J. Giapintzakis

La0.95Sr0.05CoO3: An efficient room-temperature thermoelectric oxide

We present measurements of electrical resistivity, thermal conductivity, and thermopower of polycrystalline Sr-doped LaCoO3 with composition La0.95Sr0.05CoO3. Our data show that the investigated compound exhibits a very respectable room-temperature thermoelectric figure of merit value of 0.18. Our results not only show that oxides are promising candidates for thermoelectric cooling applications, but also point toward the need for careful theoretical calculations that will serve as a guide in producing the next generation of thermoelectric materials.

Synthesis, spectroscopic, and structural characterization of the first aqueous cobalt(II)-citrate complex: toward a potentially bioavailable form of cobalt in biologically relevant fluids.

Abstract Citric acid represents a class of carboxylic acids present in biological fluids and playing key roles in biochemical processes in bacteria and humans. Its ability to promote diverse coordination chemistries in aqueous media, in the presence of metal ions known to act as trace elements in human metabolism, earmarks its involvement in a number of physiological functions. Cobalt is known to be a central element of metabolically important biomolecules, such as B12, and therefore its biospeciation in biological fluids constitutes a theme worthy of chemical and biological perusal. In an effort to unravel the aqueous chemistry of cobalt in the presence of a physiologically relevant ligand, citrate, the first aqueous, soluble, mononuclear complex has been synthesized and isolated from reaction mixtures containing Co(II) and citrate in a 1 :2 molar ratio at pH∼8. The crystalline compound (NH4)4[Co(C6H5O7)2] (1) has been characterized spectroscopically (UV/vis, EPR) and crystallographically. Its X-ray structure consists of a distorted octahedral anion with two citrate ligands fulfilling the coordination requirements of the Co(II) ion. The magnetic susceptibility measurements of 1 in the range from 6 to 295 K are consistent with a high-spin complex containing Co(II) with a ground state S=3/2. Corroborating this result is the EPR spectrum of 1, which shows a signal consistent with the presence of a Co(II) system. The spectroscopic and structural properties of the complex signify its potential biological relevance and participation in speciation patterns arising under conditions consistent with those employed for its synthesis and isolation.

Low-temperature growth of NiMnSb thin films by pulsed-laser deposition

We report the growth of thin films of the ferromagnetic half-Heusler alloy NiMnSb by pulsed-laser deposition on Si (111) and polycrystalline InAs substrates. Highly crystalline films are grown using low substrate temperatures (190–210 °C), without any postdeposition annealing. The structural, magnetic, and transport properties of the films are relatively consistent with those of the bulk NiMnSb used as target, which suggests that thin layers of this material grown by laser ablation could be used in multilayer structures as effective spin-polarized conducting layers.

Synthesis and physical properties of arc melted NiMnSb

Several polycrystalline samples of the half-Heusler alloy NiMnSb were grown by arc melting of stoichiometric and nonstoichiometric amounts of high-purity constituent elements. The structure and the phase-purity of the prepared samples were examined systematically by powder x-ray diffraction. The transport properties of the best sample, with saturation magnetization M-s(5 K)=4 mu(B)/formula unit, were studied by measuring electrical resistivity, thermal conductivity, and thermopower. Features in both magnetic and transport data are consistent with NiMnSb being in a half-metallic state at low temperatures, i.e., the conduction electrons are fully spin polarized. However, point-contact Andreev reflection measurements on the same sample at 4.2 K demonstrate only similar to45% spin polarization.

Comparative study of the magnetic and magnetotransport properties of a metallic and a semiconducting member of the solid solution LaNixCo1−xO3

We present a comparative study of both the magnetic and magnetotransport properties for two members of the perovskite solid solution LaNixCo1−xO3 (x=0.2, 0.6) located on opposite sides of the chemically induced metal-to-insulator transition. LaNi0.6Co0.4O3 exhibits metallic behavior and small but negative magnetoresistance, whereas LaNi0.2Co0.8O3 exhibits semiconducting behavior and giant negative magnetoresistance at low temperatures. On the other hand, we observe pronounced similarities in the magnetic properties of both compounds. We provide a consistent explanation regarding the origin of the magnetoresistance in the two members of the solid solution.

Thermoelectric properties of LaNi1−xCoxO3 solid solution

We present the results of a systematic investigation of the thermal conductivity, Seebeck coefficient and electrical resistivity for selected members (x=0.3, 0.7, 0.8, and 1) of the LaNi1−xCoxO3 solid solution in the temperature range of 80⩽T⩽320 K. Substitution of Ni by Co drives the system from n-type metallic phase with low thermopower values into p-type semiconducting phase with high thermopower values of the order of hundreds of μV/K. But the thermal conductivity observed varies only slightly with the composition, x, over the temperature range investigated. The latter effect also holds true for the lightly hole doped x=0.8 member (in substituting Sr for La). Our results suggest that semiconducting members of the LaNi1−xCoxO3 solid solution can be classified as “phonon glass and electron crystal” systems. The highest room temperature figure of merit was exhibited by the x=0.8 member (ZT=0.022), which is comparable to that of single crystal NaCo2O4 (ZT=0.03).

Coexistence of ferromagnetic and glassy behavior in semiconducting LaNi0.2Co0.8O3

Systematic dc and ac magnetic susceptibility studies have been performed on single-phase LaNi0.2Co0.8O3 powder samples. Evidence of hysteresis loops supports the existence of ferromagnetic correlations that lead to a rapid increase of the dc susceptibility at T⩽55 K. The frequency dependence of the ac susceptibility is suggestive of a spin-glass-like phase. The temperature dependence of the nonlinear susceptibility components, χ2 and χ3, in zero dc magnetic field demonstrates the existence of spontaneous magnetization, appears to exclude the occurrence of a canonical spin-glass phase and instead indicates that a cluster-glass phase occurs in LaNi0.2Co0.8O3. It is suggested that the spontaneous magnetization in this system is due to the existence of Ni-based clusters with strong intracluster ferromagnetic correlations, embedded randomly in a paramagnetic matrix.

Realization of La2MnVO6: search for half-metallic antiferromagnetism?

Abstract Single-phase polycrystalline La2MnVO6 samples were synthesized by arc-melting and characterized by X-ray diffraction, magnetization and resistivity measurements. We find that the compound has cubic (space group Fm3m), partly ordered double perovskite structure. The sample exhibits ferrimagnetic behavior and variable-range hopping conductivity. We conclude based on the magnetic properties that both Mn and V ions are trivalent; moreover, the Mn3+ ions are in a high spin state, which is the reason that the compound is not a half-metallic antiferromagnet.

Surface and interface study of pulsed-laser-deposited off-stoichiometric NiMnSb thin films on a Si(100) substrate

We report a detailed study of surface and interface properties of pulsed-laser-deposited NiMnSb films on a Si(100) substrate as a function of film thickness. As the thickness of films is reduced below

Very high thermoelectric power factor in a Fe3O4/SiO2/p-type Si(100) heterostructure

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