E. Royanian

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.

Type-I clathrate Ba8NixSi46−x: Phase relations, crystal chemistry and thermoelectric properties

The phase relations, crystal structure and thermoelectric properties of the type-I solid solution Ba(8)Ni(x)Si(46-x) were investigated. Based on X-ray diffraction, differential thermal analysis and electron probe microanalysis data, a partial phase diagram was constructed for the Si-rich part of ternary system Ba-Ni-Si at 800 °C. The solubility range of Ni in the clathrate-I phase at 800 °C was determined (2.9 ≤x≤ 3.8) and thermoelectric properties, namely electrical resistivity, Seebeck-coefficient and thermal conductivity, were measured in the temperature range from 300 to 850 K. A shift of the thermoelectric properties from a predominantly metallic to a more semiconducting behavior was observed for an increasing Ni-content. Density functional calculations revealed a significant decrease of the gap width in the density of states induced by the incorporation of Ni. Electrical resistivity and Seebeck coefficients for Ba(8)Ni(x)Si(46-x) with 3.3 ≤x≤ 3.8 have been modeled within the rigid band approximation.

Thermoelectric properties of PbTe with encapsulated bismuth secondary phase

Lead Telluride (PbTe) with bismuth secondary phase embedded in the bulk has been prepared by matrix encapsulation technique. X-Ray Diffraction results indicated crystalline PbTe, while Rietveld analysis showed that Bi did not substitute at either Pb or Te site, which was further confirmed by Raman and X-Ray Photoelectron Spectroscopy. Scanning Electron Microscopy showed the expected presence of a secondary phase, while Energy Dispersive Spectroscopy results showed a slight deficiency of tellurium in the PbTe matrix, which might have occurred during synthesis due to higher vapor pressure of Te. Transmission Electron Microscopy results did not show any nanometer sized Bi phase. Seebeck coefficient (S) and electrical conductivity (sigma) were measured from room temperature to 725 K. A decrease in S and sigma with increasing Bi content showed an increased scattering of electrons from PbTe-Bi interfaces, along with a possible electron acceptor role of Bi secondary phase. An overall decrease in the power factor was thus observed. Thermal conductivity, measured from 400K to 725K, was smaller at starting temperature with increasing Bi concentration, and almost comparable to that of PbTe at higher temperatures, indicating a more important role of electrons as compared to phonons at PbTe-Bi interfaces. Still, a reasonable zT of 0.8 at 725K was achieved for undoped PbTe, but no improvement was found for bismuth added samples with micrometer inclusions

Cage-forming compounds in the Ba-Rh-Ge system: from thermoelectrics to superconductivity.

Phase relations and solidification behavior in the Ge-rich part of the phase diagram have been determined in two isothermal sections at 700 and 750 °C and in a liquidus projection. A reaction scheme has been derived in the form of a Schulz–Scheil diagram. Phase equilibria are characterized by three ternary compounds: τ1-BaRhGe3 (BaNiSn3-type) and two novel phases, τ2-Ba3Rh4Ge16 and τ3-Ba5Rh15Ge36-x, both forming in peritectic reactions. The crystal structures of τ2 and τ3 have been elucidated from single-crystal X-ray intensity data and were found to crystallize in unique structure types: Ba3Rh4Ge16 is tetragonal (I4/mmm, a = 0.65643(2) nm, c = 2.20367(8) nm, and RF = 0.0273), whereas atoms in Ba5Rh15Ge36–x (x = 0.25) arrange in a large orthorhombic unit cell (Fddd, a = 0.84570(2) nm, b = 1.4725(2) nm, c = 6.644(3) nm, and RF = 0.034). The body-centered-cubic superstructure of binary Ba8Ge43□3 was observed to extend at 800 °C to Ba8Rh0.6Ge43□2.4, while the clathrate type I phase, κI-Ba8RhxGe46–x–y□y, reveals a maximum solubility of x = 1.2 Rh atoms in the structure at a vacancy level of y = 2.0. The cubic lattice parameter increases with increasing Rh content. Clathrate I decomposes eutectoidally at 740 °C: κI ⇔ (Ge) + κIX + τ2. A very small solubility range is observed at 750 °C for the clathrate IX, κIX-Ba6RhxGe25–x (x ∼ 0.16). Density functional theory calculations have been performed to derive the enthalpies of formation and densities of states for various compositions Ba8RhxGe46–x (x = 0–6). The physical properties have been investigated for the phases κI, τ1, τ2, and τ3, documenting a change from thermoelectric (κI) to superconducting behavior (τ2). The electrical resistivity of κI-Ba8Rh1.2Ge42.8□2.0 increases almost linearly with the temperature from room temperature to 730 K, and the Seebeck coefficient is negative throughout the same temperature range. τ1-BaRhGe3 has a typical metallic electrical resistivity. A superconducting transition at TC = 6.5 K was observed for τ2-Ba3Rh4Ge16, whereas τ3-Ba5Rh15Ge35.75 showed metallic-like behavior down to 4 K.

New orthorhombic modification of equiatomic CePdAl

The crystal structure of a new low-temperature modification of CePdAl was determined from single-crystal x-ray data: CePdAl-type; space group Pmmn (No. 59), Z = 14, oP42, a = 0.426 07 nm, b = 2.887 58 nm, c = 0.721 90 nm; RF = 0.048. Physical properties of orthorhombic CePdAl are governed by a mutual balance of the RKKY interaction, the Kondo effect and crystalline electric field splitting, resulting in antiferromagnetic ordering below ≈2.5 K. Electronic transport is reminiscent of a textbook-like Kondo lattice, which comes along with a significant negative magnetoresistance of more than 50% at low temperatures. Although geometrical frustration is absent when compared to the hexagonal modification of CePdAl, the ordering temperature TN is even smaller. A possible cause is enhanced Kondo interactions.

Superconductivity and Magnetism in MPt4Ge12, M = Ca, Ba, Sr, Eu

X-ray powder data for Ba 0.8 Ca 0.2 Pt 4 Ge 12 and X-ray single crystal data for EuPt 4 Ge 12 define cubic body-centered symmetry consistent with novel Ge-based skutterudites SrPt 4 Ge 12 and BaPt 4 Ge 12 (space group Im 3 ). Structural and electron microprobe analysis investigations evidence a complete filling of the icosahedral cages without large atomic displacement parameters. Ba 0.8 Ca 0.2 Pt 4 Ge 12 exhibits phonon-mediated superconductivity at T c = 5.2 K. Density functional theory (DFT) calculations (LDA+U) carried out for EuPt 4 Ge 12 proved that Eu guest atoms strongly stabilize the compound in which the calculated density of states around the Fermi energy essentially consists of hybridized Ge 4 p -like and Pt 5 d -like states. Low temperature resistivity studies evidence magnetic ordering at T m ≈1.7 K. Susceptibility measurements reveal a divalent state for europium, in excellent agreement with the DFT calculations.

From Superconductivity Towards Thermoelectricity: Ge-Based Skutterudites

We report on a substitution of Ge by Sb in \(\mathrm {LaPt_4Ge_{12}}\), which significantly changes ground state properties, from a superconducting state below \(T_c = 8\) K for \(\mathrm {LaPt_4Ge_{12}}\) to a nearly insulating state in \(\mathrm {LaPt_4Ge_{7}Sb_5}\). Evidence for this crossover is from electronic structure calculations based on VASP and is reflected in the specific heat, electrical resistivity and the Seebeck data as well. This electron-doping drives a well behaving metal with poor thermoelectric properties towards a bad metal, in the proximity of a metal-to-insulator transition with a substantially enhanced Seebeck effect. Results derived for \(\mathrm {LaPt_4Ge_{12-x}Sb_x}\) can serve as feedback to improve the thermoelectric performance of technologically interesting multicomponent skutterudites.

The system uranium–palladium–boron with U2.5Pd20.5B6, a new heavy fermion compound

Phase equilibria in the system U-Pd-B were established at 850 °C by light optical microscopy (LOM) and x-ray powder and single crystal diffraction. Whereas in as-cast alloys only one ternary compound, τ(1)-U(2 + x)Pd(21 - x)B(6), was found to form at x ∼ 0.5, a further compound τ(2) with hitherto unknown structure was observed in alloys annealed at 850 °C. Due to the formation of suitable single crystals, the crystal structures of two binary compounds, UB(12) and UPd(3) have been redetermined from high precision x-ray data. Similarly, the crystal structure of τ(1)-U(2.5)Pd(20.5)B(6) was investigated by single crystal x-ray diffraction (XRD) revealing isotypism with the Cr(23)C(6)-type, (space group [Formula: see text]; a  = 1.1687(5) nm; R(F)(2)  = Σ|F(0)(2) - F(c)(2)|/ΣF(0)(2) =  0.021). τ(1)-U(2 + x)Pd(21 - x)B(6) is a partially ordered compound where 0.37(1)U + 0.63Pd atoms randomly share the 4a site in (0, 0, 0). Whereas mutual solubility of U-borides and Pd-borides was found at 850 °C to be below 1.0 at.%, a large homogeneity region of fcc-Pd(U, B) extends into the ternary system. U(2.5)Pd(20.5)B(6) has metallic behavior; the ground state properties are determined from a balance of the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, revealing long range antiferromagnetic ordering below 6 K. An extraordinarily large Sommerfeld value (γ > 500 mJ mol(-1) K(-2)) groups U(2.5)Pd(20.5)B(6) among heavy fermion materials.