We report on Pb,Br, N,H, C andH NMR experiments for studying the local order and dynamics in hybrid perovskite lattices. Pb NMR experiments conducted at room temperature on a series of MAPbX compounds (MA = CHNH; X = Cl, Br and I) showed that the isotropic Pb NMR shift is strongly dependent on the nature of the halogen ions. ThereforePb NMR appears to be a very promising tool for the characterisation of local order in mixed halogen hybrid perovskites. Pb NMR on MAPbBrI served as a proof of concept. Proton, C andN NMR experiments confirmed the results previously reported in the literature. Low temperature deuterium NMR measurements, down to 25 K, were carried out to investigate the structural phase transitions of MAPbBr. Spectral lineshapes allow following the successive phase transitions of MAPbBr. Finally, quadrupolar NMR lineshapes recorded in the orthorhombic phase were compared with simulated spectra, using DFT calculated electric field gradients (EFG). Computed data do not take into account any temperature effect. Thus, the discrepancy between the calculated and experimental EFG evidences the fact that MA cations are still subject to significant dynamics, even at 25 K.

Polycrystalline samples and single crystals of the new compound Ag 3 In 2 Mo 15 Se 19 were synthesized by solidstate reaction in a sealed molybdenum crucible at 1300 °C. Its crystal structure (space group R3̅ c, a = 9.9755(1) Å, c = 57.2943(9) Å, and Z = 6) was determined from single-crystal X-ray diffraction data and constitutes an Ag-filled variant of the In 2 Mo 15 Se 19 structure-type containing octahedral Mo 6 and bioctahedral Mo 9 clusters in a 1:1 ratio. The increase of the cationic charge transfer due to the Ag insertion induces a modification of the Mo−Mo distances within the Mo clusters that is discussed with regard to the electronic structure. Transport properties were measured in a broad temperature range (2−1000 K) to assess the thermoelectric potential of this compound. The transport data indicate an electrical conduction dominated by electrons below 25 K and by holes above this temperature.The metallic character of the transport properties in this material is consistent with electronic band structure calculations carried out using the linear muffin-tin orbital (LMTO) method. The complex unit cell, together with the cagelike structure of this material, results in very low thermal conductivity values (0.9 W m −1 K −1 at 300 K), leading to a maximum estimated thermoelectric figure of merit (ZT) of 0.45 at 1100 K.

We present a detailed study of the evolution of the electrical, galvanomagnetic, and thermodynamic properties of polycrystalline Ag x Mo 9 Se 11 compounds for 3.4 ≤ x ≤ 3.8 at low temperatures (2−350 K). In agreement with density functional theory calculations, the collected data show an overall gradual variation in the transport properties from metallic to semiconducting behavior on going from x = 3.4 to 3.8. The results evidence subtle variations in the electronic properties with the Ag content, typified by both positive and negative phonon-drag effects together with thermopower and Hall coefficient of opposite signs. Analysis of the data suggests that these features may be due to peculiarities of the dispersion of the valence bands in the vicinity of the chemical potential. A drastic influence of the Ag content on the thermal transport was evidenced by a pronounced change in the temperature dependence of the specific heat below 10 K. Nonlinearities in the C p (T 3 ) data are correlated to the concentration of Ag atoms, with an increase in x resulting in a more pronounced departure from a Debye law. The observed behavior mirrors that of ionic conductors, suggesting that Ag x Mo 9 Se 11 for x ≥ 3.6 might belong to this class of compounds.

We report a remarkably high and stable thermoelectric zT ∼ 2 by manipulating the electronic bands in hybrid flash-SPSed Ga–Sb codoped GeTe.

The distorted perovskites NaTaO(3) and NaNbO(3) have been studied using (23)Na multiple-quantum (MQ) MAS NMR. NaTaO(3) was prepared by high temperature solid state synthesis and the NMR spectra are consistent with the expected room temperature structure of the material (space group Pbnm), with a single crystallographic sodium site. Two samples of NaNbO(3) were studied. The first, a commercially available sample which was annealed at 900 degrees C, showed two crystallographic sodium sites, as expected for the room temperature structure of the material (space group Pbcm). The second sample, prepared by a low temperature hydrothermal method, showed the presence of four sodium sites, two of which match the expected room temperature structure and the second pair, another polymorph of the material (space group P21ma). This is consistent with powder X-ray diffraction data which showed weak extra peaks which can be accounted for by the presence of this second polymorph. Density functional theory (DFT) calculations support our conclusions, and aid assignment of the NMR spectra. Finally, we discuss the measured NMR parameters in relation to other studies of sodium in high coordination sites in the solid state.

The change of multivalence band structure configuration between rhombodedral and cubic phase in GeTe offers additional dimension to modify its thermoelectric properties. Here, we report the synergetic optimization of electronic and thermal transport properties in rhombohedral GeTe doped with transition metal Ti. The Seebeck coefficient of Ge 1-x Ti x Te is significantly increased and the corresponding thermal conductivity is decreased. The structure refinement shows that Ti doping could reduce the lattice constant c/a ratio. Density functional theory calculations demonstrate that the affected crystal field rather than Ti orbitals is contributing to the valence band convergence and a Seebeck coefficient enhancement. Further optimization incorporates the effects of Bi substitution for reducing the carrier concentrations and introducing more point defects. This work not only confirms the transition metal elements as promising dopants for GeTe-based materials, but also indicates that the strategy of manipulating the crystal field effect can be exploited as a direct but effective route for improving thermoelectric performance.

The combined time-resolved photoluminescence (PL) and theoretical study performed on luminescent [Mo6Br(i)8Br(a)6](2-)-based systems unambiguously shows that their NIR-luminescence is due to at least two emissive states. By quantum chemical studies, we show for the first time that important geometrical relaxations occur at the triplet states either by the outstretching of an apex away from the square plane of the Mo6 octahedron or by the elongation of one Mo-Mo bond. Experimental PL measurements demonstrate that the external environment (counter-ions, crystal packing) of the cluster has a noticeable impact on its relaxation processes. Temperature and excitation wavelength dependence of the two components of the luminescence spectra is representative of multiple competitive de-excitation processes in contradiction with Kasha's rule. Our results also demonstrate that the relaxation processes before and after emission can be tracked via fast time-resolved spectroscopy. They also show that the surroundings of the luminescent cluster unit and the excitation wavelength could be modulated for target applications.

International audienceUltralow thermal conductivity is of great interest in a variety of fields, including thermoelectric energy conversion. We report, for the first time, experimental evidence that Ga-doping in SnTe may lower the lattice thermal conduction slightly below the theoretical amorphous minimum at high temperature. Such an effect is justified by the spontaneous formation of nanoprecipitates we characterized as GaTe. Remarkably, the introduction of Ga (2-10%) in SnTe also improves the electronic transport properties by activating several hole pockets in the multivalley valence band. Experimental results are supported by density functional theory calculations. The thermoelectric figure of merit, ZT, reaches similar to 1 at 873 K in Sn0.96Ga0.07Te, which corresponds to an similar to 80% improvement with respect to pure SnTe