Yu. Grin

Structure refinement of the iron–aluminium phase with the approximate composition Fe2Al5

Diiron pentaaluminide, Fe 2 Al 5 , M r =268.8, orthorhombic, Cmcm (No. 63), a=7.6559(8), b=6.4154(6), c=4.2184 (4) A, V=207. 19 (6) A 3 , Z=2, D x =4.23 (2), D m =4.20 (1) Mg m -3 , Mo Kα radiation, μ=8.2 mm -1 , F(000)=249.6, T=293 K, R=0.0229, wR=0.0270 for 137 unique reflections. The refined structure differs from the approximate structure proposed previously by an additional partially occupied atomic position

A new type of thermoelectric material, EuZn2Sb2.

Polycrystalline EuZn(2)Sb(2) is prepared by direct reaction of the elements. Its composition, structure, magnetism, heat capacity, and thermoelectric properties have been investigated. EuZn(2)Sb(2) crystallizes in p3m space group with a=4.4932(7) A and c=7.6170(10) A. Antiferromagnetic ordering is detected at the Neel temperature of 13.06 K, and the saturation magnetization reaches 6.87mu(B)Eu at 2 K and 7 T. Eu ion has +2 valence. Its Hall effects are characterized by a high positive Hall coefficient of +0.226 cm(3)C, proper carrier concentration of 2.77x10(19)cm(3), and high carrier mobility of 257 cm(2)V s at 300 K. This compound shows high p-type Seebeck coefficient (+122 to +181 muVK), low lattice thermal conductivity (1.60-0.40 Wm K), and high electrical conductivity (1137-524 Scm). The obtained figure of merit and powder factor reach 0.92 and 20.72 muWcm K(2), respectively. The thermoelectric properties of EuZn(2)Sb(2) are encouraging.

Atomic ordering and thermoelectric properties of the n-type clathrate Ba8Ni3.5Ge42.1□0.4

Single crystals of the type-I clathrate Ba(8)Ni(3.5)Ge(42.1)square(0.4) (space group Pm3n, no. 223, a = 10.798(2) A, l = 30 mm, slashed circle = 8 mm) were grown from the melt using the Bridgman technique. Their composition, determined by microprobe analysis, reveals a distinctly lower Ni content than previously reported for the lower limit (x = 5.4) of the homogeneity range of the clathrate-I phase Ba(8)Ni(x)Ge(46-x). From single crystal X-ray diffraction data we introduce a crystal structure model that takes point defects (vacancies) square in the Ge network into account. It reveals that both Ni and square accumulate at a single site (6c) and that, as a consequence, the Ge network distorts considerably. Ba(8)Ni(3.5)Ge(42.1)square(0.4) shows metal-like behaviour (drho/dT > 0) albeit with high resistivity at room temperature (rho(300 K) approximately 1 mOmega cm). Together with the low charge carrier concentration of 2.3 e(-)/unit cell at 300 K this is typical of a degenerate semiconductor. The lattice thermal conductivity is distinctly smaller than that of Ba(8)Ge(43)square(3), where the vacancies partially order, and smaller than those of Ba-Ni-Ge type-I clathrates without vacancies, suggesting that disordered vacancies efficiently scatter heat-transporting phonons. We provide evidence that the maximum value of the thermoelectric figure of merit reached in Ba(8)Ni(3.5)Ge(42.1)square(0.4), ZT(680 K) congruent with 0.21, can be further improved by adjusting the charge carrier concentration.

Structural chemistry and magnetic behaviour of ternary uranium gallides U{Fe,Co,Ni,Ru,Rh,Pd,Os,Ir,Pt}Ga5☆

The crystal structure of UCoGa5 was refined from single-crystal v-ray counter data. The space group was P4/mmm (No. 123) and the lattice parameters were a = 4.2357(5) A and c = 6.7278(9) A; v = 1. For 214 independent reflections (¦Fo¦>3σ) the reliability factor obtained was R − (Σ¦ΔF¦)/Σ¦Fo¦= 0.027. UCoGa5 crystallives with the HoCoGa5-type structure, combining structural units of the AuCu3- and PtHg2-type. Isotypic compounds were synthesived with all the group VIII elements: U{Fe,Co,Ni,Ru,Rh,Pd,Os,Ir,Pt}Ga5. Magnetic susceptibilities of the new compounds were studied in the temperature range of 80 K < T < 1100 K, and were found to be practically temperature independent. The crystal chemistry and magnetochemical behaviour of the new uranium gallides is discussed.

Superconductivity in the Platinum Germanides MPt4Ge12 (M = Rare-Earth or Alkaline-Earth Metal) with Filled Skutterudite Structure

New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. The crystal structure and composition were investigated by x-ray diffraction and microprobe analysis. Magnetic susceptibility, specific heat, and electrical resistivity measurements evidence superconductivity in LaPt4Ge12 and PrPt4Ge12 below 8.3 K. The parameters of the normal and superconducting states were established. Strong coupling and a crystal electric field singlet ground state is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level. Similar behavior with lower Tc was observed for SrPt4Ge12 and BaPt4Ge12.

Application of spark plasma sintering to the fabrication of binary and ternary skutterudites

Abstract Spark plasma sintering (SPS) synthesis of CoSb3 and LaFe4Sb12 from ball-milled elemental metal powders has been performed obtaining samples with high relative densities (∼99%). Fracture behaviour, the microstructure and mechanical properties were studied. From ultrasound and nano-indentation measurements, Young’s moduli of 148 GPa for CoSb3 and 141 GPa for LaFe4Sb12 were deduced. SPS has been also applied to compact alkali metal and alkaline-earth metal iron antimonides with filled skutterudite structure.

Crystal structure and transport properties of Ba8Ge43□3

The single phase clathrate-I Ba(8)Ge(43)square(3) (space group Ia3d (no. 230), a = 21.307(1) A) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2-673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (kappa(l)(300 K) = 2.7 W m(-1) K(-1)) has been evidenced.

High temperature thermoelectric properties of the type-I clathrate Ba8NixGe46-x-ysquarey

Polycrystalline samples of the type-I clathrate Ba(8)Ni(x)Ge(46-x-y)□(y) were synthesized for 0.2 ⩽ x ⩽ 3.5 by melt quenching and for 3.5<x ⩽ 6.0 by melting with subsequent annealing at 700 °C. The maximum Ni content in the clathrate framework at this temperature was found to be x ≈ 4.2 atoms per unit cell. Thermoelectric and thermodynamic properties of the type-I clathrate were investigated from 300 to 700 K by means of electrical resistivity, thermopower, thermal conductivity and specific heat measurements. As the Ni content increases, the electronic properties gradually evolve from a metallic character (x < 3.5) towards a highly doped semiconducting state (x ⩾ 3.5). Below x ≈ 4.0 transport is dominated by electrons, while further addition of Ni (x ≈ 4.2) switches the electrical conduction to p-type. Maximum value of the dimensionless thermoelectric figure of merit ZT ≈ 0.2 was achieved at 500 K and 650 K for x ≈ 2.0 and x ≈ 3.8, respectively.

Cationic Clathrate I Si46-xPxTey (6.6(1) <= y <= 7.5(1), x <= 2y): Crystal Structure, Homogeneity Range, and Physical Properties

A new cationic clathrate I Si(46-x)P(x)Te(y) (6.6(1) < or = y < or = 7.5(1), x < or = 2y at 1375 K) was synthesized from the elements and characterized by X-ray powder diffraction, thermal analysis, scanning electron microscopy, wavelength dispersive X-ray spectroscopy (WDXS), neutron powder diffraction, and (31)P NMR spectroscopy. The thermal behaviors of the magnetic susceptibility and resistivity were investigated as well. Si(46-x)P(x)Te(y) reveals a wide homogeneity range due to the presence of vacancies in the tellurium guest positions inside the smaller cage of the clathrate I structure. The vacancy ordering in the structure of Si(46-x)P(x)Te(y) causes the change of space group from Pm3n (ideal clathrate I) to Pm3 accompanied by the redistribution of P and Si atoms over different framework positions. Neutron powder diffraction confirmed that P atoms preferably form a cage around the vacancy-containing tellurium guest position. Additionally, (31)P NMR spin-spin relaxation experiments revealed the presence of sites with different coordination of phosphorus atoms. Precise determination of the composition of Si(46-x)P(x)Te(y) by WDXS showed slight but noticeable deviation (x < or = 2y) of phosphorus content from the Zintl counting scheme (x = 2y). The compound is diamagnetic while resistivity measurements show activated behavior or that of heavily doped semiconductors. Thermal analysis revealed high stability of the investigated clathrate: Si(46-x)P(x)Te(y) melts incongruently at approximately 1460 K in vacuum and is stable in air against oxidation up to 1295 K.

Are type-I clathrates Zintl phases and 'phonon glasses and electron single crystals'?

Abstract We discuss to which extent the concepts of Zintl phases and of ‘phonon glasses and electron single crystals’ apply to type-I clathrates. In (β-) Eu 8 Ga 16 Ge 30 the presence of residual charge carriers appears to be related to a slight off-stoichiometry of the samples pointing to the validity of the Zintl concept in stoichiometric samples. The low and almost stoichiometry independent mobilities of (β-) Eu 8 Ga 16 Ge 30 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Ge 30 seriously question the validity of the ‘electron single crystal’ concept for type-I clathrates. The temperature dependence of the thermal conductivity of a Ba 8 Ga 16 Ge 30 single crystal indicates that tunneling states play a central role in producing ‘phonon glass’-like thermal conductivities.