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The electron localizability indicator in momentum space is proposed as a functional of the same-spin momentum pair density. This functional yields a discrete distribution of values, which are proportional to the charge needed to form a fixed very small fraction of a same-spin electron pair. It resolves all atomic shells for the examined atoms (Li–Kr) with reasonable occupation numbers, especially in the valence region. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]

The thermoelectric performance of EuCd2Sb2 and YbCd2Sb2 was improved by mixed cation occupation. The composition, structure, and thermoelectric properties of YbxEu1-xCd2Sb2 (x=0, 0.5, 0.75, and 1) have been investigated. Polycrystalline samples are prepared by direct reaction of the elements. Thermoelectric properties were investigated after densification of the materials by spark plasma sintering. YbxEu1-xCd2Sb2 crystallizes in the P3m1 space group. The lattice parameters increase with the europium content. These materials show low electrical resistivity, high Seebeck coefficient, and low thermal conductivity together with high carrier concentration and high carrier mobility. ZT values of 0.88 and 0.97 are obtained for Yb0.5Eu0.5Cd2Sb2 and Yb0.75Eu0.25Cd2Sb2 at 650 K, respectively. [ABSTRACT FROM AUTHOR]

Starting from the nominal BixV8O16 formula, we have performed a state-of-the-art transmission electron microscopy investigation to propose a new Bi2-yVyV8O16 chemical formula for this hollandite structure. This results from the filling of the channels by main-group elements together with vanadium (V5+) species, with a variable content of Bi and V inside the channels. The influence of the Bi content and of this local disorder on the magnetic and transport properties has been investigated in polycrystalline samples of BixV8O16 with nominal x = 1.6 and x = 1.8 compositions. The rather x-independent electrical resistivity (≈5 mΩ cm) and Seebeck coefficient at high T (-35 μV K-1 at 900 K) are discussed in terms of an unchanged V oxidation state resulting from the filling of the wide channels with Bi and V. It is proposed that this local disorder hinders the charge/orbital setting below 60 K on the V ions of the V8O16 framework. Hollandites exhibit complex electronic and magnetic properties and have potential applications in the fields of batteries, photocatalysis, and nuclear waste storage, and these results show that a careful and detailed investigation of the nature and content of the cations inside the channels is crucial for improving our understanding of the impact of doping and disorder on their properties. [ABSTRACT FROM AUTHOR]

Detailed 195Pt and 171Yb nuclear magnetic resonance (NMR) studies on the heterogeneous mixed valence system YbPtGe2 are reported. The temperature dependence of the 195Pt-NMR shift 195K(T) indicates the opening of an unusual magnetic gap below 200 K. 195K(T) was analyzed by a thermal activation model which yields an isotropic gap Δ/kB ≈ 200 K. In contrast, the spin-lattice relaxation rate 195(1/T1) does not provide evidence for the gap. Therefore, an intermediate-valence picture is proposed while a Kondo-insulator scenario can be excluded. Moreover, 195(1/T1) follows a simple metallic behavior, similar to the reference compound YPtGe2. A well-resolved NMR line with small shift is assigned to divalent 171Yb. This finding supports the proposed model with two subsets of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the YbPtGe2 lattice. [ABSTRACT FROM AUTHOR]

We have investigated the structure of the Al13Fe4(010) surface using both experimental and ab initio computational methods. The results indicate that the topmost surface layers correspond to incomplete puckered (P) planes present in the bulk crystal structure. The main building block of the corrugated termination consists of two adjacent pentagons of Al atoms, each centered by a protruding Fe atom. These motifs are interconnected via additional Al atoms referred to as "glue" atoms which partially desorb above 873 K. The surface structure of lower atomic density compared to the bulk P plane is explained by a strong Fe-Al-Fe covalent polar interaction that preserves intact clusters at the surface. The proposed surface model with identified Fe-containing atomic ensembles could explain the Al13Fe4 catalytic properties recently reported in line with the site-isolation concept. [ABSTRACT FROM AUTHOR]

We report measurements of the London penetration depth Δλ(T) and the electronic specific heat Ce(T) on high-quality single crystals of the filled skutterudite superconductor PrPt4Ge12 (Tc~ 8 K). Both quantities show a weak temperature dependence at T ≪ Tc, following Δ λ ~ Tn (n = 3.2 ± 0.1) and Ce/T ~ T28. Such temperature dependences deviate from both conventional s-wave-type and nodal superconductivity. A detailed analysis indicates that the superfluid density ps(T), derived from the penetration depth, as well as the electronic specific heat can be consistently described in terms of a two-gap model, providing strong evidence of multiband superconductivity for PrPt4Ge12. [ABSTRACT FROM AUTHOR]

One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone, which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime, which contradicts the usual assumption of strong anharmonic phonon-phonon scattering processes. Within the framework of ab initio density-functional theory calculations we interpret these hybridizations as a series of anticrossings which act as a low-pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low-pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba8Ge40Ni6 and compare it to those of pure Ge and a Ge46 empty-cage model system. [ABSTRACT FROM AUTHOR]

Abstract: Cobalt ferrite exhibits a high coercivity at room temperature and a strong magnetic anisotropy compared to the other spinel ferrites and, consequently appears as an interesting material for permanent magnets and high-density recording. The magnetic properties depend also on the crystallite size. In order to keep the powder properties in a bulk material, dense nanostructured cobalt ferrite has to be sintered. A field activated sintering process like spark plasma sintering (SPS) may be promising for such challenge. The present paper deals with: (i) the preparation of cobalt ferrite by two methods: coprecipitation and hydrothermal synthesis in supercritical water; (ii) the SPS sintering of the cobalt ferrite nanopowder prepared by coprecipitation. The sintering of the as-processed powder and that obtained after a thermal treatment resulting in the spinel phase has been investigated. The influence of the starting powder and the sintering parameters such as the temperature, the duration of the SPS stage on the grain growth and the densification degree of bulk materials will be presented. [Copyright &y& Elsevier]

Abstract: Titanium materials with high strength and hardness at room temperature have been developed by introducing very fine Ti5Si3 dispersoids into an ultra-fine grained titanium matrix. After high-energy ball milling of a Ti–Si powder mixture SPS (spark plasma sintering) was used to consolidate the granules. For both process steps, an optimization of the parameters was necessary. XRD, SEM and TEM were used to investigate the mechanism of formation of Ti5Si3 dispersoids during SPS at various sintering temperatures and sintering times and furthermore, to characterize the microstructure with regards to titanium matrix grain size, dispersoid size and dispersoid distribution. The new material shows high hardness, strength and improved wear resistance in comparison to other titanium materials. [Copyright &y& Elsevier]

Unexpectedly strong magnetic coupling is observed in boride insulators containing the B12 icosahedra as a structural unit. We have successfully synthesized Dy0.74B22C2N, which takes the RB22C2N-type crystal structure. The Dy phase was previously thought to not be possible to form. Spin glass-like behavior was observed with a broad peak in the zero field cooled (ZFC) magnetic susceptibility at T ∼ 44 K, while a large divergence for ZFC and field cooled (FC) curves is observed. The Curie-Weiss constant is determined to be θ=-43.9 K, and this system is indicated to have the strongest magnetic coupling ever observed for any rare earth boride insulator. Electronic structure and chemical bonding calculations were also made. Conventional mechanisms for f-electron magnetism cannot explain the strong coupling observed, which further indicates that the B12 icosahedra is functioning as a novel mediator of magnetic interaction. [ABSTRACT FROM AUTHOR]

Abstract: A 23Na NMR study is presented on the itinerant ferromagnets NaFe4Sb12 and the isostructural Ca-substituted compound Na0.5Ca0.5Fe4Sb12 performed in 7.05 and 11.74T in the temperature range from 4 to 300K. Both compounds show a bulk ferromagnetism below 85K. Static (Knight-shift) and dynamic (spin-lattice relaxation) measurements are described in the framework of the SCR theory for itinerant d band metals. [Copyright &y& Elsevier]

With a counterintuitive approach to crystal growth, single crystals of α-TmAlB4 were successfully grown, which were indicated from transmission electron microscopy (TEM) and x-ray diffraction (XRD) measurements and analysis to be virtually free from the ubiquitous building defects, and their properties investigated. Recent developments in rare earth aluminoboride compounds with two-dimensional boron layers have attracted interest due to the heavy fermion superconductivity in β-YbAlB4, multiple anomalies manifesting below the Néel temperatures in α-TmAlB4 attributed to intrinsic building defects, and field stable state in Tm2AlB6. The physical properties of the obtained α-TmAlB4 crystals show a striking difference from the properties of conventional α-TmAlB4 crystals containing building defects. TN = 6.8 K is revealed to be higher than the 5.8 K formerly thought, and the multiple anomalies below TN are replaced by a sharp λ-like peak in the specific heat C. The large effect of the building defects on the physical properties could be directly confirmed, and strikingly it is shown how the large peak structure in the specific heat of the high quality sample could be destroyed by the presence of a small amount (one or two percent) of building defects. [ABSTRACT FROM AUTHOR]

We investigated the superconducting order parameter of the filled skutterudite LaPt4Ge12, with a transition temperature of Tc=8.3 K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting s-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series Pr1-xLaxPt4Ge12. They suggest compatible order parameters for the two end compounds LaPt4Ge12 and PrPt4Ge12. This is not consistent with a single s-wave gap in LaPt4Ge12 considering previous reports of unconventional and/or multiband superconductivity in PrPt4Ge12. [ABSTRACT FROM AUTHOR]

71Ga NQR, magnetization, and specific-heat measurements have been performed on polycrystalline Ge-doped FeGa3 samples. A crossover from an insulator to a correlated local moment metal in the low-doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, xC=0.15, 71(1/T1T) exhibits a pronounced T-4/3 power law over two orders of magnitude in temperature, which indicates three-dimensional quantum critical ferromagnetic fluctuations. Furthermore, for the ordered x=0.2 sample (TC≈6 K), (1/T1T)71 could be fitted well in the frame of Moriya's self-consistent renormalization theory for weakly ferromagnetic systems with 1/T1T∼χ. In contrast to this, the low-doping regime nicely displays local moment behavior where 1/T1T∼χ² is valid. For T→0, the Sommerfeld ratio γ=(C/T) is enhanced (70mJ/moleK² for x=0.1), which indicates the formation of heavy 3d electrons. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution photoemission and X-ray absorption measurements of YbFe4Sb12 single crystals are performed. The experimental data are compared to the results of LDA+ U calculations. Ytterbium was found to be mainly divalent in the bulk of the filled skutterudite. [Copyright &y& Elsevier]

The newly synthesized alkali metal iron antimonides with a filled skutterudite structure NaFe4Sb12 and KFe4Sb12 together with the known BaFe4Sb12 have been investigated by means of specific heat and AC susceptibility measurements. The alkali metal samples exhibit ferromagnetism below 85 K and also show additional magnetic anomaly at 40 K. [Copyright &y& Elsevier]