Modeling interfaces of fluorite-structure compounds using slab charge distribution

Automated generation of reasonable atomic-level interface models, for example at a grain boundary, is often a difficult task. The interface modeling algorithm for elementary substances based on charge densities of slab surfaces by Hinuma et al. [AIP Advances 11, 115020 (2021)] was applied to obtain Σ3 (111)/(111¯) and Σ5 (310)/(31¯0) interface models of fluorite structure compounds reported in the ICSD database. The algorithm found only one type of in-plane rigid-body translation (RBT) in the former. In contrast, there were diverse interfaces with various RBTs in the latter; the RBT for each compound was identified by also testing a set of RBTs, given by the algorithm, from other compounds. The algorithm by Hinuma et al. can therefore be used, although with caveats, as a complementary tool to estimate the atom configuration at interfaces of compounds.</p

CO₂ Adsorption on the (111) Surface of Fcc-structure High Entropy Alloys

High entropy alloys (HEAs), obtained by alloying five or more elements, can exhibit unique characteristics. The CO2 adsorption capabilities of fcc structure HEAs consisting of five elements among Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt, and Au were evaluated by conducting first-principles calculations of the CO2 adsorption energy. HEAs could be categorized into “binding”, “less binding”, and “inconclusive” HEAs, where there were 27, 23, and 10 HEAs each, respectively, out of 60 randomly chosen HEAs. “binding” HEAs are defined as having low CO2 adsorption energy sites of less than -0.08 eV, which is difficult to attain with elementary substances or binary alloys. These low adsorption energy, or more active, sites are found near on-top positions of the HEA surface, whereas CO2 does not adsorb at such positions in “less binding” HEAs. Calculating CO2 adsorption energies could be a useful tool to check whether a specific HEA is “binding” or “less binding” prior to conducting extensive experiments.</p

First-Principles Study on Relaxor-Type Ferroelectric Behavior without Chemical Inhomogeneity in BaTaO₂N and SrTaO₂N

The wide range of applications attracts interest in oxynitride perovskites. BaTaO₂N and SrTaO₂N have relaxor-type high dielectric permittivities and are promising candidates in many applications especially because Pb is not included, unlike in many relaxor ferroelectrics. There is an urgent need to understand the relation between the anion ordering and the permittivity to facilitate screening and designing materials with higher permittivity, and the chemistry that results in relaxor-type behavior without chemical inhomogeneity. We show using systematic first-principles calculations that stable anion orderings in BaTaO₂N and SrTaO₂N have two kinds of similar, 3D −Ta–N– coiled chain motifs that can switch to each other, forming a mechanism to break long-range order and increasing the diversity of anion orderings around the pentavalent Ta. Both materials have two sets of low-energy displacements forming opposite polarization directions that can be easily alternated at the picosecond scale. This explanation of the origin of relaxor-type behavior without chemical inhomogeneity currently found only in these two materials will fuel further searching of similar materials

Electronic Spin Transition in Nanosize Stoichiometric Lithium Cobalt Oxide

A change in the electronic spin state of the surfaces relevant to Li (de)­intercalation of nanosized stoichiometric lithium cobalt oxide LiCo­(III)­O₂ from low-spin to intermediate and high spin is observed for the first time. These surfaces are the ones that are relevant for Li (de)­intercalation. From density functional theory calculations with a Hubbard U correction, the surface energies of the layered lithium cobalt oxide can be significantly lowered as a consequence of the spin change. The crystal field splitting of Co d orbitals is modified at the surface due to missing Co–O bonds. The electronic spin transition also has a significant impact on Co­(III)–Co­(IV) redox potential, as revealed by the change in the lithium (de)­intercalation voltage profile in a lithium half cell

Lithium Diffusion in Graphitic Carbon

Graphitic carbon is currently considered the state-of-the-art material for the negative electrode in lithium ion cells, mainly due to its high reversibility and low operating potential. However, carbon anodes exhibit mediocre charge/discharge rate performance, which contributes to severe transport-induced surface structural damage upon prolonged cycling and limits the lifetime of the cell. Lithium bulk diffusion in graphitic carbon is not yet completely understood, partly due to the complexity of measuring bulk transport properties in finite-sized nonisotropic particles. To solve this problem for graphite, we use the Devanathan−Stachurski electrochemical methodology combined with ab initio computations to deconvolute and quantify the mechanism of lithium ion diffusion in highly oriented pyrolytic graphite (HOPG). The results reveal inherent high lithium ion diffusivity in the direction parallel to the graphene plane (∼10⁻⁷−10⁻⁶ cm² s⁻¹), as compared to sluggish lithium ion transport along grain boundaries (∼10⁻¹¹ cm² s⁻¹), indicating the possibility of rational design of carbonaceous materials and composite electrodes with very high rate capability