Dynamics of Interlayer Na-Ions in Ga-Substituted Na2Zn2TeO6 (NZTO) Studied by Variable-Temperature Solid-State 23Na NMR Spectroscopy and DFT Modeling.

Local Na-coordination and dynamics of Na2-xZn2-xGaxTeO6; x = 0.00 (NZTO), 0.05, 0.10, 0.15, 0.20, were studied by variable-temperature, 23Na NMR methods and DFT AIMD simulations. Structure and dynamics were probed by NMR in the temperature ranges of 100-293 K in a magnetic field of 18.8 T and from 293 up to 500 K in a magnetic field of 11.7 T. Line shapes and T1 relaxation constants were analyzed. At 100 K, the otherwise dynamic Na-ions are frozen out on the NMR time scale, and a local structure characterization was performed for Na-ions at three interlayer sites. On increasing the temperature, complex peak shape coalescences occurred, and at 293 K, the Na NMR spectra showed some averaging due to Na-ion dynamics. A further increase to 500 K did not reveal any new peak shape variations until the highest temperatures, where an apparent peak splitting was observed, similar to what was observed in the 18.8 T experiments at lower temperatures. A three-site exchange model coupled with reduced quadrupolar couplings due to dynamics appear to explain these peak shape observations. The Ga substitution increases the Na-jumping rate, as proved by relaxation measurements and by a decrease in temperature for peak coalescence. The estimated activation energy for Na dynamics in the NZTO sample, from relaxation measurements, corresponds well to results from DFT AIMD simulations. Upon Ga substitution, measured activation energies are reduced, which is supported, in part, by DFT calculations. Addressing the correlated motion of Na-ions appears important for solid-state ion conductors since benefits can be gained from the decrease in activation energy upon Ga substitution, for example.

Magnetic Dynamo Caused by Axions in Neutron Stars.

The coupling between axions and photons modifies Maxwell's equations, introducing a dynamo term in the magnetic induction equation. In neutron stars, for critical values of the axion decay constant and axion mass, the magnetic dynamo mechanism increases the total magnetic energy of the star. We show that this generates substantial internal heating due to enhanced dissipation of crustal electric currents. These mechanisms would lead magnetized neutron stars to increase their magnetic energy and thermal luminosity by several orders of magnitude, in contrast to observations of thermally emitting neutron stars. To prevent the activation of the dynamo, bounds on the allowed axion parameter space can be derived.

Effects of Ga Substitution on the Local Structure of Na2Zn2TeO6.

In the work presented here, we prepared Ga-substituted NZTO (Na2-xZn2-xGaxTeO6, x = 0.00, 0.05, 0.10, 0.15, 0.20) layered materials with a soft chemical, citric acid-based synthesis method and characterized these by means of X-ray diffraction (XRD), 23Na and 125Te NMR, and by density functional theory (DFT) modeling. The influence of randomly distributed Ga cations on the 125Te NMR spectra confirms the successful synthesis. With DFT-based linear response computations, we show that the local distribution of Na ions in the two neighboring interlayers influences the 125Te chemical shift, consistent with observations. DFT modeling suggests that some of the Na sites are rarely occupied in pure NZTO but become favorable upon Ga substitution. There are clear indications that Ga substitution gives an uneven distribution of Na ions in neighboring interlayers and that the Na structure in one layer affects the adjacent layers.

Antifluorite-type Na5FeO4 as a low-cost, environment-friendly cathode with combined cationic/anionic redox activity for sodium ion batteries: a first-principles investigation.

The rapid electrification of our society and the transition towards a larger share of intermittent renewable energy sources in our electricity grids will dramatically increase the demand for cheap energy storage. Sodium ion batteries (SIBs) show a lot of promise to provide the required stationary storage at the grid level at low cost owing to the natural abundance and geographical availability of sodium. In addition, alkali-rich cathode materials exhibiting anionic redox contributions have garnered much attention over the past decade as a strategy to increase the specific capacity. In this work, we investigate for the first time the sodium-rich compound Na5FeO4 as a potential low-cost, environment-friendly cathode for sodium ion batteries from first principles using density functional theory. We investigate three low-energy polymorphs related to the antifluorite structure, verify their dynamical and mechanical stabilities, and show that they exhibit promising ion diffusive properties. As alkali-rich cathode materials are prone to oxygen loss during cycling, we investigate cycling stability with respect to phase transformations and oxygen loss and identify in particular one promising cycling interval that can reversibly shuttle 1.5 Na+ per formula unit between Na5FeO4 and Na3.5FeO4 with a gravimetric energy density exceeding 360 W h kg-1. Investigations into possible redox mechanisms reveal that the charge compensation occurs simultaneously on Fe- and O-atoms in FeO4-tetrahedra, which suggests that Na5FeO4, if realised experimentally as a cathode material, would join the family of combined cationic/anionic redox compounds.

Near-Broken-Gap Alignment between FeWO4 and Fe2WO6 for Ohmic Direct p-n Junction Thermoelectrics.

We report a near-broken-gap alignment between p-type FeWO4 and n-type Fe2WO6, a model pair for the realization of Ohmic direct junction thermoelectrics. Both undoped materials have a large Seebeck coefficient and high electrical conductivity at elevated temperatures, due to inherent electronic defects. A band-alignment diagram is proposed based on X-ray photoelectron and ultraviolet-visible light reflectance spectroscopy. Experimentally acquired nonrectifying I-V characteristics and the constructed band-alignment diagram support the proposed formation of a near-broken-gap junction. We have additionally performed computational modeling based on density functional theory (DFT) on bulk models of the individual compounds to rationalize the experimental band-alignment diagram and to provide deeper insight into the relevant band characteristics. The DFT calculations confirm an Fe-3d character of the involved band edges, which we suggest is a decisive feature for the unusual band overlap.

Insights into Crystal Structure and Diffusion of Biphasic Na2Zn2TeO6.

The layered oxide Na2Zn2TeO6 is a fast Na+ ion conductor and a suitable candidate for application as a solid-state electrolyte. We present a detailed study on how synthesis temperature and Na-content affect the crystal structure and thus the Na+ ion conductivity of Na2Zn2TeO6. Furthermore, we report for the first time an O'3-type phase for Na2Zn2TeO6. At a synthesis temperature of 900 °C, we obtain a pure P2-type phase, providing peak performance in Na+ ion conductivity. Synthesis temperatures lower than 900 °C produce a series of mixed P2 and O'3-type phases. The O'3 structure can only be obtained as a pure phase by substituting Li on the Zn-sites to increase the Na-content. Thorough analysis of synchrotron data combined with computational modeling indicates that Li enters the Zn sites and, consequently, the amount of Na in the structure increases to balance the charge according to the formula Na2+xZn2-xLixTeO6 (x = 0.2-0.5). Impedance spectroscopy and computational modeling confirm that reducing the amount of the O'3-type phase enhances the Na+ ion mobility.

Properties of Novel Non-Silicon Materials for Photovoltaic Applications: A First-Principle Insight.

Due to the low absorption coefficients of crystalline silicon-based solar cells, researchers have focused on non-silicon semiconductors with direct band gaps for the development of novel photovoltaic devices. In this study, we use density functional theory to model the electronic structure of a large database of candidates to identify materials with ideal properties for photovoltaic applications. The first screening is operated at the GGA level to select only materials with a sufficiently small direct band gap. We extracted twenty-seven candidates from an initial population of thousands, exhibiting GGA band gap in the range 0.5⁻1 eV. More accurate calculations using a hybrid functional were performed on this subset. Based on this, we present a detailed first-principle investigation of the four optimal compounds, namely, TlBiS2, Ba3BiN, Ag2BaS2, and ZrSO. The direct band gap of these materials is between 1.1 and 2.26 eV. In the visible region, the absorption peaks that appear in the optical spectra for these compounds indicate high absorption intensity. Furthermore, we have investigated the structural and mechanical stability of these compounds and calculated electron effective masses. Based on in-depth analysis, we have identified TlBiS2, Ba3BiN, Ag2BaS2, and ZrSO as very promising candidates for photovoltaic applications.

Real-time imaging of adatom-promoted graphene growth on nickel.

Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

Temperature-driven changes of the graphene edge structure on Ni(111): substrate vs hydrogen passivation.

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 °C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges.

Atomic Scale Identification of Coexisting Graphene Structures on Ni(111).

Through a combined scanning tunneling microscopy (STM) and density functional theory (DFT) approach, we provide a full characterization of the different chemisorbed configurations of epitaxial graphene coexisting on the Ni(111) single crystal surface. Top-fcc, top-hcp, and top-bridge are found to be stable structures with comparable adsorption energy. By comparison of experiments and simulations, we solve an existing debate, unambiguously distinguishing these configurations in high-resolution STM images and characterizing the transitions between adjacent domains. Such transitions, described in detail through atomistic models, occur not only via sharp domain boundaries, with extended defects, but predominantly via smooth in-plane distortions of the carbon network, without disruption of the hexagonal rings, which are expected not to significantly affect electron transport.