPt2Al2, was synthesized by melting of the elements in a sealed niobium tube in a high frequency furnace. The structure of SrPt2Al2 was determined by single-crystal X-ray diffraction on the basis of data sets recorded at 90, 150, 210, 270 and 300 K. The compound crystallizes in an incommensurately modulated (3+2)D structure being related to the tetragonal primitive CaBe2Ge2 type structure. The structure was refined in the orthorhombic superspace group Pmmn(alpha, 0,0) 0s0(0,beta,0) s00 (alpha = 0.222(1) a*, beta = 0.224(1) b*, a = b = 436.89(5), c = 1010.16(13) pm at 90 K) with R-M = 0.0403, R-S,R-1 = 0.0597 and R-S,R-2 = 0.2046. The structural relation with CaBe2Ge2 along with a refinement in the tetragonal superspace group P4/nmm(alpha, 0,0)-0000(0,alpha,0)00s0 is discussed with respect to the temperature dependence of the q-vectors and by group-subgroup relations. The modulation, from the standpoint of the crystal chemistry, is mainly caused by the distortion of the Pt arrangements within the pseudo-tetragonal ab plane.
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  • Magnetic Anomalies and Electronic Structure of Ce(2)Cu(2)Mg and Ce(2)Pd(2)Mg

    Hermes, Wilfried   Linsinger, Stefan   Rayaprol, Sudhindra   Tuncel, Selcan   Hoffmann, Rolf-Dieter   Kremer, Reinhard K.   Jepsen, Ove   Poettgen, Rainer  

    The intermetallic compounds RE (2)Cu(2)Mg and RE (2)Pd(2)Mg (RE=La, Ce) were prepared and characterized by magnetic susceptibility and heat-capacity measurements. They crystallize with an ordered U(3)Si(2) type structure (space group P4/mbm) and the cerium compounds contain stable trivalent cerium ions, which undergo antiferromagnetic ordering at similar to 7.5 and similar to 4 K, respectively. In high magnetic fields Ce(2)Cu(2)Mg exhibits spin reorientation with critical field strength of 40 kOe at 5 K. The data are compared to the non-magnetic isotypic compounds La(2) T (2)Mg (T=Cu, Ni, Pd) and to the already reported intermediate valent Ce(2)Ni(2)Mg.
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  • SrPt2Al2 – A (3+2)D-incommensurately modulated variant of the CaBe2Ge2 type structure

    Hoffmann, Rolf-Dieter   Stegemann, Frank   Janka, Oliver  

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  • Magnetic properties of the germanides RE3Pt4Ge6 (RE=3DY,Pr,Nd,Sm,Gd Dy)

    Eustermann, Fabian   Eilers-Rethwisch, Matthias   Renner, Konstantin   Hoffmann, Rolf-Dieter   Poettgen, Rainer   Janka, Oliver  

    The germanides RE3Pt4Ge6 (RE =3DY, Pr, Nd, Sm, Gd Dy) have been synthesized by arc-melting of the elements followed by inductive annealing to improve the crystallinity and allow for structural order. The compounds have been studied by powder X-ray diffraction; additionally the structure of Y3Pt4Ge6 has been refined from single-crystal X-ray diffractometer data. It exhibits a (3 + 1)D modulated structure, indicating isotypism with Ceyt Ge,. The crystal structure can be described as an intergrowth between YIrGe2- and CaBe2Ge2-type slabs along [100]. Temperature-dependent magnetic susceptibility measurements showed Pauli paramagnetism for Y3PE,Ge, and Curie-Weiss paramagnetism for Pr3Pt4Ge6 and Nd3PT4Ge6. Sm3Pt4Ge6 exhibits van Vleck paramagnetism, while antiferromagnetic ordering at T-N=3D8.1(1.) K and T-N =3D 11.0(1) K is observed for Gdyt,Ge, and Tb3Pt4Ge6, respectively.
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