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S.R.P. and J.W.G.B. acknowledge the EPSRC (grant EP/N01717X/1) and Leverhulme Trust (grant RPG-2012-576) for financial support and the STFC for the provision of beam time at ISIS. F.D.M. acknowledges the EPSRC for financial support (grant EP/P024637/1). M.S.S. acknowledges the Royal Society for a University Research Fellowship (grant UF160265). The phonon-glass electron crystal concept is one of the key guiding principles for the development of efficient thermoelectric materials. Here, we confirm that SrTiO 3 becomes a phonon-glass for large numbers of A-site vacancies in the Sr 1-x La 0.67x → 0.33x TiO 3 series and show that its electron crystal properties are stymied by the presence of a core-shell grain structure. Thermal conductivity, heat capacity, and neutron powder diffraction, complemented by representational analysis and phonon calculations, were used to investigate the thermal transport. This reveals that the heat carrying modes are dominated by Sr motions and that these become more localized upon the introduction of the A-site vacancies, consistent with the observed phonon-glass state. Impedance spectroscopy and direct current electrical measurements were used to probe the electrical properties of insulating and conducting samples. This reveals the coring of grains due to oxidation on cooling from sintering temperatures. The resultant insulating shell limits the thermoelectric power factor to S 2 /ρ = 0.45 mW m -1 K -2 and the figure-of merit to ZT = 0.15 at 900 K for Sr 0.20 La 0.53 → 0.27 Ti 0.95 Nb 0.05 O 3?δ . The thermal properties of these materials are, therefore, controlled by an intrinsic feature of the microstructure (i.e., the A-site vacancies), whereas the electrical properties are grain boundary limited, which in principle can be controlled independently to raise S 2 /ρ and ZT. Postprint

International audience; Lamellar AxCoO2 cobalt double oxides with A = Li, Na, and K (x 0.6) have been synthesized and their chemical (alkali content, oxidation state, and structure) and physical (resistivity, thermopower, magnetization, and specific heat) properties have been studied. All the three materials exhibit strong electron correlation emphasized by their behavior ranging from Fermi liquid to spin-polarized system. Our results show that both the dimensionality of the interactions and the nature of the alkali play a determining role on the properties.

International audience; Well crystallized VO2 (A) microrods were grown via a single step hydrothermal reaction in the presence of V2O5 and oxalic acid. With the advantage of high crystalline samples, we propose P4/ncc as an appropriate space group at room temperature. From morphological studies, we found that the oriented attachment and layer by layer growth mechanisms are responsible for the formation of VO2 (A) micro rods. The structural and electronic transitions in VO2 (A) are strongly first order in nature, and a marked difference between the structural transition temperatures and electronic transitions temperature was evidenced. The reversible intra- (LTP-A to HTP-A) and irreversible inter- (HTP-A to VO2 (M1)) structural phase transformations were studied by in-situ powder X-ray diffraction. Attempts to increase the size of the VO2 (A) microrods are presented and the possible formation steps for the flower-like morphologies of VO2 (M1) are described.

International audience; The possible existence of an intermediate spin state is now a few decades fascinating solid state chemists and physicists, experimentalists, and theoreticians. In this article, we revise some recent results on the stability diagrams of spin states, low, high, or intermediate, in distorted environments and extend their approach to redraw more realistic diagrams for d(4), d(5), and d(6) ions in a tetragonally distorted 6-fold oxygen environment (D4h, D2d, and C4ν). The model relies on a point charge model and further uses effective parameters to account for the cubic field drift on spin state change and for suitable values for solid state of the expectation values of the 3d-radial wave functions; additionally the model uses rational parameters to characterize the distortion; finally, we also consider the possible existence of states' combinations to propose reliable stability diagrams. Whatever the representation involved in the distortion, the existence domain of the intermediate spin state appears very small and more likely replaced with mixtures of cations in low and high spin states; the opportunity of induced distortive ordering is discussed.

International audience; The magnetic properties and the magnetocaloric effect are presented for the perovskite-related oxide SrFe0.5Co0.5O3 prepared using electrochemical oxidation. SrFe0.5Co0.5O3 exhibits a second order paramagnetic-ferromagnetic transition close to room temperature (TC=330 K). The maximal magnetic entropy change ΔSMMax , the maximal adiabatic temperature change ΔTad and the refrigerant capacity are found to be equal to respectively 4.0 J/kgK, 1.8 K and 258 J/kg while raising the B-field change from 0 to 5 T.

Various P2 and P'3-Na(x)CoO(2) phases, with x ranging approximately from 0.6 to 0.75, have been studied by variable-temperature (23)Na magic angle spinning (MAS) NMR. Signal modification versus temperature plots clearly show that Na(+) ions are not totally mobile at room temperature on the NMR time scale. As the temperature increases, the line shape change of the (23)Na MAS NMR signal differs for the P2 and P'3 stackings and is interpreted by the differences of Na(+) ion sites and of sodium diffusion pathways in the two structures.

International audience; A polytype of LiCoO2 has been synthesized through an ion-exchange reaction. By exchanging Na+ for Li+ ions from an OP4-(Li/Na)CoO2-mixed cobaltite, a LiCoO2 stacking is obtained. It alternatively combines O2- and O3-LiCoO2 polytypes and corresponds to an O4 stacking. The phase crystallizes in the hexagonal system with cell parameters ahex = 2.802(1) Å and chex = 18.89(2) Å. As the O2 phase, the O4-LiCoO2, is metastable; it transforms into the thermodynamically stable O3-LiCoO2at around 350–400°C. The crystallographic data, the thermal properties, and the electrochemical behavior of this phase are presented.

This paper is devoted to a comparative study of the AxCoO2 (A = Li, Na and K) systems. For x ~ 0.6, all the investigated alkali cobaltites exhibit a metallic-like behavior. Potassium cobaltites exhibit a T2 dependence of the resistivity - suggesting a Fermi liquid character - and an enhanced Pauli type paramagnetism ascribed to electronic correlations. On the other hand lithium and sodium compounds show linear temperature dependence for the resistivity down to 4 K, a large thermopower above room temperature and a Curie-Weiss paramagnetism denoting larger electronic correlations. Physical properties of these cobaltites are discussed in details especially in the case of K-ordered and disordered K0.6CoO2 phases.