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Now showing items 1 - 16 of 82

  • The role of beryllium in alloys,Zintl phases and intermetallic compounds

    Janka, Oliver   Pottgen, Rainer  

    Although beryllium is widely used as alloying component in diverse light-weight alloys, the crystal chemistry of beryllium containing Zintl phases and intermetallic compounds is only scarcely developed and only few phase diagrams, mostly the industrially relevant ones, have been studied in detail. The present review summarizes the crystal chemical data of binary and ternary beryllium intermetallic compounds along with the results of the few documented physical property studies.
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  • Fe2Si5N8: Access to Open-Shell Transition-Metal Nitridosilicates

    Bielec, Philipp   Janka, Oliver   Block, Theresa   Poettgen, Rainer   Schnick, Wolfgang  

    Highly condensed nitridosilicates doped with Eu2+ or Ce3+ play an important role in saving energy by converting the blue light of (In,Ga)N-LEDs. Although nitridosilicates are known for great structural variety based on covalent anionic Si-N networks, elemental variety is restricted. Presenting a significant extension of the latter, this work describes a general access to open-shell transition-metal nitridosilicates. As a proof-of-principle, the first iron nitridosilicate, namely Fe2Si5N8, was prepared by exchanging Ca2+ in alpha-Ca2Si5N8 applying a FeCl2 melt (salt metathesis). The title compound was analyzed by powder X-ray diffraction, EDX, ICP-OES, combustion analysis, TG/DSC, Mossbauer spectroscopy and magnetic susceptibility measurements. Furthermore, the structure of alpha-Ca2Si5N8 was determined at 1073 and 1173 K confirming the anionic network of alpha-Ca2Si5N8 providing possible migration pathways for the ion-exchange reaction.
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  • Binary Polyazides of Cerium and Gadolinium

    Rosenstengel, Kati   Schulz, Axel   Niehaus, Oliver   Janka, Oliver   Poettgen, Rainer   Villinger, Alexander  

    Herein, we report on the preparation and full characterization (melting point, ATR-IR/Raman, elemental analysis, N-14 NMR) of binary gadolinium(III) azide as DMSO solvate Gd(N-3)(3)(8)/3DMSO (DMSO =3D dimethylsulfoxide), along with the partially hydroxide substituted species Gd-4(OH)(2)(N-3)(10)nDMSO (n =3D 8, 10). Moreover, the isolation and comprehensive characterization of hexaazido lanthanate(III) anions [Ln(N-3)(6)](3-) (Ln =3D Ce, Gd) in a series of [Ph4P](+) and [EtPh3P](+) salts is presented. Single-crystal structures of all compounds were determined, and the magnetic susceptibility in solution (NMR, Evans Method), as well as in the solid state (VSM), was investigated.
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  • Unusual Valence State - Trivalent Europium in Intermetallic Eu2Ir3Al9

    Stegemann, Frank   Block, Theresa   Klenner, Steffen   Janka, Oliver  

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  • Mo2B4O9-Connecting Borate and Metal-Cluster Chemistry

    Schmitt, Martin K.   Janka, Oliver   Poettgen, Rainer   Benndorf, Christopher   de Oliveira, Marcos, Jr.   Eckert, Hellmut   Pielnhofer, Florian   Tragl, Amadeus-Samuel   Weihrich, Richard   Joachim, Bastian   Johrendt, Dirk   Huppertz, Hubert  

    We report on the first thoroughly characterized molybdenum borate, which was synthesized in a high-pressure/high-temperature experiment at 12.3 GPa/1300 degrees C using a Walker-type multianvil apparatus. Mo2B4O9 incorporates tetrahedral molybdenum clusters into an anionic borate crystal structure-a structural motif that has never been observed before in the wide field of borate crystal chemistry. The six bonding molecular orbitals of the [Mo-4] tetrahedron are completely filled with 12 electrons, which are fully delocalized over the four molybdenum atoms. This finding is in agreement with the results of the magnetic measurements, which confirmed the diamagnetic character of Mo2B4O9. The two four-coordinated boron sites can be differentiated in the B-11 MAS-NMR spectrum because of the strongly different degrees of local distortions. Experimentally obtained IR and Raman bands were assigned to vibrational modes based on DFT calculations.
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  • The high-pressure phase of CePtAl

    Heymann, Gunter   Heying, Birgit   Rodewald, Ute Ch.   Janka, Oliver  

    The intermetallic aluminum compound HP-CePtAl was synthesized by arc melting of the elements with subsequent high-pressure/high-temperature treatment at 1620 K and 10.5 GPa in a multianvil press. The compound crystallizes in the hexagonal MgZn2-type structure (P6(3)/mmc) with lattice parameters of a =3D 552.7(1) and c =3D 898.8(2) pm refined from powder X-ray diffraction data. With the help of single crystal investigations (wR =3D 0.0527, 187 F-2 values, 13 variables), the proposed structure type was confirmed and the mixed Pt/Al site occupations could be refined. Magnetic susceptibility measurements showed a disappearance of the complex magnetic ordering phenomena, which are observed in NP-CePtAl.
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  • Hydrogenation-induced cerium valence change in CeNiZn

    Tence, Sophie   Heying, Birgit   Janka, Oliver   Poettgen, Rainer   Chevalier, Bernard  

    Polycrystalline CeNiZn (ZrNiAl type, P (6) over bar 2m, a =3D 715.6 (4), c =3D 388.4 (2) pm) forms a new hydride CeNiZnH1.2( 1) under hydrogen exposure (10 bar) at 423 K. The resulting hydride crystallizes in a hexagonal structure (P6/mmm) with the unit cell parameters a - 433.19 (2) and c - 424.37 (2) pm. Thus, hydrogen atom insertion yields a structural transition from a ZrNiAl-to an AlB2-type structure with a high volume cell expansion of +20.1%. The magnetic, thermal and transport properties of CeNiZnH1.2(1) were investigated. Susceptibility and specific heat measurements do not reveal magnetic order down to 1.8 K but most likely the presence of ferromagnetic correlations due to the proximity of a ferromagnetic order or a nonmagnetic atom disorder spin-glass state. Moreover, the cerium valence is modified from an intermediate to a trivalent state, a change that results from the decrease of the hybridization strength between conduction and Ce (4f) electrons upon hydrogenation. Transport measurements also evidence a deviation from the typical resistivity behavior of metallic systems. Several scenarios are conceived to explain this change, in particular weak localization effect, knowing that it rarely occurs upon hydrogenation of intermetallics. (C) 2017 Elsevier B.V. All rights reserved.
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  • Abrupt Europium Valence Change in Eu2Pt6Al15 around 45 K

    Radzieowski, Mathis   Stegemann, Frank   Block, Theresa   Stahl, Juliane   Johrendt, Dirk   Janka, Oliver  

    Eu2Pt6Al15 has been prepared from the elements via arc-melting and subsequent temperature treatment; the structure was refined from single crystal X-ray diffraction data. The compound crystallizes in an orthorhombic (3 + 1)D commensurately modulated structure (Sc2Pt6Al15 type) with space group Cmcm(alpha,0,0)0s0 (alpha =3D 2/3). Full ordering of the Pt and Al atoms within the [Pt6Al15](delta-) polyanion was observed. Magnetic measurements revealed an anomaly in the susceptibility data at T =3D 41.6(1) K, which was also observed as lambda-type anomaly in heat capacity measurements (T =3D 40.7(1) K). Temperature dependent powder X-ray diffraction experiments indicated a drastic shortening of the c axis (-18 pm, -1.1%) around 45 K, while the a axis nearly remains the same (-1 pm, -0.2%). Measurements of the electrical resistivity verified the anomaly, indicating a clear change in the electronic structure of the material. The observed anomalies in the physical measurements can be explained by a temperature driven first order valence change from Eu2+ at higher temperatures (>55 K) to Eu3+ at low temperatures. This valence change was proven by temperature dependent Eu-151 Mossbauer spectroscopic investigations. Isostructural Eu2Pt6Ga15 was prepared in comparison, and it shows divalent Eu atoms down to 2.5 K along with antiferromagnetic ordering at T-N =3D 13.1(1) 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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  • HP-MoO2: A High-Pressure Polymorph of Molybdenum Dioxide

    Luedtke, Tobias   Wiedemann, Dennis   Efthimiopoulos, Ilias   Becker, Nils   Seidel, Stefan   Janka, Oliver   Poettgen, Rainer   Dronskowski, Richard   Koch-Mueller, Monika   Lerch, Martin  

    High-pressure molybdenum dioxide (HP-MoO2) was synthesized using a multianvil press at 18 GPa and 1073 K, as motivated by previous first-principles calculations. The crystal structure was determined by single crystal X-ray diffraction. The new polymorph crystallizes isotypically to HP-WO2 in the orthorhombic crystal system in space group Pnma and was found to be diamagnetic. Theoretical investigations using structure optimization at density-functional theory (DFT) level indicate a transition pressure of 5 GPa at 0 K and identify the new compound as slightly metastable at ambient pressure with respect to the thermodynamically stable monoclinic MoO2 (alpha-MoO2; Delta E-m =3D 2.2 kJ.mol(-1)).
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  • In celebration of Thomas Schleid's 60(th) birthday

    Hartenbach, Ingo   Janka, Oliver   Lissner, Falk  

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  • Cerium intermetallics with TiNiSi-type structure

    Janka, Oliver   Niehaus, Oliver   Poettgen, Rainer   Chevalier, Bernard  

    Intermetallic compounds with the equiatomic composition CeTX that crystallize with the orthorhombic TiNiSi-type structure can be synthesized with electron-rich transition metals (T) and X = Zn, Al, Ga, Si, Ge, Sn, As, Sb, and Bi. The present review focusses on the crystal chemistry and chemical bonding of these CeTX phases and on their physical properties, Sn-119 and Sb-121 Mossbauer spectra, high-pressure effects, hydrogenation reactions and the formation of solid solutions in order to elucidate structure-property relationships. This paper is the final one of a series of four reviews on equiatomic intermetallic cerium compounds [Part I: Z. Naturforsch. 2015, 70b, 289; Part II: Z. Naturforsch. 2015, 70b, 695; Part III: Z. Naturforsch. 2016, 71b, 165].
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  • Cerium intermetallics CeTX - review III

    Poettgen, Rainer   Janka, Oliver   Chevalier, Bernard  

    The structure-property relationships of CeTX intermetallics with structures other than the ZrNiAl and TiNiSi type are systematically reviewed. These CeTX phases form with electron-poor and electron-rich transition metals (T) and X = Mg, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn, Pb, P, As, Sb, and Bi. The review focusses on the crystal chemistry, the chemical bonding peculiarities, and the magnetic and transport properties. Furthermore 119Sn Mossbauer spectroscopic data, high-pressure studies, hydrogenation reactions and the formation of solid solutions are reviewed. This paper is the third of a series of four reviews on equiatomic intermetallic cerium compounds
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  • Inorganic Double Helices in Semiconducting SnIP

    Pfister, Daniela   Schaefer, Konrad   Ott, Claudia   Gerke, Birgit   Poettgen, Rainer   Janka, Oliver   Baumgartner, Maximilian   Efimova, Anastasia   Hohmann, Andrea   Schmidt, Peer   Venkatachalam, Sabarinathan   van Wuellen, Leo   Schuermann, Ulrich   Kienle, Lorenz   Duppel, Viola   Parzinger, Eric   Miller, Bastian   Becker, Jonathan   Holleitner, Alexander   Weihrich, Richard   Nilges, Tom  

    SnIP is the first atomic-scale double helical semiconductor featuring a 1.86 eV bandgap, high structural and mechanical flexibility, and reasonable thermal stability up to 600 K. It is accessible on a gram scale and consists of a racemic mixture of right-and left-handed double helices composed by [SnI] and [P] helices. SnIP nanorods <20 nm in diameter can be accessed mechanically and chemically within minutes.
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  • Cerium intermetallics with TiNiSi-type structure

    Janka, Oliver   Niehaus, Oliver   P?ttgen, Rainer   Chevalier, Bernard  

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  • Magnetic Ordering in the Solid Solution CePt1-xPdxAl (x=0.1-0.8)

    Eilers-Rethwisch, Matthias   Niehaus, Oliver   Janka, Oliver  

    In order to investigate the magnetic properties of the solid solution CePt1-xPdxAl the individual compounds were synthesized from the elements using an arc furnace. From x = 0-0.8 the solid solution crystallizes in the orthorhombic TiNiSi-type structure (Pnma, no. 62; a = 721-722, b = 448-453 and c = 778-779 pm) and for x = 0.9 both the TiNiSi- as well as the hexagonal ZrNiAl-type structures were observed. The solid solution exhibits an interesting magnetic behavior as for small values of x ferromagnetic ordering can be found. At higher palladium content this changes towards an antiferromagnetic ordering. Furthermore the solid solution exhibits a spin-reorientation (meta-magnetic step) for x = 0.2-0.7.
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