H. Michor

Ternary clathrates Ba–Zn–Ge: phase equilibria, crystal chemistry and physical properties

The present paper describes the formation, phase relations at subsolidus temperatures and at 800 °C, crystal chemistry and physical properties of a series of ternary clathrates as part of the solid solution , derived from binary with a solubility limit of 8 Cd per formula unit at 800 °C. Structural investigations in all cases confirm cubic primitive symmetry with a lattice parameter a≈1.1 nm, consistent with the space group type . Both the temperature dependent x-ray spectra and the heat capacity define a low-lying, almost localized, phonon branch. Studies of transport properties show electrons to be the majority charge carriers in the systems. As the Cd content increases, the system is driven towards a metal-to-insulator transition, causing , for example, to show metallic behaviour at low temperatures while at high temperatures semiconducting features become obvious. A model based on a gap of the electronic density of states slightly above the Fermi energy perfectly explains such a scenario. Thermal conductivity exhibits a pronounced low temperature maximum, dominated by the lattice contribution, while at higher temperatures the electronic part becomes more important.

Superconductivity in novel Ge-based skutterudites: {Sr,Ba}pt4Ge12.

Combining experiments and ab initio models we report on SrPt4Ge12 and BaPt4Ge12 as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge atoms. Below T(c)=5.35 and 5.10 K for BaPt4Ge12 and SrPt4Ge12, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-p states dominate the electronic structure at the Fermi energy.

Superconductivity in Y-Ni-B base compounds

Carbon introduced in Y-Ni-B ternary alloy material was shown to stabilise aquaternary compound YNi2B2C which is also responsible for the appearance of superconductivity in Y-Ni-B alloys YNi4−xB1+x. Rietveld refinement of a nearly single-phase material of YNi2B2C confirmed isotypism with the filled ThCr2Si2-type (zB=0.353(1)). Magnetic and specific heat measurements indicate type-II superconductivity in YNi2B2C with a Tc-onset of 15.2 K and a normalised specific heat discontinuity ΔC/γTc=1.57. We present a first estimate of the upper critical field μ0Hc2(0)=3.7 T, the coherence length ξGL=9.4 nm and the Ginzburg-Landau parameter κ=10.5 of this new compound.

A novel skutterudite phase in the Ni–Sb–Sn system: phase equilibria and physical properties

A novel ternary phase, SnyNi4Sb12−xSnx, has been characterized and found to exhibit a wide range of homogeneity (at 250 °C, 2.4 ≤ x ≤ 5.6, 0 ≤ y ≤ 0.31; at 350 °C, 2.7 ≤ x ≤ 5.0, 0 ≤ y ≤ 0.27). SnyNi4Sb12−xSnx crystallizes in a skutterudite-based structure in which Sn atoms are found to occupy two crystallographically inequivalent sites: (a) Sn and Sb atoms randomly share the 24g site; and (b) a small fraction of Sn atoms occupy the 2a (0, 0, 0) position, with an anomalously large isotropic atomic displacement parameter. Eu0.8Ni4Sb5.8Sn6.2, Yb0.6Ni4Sb6.7Sn5.3 and Ni4As9.1Ge2.9 are isotypic skutterudites. Depending on the particular composition, metallic as well as semiconducting states appear. The crossover from semiconducting to metallic behaviour is discussed in terms of a temperature-dependent carrier concentration employing a simple model density of states with the Fermi energy slightly below a narrow energy gap. This model accounts for the peculiar temperature-dependent electrical resistivity. These skutterudites are characterized by a number of lattice vibrations, which were elucidated by Raman measurements and compared to the specific heat data. The Eu-containing compound exhibits long-range magnetic order at Tmag ≈ 6 K, arising from the Eu2+ ground state.

Magnetic behaviour of RCuAl compounds

Abstract We studied structural and magnetic properties of the RCuAl (R=rare earth) intermetallic compounds by means of X-ray diffraction, susceptibility, magnetization and specific heat measurements. Magnetic ordering at low temperatures was observed in most of these materials. Results provide indications of ferromagnetic coupling in the heavy rare-earth compounds (R=Gd–Er), while PrCuAl and NdCuAl show antiferromagnetic behaviour. Additional magnetic phase transition in the ordered state was found in GdCuAl and DyCuAl. A metamagnetic transition was observed in DyCuAl.

Structural chemistry, magnetism and thermodynamic properties of R2Pd2In

We report on magnetisation, resistivity and specific heat measurements of R2Pd2In compounds synthesised with the nominal composition R40Pd41In19 (R=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y). Magnetic and thermodynamic measurements reveal antiferromagnetic order below 32 K for this series except for Y, La, Yb and Lu. An appreciably wide homogeneity range is found for Ce2Pd2+xIn1−x where ferro- or antiferromagnetic order or both occur with typical features of a Kondo lattice. Yb2Pd2In exhibits intermediate valent behaviour and no magnetic order could be detected down to 0.3 K.

Unconventional superconducting phase in the weakly correlated noncentrosymmetric Mo 3 Al 2 C compound

Electrical resistivity, specific-heat, and NMR measurements classify noncentrosymmetric Mo(3)Al(2)C (beta-Mn type, space group P4(1)32) as a strong-coupled superconductor with T(c)=9 K deviating notably from BCS-type behavior. The absence of a Hebbel-Slichter peak, a power-law behavior of the spin-lattice relaxation rate (from (27)Al NMR), an electronic specific heat strongly deviating from BCS model and a pressure enhanced T(c) suggest unconventional superconductivity with possibly a nodal structure of the superconducting gap. Relativistic density-functional theory calculations reveal a splitting of degenerate electronic bands due to the asymmetric spin-orbit coupling, favoring a mix of spin-singlet and spin-triplet components in the superconducting condensate, in absence of strong correlations among electrons.

Thermal conductivity of superconducting MgB2

Thermal conductivity of superconducting MgB2 was studied in both the superconducting and the normal state region. The latter is almost equally determined by the electronic and the lattice contribution to the total thermal conductivity. In the superconducting state, however, the lattice contribution is larger. The electronic thermal conductivity below Tc was derived from the experimental data considering the Bardeen-Rickayzen-Tewordt theory together with the model of Geilikman. The analysis shows that electron scattering on static imperfections dominates.

Magnetic structure study of ErCuAl and ErNiAl

Abstract Polycrystalline samples of ErCuAl and ErNiAl have been studied by powder neutron diffraction and, in the case of ErCuAl, by specific heat and AC-susceptibility measurements. ErCuAl orders ferromagnetically with Er magnetic moments aligned along the c-axis below Tc = 6.8 K. In the case of ErNiAl, magnetic ordering below TN = 5.4 K and indications of an additional phase transition at 4.2 K have been observed. The magnetic structure of ErNiAl is described by the propagation vector ( 1 2 , 0, 1 2 ). The magnetic moments lie in the basal plane at 1.8 K.

New magnetic phenomena in TbNi2

Structural investigations at room temperature revealed that TbNi2 does not crystallize in the Laves phase structure, but shows a superstructure of the Laves phase with the space group F-43m. Susceptibility, specific heat, magnetostriction and magnetoresistance measurements on polycrystalline specimens showed an additional magnetic phase transition at TR = 14 K below the Curie temperature of TC = 36±0.2 K. In order to clarify the nature of this magnetic phase transition at 14 K, elastic neutron diffraction below and above TR and TC was performed. The analysis of these data showed that this transition at TR is due to the rotation of the Tb moments on three of the total of eight non-equivalent Tb sites in the rhombohedrally distorted unit cell in the magnetic ordered state. This rotation of these Tb moments is out of the [111] direction into a plane perpendicular to the space diagonal. The cause for this magnetic instability is due to an interplay of the regularly arranged vacancies in the superstructure and the crystal field level position which has been studied by inelastic neutron scattering.