Wilfried Hermes

Publications of Wilfried Hermes

• Ternary Antmonides RE2Ir3Sb4 (RE = La, Ce, Pr, Nd)

  Authors: Konrad Schäfer, Wilfried Hermes, Ute Ch. Rodewald, Rolf-Dieter Hoffmann, Rainer Pöttgen

  06/2011;

  The antimonides RE2Ir3Sb4 (RE = La, Ce, Pr, Nd) were synthesized by arc-melting of the elements and subsequent annealing or via high-frequence melting. The samples were characterized by X-ray powderThe antimonides RE2Ir3Sb4 (RE = La, Ce, Pr, Nd) were synthesized by arc-melting of the elements and subsequent annealing or via high-frequence melting. The samples were characterized by X-ray powder diffraction and the four structures were refined from sin-gle-crystal X-ray diffraction data: Pr2Ir3Sb4-type, Pnma, Z = 4, a = 1621.9(2), b = 458.60(8), c = 1099.8(1) pm, wR2 = 0.036, 1558 F2 values for La2Ir3Sb4, a = 1616.6(8), b = 456.5(2), c = 1094.8(5) pm, wR2 = 0.092, 1080 F2 values for Ce2Ir3Sb4, a = 1613.0(5), b = 454.9(2), c = 1094.1(5) pm, wR2 = 0.057, 1428 F2 values for Pr2Ir3Sb4, and a = 1609.8(6), b = 452.9(2), c = 1092.3(5) pm, wR2 = 0.052, 1472 F2 values for Nd2Ir3Sb4, with 56 parameters per refinement. The Sb1 atoms show enhanced displace-ment off the mirror planes at y = 1/4 and y = 3/4. A series of temperature dependent structure refinements of Pr2Ir3Sb4 down to 90 K are indicative of static disorder. The iridium and antimony atoms build up complex covalently bonded three-dimensional [Ir3Sb4] networks with Ir-Sb distances ranging from 256-269 pm (Nd2Ir3Sb4). The two crystallographically independent rare earth sites fill cavities of coordination numbers 17 (8 Ir + 9 Sb) and 15 (6 Ir + 9 Sb) within the [Ir3Sb4] polyanions. Temperature dependent magnetic susceptibility measurements indicate a stable trivalent ground state for the ce-rium compound. No magnetic ordering was evident down to 3 K.

• On the Strongly Correlated Electron Hydride Ce2Ni2MgH7.7.

  Authors: Bernard Chevalier, Aleksandra Krolak, Jean-Louis Bobet, Etienne Gaudin, François Weill, Wilfried Hermes, Rainer Pöttgen

  Inorganic chemistry. 11/2008;

  The intermediate valence compound Ce 2Ni 2Mg absorbs irreversibly hydrogen when exposed under 1 MPa of H 2 pressure at room temperature. The resulting hydride Ce 2Ni 2MgH 7.7 is stable in air andThe intermediate valence compound Ce 2Ni 2Mg absorbs irreversibly hydrogen when exposed under 1 MPa of H 2 pressure at room temperature. The resulting hydride Ce 2Ni 2MgH 7.7 is stable in air and crystallizes as the deuteride La 2Ni 2MgD 8 in a monoclinic structure (space group P2 1 /c) with the unit cell parameters a = 11.7620(2), b = 7.7687(2), and c = 11.8969(2) A and beta = 92.75 degrees . The H-insertion in Ce 2Ni 2Mg induces a structural transition from a tetragonal to a monoclinic symmetry with an unit cell volume expansion Delta V m/ V m approximately 24.9%. The investigation of the hydride by magnetization, electrical resistivity, and specific heat measurements indicates a change from an intermediate valence behavior to a non-magnetic strongly correlated electron system. This transition results from a change of the coupling constant J cf between 4f(Ce) and conduction electrons induced by the hydrogenation.

• 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.

• Spin density wave anomaly at 140 K in the ternary iron arsenide BaFe2As2

  Authors: Marianne Rotter, Marcus Tegel, Inga Schellenberg, Wilfried Hermes, Rainer Pöttgen, Dirk Johrendt

  06/2008;

  The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to LaFeAsO, the parent compound of the iron arsenideThe ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (I4/mmm) to orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a single signal at room temperature and full hyperfine field splitting below the phase transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can serve as a new parent compound for oxygen-free iron arsenide superconductors.

• Structure, magnetic and electrical properties of CeIrSb and its hydride CeIrSbH_0.8

  Authors: Etienne Gaudin, Bernard Chevalier, Wilfried Hermes, Ute Ch. Rodewald, Rainer Pöttgen

  The ternary antimonide CeIrSb absorbs hydrogen under moderate temperature and pressure conditions (4 MPa and 573 K), leading to the hydride CeIrSbH_0.8. The crystal structures of bothThe ternary antimonide CeIrSb absorbs hydrogen under moderate temperature and pressure conditions (4 MPa and 573 K), leading to the hydride CeIrSbH_0.8. The crystal structures of both compounds have been investigated by X-ray diffraction on powders and single crystals: TiNiSi type, space group Pnma , a =735.07(7), b =456.93(4), c =792.8(1) pm, R 1/w R 2=0.0206/0.0395, 601 F ² values for CeIrSb and a =728.16(14), b =460.35(6), c =825.87(2) pm, R 1/w R 2=0.0322/0.0735, 528 F ² values for CeIrSbH_0.8 with 20 variables per refinement. Hydrogenation induces both an increase of the cell volume V (+4%) and a strongly anisotropic expansion of the unit cell with a maximum of 4.3% in the c direction, leading to a significant increase of the Ce–Ir and Ce–Ce distances in this direction. The H-insertion into CeIrSb leads to a magnetic transition from intermediate valence to antiferromagnetic behavior ( T _N=7.0 K) evidenced by magnetization, electrical resistivity and specific heat measurements. This transition can be explained on the basis of the Doniach diagram considering the J_cf interaction between the 4 f (Ce) and conduction electrons.

• New quaternary hydride CeZnSnH_1.5 : structure, Magnetism, and chemical bonding

  Authors: Wilfried Hermes, Bernard Chevalier, Ute Ch. Rodewald, Samir F. Matar, François Weill, Inga Schellenberg, Rainer Pöttgen, Heiko Lueken, Manfred Speldrich

  Chemistry of Materials.

  The quaternary hydride CeZnSnH1.5 was synthesized from a CeZnSn precursor upon hydrogenation at 553 K and 4 MPa H2 pressure. Single-crystal X-ray diffraction data reveal that the metal atoms adopt anThe quaternary hydride CeZnSnH1.5 was synthesized from a CeZnSn precursor upon hydrogenation at 553 K and 4 MPa H2 pressure. Single-crystal X-ray diffraction data reveal that the metal atoms adopt an YPtAs-type structure: P63/mmc, a = 449.34(9) and c = 1731.3(4) pm, wR2 = 0.0493, 218 F2 values, and 12 variables. The hydrogen atoms fill three crystallographically independent ZnCe3 tetrahedral sites, leading to the composition CeZnSnH1.5. Hydrogenation anisotropically modifies the unit cell parameters, i.e., 456.7 → 449.34 pm (−1.6%) for a and 1673.8 → 1731.3 pm (+3.4%) for c. This leads to a drastic flattening of the two-dimensional [ZnSn] layers and a decrease of the Zn−Sn distances from 271 to 260 pm. Hydrogenation increases the Curie temperature from 4.8 K for CeZnSn to 7.3 K for CeZnSnH1.5.

• Hysteretic behavior and magnetic ordering in CeRuSn

  Authors: John A. Mydosh, Andre M. Strydom, Michael Baenitz, Bernard Chevalier, Wilfried Hermes, Rainer Pöttgen

  Physical Review B.

  We report the thermodynamic and transport properties of the newly synthesized Ce-intermetallic compound CeRuSn. This ternary stannide possesses an unconventional structure with two Ce sites at roomWe report the thermodynamic and transport properties of the newly synthesized Ce-intermetallic compound CeRuSn. This ternary stannide possesses an unconventional structure with two Ce sites at room temperature which exhibit different valencies. Just below room temperature there are large thermal hysteretic effects in the magnetic susceptibility, in the specific heat, as well as in electronic and heat transport properties suggesting the formation of an incommensurate charge density wave modulation whose q vector changes as a function of temperature. Our measurements indicate that one site displays magnetic Ce3+ behavior while the other is a valence fluctuator. At 2.7 K antiferromagnetic long-range order occurs within one-half of the Ce sites, e.g., the magnetic entropy of the transition is 1/2Rln2. Below TN a series of metamagnetic transitions takes place in rather small fields (~1–2 T), leaving a magnetically fluctuating background. Such behavior is unique among the many Ce–transition-metal compounds.

• New hydrides REScSiH and REScGeH (RE = La, Ce) : structure, magnetism, and chemical bonding

  Authors: Bernard Chevalier, Wilfried Hermes, Birgit Heying, Ute Ch. Rodewald, Adrienne Hammerschmidt, Samir F. Matar, Etienne Gaudin, Rainer Pöttgen

  Chemistry of Materials.

  The La2Sb type silicides and germanides REScSi and REScGe (RE = La, Ce) were synthesized from the elements by arc-melting and subsequent annealing at 1170 K. The structures of LaScSi and LaScGe wereThe La2Sb type silicides and germanides REScSi and REScGe (RE = La, Ce) were synthesized from the elements by arc-melting and subsequent annealing at 1170 K. The structures of LaScSi and LaScGe were refined on the basis of single-crystal X-ray diffraction data. The structures consist of a stacking of two-dimensional [ScSi] networks with ScSi4/4 rectangles, which are separated by rare earth atoms, which leave RE4/4 tetrahedral voids. The latter can be completely filled by hydrogenation leading to the quaternary hydrides REScSiH and REScGeH (RE = La, Ce). Hydrogenation is accompanied by an anisotropic unit cell expansion, i.e., a decrease in the a lattice parameter and a strong increase in c. The H-insertion into the compounds based on cerium induces for both a quasi-2D structure, a strong decrease of their antiferromagnetic ordering; for instance TN decreases from 26 to 3.0 K in the sequence CeScSi → CeScSiH and the occurrence of the influence of the Kondo effect evidenced by electrical resistivity and specific heat measurements. The electronic structure calculation applied to CeScSi and its hydride reveals strong Ce−H bonding influencing the magnetic properties of CeScSiH.