Ch. Paul

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.

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.

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.

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.

Magnetic behaviour of PrFe4Sb12 and NdFe4Sb12 skutterudites

Abstract Skutterudites Pr0.73Fe4Sb12 and Nd0.72Fe4Sb12 order magnetically below 5 and 16.5 K , respectively. Pr0.73Fe4Sb12 exhibits an unusual high electronic contribution to the specific heat Cp/T of about 1000 mJ / mol K 2 ; moreover, magnetic fields of the order of 3 T are able to suppress long range magnetic order and at some critical field Cp/T behaves proportional to − ln T . Additionally, the low temperature resistivity changes from a T2 behaviour at μ 0 H=0 T to an almost linear dependence for μ0H≈3– 4 T .

Novel Zn9-cluster compounds RE2Zn6Ge3 (RE: La, Ce, Pr, Nd, Sm, Gd): crystal structure and physical properties*

A novel ternary structure type has been determined from single crystals of Ce2Zn6Ge3 grown from indium?zinc flux solvent. The Ce2Zn6Ge3 type is hexagonal (a = 0.767 69(2) nm; c = 0.411 59(2) nm) with space group P 2m, Z = 1. Isotypic compounds with La, Pr, Nd, Sm and Gd were synthesized by reaction sintering, and their isotypic crystal structures were confirmed from Rietveld refinements. These novel ternaries show metallic behaviour and their ground state depends on the particular rare earth ion. Long-range magnetic order was deduced for the compounds from Ce to Gd, with a maximum transition temperature TN ~ 29 K for Gd2Zn6Ge3. While the compounds with Ce and Pr exhibit a spontaneous magnetic type of order, those with Nd, Sm and Gd are antiferromagnetic. The magnetic structures of Pr2Zn6Ge3 and Nd2Zn6Ge3 were resolved on the basis of neutron powder diffraction performed at 1.5 K.

The magnetic instability of Yb2Pd2(In,Sn) in a non-Fermi liquid environment

Various concentrations of the solid solution Yb 2 Pd 2 In 1-x Sn x ranging from x = 0 to 1 with the tetragonal Mo 2 FeB 2 -type structure were prepared and their physical properties were studied by means of bulk and spectroscopic measurements as a function of temperature, pressure and magnetic fields. Results deduced from these studies indicate a slight variation of the valency of the Yb ions, from v 2.9 for both border compounds to v = 3 at x = 0.6. This variation gives rise to the appearance of long range magnetic order in a narrow concentration region around x = 0.6, while both border compounds remain non-magnetic. The outstanding concentration dependence for the ordered regime may be indicative of two quantum critical points, a novel feature not yet observed in Yb compounds.

Novel thermoelectric skutterudites SnyNi4Sb12-xSnx

Abstract SnyNi4Sb12−xSnx (2.4⩽x⩽5.6; 0⩽y⩽0.31 at 250°C) belongs to a new class of skutterudites in which the 8c site is fully occupied by Ni atoms. Sn atoms occupy two inequivalent sites (24g and 2a) and have a large thermal displacement parameter in 2a. Within the homogeneity range, electronic transport is primarily governed by the number of charge carriers, revealing a crossover from metallic- to a semiconducting behaviour with increase in x.

Effect of ionic valence and electronic correlations on the thermoelectric power in some filled skutterudites

Abstract Electron correlations and intermediate valence distinctly influence electronic transport in skutterudites and hence modify the thermoelectric performance. Kondo interaction remarkably enhances thermopower, and electrons provided, e.g., by intermediate valent Yb may compensate holes of the transition metal–pnictogen units, which then can show thermopower values well above 100 μV / K .