Pooja M. Panchmatia

The lithium intercalation process in the low-voltage lithium battery anode Li1+xV1−xO2

Lithium can be reversibly intercalated into layered Li(1+x)V(1-x)O(2) (LiCoO(2) structure) at ~0.1 V, but only if x>0. The low voltage combined with a higher density than graphite results in a higher theoretical volumetric energy density; important for future applications in portable electronics and electric vehicles. Here we investigate the crucial question, why Li cannot intercalate into LiVO(2) but Li-rich compositions switch on intercalation at an unprecedented low voltage for an oxide? We show that Li(+) intercalated into tetrahedral sites are energetically more stable for Li-rich compositions, as they share a face with Li(+) on the V site in the transition metal layers. Li incorporation triggers shearing of the oxide layers from cubic to hexagonal packing because the Li(2)VO(2) structure can accommodate two Li per formula unit in tetrahedral sites without face sharing. Such understanding is important for the future design and optimization of low-voltage intercalation anodes for lithium batteries.

Oxygen Defects and Novel Transport Mechanisms in Apatite Ionic Conductors: Combined 17O NMR and Modeling Studies

Germanium-based apatite compounds are fast oxide-ion conductors for potential use in fuel cells. A combination of solid-state 17O NMR spectroscopy, atomistic modeling, and DFT techniques help to elucidate oxygen defect sites and novel cooperative mechanisms of ion conduction. The picture shows oxygen diffusion in the studied apatite compound from molecular dynamics simulations.

Protonic defects and water incorporation in Si and Ge-based apatite ionic conductors

Apatite-type oxide-ion conductors have attracted considerable interest as potential fuel cell electrolytes. Atomistic modelling techniques have been used to investigate oxygen interstitial sites, protonic defects and water incorporation in three silicate and three germanate-based apatite-systems, namely La8Ba2(SiO4)6O2, La9.33(SiO4)6O2, La9.67(SiO4)6O2.5, La8Ba2(GeO4)6O2, La9.33(GeO4)6O2, and La9.67(GeO4)6O2.5. The simulation models reproduce the complex experimental structures for all of these systems. The interstitial defect simulations have examined the lowest energy configuration and confirm this site to be near the Si/GeO4 tetrahedra. The water incorporation calculations identify the O–H protonic site to be along the O4 oxygen channel as seen in naturally occurring hydroxy-apatites. The results also show more favourable and exothermic water incorporation energies for the germanate-based apatites. This is consistent with recent experimental work, which shows that Ge-apatites take up water more readily than the silicate analogues.

The Shape of TiO2-B Nanoparticles

The shape of nanoparticles can be important in defining their properties. Establishing the exact shape of particles is a challenging task when the particles tend to agglomerate and their size is just a few nanometers. Here we report a structure refinement procedure for establishing the shape of nanoparticles using powder diffraction data. The method utilizes the fundamental formula of Debye coupled with a Monte Carlo-based optimization and has been successfully applied to TiO2-B nanoparticles. Atomistic modeling and molecular dynamics simulations of ensembles of all the ions in the nanoparticle reveal surface hydroxylation as the underlying reason for the established shape and structural features.

Halide Ligated Iron Porphines: A DFT+U and UB3LYP Study

We apply the density functional theory + U (DFT+U) and unrestricted hybrid functional DFT-UB3LYP methods to study the electronic structure and magnetic properties of two prototypical iron porphines: iron(III) porphine chloride (FePCl) and difluoro iron(III-IV) porphine. Plain DFT within the generalized gradient approximation (GGA) implementation fails in describing the correct high-spin ground state of these porphine molecules, whereas DFT+U and UB3LYP provide an improved description. For a range of U values (4-8 eV), we compare the results of the DFT+U approach to those obtained previously with the hybrid functional (B3LYP) and with the CASPT2 approach. The DFT+U and UB3LYP methods successfully predict the molecular high spin (S = 5/2) ground state of FePCl, and also provide the nontrivial S = 3 high spin ground state for FePF(2). For the latter six-coordinated Fe porphine, our DFT+U calculations show that the S = 2, S = 5/2, and S = 3 states are energetically very close together (differences of 30 meV). Nonetheless, S = 3 is obtained as the ground state of the whole molecule, in accordance with the spin expected from the electron count. Our DFT+U calculations show furthermore that the Fe 3d occupancy is similar for FePF(2) and FePCl, i.e., DFT+U does not support Fe(IV) for FePF(2), but rather an Fe(III) porphyrin π-cation radical species, with an Fe high spin S(Fe) = 5/2, and an additional S = 1/2 stemming from spin density distributed over the porphine ring. This observation is also supported by our UB3LYP calculations.

Concurrent La and A-Site Vacancy Doping Modulates the Thermoelectric Response of SrTiO3: Experimental and Computational Evidence

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1-xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration.

Ab initio calculations of the electronic structure and magnetism of iron porphyrin-type molecules: a benchmarking study

The iron porphyrin molecule is one of the most important biomolecules. In spite of its importance to life science, on a microscopic scale its electronic properties are not yet well-understood. In order to achieve such understanding we have performed an ab initio computational study of various molecular models for the iron porphyrin molecule. Our ab initio electronic structure calculations are based on the density functional theory (DFT) and have been conducted using both the Generalised Gradient Approximation (GGA) and the GGA+U approach, in which an additional Hubbard-U term is added for the treatment of on-site electron-electron correlations. In our investigations we have, first, optimised the molecular structures by computing the minimal-energy atomic distances, and second, benchmarked our computational approach by comparison to existing calculated results obtained by quantum-chemical methods. We have considered several models of ligated porphyrin (Cl and NH3 ligated), as well as charged and non-charged molecules. In this way, the changes in the electronic, structural, and magnetic properties of the iron atom have been investigated as a function of the oxidation state and local environment of the iron atom. Our results for some of the model molecules reproduce the earlier quantum-chemical calculations done by Johansson and Sundholm [J. Chem. Phys. 120 (2003) 3229]. We find that the GGA+U approach provides a better description of the molecular electronic properties, which indicates that electron correlation effects on the iron are important and play an essential role, particularly for the spin moment on the iron atom. Also, we proceed beyond the relatively small molecular models to a larger, more realistic porphyrin molecule, for which we also find that the GGA+U results are in better agreement with experiments.

A computational study of doped olivine structured Cd2GeO4: local defect trapping of interstitial oxide ions

Computational modelling techniques have been employed to investigate defects and ionic conductivity in Cd2GeO4. We show due to highly unfavourable intrinsic defect formation energies the ionic conducting ability of pristine Cd2GeO4 is extremely limited. The modelling results suggest trivalent doping on the Cd site as a viable means of promoting the formation of the oxygen interstitial defects. However, the defect cluster calculations for the first time explicitly suggest a strong association of the oxide defects to the dopant cations and tetrahedral units. Defect clustering is a complicated phenomenon and therefore not trivial to assess. In this study the trapping energies are explicitly quantified. The trends are further confirmed by molecular dynamic simulations. Despite this, the calculated diffusion coefficients do suggest an enhanced oxide ion mobility in the doped system compared to the pristine Cd2GeO4.

Synthesis, conductivity and structural aspects of Nd 3 Zr 2 Li 7−3x Al x O 12

In this paper we report the synthesis, structure and Li ion conductivity of a new tetragonal garnet phase Nd3Zr2Li7O12. In line with other tetragonal garnet systems, the Li is shown to be ordered in the tetrahedral and distorted octahedral sites, and the Li ion conductivity is consequently low. In an effort to improve the ionic conductivity of the parent material, we have also investigated Al doping to reduce the Li content, Nd3Zr2Li5.5Al0.5O12, and hence introduce disorder on the Li sublattice. This was found to be successful leading to a change in the unit cell symmetry from tetragonal to cubic, and an enhanced Li ion conductivity. Neutron diffraction studies showed that the Al was introduced onto the ideal tetrahedral garnet site, a site preference also supported by the results of computer modelling studies. The effect of moisture on the conductivity of these systems was also examined, showing significant changes at low temperatures consistent with a protonic contribution in humid atmospheres. In line with these observations, computational modelling suggests favourable exchange energy for the Li+/H+ exchange process.