Showing papers by "Silke Paschen published in 2000"

Thermal-transport properties of CeNiSn

Abstract: We present thermal-conductivity $\ensuremath{\kappa}$ and thermopower S measurements on high-quality single crystalline CeNiSn along the three crystallographic axes $a, b,$ and c in the temperature range between 100 mK and 7 K and in magnetic fields up to 8 T applied along the a axis. Both $\ensuremath{\kappa}$ and S are highly anisotropic. However, characteristic features that may be attributed to the opening of a pseudogap in the electronic density of states (DOS) at the Fermi energy $\ensuremath{\eta}$ below 10 K are seen for all three crystallographic directions. These features are strongly suppressed by a magnetic field of 8 T applied along the a axis. At the lowest temperatures we have evidence for the presence of a residual metallic DOS at $\ensuremath{\eta},$ again for all three directions. The saturation of the reduced Lorenz number ${L/L}_{0}$ to a value distinctly higher than one is an interesting feature which deserves further investigation.

Crystal structure of hexabarium pentacosagermanide, Ba6Ge25

Abstract: Ba6Ge25, cubic, P4i32 (No. 213), a = 14.5564(2) A, V = 3075.8 A, Ζ = 4, pm = 5.698 g -cm", R g t ( F ) = 0.089, wR(F) = 0.086, Γ =293 Κ. Source of material The compound was prepared from the elements by melting in an open glassy carbon crucible (HF furnace, argon atmosphere) and annealing at 923 Κ (12 d). Ba6Ge25 is silvery metallic, brittle, stable in air and moisture. Discussion BaaGe25 belongs to the chiral clathrate structure family cP 124, being the second binary prototype of the BaoIn4Ge2i type [1-4]. The structure is characterized by a chiral 3D framework of condensed Ge2o pentagondodecahedra (pdod) embedded in a 3D channel labyrinth. Each pdod ist centered by Bal and the rest Ba atoms occupy the cavities in the zeolite-like labyrinth. The rather large elongation of the displacement ellipsoid of Ba2 (M2) is typical for the structure family. The Ge—Ge bond lengths vary between 2.47 A and 2.56 A. Per formula unit, there are seventeen fourfold bonded (4b) Ge° atoms and eight Ge are threefold bonded (3b)Ge : (Ba)6((3b)Ge")8((4b)Ge°)i7(4el. There are no indications of vacancies in the Ge framework. The compound is a Zintl phase with few conduction electrons. Below 180 K, the compound is a semiconducting Zintl phase with 4e in localized antibonding states of the Ge framework and it becomes metallic above 250 Κ [1], Table 1. Data collection and handling. Crystal: silvery block, size 0.10 χ 0.08 χ 0.02 mm Wavelength: Mo Ka radiation (0.71073 A) μ·· 315.23 cm" Diffractometer, scan mode: Nicolet R3m/V, ω 20max: 65° N(hkl)„,easured, N(hkl)u„ique: 1750, 1405 Criterion for F0bs, N(hkl)gC. Fobs > 4 o(Fobs), 630 N(param)Kiinea'· 49 Programs: SHELXTL-plus [5], ATOMS [6] Table 2. Atomic coordinates and displacement parameters (in A). Atom Site χ y ζ U11 U22 C/33 U12 U13 i/23 Ba(l) 8c 0.0607(2) X X 0.017(1) U11 Uu 0.002(1) U12 U12 Ba(2) 12d 1/8 0.1887(3) y+1/4 0.107(6) 0.031(2) U21 0.276(3) 0.005(3) -U13 Ba(3) 4 a 3/8 3/8 3/8 0.025(2) Uu Uu 0.010(2) Uu U12 Ge(l) 24e 0.2929(4) 0.9538(4) 0.7501(4) 0.016(3) 0.015(2) 0.017(3) 0.002(2) 0.003(2) 0.001(2) Ge(2) 8c 0.9251(4) X X 0.015(2) Uu υ η 0.002(2) U\2 U12 Ge(3) 8c 0.2182(5) X X 0.027(2) V11 Uu 0.005(3) Un Un Ge(4) 12d 1/8 0.8309(4) y+1/4 0.010(3) 0.017(2) U22 0.156(2) -0.004(2) U n Ge(5) 24e 0.9166(4) 0.0847(4) 0.8532(4) 0.015(3) 0.020(3) 0.015(2) 0.001(2) 0.001(2) 0.000(2) Ge(6) 24e 0.1853(4) 0.9899(3) 0.8767(4) 0.015(2) 0.013(2) 0.017(3) -0.004(2) -0.003(2) 0.003(2) * Correspondence author (e-mail: carrillo@cpfs.mpg.de) 10.1515/ncrs-2000-0208 Downloaded from PubFactory at 09/02/2016 06:03:31AM via free access 208 Hexabarium pentacosagermanide

Crystal structure of the defect clathrate-I, Ba8Ge43

Abstract: BagGe43, cubic, Pmln (No. 223), a = 10.6565(2)Ä,V= 1210.2 Ä, Z= 1, R&l(F) = 0.036, wRrei(F) = 0.055, T= 293 K. Source of material BasGe43 was prepared as single phase by melting the elements in an open glassy carbon crucible (HF furnace, argon atmosphere) and annealing at 1068 Κ (6 d). The single crystals were obtained from the as-cast sample. The substance is silvery metallic, brittle, relatively stable in air and moisture. EDX analysis of bulk samples resulted in the chemical composition Ba8.o(i)Ge42.9(i). Experimental details The lattice parameters were determined from the least-squares refinement of the 2Θ values of 54 reflections (18° < 2Θ < 100°, powder data). The X-ray powder data were obtained on the sample with nominal composition BasGe43 (λ(ΟιΚαι) = 1.540598 Ä; LaB6 standard, a = 4.15695(6) Ä). Discussion Ba8Ge43 was first obtained as a by-product of the decomposition of Ba3Ge4C2 [ 1 ] at 1123 Κ (1 h) under high pressure (ca. 40 kb). The BagGe43 structure (upper part of the figure) belongs to the clathrate-I type with Pearson symbol cP(54-3). Refined occupancies of Ge( 1), Ge(31) and Ge(32) positions resulted in the chemical composition BagGe42.93(5). Only the Ge(l) site is deficient in germanium. Consequently, the neighbouring Ge(3) position splits into the two near sites Ge(31) and Ge(32) (Table 2). The latter occurrs only if Ge(l) is empty. Similar results were obtained for the A8Sn44a2 clathrates, A = K, Rb, Cs [2,3]. A short range ordered model of the vacancies (•) in the germanium environment of Ba(2) atoms is shown in the lower part of the figure (atomic distances in Ä). The investigation of the possible long range ordering in long time annealed samples is in progress. Per formula unit, 31 germanium atoms are fourfold bonded (4b) and 12 germanium atoms in the Ge(31) sites are threefold bonded (3b). The compound may be formulated as (Ba)8((3b)Ge-)i2((4b)Ge°)3i(4e-). According to this, BagGc43D3 is a Zintl phase with a few electrons in not localized antibonding states of the network or in conduction bands. Semimetallic or metallic properties are expected [4, 5], Table 1. Data collection and handling. Crystal: silvery metallic, platelike, size 0 . 0 2 x 0 . 1 5 x 0 . 2 5 mm Wavelength: Mo Ka radiation (0.71073 Ä) μ: 326.16 cm\" Diffractometer, scan mode: Nicolet R3m/V, ω 2Gmax: 55.04° N(hkl)measured, N(hkl)unique: 1892,273 Criterion for /0bs, N(hkl)gL. /obs > 2 <7(/obsJ, 209 N(param)Kiined'· 19 Programs: SHELXL-97 [6], ATOMS [7] Table 2. Atomic coordinates and displacement parameters (in Ä). Atom Site Occ. χ y ζ fiso Ge(31) 24k 0.505(4) 0 Ge(32) 24* 0.495 0 0.3073(3) 0.1124(3) 0.0133(8) 0.3332(3) 0.1314(3) 0.0190(9) * Correspondence author (e-mail: carrillo@cpfs.mpg.de)

Low-temperature transport, thermal, and optical properties of single-grain quasicrystals of icosahedral phases in the Y-Mg-Zn and Tb-Mg-Zn alloy systems

Abstract: We present a comprehensive series of results of electrical transport ~electrical conductivity, magnetoconductivity, Hall effect!, thermal ~specific heat!, and optical ~reflectivity ! measurements in varying temperature ranges between 1.5 and 300 K on high-quality single-grain quasicrystals of icosahedral Y-Mg-Zn. This data set is augmented by the specific-heat and optical-reflectivity data obtained from a single-grain quasicrystal of icosahedral Tb-Mg-Zn. For Y-Mg-Zn, both the electrical conductivity s(T) and magnetoconductivity ds(H) may be described by calculations considering quantum interference effects. A detailed comparison of the weak-localization contributions to s(T) and ds(H) with our experimental data provides estimates of the inelastic and spin-orbit relaxation rates. The inelastic relaxation rate is found to be proportional to T 3 . The dominant contributions to the optical conductivity s 1(v) spectrum, obtained from the reflectivity R(v) data in the frequency range between 16 and 9.7310 4 cm 21 , are a strong Drude feature at low frequencies and a prominent absorption signal centered at approximately 6 310 3 cm 21 . A comparison of the spectral weight of the Drude contribution to s 1(v) with the magnitude of the linear-in-T term gT of the low-temperature specific heat Cp(T) yields the itinerant charge-carrier density ni57.62310 21 cm 23 or 0.13 charge carriers per atom. The low ni value is corroborated by the results of the Hall effect measurements. For Tb-Mg-Zn, the optical conductivity s 1(v) spectrum reveals features similar to those of Y-Mg-Zn. The low-temperature specific heat Cp(T) of Tb-Mg-Zn is strongly influenced by a spin-glass-type freezing of Tb moments and by crystalelectric-field effects.

Thermal conductivity of EuB 6

Abstract: The thermal conductivityl of EuB6 has been measured in the temperature range between 0.4 and 50 K. In this temperature regime the thermal conductivity is mainly due to phonons. A broad maximum with a relatively large absolute value of about 50 W/Km is observed at 17 K. Below this temperature the thermal conductivity follows a T behavior, which may be attributed to phonon-electron scattering. At temperatures above 17 K a surprisingly weakT dependence of l is observed which we relate to the peculiar phonon dispersion relation of divalent hexaborides.

Electronic transport and thermal properties of CaB6 and Eu1−xCaxB6

Abstract: By substituting Ca for Eu in EuB 6 , a local-moment ferromagnet, the transition temperature of 16 K for the binary compound, is lowered substantially with increasing x . In spite of the divalency of both Eu and Ca, significant changes in the electronic properties are indicated by changes in the temperature and magnetic-field dependences of the electrical resistivity, particularly at low temperatures. For pure CaB 6 similar anomalies in the resistivity and the specific heat at very low temperatures as previously observed for SrB 6 , have been identified. Most of these properties most likely originate in the particular electronic structure of these hexaborides.

Crystal structure of ytterbium-dialuminium-digermanium, YbAl2Ge2

Abstract: A l 2 G e 2 Y b , trigonal , P3m\ ( N o . 1 6 4 ) , a = 4 . 1 7 8 9 ( 6 ) Ä , c = 7 . 0 5 5 ( 2 ) Ä , V= 1 0 6 . 7 Ä 3 , Z = 1, Rgt(F) = 0 . 0 1 4 , wR,ef(F2) = 0 . 0 3 5 , Τ = 2 9 3 Κ. Source of material T h e title c o m p o u n d h a s b e e n s y n t h e s i z e d b y h i g h f e q u e n c y ind u c t i o n m e l t i n g f r o m e l e m e n t s ( Y b : 9 9 . 9 % , Al: 9 9 . 9 9 9 % a n d Ge: 9 9 . 9 9 9 9 % ) f o l l o w e d b y s u b s e q u e n t annea l ing at 8 7 3 Κ for 10 d. T h e crystal structure w a s character ized b y s ing le crysta l m e t h o d s and unit ce l l parameters w e r e r e f i n e d f r o m the Guin i er p o w d e r pattern wi th Si as internal standard. Table 2. Atomic coordinates and displacement parameters (in Ä).