Y. Bréard

Transport and thermoelectric properties in Copper intercalated TiS2 chalcogenide

We report on the thermoelectric properties of CuxTiS2 bulk compounds. Copper cations have been intercalated into the layered chalcogenide TiS2 by spark plasma sintering. X-ray diffraction analysis coupled to transmission electron microscopy shows that the lattice constant c expands linearly as the Cu content x increases. The Cu-intercalation into TiS2 leads to substantial decrease in both electrical resistivity and lattice thermal conductivity as compared to those of pristine TiS2. The figure of merit, ZT, is increased up to 0.45 at 800 K for x = 0.02. The power factor, PF, reaches 1.7 mW/mK2 in TiS2 at 325 K.

Revisiting some chalcogenides for thermoelectricity

Abstract Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (∼10−4 W m−1 K−2) to yield a satisfactory figure of merit zT. Changing the anions from O2− to S2− and then to Se2− is a way to increase the covalency. In this review, some examples of sulfides (binary Cr–S or derived from layered TiS2) and an example of selenides, AgCrSe2, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe2 selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS2 and Cu0.1TiS2 are higher, reaching ∼10−3 W m−1 K−2. Apparently the magnetism related to the Cr–S network is detrimental for the PF when compared to the d0 character of the Ti4+ based sulfides. Finally, the very low PF in AgCrSe2 (PF = 2.25 × 10−4 W m1 K−2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m−1 K−1 from the measured Cp) leading to the highest zT value among the reviewed compounds (zT700K = 0.8). The existence of a glassy-like state for the Ag+ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials.

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.

Transmission Electron Microscopy Study of CaFe5O7 : Evidence of a Monoclinic Superstructure at Room Temperature

An efficient synthesis route is proposed to obtain single powder CaFe5O7 ceramic. This complex structure can be described as an intergrowth between one CaFe2O4 unit and n= 3 slices of FeO Wustite-type structures. A fine structural study has been carried out at room temperature by transmission electron microscopy (TEM) observations (ED and HREM). The analysis of these data has revealed a supercell with a monoclinic symmetry associated to some twinning phenomena. From the hkl conditions deduced to electron diffraction study, the centrosymmetric P21/m setting can be proposed. This monoclinic cell exhibit close relationships with the previous one reported as orthorhombic (ao =3.05 Å, bo = 10.05 Å and co = 18 Å) according to the following metric am=2co, bm=co, cm=bo/2sinβ and β=106.74°.

The oxycarbonates Sr4(Fe2-xMnx)1+y(CO3)1-3yO6(1+y): nano, micro and average structural approach

Abstract The possibility to synthesize layered oxycarbonates, with nominal composition Sr 4 Fe 2− x Mn x O 6 CO 3 involving trivalent manganese, with 0≤ x ≤1.5, is reported for the first time. The structural study of Sr 4 FeMnO 6 CO 3 using NPD, HREM, Mossbauer and XANES, shows that this phase is closely related to n =3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO 3 groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr 4 Fe 2 O 6 CO 3 .

Jumps in entropy and magnetic susceptibility at the valence and spin-state transition in a cobalt oxide

A wide family of cobalt oxides of formulation (Pr(1-y)Ln(y))(1-x)Ca(x)CoO3 (Ln being a lanthanide) exhibits a coupled valence and spin-state transition (VSST) at a temperature T*, which involves two concomitant modifications: (i) a change in the spin state of Co(3+) from low-spin (T < T*) to a higher spin state (T > T*) and (ii) a change in the valence state of Pr, from a mixed Pr(4+)/Pr(3+) state (T < T*) to a purely trivalent state (T > T*), accompanied by an ~ 90 K is investigated by magnetization and heat capacity measurements.First, we quantitatively characterized the jumps in magnetic susceptibility (χ) and entropy (S) around T*. Then, these values were compared to those calculated as a function of the variations in the population of the different cationic species involved in the VSST. X-ray absorption spectroscopy experiments recently showed that the higher spin state above T* should be regarded as an inhomogeneous mixture between low-spin (LS) and high-spin (HS) states. In the frame of this description, we demonstrate that the jumps in both χ and S can be associated with the same change in the Co(3+) HS content around T*. This result lends further support to the relevance of the LS/HS picture for the VSST, challenging the currently dominant interpretation based on the occurrence of an intermediate-spin (IS) state of Co(3+) above T*.

Mixed “cobalt-nickel” layered oxides and oxyhydroxides of n = 2 members of the Ruddlesden–Popper family Sr2.5La0.5Co1.3Ni0.7(O,OH)7−y

The crystal structure, transport and magnetic properties of Sr2.5La0.5Co1.3Ni0.7O6.4 and Sr2.5La0.5Co1.3Ni0.7Ox(OH)y·zH2O samples are detailed. Contrary to the first compound, the second one is not strictly speaking a second member of the well-known Ruddlesden–Popper (RP) series, but it exhibits very close relationships. Indeed, due to topotactical reactions with air (hydrolysis and hydration), water molecules are included within the rock salt layer of the “perfect” RP oxide, Sr2.5La0.5Co1.3Ni0.7O6.4, to form the hydrated oxyhydroxide phase presented in this paper. Cationic homogeneity of the samples was confirmed by energy dispersive spectroscopy analyses performed with a transmission electron microscope. The crystal structure of samples was determined by combined refinements of powder X-ray and electronic diffraction data. Physical properties of compounds are discussed, for instance the hydrated compound and the “perfect” RP oxide both exhibit typical magnetic frustrated systems and no long range order prevails at low temperature. Transport property measurements reveal a semi-conducting behavior for Sr2.5La0.5Co1.3Ni0.7O6.4. Finally, relationships between structure and physical properties are described herein.

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.