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Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon free paths are short and the electron free paths are long. We analysed the relaxed structure of Sn clathrates using four different Density Funtional Exchange-Correlation functionals. The phonon structures were investigated as a first step in order to determine the phonon contribution to the thermal conductivity. We determined the Seebeck coefficient and electrical conductivity of the clathrate compound and the thermoelectric figure of merit. A glimpse into the dynamics of the system for the evaluation of the thermoelectric and electronic properties is presented.

The theoretical energy density, stability and cost of Zn-air batteries (ZAB) are far inferior to the most advanced lithium-ion batteries due to the irreversibility of zinc electrodes and sluggish electrocatalytic performance of oxygen reduction (ORR) and oxygen evolution (OER) processes. Herein, we designed a simple strategy to fabricate a quaternary Zn–Co–N–S co-doped tubular carbon (0.3-Zn-Co-N/S–C) via direct pyrolysis of various mixtures of precursors. The high porosity of 3D nanostructure not only provides better mass transfer but also offers a large number of active sites thereof superior ORR and OER performances. As a result, the rechargeable ZAB fabricated using optimized 0.3-Zn-Co-N/S–C as cathode exhibited a higher open-circuit voltage, stable discharge rate, and better reversibility, even exceeding the state-of-the-art Pt/C catalyst in alkaline solution. The catalytic mechanism indicates that the synergistic effect of Co metal nanoparticles, Co–N/Zn–N active sites, and the high-yield of N/S-doped tubular carbon dominate the outstanding catalytic performance.

Electrochemical reduction of CO2 by Cu hollow fibers to CO with high selectivity has previously been reported but selective conversion of CO2 to formic acid at high current densities, although highly desirable, is still challenging. Herein, a Cu hollow fiber with an interconnected pore structure is fabricated via a facile method and used as a stand-alone cathode for highly efficient electrochemical reduction of CO2 to formate. We obtain a high selectivity for CO2 reduction to formate with a maximum FE of 77.1% at a high current density of 34.7 mA cm−2, one of the highest FE on Cu-based materials. Our results suggest that delivering the CO2 gas into the inner space of the hollow fiber leads to a higher CO2 partial pressure in the pores due to the pressure drop across the wall of the Cu hollow fiber. As both the CO2 and H+ ions (from the electrolyte) compete for adsorption on the Cu hollow fiber active sites, the higher CO2 partial pressure makes CO2 adsorption more favorable, thereby reducing the concentration of the H+ on the active sites. This effectively suppresses the major competing reaction, hydrogen evolution reaction (HER), from 46.9% Faradaic efficiency (FE) to 15.0%. Furthermore, our studies reveals the impotency of the co-existence of Cu(100) and Cu(110) active site facets towards excellent selectivity of formate formation under high CO2 partial pressure. Additionally, the designed catalyst also exhibits out-standing long-term stability at high current density, demonstrating potential for large-scale practical applications.

We have applied ab initio molecular dynamics simulations to study metal–metal coupling on the alkali-metal sublattice in the β-pyrochlore osmates, AOs2O6 (A = K, Rb, Cs) at 300 K. We find that the dynamics of the alkali-metal atoms (rattlers) exhibit stronger rattler–rattler correlations than rattler–cage correlations, and that, at 300 K, this correlation is strongest for Cs. We show that the rattler–rattler correlations control the dominant dynamics in the rattling of these atoms. We provide preliminary evidence that the rattler correlated motion occurs primarily through two somewhat distinct vibrational modes: a high-energy mode (peak A) couples the rattlers to each other and a low-energy mode (peak B) couples the rattlers to the cage modes. Rattler–rattler correlated motion through the high-energy mode provides insight into the trend in spectral broadening from Cs to K. The spectral broadening is inversely proportional to the strength of the dynamical correlations on the alkali-metal sublattice which in turn depend on the atomic size of the rattler, decreasing from Cs to K. Thus, the broadest spectrum exhibited by the K is partly a consequence of the small size of this rattler which permits a greater range of motions involving combinations of both correlated and anti-correlated dynamics. We emphasize that the identification of the somewhat distinct roles of the high-energy (peak A) and low-energy (peak B) modes in rattler coupling reported in this work is a significant step toward a complete fundamental mechanism of rattler dynamical coupling in these osmates. We believe that such a mechanism will have profound implications for a broad class of cage compounds, including clathrates and skutterudites.

Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.

Rattling dynamics have been identified as fundamental to superconductivity in defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low thermal conductivity in clathrates and filled skutterudites. Combining inelastic neutron scattering (INS) measurements and ab initio molecular dynamics (MD) simulations, we use a new approach to investigate rattling in the Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find that although all the alkali metals rattle, the rattling of the K atoms is unique, involving local diffusion within a cage as well as vibrations at a range of frequencies. We show that the novel K rattling is driven by the strong anharmonicity and anisotropy of the local potentials around the K atoms, both of which are a consequence of the small size of the K rattler. Our findings may open new possibilities for phonon engineering in thermoelectric materials., Comment: 17 pages, 12 figures and 3 tables

Understanding the electronic charge distribution around oxygen vacancies in transition metal and rare earth oxides is a scientific challenge of considerable technological importance. We show how significant information about the charge distribution around vacancies in cerium oxide can be gained from a study of high resolution crystal structures of higher order oxides which exhibit ordering of oxygen vacancies. Specifically, we consider the implications of a bond valence sum analysis of Ce₇O₁₂ and Ce₁₁O₂₀. To illuminate our analysis we show alternative representations of the crystal structures in terms of orderly arrays of coordination defects and in terms of fluorite-type modules. We found that in Ce₇O₁₂, the excess charge resulting from removal of an oxygen atom delocalizes among all three triclinic Ce sites closest to the O vacancy. In Ce₁₁O₂₀, the charge localizes on the next nearest neighbour Ce atoms. Our main result is that the charge prefers to distribute itself so that it is farthest away from the O vacancies. This contradicts the standard picture of charge localization which assumes that each of the two excess electrons localizes on one of the cerium ions nearest to the vacancy. This standard picture is assumed in most calculations based on density functional theory (DFT). Based on the known crystal structure of Pr₆O₁₁, we also predict that the charge in Ce₆O₁₁ will be found in the second coordination shell of the O vacancy. We also extend the analysis to the Magnéli phases of titanium and vanadium oxides (M(n)O(₂n₋₁), where M = Ti, V) and consider the problem of metal-insulator transitions (MIT) in these oxides. We found that the bond valence analysis may provide a useful predictive tool in structures where the MIT is accompanied by significant changes in the metal-oxygen bond lengths. Although this review focuses mainly on bulk cerium oxides with some extension to the Magnéli phases of titanium and vanadium, our approach to characterizing electronic properties of oxygen vacancies and the physical insights gained should also be relevant to surface defects and to other rare earth and transition metal oxides.

We have applied the bond valence method to cerium oxides to determine the oxidation states of the Ce ion at the various site symmetries of the crystals. The crystals studied include cerium dioxide and the two sesquioxides along with some selected intermediate phases which are crystallographically well characterized. Our results indicate that cerium dioxide has a mixed-valence ground state with an f-electron population on the Ce site of 0.27 while both the A- and C-sesquioxides have a nearly pure f1 configuration. The Ce sites in most of the intermediate oxides have nonintegral valences. Furthermore, many of these valences are different from the values predicted from a naive consideration of the stoichiometric valence of the compound.