Rainer Poettgen

Publications of Rainer Poettgen

• Low Temperature Crystal Structure and 57Fe Moessbauer Spectroscopy of Sr3Sc2O5Fe2As2

  Authors: Marcus Tegel, Inga Schellenberg, Franziska Hummel, Rainer Poettgen, Dirk Johrendt

  05/2009;

  The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to delta(c)/c :The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to delta(c)/c : delta(a)/a = 4.2, which results in a slight flattening of the As-Fe-As bond angle within the FeAs layers. No indication of a structural instability could be detected. 57Fe Moessbauer spectroscopic data show a single signal at 4.2, 77, and 298 K, respectively, subjected to quadrupole splitting. The isomer shift increases from 0.36(1) mm/s at 298 K to 0.49(1) mm/s at 4.2 K. No indication for magnetic ordering was found. Comment: 12 pages, 4 figures

• The layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs

  Authors: Marcus Tegel, Franziska Hummel, Sebastian Lackner, Inga Schellenberg, Rainer Poettgen, Dirk Johrendt

  04/2009;

  Polycrystalline samples of the layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs were synthesized by high temperature solid state reactions and their crystal structures determined by the X-rayPolycrystalline samples of the layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs were synthesized by high temperature solid state reactions and their crystal structures determined by the X-ray powder diffraction. Their structures are tetragonal (P4/nmm; Sr2CrO3FeAs: a = 391.12(1) pm, c = 1579.05(3) pm; Ba2ScO3FeAs: a = 412.66(5) pm, c = 1680.0(2) pm, Z = 2) and isotypic to Sr2ScO3CuS. Iron arsenide layers are sandwiched between perowskite-like oxide blocks and separated by ~1600 pm, which is much larger compared to the 1111 iron arsenide superconductors. The bond length and angles within the FeAs layers are adapted to the space requirements of the oxide blocks. Measurements of the magnetic susceptibility and electrical resistivity show no hint for a SDW-like anomaly in both compounds. Sr2CrO3FeAs shows Curie-Weiss paramagnetism above 160 K with an effective magnetic moment of 3.83(3) muB in good agreement with the theoretical value of 3.87 muB for Cr3+. Antiferromagnetic ordering was detected below TN ~ 31 K. 57Fe Moessbauer spectra of Sr2CrO3FeAs show a single signal that broadens below the magnetic ordering temperature due to a small transferred hyperfine field induced by the magnetic ordering of the chromium atoms. 57Fe-Moessbauer spectra of Ba2ScO3FeAs show single signals at 298, 77, and 4.2 K which are only subject to weak quadrupole splitting. Comment: substantial addititions and corrections, references updated 19 pages, 7 figures

• Competition of magnetism and superconductivity in underdoped (Ba1-xKx)Fe2As2

  Authors: Marianne Rotter, Marcus Tegel, Inga Schellenberg, Falko M. Schappacher, Rainer Poettgen, Joachim Deisenhofer, Axel Guenther, Florian Schrettle, Alois Loidl, Dirk Johrendt

  01/2009;

  Polycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. ThePolycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity and magnetic susceptibility measurements. Specific heat data show a broad SDW phase transition in (Ba0.9K0.1)Fe2As2, which is hardly discernible in (Ba0.8K0.2)Fe2As2. No SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4)Fe2As2, where C changes by 0.1 J/K at Tc = 37.3 K. 57Fe-Moessbauer-spectra show full magnetic hyperfine field splitting, indicative of antiferromagnetic ordering at 4.2 K in samples with x = 0-0.2, but zero magnetic hyperfine field in samples with x = 0.3. The spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2)Fe2As2 in the phase transition regions are temperature-dependent superpositions of magnetic and non-magnetic components, caused by inhomogeneous potassium distribution. Our results suggest the co-existence of AF magnetic ordering and superconductivity without mesoscopic phase separation in the underdoped region and show unambiguously homogeneous superconducting phases close to optimal doping. This is in contrast to recently reported results about single crystal (Ba1-xKx)Fe2As2.

• Magnetic ordering in the static intermediate-valent cerium compound Ce2RuZn4

  Authors: Volker Eyert, Ernst-Wilhelm Scheidt, Wolfgang Scherer, Wilfried Hermes, Rainer Poettgen

  09/2008;

  The low-temperature behavior of Ce2RuZn4 has been investigated. Specific heat and magnetic susceptibility data reveal an antiferromagnetic transition at a Neel temperature of 2 K. Ce2RuZn4 is aThe low-temperature behavior of Ce2RuZn4 has been investigated. Specific heat and magnetic susceptibility data reveal an antiferromagnetic transition at a Neel temperature of 2 K. Ce2RuZn4 is a static intermediate-valent compound with two crystallographically independent cerium atoms. The magnetic data clearly show that only one cerium site is magnetic (Ce^3+), while the second one carries no magnetic moment. The experimental data are interpreted with the help of first principles electronic structure calculations using density functional theory and the augmented spherical wave method. The calculations reveal the occurrence of two different cerium sites, which are characterized by strongly localized magneticmoments and strong Ce-Ru bonding, respectively.

• Structural and magnetic phase transitions in the ternary iron arsenides SrFe2As2 and EuFe2As2

  Authors: Marcus Tegel, Marianne Rotter, Veronika Weiss, Falko M. Schappacher, Rainer Poettgen, Dirk Johrendt

  07/2008;

  The structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray powder diffraction and 57-Fe Moessbauer spectroscopy.The structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray powder diffraction and 57-Fe Moessbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr2Si2-type structure at room temperature and exhibit displacive structural transitions at 203 K (SrFe2As2) or 190 K (EuFe2As2) to orthorhombic lattice symmetry in agreement with the group-subgroup relationship between I4/mmm and Fmmm. 57-Fe Moessbauer spectroscopy experiments with SrFe2 As2 show full hyperfine field splitting below the phase transition temperature (8.91(1) T at 4.2 K). Order parameters were extracted from detailed measurements of the lattice parameters and fitted to a simple power law. We find a relation between the critical exponents and the transition temperatures for AFe2As2 compounds, which shows that the transition of BaFe2As2 is indeed more continuous than the transition of SrFe2As2 but it remains second order even in the latter case.

• Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds

  Authors: Frederick Casper, Claudia Felser, Ram Seshadri, C. Peter Sebastian, Rainer Poettgen

  11/2007;

  The XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited toThe XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited to understand the electronic structure of semiconducting 8-electron compounds such as LiAlSi (formulated Li$^+$[AlSi]$^-$) or semiconducting 18-electron compounds such as TiCoSb (formulated Ti$^{4+}$[CoSb]$^{4-}$). The basis for this is that [AlSi]$^-$ (with the same electron count as Si$_2$) and [CoSb]$^{4-}$ (the same electron count as GaSb), are both structurally and electronically, zinc-blende semiconductors. The electronic structure of half-metallic ferromagnets in this structure type can then be described as semiconductors with stuffing magnetic ions which have a local moment: For example, 22 electron MnNiSb can be written Mn$^{3+}$[NiSb]$^{3-}$. The tendency in the 18 electron compound for a semiconducting gap -- believed to arise from strong covalency -- is carried over in MnNiSb to a tendency for a gap in one spin direction. Here we similarly propose the systematic examination of 18-electron hexagonal compounds for semiconducting gaps; these would be the "stuffed wurtzite" analogues of the "stuffed zinc blende" half-Heusler compounds. These semiconductors could then serve as the basis for possibly new families of half-metallic compounds, attained through appropriate replacement of non-magnetic ions by magnetic ones. These semiconductors and semimetals with tunable charge carrier concentrations could also be interesting in the context of magnetoresistive and thermoelectric materials.

• Variation of the bonding interactions and magnetism in GdAuX (X = Mg, Cd, and In)

  Authors: Hem Chandra Kandpal, Frederick Casper, Gerhard H. Fecher, Claudia Felser, Juergen Kuebler, Rainer Poettgen

  10/2007;

  Results of first-principles electronic structure calculations for the isotypic compounds GdAuX (ZrNiAl type, X = Mg, Cd, and In) are presented. We report on a systematic examination of the electronicResults of first-principles electronic structure calculations for the isotypic compounds GdAuX (ZrNiAl type, X = Mg, Cd, and In) are presented. We report on a systematic examination of the electronic structure and nature of the bonding in these intermetallics. Our calculations indicate a metallic state for all of the compounds. We find that the indium in GdAuIn and magnesium in GdAuMg have significant bonding interactions with Au. We have also identified In s lone pair in GdAuIn has more localized behaviour as compared with Mg s in GdAuMg. The magnetic properties are well described within the local density approximation.