G. Hilscher

Thermoelectricity of clathrate I Si and Ge phases

We report on investigations of type I clathrate Si and Ge compounds with Ba partially substituted by rare earth atoms. Novel compounds from framework-deficient solid solutions Ba8 Alx Si42−3/4x 4−1/4x and Ba8 Gax Si42−3/4x 4−1/4x (x = 8, 12, 16; , open square... lattice defect) have been prepared and characterized. All x-ray intensity data are consistent with the standardized clathrate I-Ba8Al16Ge30 type structure (space group Pmn). In rare earth substituted clathrates, Eu2Ba6MxSi46−x (M = Cu, Al, Ga), rare earth atoms completely occupy the 2a position and thus form a new quaternary ordered version of the Ba8Al16Ge30 structure type. From a geometrical analysis of clathrate crystal structures, a systematic scheme for all known clathrate compounds is proposed. All clathrates studied are metals with low electrical conductivity. The highest Seebeck coefficient in the present series is deduced for Ba8In16Ge30, S = −75μV K−1, indicating transport processes dominated by electrons as carriers. The Eu-based clathrates investigated exhibit long-range magnetic order as high as 32 K for Eu2Ba6Al8Si36 of presumably ferromagnetic type. Magnetic susceptibility indicates in all cases a 2+ ground state for the Eu ions, in fine agreement with LIII absorption edge spectra.

catena-[μ-Tris(1,2-bis(tetrazol-1-yl)ethane-N4,N4′)iron(II)] bis(tetrafluoroborate): synthesis, structure, spectroscopic and magnetic characterization of a chain-type coordination polymer spin-crossover compound

In analogy to a common synthesis of 1-substituted 5-H tetrazoles (Tetrahedron Lett. 36 (1995)1759; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 11 (1985) 1521; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 1 (1991) 66; BGU, Belarus. Vestsi Akad. Navuk Belarusi, Ser. Khim. Navuk 1 (1992) 73), the new bidentate ligand 1,2-bis(tetrazol-1-yl)ethane [endi] was synthesized and characterized by X-ray diffraction, NMR, IR and UV–Vis spectroscopy. By using iron(II) tetrafluoroborate hexahydrate the complexation with this ligand yields a 1-dimensional linear coordination polymer similar to the recently published chain compound (Inorg. Chem. 39 (2000) 1891) exhibiting a thermally induced spin-crossover phenomenon. Similar to the 1,2-bis(tetrazol-1-yl)propane-bridged compound, our 1,2-bis(tetrazol-1-yl)ethane-bridged compound shows a gradual spin transition, but the spin-crossover temperature T1/2≈140 K is found to be 10 K above the other T1/2. The T1/2 was determined by temperature-dependent 57Fe-Mossbauer, far FT-IR and UV–Vis spectroscopy as well as by temperature-dependent magnetic susceptibility measurements. Single crystals of the complex were grown in situ from a solution of the ligand and iron(II) tetrafluoroborate. The X-ray structure determinations of both the high spin as well as the low spin state of the compound revealed a solid state structure, which is comparable to that of catena-[Fe(1,2-bis(tetrazole-1-yl)propane)3](ClO4)2 (Inorg. Chem. 39 (2000) 1891; 2nd TMR-TOSS Meeting, 4th Spin Crossover Family Meeting, Lufthansa Training Center, Seeheim/Germany, April 30–May 2, 1999). Both the 1,2-bis(tetrazol-1-yl)propane-bridged and our compound do not show a thermal hysteresis effect (J. Am. Chem. Soc. 115 (1993) 9810; Inorg. Chim. Acta 37 (1979) 169; Chem. Phys. Lett. 93 (1982) 567). The synthesis of the complex described in the experimental section yielded a fine powdered product being poorly soluble in most common solvents. The single crystal measurements were done with crystals obtained by various diffusion methods. Most of them yielded either thin needles or small hexagonal prism crystals depending on the specific conditions.

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 and anisotropy studies of Nd-Fe-B based permanent magnets

Abstract Magnetocrystalline anisotropy and coercivity of sintered and rapidly solidified Nd-Fe-B magnets are compared. The coercivity for melt-spun ribbons with optimum cooling rate appears to be dominated by domain wall pinning, while for sintered Nd-Fe-B the coercivity is mainly controlled by nucleation of reverse domains.

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.

Magnetic properties of RNi4B (R=Y, Gd) : magnetic and specific-heat measurement

YNi4B is a paramagnetic compound with the susceptibility equal to 2.7×10−6 emu/g at 5 K. From the low‐temperature specific heat, values of 458 K for the Debye temperature and 14.1 mJ/K2 mol for the coefficient of the electronic specific heat are obtained. GdNi4B is ferromagnetic below Tc=35.0 K where the λ‐type anomaly is observed in the specific heat. Magnetic and specific‐heat measurements provide strong evidence that the 3d band in YNi4B and GdNi4B is completely filled.

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.

Crystal chemistry and thermoelectric properties of clathrates with rare-earth substitution

Based on an analysis of the geometric crystallographic relations a general classification scheme is presented for intermetallides with four-coordinated networks isomorphous with hydrate clathrates. We prepared novel europium substituted clathrates, Eu 2-x (Sr,Ba) 6-x M y Si 46-y (M=Al, Ga), consistent with the standardized clathrate I-Ba 8 Al 16 Ge 30 type structure (space group Pm3n). Europium atoms in Ba compounds preferentially occupy the 2a position and thus form a new quaternary version of the Ba 8 Al 16 Ge 30 structure type. All clathrates studied are metals with low electrical conductivity. The negative Seebeck coefficients indicate transport processes dominated by electrons as carriers. Eu 2 Ba 6 Al 8 Si 36 and Eu 2 Ba 6 Ga 8 Si 36 exhibit long-range magnetic order below 32 and 38K of presumably ferromagnetic type. Magnetic susceptibilities indicate an Eu 2+ ground state, in fine agreement with L III absorption edge spectra.