TiNb2O7 is a Wadsley–Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li0.60TiNb2O7. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in Li x TiNb2O7) is rapid with low activation barriers from NMR and D Li = 10–11 m2 s–1 at the temperature of the observed T 1 minima of 525–650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100–200 meV down the tunnels but ca. 700–1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in Li x TiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 reveals high diffusion barriers of 1–3 eV, indicating that this structure is specifically suited to Li+ mobility.

The effect of lattice strain on the catalytic properties of Pd nanoparticles is systematically studied. Synthetic strategies for the preparation of a series of shape-controlled Pd nanocrystals with lattice strain generated from different sources has been developed. All of these nanocrystals were created with the same capping agent under similar reaction conditions. First, a series of Pd nanoparticles was synthesized that were enclosed in {111} surfaces: Single-crystalline Pd octahedra, single-crystalline AuPd core-shell octahedra, and twinned Pd icosahedra. Next, various {100}-terminated particles were synthesized: Single-crystalline Pd cubes and single-crystalline AuPd core-shell cubes. Different extents of lattice strain were evident by comparing the X-ray diffraction patterns of these particles. During electrocatalysis, decreased potentials for CO stripping and increased current densities for formic-acid oxidation were observed for the strained nanoparticles. In the gas-phase hydrogenation of ethylene, the activities of the strained nanoparticles were lower than those of the single-crystalline Pd nanoparticles, perhaps owing to a larger amount of cetyl trimethylammonium bromide on the surface.

Developing syntheses of more sophisticated nanostructures comprising late transition metals broadens the tools to rationally design suitable heterogeneous catalysts for chemical transformations. Herein, we report a synthesis of Pd-Rh nanoboxes by controlling the migration of metals in a core-shell nanoparticle. The Pd-Rh nanobox structure is a grid-like arrangement of two distinct metal phases, and the surfaces of these boxes are {100} dominant Pd and Rh. The catalytic behaviors of the particles were examined in electrochemistry to investigate strain effects arising from this structure. It was found that the trends in activity of model fuel cell reactions cannot be explained solely by the surface composition. The lattice strain emerging from the nanoscale separation of metal phases at the surface also plays an important role.

Half-Heusler XYZ compounds with an 18 valence electron count are promising thermoelectric materials, being thermally and chemically stable, deriving from relatively earth-abundant components, and possessing appropriate electrical transport properties. The typical drawback with this family of compounds is their high thermal conductivity. A strategy for reducing thermal conductivity is through the inclusion of secondary phases designed to minimize negative impact on other properties. Here, we achieve this through the addition of excess Co to half-Heusler NbCoSn, which introduces precipitates of a semi-coherent NbCo2Sn Heusler phase. A series of NbCo1+xSn materials are characterized here using X-ray and neutron diffraction studies and electron microscopy. Electrical and thermal transport measurements and electronic structure calculations are used to understand property evolution. We find that annealing has an important role to play in determining antisite ordering and properties. Antisite disorder in the as-prepared samples improves thermoelectric performance through the reduction of thermal conductivity, but annealing during the measurement degrades properties to resemble those of the annealed samples. Similar to the more widely studied TiNi1+xSn system, Co addition to the NbCoSn phase results in improved thermoelectric performance through a decrease in thermal conductivity which results in a 20% improvement in the thermoelectric figure of merit, zT.

PbPdO 2 is a band semiconductor with a band gap arising from the filled d 8 nature of square-planar Pd 2+ . We establish that hole doping through Li substitution for Pd in PbPdO 2 results in a p-type metallic oxide with a positive temperature coefficient of resistance for substitution amounts as small as 2 mol % of Li for Pd. Furthermore, PbPd 1−x Li x O 2 demonstrates a high Seebeck coefficient, and is therefore an oxide thermoelectric material with high thermopower despite the metallic conductivity. Up to 4 mol % Li is found to substitute for Pd as verified by Rietveld refinement of neutron diffraction data. At this maximum Li-substitution, the resistivity is driven below the Mott metallic maximum to 3.5×10 −3 Ω cm with a Seebeck coefficient of 115 µV/K at room temperature which increases to 175 µV/K at 600 K. These electrical properties are almost identical to the well-known p-type oxide thermoelectric Na x CoO 2 . Nonmagnetic Li-substituted PbPdO 2 does not possess a correlated, magnetic state with high spin degeneracy as found in some complex cobalt oxides. This suggests that there are other avenues to achieving high Seebeck coefficients with metallic conductivities in oxide thermoelectrics. The electrical properties coupled with the moderately low lattice thermal conductivities allow for a zT = 0.12 at 600 K; the maximum temperature measured here. The trend suggests yet higher values at elevated temperatures.First-principles calculations of the electronic structure and electrical transport provide insight into the observed properties.