A. Grytsiv

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.

High thermoelectric performance of triple-filled n-type skutterudites (Sr,Ba,Yb)yCo4Sb12

The influences of single, double and triple filling of Yb, Ba and Sr in CoSb3 on thermoelectric performance were evaluated. A significant suppression of lattice thermal conductivity κl is achieved by the increase in the number of filler elements. The highest power factor appears in triple-filled (Ba0.1Yb0.1)0.25(Sr0.1Yb0.1)0.75Co4Sb12, whereas single-filled Ba0.23Co4Sb12 has higher S2/ρ than double-filled (Ba,Yb)0.2Co4Sb12 forming a sequence S2/ρ(triple filling) > S2/ρ(single filling) > S2/ρ(double filling). However, the respective figures of merit follow a sequence ZT(triple filling) > ZT(double filling) > ZT(single filling) proving a strong correlation with κl that appears to be a predominant factor in the efficiency of filled skutterudites. Thus, κ and ZT increase with increasing lattice thermal resistivity (1/κl) and maximum efficiencies may be obtained for 1/κl > 0.2 cm K mW−1.

Thermal expansion of skutterudites

The current paper gives an overview of the newly obtained thermal expansion coefficients of skutterudites as well as those so far available in literature. Thermal expansion was determined for CoSb3, Pt4Sn4.4Sb7.6, for As- and Ge-based skutterudites as well as for various high-ZT skutterudites (micro- and nanostructured) with didymium (DD) and mischmetal (Mm) as filler atoms in frameworks of (Fe1−xCox)4Sb12 and (Fe1−xNix)4Sb12, and for double and triple-filled skutterudites such as Ca0.07Ba0.23Co3.95Ni0.05Sb12 and Sr0.025Ba0.075Yb0.1Co4Sb12. For low temperatures, a capacitance dilatometer was used (4–300 K), whereas for temperatures 300<T<750 K, a dynamic mechanical analyzer was employed. For a set of Ge-, P-, and Sb-based skutterudites, lattice parameters of single crystals, measured at three different temperatures, were used to derive the thermal expansion coefficient. The semiclassical model of Mukherjee [Phys. Rev. Lett. 76, 1876 (1996)] has been successfully used to quantitatively describe the thermal...

Structure and physical properties of type-I clathrate solid-solution Ba 8 Pt x Ge 46 − x − y ◻ y ( ◻ = vacancy )

Formation, crystal chemistry, and physical properties were investigated for the solid-solution Ba{sub 8}Pt{sub x}Ge{sub 46-x-y}{open_square}{sub y} ({open_square} is a vacancy) deriving from binary clathrate Ba{sub 8}Ge{sub 43}{open_square}{sub 3} with a solubility limit of {approx}3.5 Pt atoms/f.u. at T=800 deg. C. Structural investigations throughout the homogeneity region confirm isotypism with the cubic primitive clathrate type-I structure (space group type Pm3n) and lattice parameters ranging from a=1.0657(2) nm for Ba{sub 8}Ge{sub 43}{open_square}{sub 3} to a=1.0752(2) nm for Ba{sub 8}Pt{sub 3.5}Ge{sub 41.5}{open_square}{sub 1.0}. Phase relations for the region concerning the clathrate solution were derived at subsolidus temperatures as well as at 800 deg. C. Transport properties evidence electrons as the majority charge carriers in the system with a slight dependency on the Pt content. The system is located close to a semiconducting regime with a gap in the electronic density of states of a few thousand K. No low temperature maximum is obvious from thermal conductivity which is dominated by the lattice contribution. Thermal conductivity furthermore documents a high efficiency of phonon scattering on vacancies.

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.

Compositional dependence of the thermoelectric properties of (SrxBaxYb1 ? 2x)yCo4Sb12 skutterudites

High temperature thermoelectric (TE) properties for triple-filled skutterudites (Sr(x)Ba(x)Yb₁₋₂x)(y)Co₄Sb₁₂ were investigated for alloy compositions in two sections of the system: (a) for x = 0.25 with a filling fraction y ranging from 0.1 to 0.25 and (b) for 0 < x < 0.5 and y = 0.11 + 0.259x. The representation of the figure of merit, ZT, as a function of skutterudite composition, defined the compositional range (0.25 < x < 0.4; 0.18 < y < 0.24) with ZT over 1.4 at 800 K. It was shown that an enhanced TE performance for these triple-filled skutterudites is caused by low electrical resistivities and low lattice thermal conductivities, as well as by a fine tuning of the chemical composition. Low temperature measurements for the samples with the highest ZT values showed that even a small change of the filler ratios changes the contribution of scattering effects, the carrier concentration and the mobility.

Crystal chemistry of the G-phases in the systems Ti–{Fe, Co, Ni}–Al with a novel filled variant of the Th6Mn23-type

Abstract The crystal structures of the compounds Ti6M7Al17 with M=Fe,Co,Ni have been investigated by X-ray powder and single crystal, neutron powder and electron diffraction. These compounds crystallize with a new filled variant of the Mg6Cu16Si7-type (space group Fm 3 m ). A close structural relation is encountered for the series of crystal structures: Th6Mn16Mn7?, Mg6Cu16Si7?, Sc6Sc16Ir7Ir, Zr6Zn16Zn7Si, Ti6Al16M7Al, and Ti6Ni16Si7Si. Electrical resistivity of the Fe, Co-based compounds is typical metallic and temperature dependence follows the Bloch Gruneisen relation with a Debye temperature of about 300 K. No hydrogen was found to be absorbed in the Fe-, and Co-based compounds at room temperature under a H2 pressure of 5 MPa.

Physical properties and superconductivity of skutterudite-related Yb3Co4.3Sn12.7 and Yb3Co4Ge13

Rietveld analysis was performed for the intermetallics Yb3Co4.3Sn12.7 and Yb3Co4Ge13 crystallizing with the closely related structure types, Yb3Rh4Sn13 and Yb3Co4Ge13. Below Tc = 3.4 K Yb3Co4.3Sn12.7 crosses over into a type-II superconducting ground state with Hc2(0)~2.5 T. Yb3Co4Ge13 stays in the normal state down to 300 mK. The γ value of 2.3(2) mJ gat-1 K-2 and the Debye temperature ΘD = 207(5) K deduced from the specific heat as well as Tc correspond to that of elementary Sn, thus indicating conventional BCS superconductivity. Hydrostatic pressure applied to Yb3Co4.3Sn12.7 reveals both an overall decrease of the absolute resistivity values, as well as a decrease of Tc, which vanishes for a critical pressure below 10 kbar. The magnetoresistance of both Yb-based compounds is positive at low temperature but does not exceed 8% in fields of 12 T. The Seebeck coefficient has a maximum value of about 18 µV K-1 at T~250 K. LIII and magnetic susceptibility measurements reveal intermediate valence: 2.66(3) and 2.18(3) for Yb3Co4Ge13 and Yb3Co4.3Sn12.7, respectively.

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 .

Formation of clathrates Ba-M-Ge (M = Mn, Fe, Co)

Abstract In order to define the ability of magnetic elements M = Mn, Fe, Co to stabilise clathrate structures, alloys of the Ba – M – Ge system were investigated in the as-cast state and after annealing at 700°C and 800°C by means of X-ray powder diffraction, light optical and electron-probe microanalysis. Temperatures of phase transformations were derived from differential thermal analysis. Results are summarised in (i) isothermal sections at 700°C and 800°C, (ii) solidus and liquidus surfaces covering the region of existence for both clathrate phases in these systems. Invariant reactions during crystallisation are presented in form of Schultz – Scheil diagrams. In all three cases only limited solubility of the M element was found for clathrate IX (Ba6Ge25) i. e. the Ge-framework in the crystal structure of Ba6MxGe25 – x dissolves 0.6 atom of Mn, and about 1 atom of Fe and Co per unit cell. The maximum solubility of iron in clathrate type I (Ba8Ge46 – x) was found to be less than 0.5 Fe atom per unit cell, and clathrates with Mn and Co contain up to 1.0 and 2.5 atoms in the unit cell, respectively. Whilst Fe does not decrease the formation temperature of the clathrate phase, Mn and Co decrease it from 770°C (for binary Ba8Ge43) to 766°C and 749°C, respectively.