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ABSTRACT: In this paper, we show how the thermoelectric properties can be modified in crystallographic structures based on the CdI2 type layer, by changing the block layers between these CdI2 type layers or by going from oxides to selenides and sulfides. In the case of oxides, the unique properties generated in these structures will be demonstrated in Bi-based misfit compounds. By combining Hall effect, resistivity, and Seebeck coefficients in single crystals of this family, the importance of doping and of spin and orbital degeneracy term on the Seebeck coefficient will be shown. From this single crystal investigation, the power factor at 300 K is found to be unexpectedly constant as a function of doping. To further enhance the power factor and thus ZT, it is necessary to modify either the block layer or to perform anionic substitutions. By going from oxides to selenides and sulfides, the decrease of the ionic character can induce a decrease of electrical resistivity. Compared to oxides, the properties can generally be described in a more classical way using Boltzmann transport theory. For these materials, the critical parameter is then thermal conductivity and this quantity can be decreased as shown here by intercalating Cu between the layers (CuxTiS2), or by making solid solution such as TiS2 − xSex. These two approaches will be described here, leading to ZT close to 0.5 and 0.4, at 800 and 700 K, respectively.
physica status solidi (a) 01/2013; 210(1):69-81. · 1.21 Impact Factor
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ABSTRACT: We have synthesized single crystals of the misfit-layered cobalt oxide, [Bi(1.5)Pb(0.5)Sr(2)O(4-δ)][CoO(2)](1.86), with quadruple NaCl-type layers, using a flux method, and measured their transport properties. From structural refinements, it is found that the modulation in the BiO layer observed in [Bi(1.74)Sr(2)O(4-δ)](RS)[CoO(2)](1.82) is suppressed by Pb substitution. The in-plane resistivity, thermopower, and Hall coefficient are 4.3 mΩ cm, 101 µV K(-1), and 1 × 10(-2) cm(3) C(-1) at 300 K, respectively; these are consistent with those of the misfit-layered cobalt oxides. All of these values are smaller than those of [Bi(1.74)Sr(2)O(4)][CoO(2)](1.82), indicating that the carrier concentration is larger than that of the undoped crystal. Moreover, the low T upturn of resistivity observed for most of the cobalt misfit oxides is replaced by a metallic behavior, following a T(2) dependence, indicating strong correlations in the Pb-doped crystal. Also, the magnetoresistance, usually negative for misfit oxides, is replaced by a small positive magnetoresistance. Doping with Pb is thus an efficient way to suppress the low T localized behavior usually observed for misfits.
Journal of Physics Condensed Matter 06/2009; 21(23):235404. · 2.55 Impact Factor
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ABSTRACT: The search for multifunctional materials as multiferroics to be applied in microelectronic or for new, chemically stable and nontoxic, thermoelectric materials to recover waste heat is showing a common interest in the oxides whose structures contain a triangular network of transition-metal cations. To illustrate this point, two ternary systems, Ba-Co-O and Ca-Co-O, have been chosen. It is shown that new phases with a complex triangular structure can be discovered, for instance, by introduction of Ga (3+) into the Ba-Co-O system to stabilize Ba 6Ga 2Co 11O 26 and Ba 2GaCo 8O 14, which both belong to a large family of compounds with formula [Ba(Co,Ga)O 3-delta] n [BaCo 8O 11]. In the latter, both sublattices contain triangular networks derived from the hexagonal perovskite and the spinel structure. Among the hexagonal perovskite, the Ca 3Co 2O 6 crystals give clear evidence where the coupling of charges and spins is at the origin of a magnetocapacitance effect. In particular, the ferrimagnetic to ferromagnetic transition, with a one-third plateau on the M( H) curve characteristic of triangular magnetism, is accompanied by a peak in the dielectric constant. A second class of cobaltites is the focus of much interest. Their 2D structure, containing CoO 2 planes isostructural to a CdI 2 slice that are stacked in an incommensurate way with rock salt type layers, is referred to misfit cobaltite. The 2D triangular network of edge-shared CoO 6 octahedra is believed to be responsible for large values of the Seebeck coefficient and low electrical resistivity. A clear relationship between the structuresincommensurability ratiosand the electronic properties is evidenced, showing that the charge carrier concentration can be tuned via the control of the ionic radius of the cations in the separating layers.
Inorganic Chemistry 11/2008; 47(19):8553-61. · 4.60 Impact Factor
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ABSTRACT: Single crystals of different phases of misfit cobalt oxides have been synthesized. Using these materials, the intrinsic power factor has been determined. By combining thermopower S and Hall coefficient measurements, we will show how S can be tuned by doping in this family of materials, the doping being strongly dependent on the misfit ratio p=b<sub>1</sub>/b<sub>2</sub> ([Bi<sub>2</sub>A<sub>2</sub>O<sub>4</sub>][CoO<sub>2</sub>]<sub>b1/b2</sub> with A=Ca, Sr, Ba, b<sub>1</sub>/b<sub>2</sub>=1.65, 1.82, 1.98). The in-plane resistivity also strongly depends on the doping : the more metallic samples are obtained for the largest carrier concentration, i.e. the smaller S value. The combination of these two factors leads to an important conclusion that the power factor is independent of the doping in these Bi-based materials converging to a unique value of 2.2plusmn0.3 muW/cmK<sup>2</sup> at room temperature, which is not so trivial. We will discuss the origin of the almost unique value of 2.2plusmn0.3 muW/cmK<sup>2</sup>.
Thermoelectrics, 2007. ICT 2007. 26th International Conference on; 07/2007
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ABSTRACT: We present the first angle-resolved photoemission (ARPES) measurement of Fermi Surface in the "misfit" cobaltate [Bi2Ba2O4].[CoO2]~2. This compound contains the same triangular Co planes as Na cobaltates, but in a different 3D environment. Our data establish the similarity of their electronic structure. We propose that the peculiar lineshape of all cobaltates is of the "peak-dip-hump" type, due to strong many-body effects. We detect a progressive transfer of spectral weight from the quasiparticle feature near Ef to a broad hump in misfit phases where Ba is replaced by Sr or Ca. This indicates stronger many-body interactions in proximity of the band insulator regime, which we attribute to the presence of unusual magnetic excitations. Comment: 4 pages, 4 figures
06/2007;
