Nataliya Melnychenko-Koblyuk

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.

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.

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.

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.

Thermoelectric power factor in intermetallic semiconductors with MgAgAs type of structure: requirements for highest values and thermal stability achievement

A condition of the maximum thermoelectric power factor (Z*) arising in the intermetallic semiconductors with the crystal structure of the MgAgAs type is unambiguously a heavy doping these materials with both the acceptor and/or donor impurities up to the concentrations, when the Fermi level becomes fixed by the mobility edge of one of the bands with the continuous energies (i.e. conduction or valence band)