Searching for new thermoelectric materials: some examples among oxides, sulfides and selenides.

Different families of thermoelectric materials have been investigated since the discovery of thermoelectric effects in the mid-19th century, materials mostly belonging to the family of degenerate semi-conductors. In the last 20 years, new thermoelectric materials have been investigated following different theoretical proposals, showing that nanostructuration, electronic correlations and complex crystallographic structures (low dimensional structures, large number of atoms per lattice, presence of 'rattlers'…) could enhance the thermoelectric properties by enhancing the Seebeck coefficient and/or reducing the thermal conductivity. In this review, the different strategies used to optimize the thermoelectric properties of oxides and chalcogenides will be presented, starting with a review on thermoelectric oxides. The thermoelectric properties of sulfides and selenides will then be discussed, focusing on layered materials and low dimensional structures (TiS2 and pseudo-hollandites). Some sulfides with promising ZT values will also be presented (tetrahedrites and chalcopyrites).

Two components for one resistivity in LaVO3/SrTiO3 heterostructure.

A series of 100 nm LaVO3 thin films have been synthesized on (0 0 1)-oriented SrTiO3 substrates using the pulsed laser deposition technique, and the effects of growth temperature are analyzed. Transport properties reveal a large electronic mobility and a non-linear Hall effect at low temperature. In addition, a cross-over from a semiconducting state at high-temperature to a metallic state at low-temperature is observed, with a clear enhancement of the metallic character as the growth temperature increases. Optical absorption measurements combined with the two-bands analysis of the Hall effect show that the metallicity is induced by the diffusion of oxygen vacancies in the SrTiO3 substrate. These results allow us to understand that the film/substrate heterostructure behaves as an original semiconducting-metallic parallel resistor, and electronic transport properties are consistently explained.

Structural transition at 360 K in the CaFe5O7 ferrite: toward a new charge ordering distribution.

An efficient synthesis route is proposed to obtain single phase powder ceramic of CaFe5O7. This complex structure can be described as an intergrowth between one CaFe2O4 unit and n = 3 slices of FeO Wüstite-type structure. A detailed structural study has been carried out at room temperature combining transmission electron microscopy (TEM) observations (ED, HREM), scanning transmission electron microscopy (STEM-HAADF), and X-ray diffraction data. The analysis of these data has revealed an unexpected supercell with a monoclinic symmetry. From the hkl conditions deduced from the electron diffraction study and the analysis of X-ray diffraction data by simulated annealing, a structural model considering the centrosymmetric P21/m setting can be proposed. In addition the first magnetic and electrical transport measurements are reported showing a sharp peak in magnetic susceptibility and a strong localization around 360 K, associated to a structural change from monoclinic setting to orthorhombic one.

Sr7Co4(CO3)O(13-δ) (δ = 1.64), an original cobaltite derivative of the Ruddlesden-Popper series.

The oxycarbonate Sr7Co4(CO3)O11.36 exhibits a peculiar structure that has been characterized by combining transmission electron microscopy analyses and neutron diffraction. It consists of a regular intergrowth between the m = 2 and carbonated m = 3 members of the Sr(m+1)Co(m)O3(m+1) Ruddlesden-Popper (RP) series, Sr3Co2O5.87 and Sr4Co2(CO3)O5.49, respectively. A description of the structure is proposed to provide identification of the different building blocks. This material is semiconducting and presents a complex magnetic behavior, characteristic of what is observed for the RP(2) or RP(3) series, with a cobalt valency close to 2.7.

Large oxygen nonstoichiometry in La(0.77)Sr(3.23)Co(2.75)C(0.25)O(8.40+δ) oxide (δ = 0, 1.3) related to n = 3 RP series.

An original Ruddlesden-Popper phase, La(0.77)Sr(3.23)Co(2.75)C(0.25)O(8.40+δ), was isolated and studied by electron, X-ray, and neutron diffraction. This structure has complex crystal chemistry resulting from a high degree of flexibility in the structure, comprising the disordered introduction of carbonates into a cobalt layer and an important oxygen deficiency with a preferential repartition of vacancies along the layers stacking sequence. The former is necessary for the stabilization of the system, while the latter can be tuned by postsynthetic treatment, yielding in a large variety of cobalt species formal oxidation states ranging from Co(2+)/Co(3+) in the as-made phase to Co(3+)/Co(4+) when annealed under oxygen pressure. The potential richness deriving from this flexibility is illustrated in terms of the magnetotransport properties and includes a resistivity that varies within a range of 5 orders of magnitude after modulation of the oxygen content with the appearance of negative magnetoresistance and ferromagnetic interactions due to Co(3+)/Co(4+) mixed-valence state.

Transition-metal oxides with triangular lattices: generation of new magnetic and electronic properties.

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.

Re-entrant metallicity and magnetoresistance induced by Ce for Sr substitution in SrCoO(3-δ).

Cerium for strontium substitution allows an oxygen deficient perovskite Sr(1-x)Ce(x)CoO(3-δ) to be stabilized with a cerium solubility limited to x≤0.15 (Trofimenko et al 1997 Solid State Ion. 100 183). For these samples, the magnetic properties depend clearly upon the oxygen content: Sr(0.9)Ce(0.1)CoO(2.74) and Sr(0.9)Ce(0.1)CoO(2.83) are weak and strong ferromagnets (T(C) = 160 K), respectively, the maximum ac-magnetic susceptibility of the latter being larger by two orders of magnitude than that of the former. In contrast to other Sr(1-x)L(x)CoO(3-δ) series (L =  lanthanide, Y(3) or Th(4+)), the electrical resistivity (ρ) behaviour does not simply reflect the magnetic behaviour. For x = 0.10 the re-entrant ρ feature becomes more pronounced whereas the ferromagnetic fraction and cobalt oxidation state increase. This unexpected behaviour could be related to the Ce(3+)/Ce(4+) mixed valency, the 4f localized moment of the Ce(3+) cations interacting with the conduction electrons through a Kondo-like mechanism. It is also found that the increase of oxygen vacancies favours the appearance of magnetoresistance at low T, reaching -50% at 5 K in 7 T for the Sr(0.95)Ce(0.05)CoO(2.61) sample prepared in a sealed tube.

Single-crystal study of the m = 2 tubular cobalt oxide, Bi(4)Sr(12)Co(8)O(30-delta).

The structure of the m = 2 tubular compound Bi(4)Sr(12)Co(8)O(30-delta), bismuth strontium cobalt oxide, was determined by single-crystal X-ray diffraction. This phase of orthorhombic symmetry exhibits a very strong tetragonal pseudosymmetry. The structure consists of 90 degrees -oriented Bi(2)Sr(2)CoO(6+delta) slices, four Co atoms wide, forming [Sr(4)Co(4)O(13)](infinity) pillars at their intersection. The Co atoms in these pillars form four corner-sharing CoO(5) bipyramids. In the resulting [Co(4)O(13)] cluster, an anionic disorder is evidenced and discussed. Then, an accurate description of the particular structure of the pillars is given. Finally, a comparison with the Mn tubular compound Bi(3.6)Sr(12.4)Mn(8)O(30-delta) is carried out.