The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

No abstract

The material class of rare earth nickelates with high Ni3+ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO3 thin films has also triggered intensive research efforts. LaNiO3 seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO3 single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni2+ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.

No abstract

Thisis an author version of the contribution published on:Questa è la versione dell'autore dell'opera: By Federico Cesano, Serena Bertarione, Andrea Piovano, Giovanni Agostini, Mohammed Mastabur Rahman, Elena Groppo, Francesca Bonino, Domenica Scarano, Carlo Lamberti, Silvia Bordiga, Luciano Montanari, Lucia Bonoldi, Roberto Millini and XRPD, HRTEM, Raman and UV-Vis characterization methods have been applied to obtain information on the morphology and the structure of the catalysts (including degree of staking and structural disorder) as well as on the vibrational and spectroscopic properties. It is shown that, when compared with HRTEM results, XRPD, Raman and UV-Vis data give a realistic information on the staking degree, on the particle size distribution and on the heterogeneity of supported MoS 2 particles on the various supports. EXAFS and XANES spectroscopies have been also used to set up the best sulfidation procedure on the MoS 2 /SiO 2 system, and the obtained recipe has been adopted for the MoS 2 /γ-Al 2 O 3 and MoS 2 /MgO systems. UV-vis analysis under controlled atmosphere has been performed to understand the effect of reductive and sulfiding treatments on the presence of sulfur vacancies and on the valence state of Mo ions associated with them. To explore the structure of coordinatively unsaturated Mo sites after reducing or sulfiding treatments (with CS 2 or, occasionally, with H 2 S), in situ FTIR of CO adsorbed at low-temperature has been performed on all samples. It is demonstrated that CO is a sensitive probe for coordinatively unsaturated sites and that the formation of sulfur vacancies on the MoS 2 surface upon reduction in pure H 2 at 673 K is accompanied by an increase of the coordinative unsaturation and a decrease of the valence state of a fraction of surface Mo cations, mainly located on corner and edge sites. Considering the non planarity of a consistent fraction of lammelae (as revealed in particular by HRTEM of MoS 2 on Al 2 O 3 and MgO), the presence of reducible Mo ions, located in defective positions on basal planes, cannot be excluded. Furthermore, it is demonstrated that this process can be reversed upon interaction with the sulfiding agent and that this reversible behavior is really mimicking some of the elementary acts occurring in the HDS process. Comparing the results obtained by all the adopted characterization techniques, it is concluded that the reductive effect of H 2 preferentially affects the particles characterized by the lowest staking degree. The complexity of the IR results suggests that the adopted reduction procedure in pure H 2 at 673 K induces the formation of several type of sulfur vacancies, presumably located in different crystallographic positions of the MoS 2 particles. In conclusion, the IR results obtained with the CO probe fully support the idea that the adopted reduction procedure in pure H 2 at 673 K is not only inducing the formation of several families of sulfur vacancies but that a continuity exists between the stages associated with the reducti...