Transition Metal Hosts Research Articles

Multi-Site Redox Activities in Chemically Complex Sodium Layered Oxide Cathode Materials

Sodium layered oxide cathode materials with high specific capacity, energy density, and long cycle life have been the bottleneck for the widespread market implementation of sodium-ion batteries. Even though sodium layered oxide materials share many similarities with their Li-ion counterpart, there are many dissimilarities between the two which can be utilized to design high performance sodium layered oxide cathode materials. Sodium layered oxide materials can incorporate almost all of the first row transition metals in the transition metal oxide layer. Herein, we have synthesized a sodium layered oxide material with five transition metals to demonstrate such concept. The material is synthesized following a simple co-precipitation method and is a mixture of P2 and P3 type crystal structures (P2/P3=1). The material has good electrochemical performance with excellent stability on long term cycling. It delivers a specific capacity in the range of 95 mAh/g at C/10 with no capacity fading after 100 cycles, 72 mAh/g at 1C with 95% capacity retention after 500 cycles, and 55 mAh/g at 5C with 85% capacity retention after 1000 cycles. We further demonstrate the surprisingly good performance of the material at low temperatures. For example, the material delivers a 60 mAh/g capacity at 1C and 0°C with no capacity fading after 100 cycles. We further studied the structural evolution and redox reaction mechanism of the cathode material at different states of charge. The reversibility of the Na (de)insertion is evident from the reversibility of the XRD pattern upon cycling. Hard XAS study has revealed that most transition metals contribute to some degree towards the redox reaction of the cathode material, with most energy derived from the redox of Ni2+ and Co3+. This study has demonstrated that selecting the ideal transition metal hosts and building up the compositional complexity can be a path towards further improving sodium cathode materials.

Electronic structure and magnetic properties of dilute U impurities in metals

The electronic structure and magnetic moment of dilute U impurity in metallic hosts have been calculated from first principles. The calculations have been performed within local density approximation of the density functional theory using Augmented plane wave+local orbital (APW+lo) technique, taking account of spin–orbit coupling and Coulomb correlation through LDA+U approach. We present here our results for the local density of states, magnetic moment and hyperfine field calculated for an isolated U impurity embedded in hosts with sp-, d- and f-type conduction electrons. The results of our systematic study provide a comprehensive insight on the pressure dependence of 5f local magnetism in metallic systems. The unpolarized local density of states (LDOS), analyzed within the frame work of Stoner model suggest the occurrence of local moment for U in sp-elements, noble metals and f-block hosts like La, Ce, Lu and Th. In contrast, U is predicted to be nonmagnetic in most transition metal hosts except in Sc, Ti, Y, Zr, and Hf consistent with the results obtained from spin polarized calculation. The spin and orbital magnetic moments of U computed within the frame of LDA+U formalism show a scaling behavior with lattice compression. We have also computed the spin and orbital hyperfine fields and a detail analysis has been carried out. The host dependent trends for the magnetic moment, hyperfine field and 5f occupation reflect pressure induced change of electronic structure with U valency changing from 3+ to 4+ under lattice compression. In addition, we have made a detailed analysis of the impurity induced host spin polarization suggesting qualitatively different roles of f-band electrons on moment stability. The results presented in this work would be helpful towards understanding magnetism and spin fluctuation in U based alloys.

Theoretical probing of inelastic spin-excitations in adatoms on surfaces

We review our recent work on the simulation, description and prediction of spin-excitations in adatoms and dimers deposited on metallic surfaces. This work done together with Douglas L. Mills, is an extension of his seminal contribution (with Pascal Lederer) published 50years ago on the spin-dynamics of transition metal impurities embedded in transition metal hosts [Lederer et al. (1967)]. The main predictions of his model were verified experimentally with state of the art inelastic scanning tunneling spectroscopy on adatoms. Our formalism, presented in this review, is based on time-dependent density functional theory, combined with the Korringa–Kohn–Rostoker Green function method. Comparison to experiments is shown and discussed in detail. Our scheme enables the description and prediction of the main characteristics of these excitations, i.e. their resonance frequency, their lifetime and their behavior upon application of external perturbations such as a magnetic field.

Evolution of Strategies for Modern Rechargeable Batteries

This Account provides perspective on the evolution of the rechargeable battery and summarizes innovations in the development of these devices. Initially, I describe the components of a conventional rechargeable battery along with the engineering parameters that define the figures of merit for a single cell. In 1967, researchers discovered fast Na(+) conduction at 300 K in Na β,β''-alumina. Since then battery technology has evolved from a strongly acidic or alkaline aqueous electrolyte with protons as the working ion to an organic liquid-carbonate electrolyte with Li(+) as the working ion in a Li-ion battery. The invention of the sodium-sulfur and Zebra batteries stimulated consideration of framework structures as crystalline hosts for mobile guest alkali ions, and the jump in oil prices in the early 1970s prompted researchers to consider alternative room-temperature batteries with aprotic liquid electrolytes. With the existence of Li primary cells and ongoing research on the chemistry of reversible Li intercalation into layered chalcogenides, industry invested in the production of a Li/TiS2 rechargeable cell. However, on repeated recharge, dendrites grew across the electrolyte from the anode to the cathode, leading to dangerous short-circuits in the cell in the presence of the flammable organic liquid electrolyte. Because lowering the voltage of the anode would prevent cells with layered-chalcogenide cathodes from competing with cells that had an aqueous electrolyte, researchers quickly abandoned this effort. However, once it was realized that an oxide cathode could offer a larger voltage versus lithium, researchers considered the extraction of Li from the layered LiMO2 oxides with M = Co or Ni. These oxide cathodes were fabricated in a discharged state, and battery manufacturers could not conceive of assembling a cell with a discharged cathode. Meanwhile, exploration of Li intercalation into graphite showed that reversible Li insertion into carbon occurred without dendrite formation. The SONY corporation used the LiCoO2/carbon battery to power their initial cellular telephone and launched the wireless revolution. As researchers developed 3D transition-metal hosts, manufacturers introduced spinel and olivine hosts in the Lix[Mn2]O4 and LiFe(PO4) cathodes. However, current Li-ion batteries fall short of the desired specifications for electric-powered automobiles and the storage of electrical energy generated by wind and solar power. These demands are stimulating new strategies for electrochemical cells that can safely and affordably meet those challenges.

Computational design of interstitial alloys: Effect of metal–interstitial interactions on atom distributions

Abstract Mutual interaction of (metal) host atoms and interstitial atoms can induce phase transitions and order–disorder transitions. Therefore, such correlated site occupations need to be taken into account in computational design of material like steels and membranes for hydrogen purification. In this paper, the cluster variation method based simple cube approximation is applied to predict phase transformations in a hypothetical system and the effect of alloying elements on the distribution of atoms and vacancies in fcc structured transition metal hosts. The cube cluster is composed of both metal and interstitial sublattice sites of which the mutual interaction is expressed in terms of pair potentials.

Surface segregation energies in low-index open surfaces of bimetallic transition metal alloys

We present a database of 24×24 segregation energies of single transition metal impurities in low-index surfaces of transition metal hosts, calculated using the localized self-consistent Green’s function (LSGF) method, in combination with the atomic sphere approximation including a multipole correction to the electrostatic potential and energy. The surface energies of {100} facets for fcc and bcc transition metals, and the more stable of the two {101¯0} facets of hcp transition metals are also calculated and compared with available theoretical results. Insights derived should be useful for determining the nature of active sites in a variety of catalytic reactions employing bimetallic catalysts.

A DFT study of pseudomorphic monolayer Pt and Pd catalysts for NO x storage reduction applications

Pseudomorphic monolayer catalysts involving Pd and Pt monolayers above late transition metal (TM) hosts (Cu, Ru, Rh, Ag, Ir and Au) were examined for the oxidation of NO with atomic oxygen. It is found that for systems in which the lattice constant of Pt (or Pd) is expanded, the reaction enthalpy is more endothermic and the reaction barrier is higher. Conversely, for systems under compression, NO 2 formation is more exothermic and has a lower barrier. These systems were also examined for their thermodynamic stability upon adsorption of NO and oxygen. Although, segregated systems with a pseudomorphic monolayer of Pt and Pd above a transition metal were found to be stable for systems involving Ru, Ir, Rh and Au even in the presence of adsorbates, oxygen was found to be capable of reversing the position of the supported monolayer and the host metal for systems involving Ag or Cu.

Atomic displacements due to interstitial hydrogen in Cu and Pd

The density functional theory (DFT) is used to study the atomic interactions in transition metal-based interstitial alloys. The strain field is calculated in the discrete lattice model using Kanzaki method. The total energy and hence atomic forces between interstitial hydrogen and transition metal hosts are calculated using DFT. The norm-conserving pseudopotentials for H, Cu and Pd are generated self-consistently. The dynamical matrices are evaluated considering interaction up to first nearest neighbors whereas impurity-induced forces are calculated with M32H shell (where M = Cu and Pd). The atomic displacements produced by interstitial hydrogen at the octahedral site in Cu and Pd show displacements of 7.36% and 4.3% of the first nearest neighbors respectively. Both Cu and Pd lattices show lattice expansion due to the presence of hydrogen and the obtained average lattice expansion ΔV/V = 0.177 for Cu and 0.145 for Pd.

Atomic displacements in bcc dilute alloys

We present here a systematic investigation of the atomic displacements in bcc transition metal (TM) dilute alloys. We have calculated the atomic displacements in bcc (V, Cr, Fe, Nb, Mo, Ta and W) transition metals (TMs) due to 3d, 4d and 5d TMs at the substitutional site using the Kanzaki lattice static method. Wills and Harrison interatomic potential is used to calculate the atomic force constants, the dynamical matrix and the impurity-induced forces. We have thoroughly investigated the atomic displacements using impurities from 3d, 4d and 5d series in the same host metal and the same impurity in different hosts. We have observed a systematic pattern in the atomic displacements for Cr-, Fe-, Nb-, Mo-, Ta-and W-based dilute alloys. The atomic displacements are found to increase with increase in the number of d electrons for all alloys considered except for V dilute alloys. The 3d impurities are found to be more easily dissolved in the 3d host metals than 4d or 5d TMs whereas 4d and 5d impurities show more solubility in 4d and 5d TMs. In general, the relaxation energy calculation suggests that impurities may be easily solvable in 5d TM hosts when compared to 3d or 4d TMs.

Steps, kinks, and segregation at metallic surfaces

We have used density functional theory to establish databases of surface energies for low index surfaces of monoatomic metals and of surface segregation energies of single transition metal impurities in transition metal hosts. The data form one consistent starting point for models of surface science phenomena.

Surface segregation energies in transition-metal alloys

We present a database of $24\ifmmode\times\else\texttimes\fi{}24$ surface segregation energies of single transition metal impurities in transition-metal hosts obtained by a Green's-function linear-muffin-tin-orbitals method in conjunction with the coherent potential and atomic sphere approximations including a multipole correction to the electrostatic potential and energy. We use the database to establish the major factors which govern surface segregation in transition metal alloys. We find that the calculated trends are well described by Friedel's rectangular state density model and that the few but significant deviations from the simple trends are caused by crystal structure effects.

Foreword: Nuclear methods in magnetism

Usually, the occurrence of magnetism on isolated, substitutional 3d, 4d and 5d impurity ions in metals is restricted to certain 3d ions (mainly Cr, Mn, Fe, Co, Ni) in alloying metallic systems. The application of the perturbed γ-ray distribution method following heavy ion reactions and recoil implantation has offered an experimental technique for producing and investigating new magnetic systems. Of special importance are nonalloying systems, which can exhibit extreme variations of e.g. density of states and atomic volume in the probe–host combinations produced by recoil implantation. Recent developments in this field include the following: Magnetism and the Kondo effect observed for 43Sc ions in alkali metal hosts are found to be consistent with a nearly localised, ionic 3d1 single-electron configuration, and parallel the behaviour observed in certain Ce systems. More generally, essential features of the magnetism of 3d and 4d ions in sp metal hosts are similar to those of 4f systems. Recent experimental and theoretical studies of 54Fe in d-band metal hosts are of key importance for an understanding of the basic features of local moment formation on substitutional Fe ions in transition metal hosts in general. In many nonalloying 54Fe probe-host combinations, (at least) two different magnetic responses have been detected. These components correspond to substitutional and interstitial sites of the implanted probes, as has been verified by in-beam Mössbauer spectroscopy of 57Fe in a series of host metals. This provides new insight into lattice site occupation as a function of host properties and allows directed investigations of the magnetic behaviour of Fe (and Mo) ions on interstitial lattice sites. Depending on the host metal, interstitial Fe is found to be nonmagnetic, e.g. in Zr, or magnetic, e.g. in Yb. Surprisingly, even the 4d ion Mo can be magnetic on interstitial sites. The experimental results for the substitutional as well as the interstitial sites can be compared to extensive theoretical work within the framework of local spin density calculations.

Magnetic Behaviour of Implanted Transition Metal Probes at Different Lattice Sites in Metals

Usually, the occurrence of magnetism on isolated, substitutional 3d, 4d and 5d impurity ions in metals is restricted to certain 3d ions (mainly Cr, Mn, Fe, Co, Ni) in alloying metallic systems. The application of the perturbed γ-ray distribution method following heavy ion reactions and recoil implantation has offered an experimental technique for producing and investigating new magnetic systems. Of special importance are nonalloying systems, which can exhibit extreme variations of e.g. density of states and atomic volume in the probe–host combinations produced by recoil implantation. Recent developments in this field include the following: Magnetism and the Kondo effect observed for 43Sc ions in alkali metal hosts are found to be consistent with a nearly localised, ionic 3d1 single-electron configuration, and parallel the behaviour observed in certain Ce systems. More generally, essential features of the magnetism of 3d and 4d ions in sp metal hosts are similar to those of 4f systems. Recent experimental and theoretical studies of 54Fe in d-band metal hosts are of key importance for an understanding of the basic features of local moment formation on substitutional Fe ions in transition metal hosts in general. In many nonalloying 54Fe probe-host combinations, (at least) two different magnetic responses have been detected. These components correspond to substitutional and interstitial sites of the implanted probes, as has been verified by in-beam Mössbauer spectroscopy of 57Fe in a series of host metals. This provides new insight into lattice site occupation as a function of host properties and allows directed investigations of the magnetic behaviour of Fe (and Mo) ions on interstitial lattice sites. Depending on the host metal, interstitial Fe is found to be nonmagnetic, e.g. in Zr, or magnetic, e.g. in Yb. Surprisingly, even the 4d ion Mo can be magnetic on interstitial sites. The experimental results for the substitutional as well as the interstitial sites can be compared to extensive theoretical work within the framework of local spin density calculations.

Accurate densities of states for metallic compounds from parametrized tight-binding calculations?

The local density of states is calculated for an Fe atom in various 4d transition-metal hosts on the one hand by the linear-muffin-tin-orbital method in the atomic-sphere approximation, and on the other hand by a two-centre orthogonal Slater - Koster tight-binding method supplemented by the demand for local charge neutrality. The comparison demonstrates that the latter method is able to yield accurate densities of states for metallic compounds.

Magnetic Behavior of Fe Impurities in Tc and Re, and Its Relevance to the General Problem of the Magnetism of Fe in d-Band Metal Hosts.

We have observed implanted Fe ions to be nonmagnetic in the group VIIb hosts Tc and Re, by applying the in-beam perturbed angular {gamma}-ray distribution method. This result is in agreement with local spin density calculations for Fe in Tc using both supercell and single impurity approaches. The results are of key importance for a reasonable and consistent understanding of the basic, general features of local magnetic moment formation on Fe ions in transition metal hosts. {copyright} {ital 1996 The American Physical Society.}

Effects of Ce vs. Lu substitution on the electronic structure of rare earth-transition metal compounds

We report the results of a combined UV photoemission and inverse photoemission spectroscopy investigation of some isostructural rare earth (R) intermetallic compounds, i.e. CeCo 2, LuCo 2, CeRh 2, LuRh 2, Ce 7Rh 3 and Lu 7Rh 3, aiming to highlight the effects, on both the occupied and unoccupied d-derived partial density of states, of R substitution on different transition metal (M) hosts. These effects cannot be detected by X-ray spectroscopies because of the sizeable f-related contribution while, for cross-section reasons, UV spectroscopies may allow direct access to the d-like states. The results are discussed in terms of the different spatial extents of Ce and Lu 5d wavefunctions and of the degree of intermixing with the M partner d valence states.

Formation of magnetic moments in transition metal hosts

Abstract The role of crystal symmetry for the formation of magnetic moments on iron impurities in d metal hosts is elucidated by ab initio calculations in the local-spin-density approximation for 3d, 4d and 5d metal hosts, both for hypothetical bcc structure and for the real structures of the hosts.

Formation of magnetic moments on iron impurities in transition-metal hosts.

The role of crystal symmetry for the formation of magnetic moments on iron impurities in transition-metal hosts is elucidated. To do this, ab initio calculations within the framework of the local-spin-density approximation are performed for iron atoms in 3d, 4d, and 5d transition-metal hosts, both for hypothetical bcc structures as well as for the real structures of the hosts. It is shown that the systematics of the formation of magnetic moments is related to the fact that the degree of hybridization among the ${\mathit{T}}_{2\mathit{g}}$ and ${\mathit{E}}_{\mathit{g}}$ orbitals depends on the crystal symmetry. The results for the magnetic moments and hyperfine fields are compared with experimental data and with results from other calculations.

Impurity-impurity interactions in Cu, Ni, Ag, and Pd.

We present systematic ab initio calcultions for the interaction energies of impurity pairs in Cu, Ni, Ag, and Pd. The calculations are based on local-density theory and apply the Korring-Kohn-Rostoker Green's-function method for spherical potentials. The full nonspherical charge density is used to calculate the double-counting contributions to the total energy. In particular, we calculate the nearest- and next-nearest-neighbor interaction energies of impurity pairs of the 3d and 4sp elements in Cu and Ni as well as of similar pairs of the 4d and 5sp elements in Ag and Pd: these interactions determine the ordering or segregation behavior of the alloys at finite temperatures. Comparisons are drawn with experimental phase diagrams, solid solubilities of the impurities, and also with the available information about the interaction from diffuse-scattering experiments. The physical mechanisms determining the nearest-neighbor interactions of the impurity pairs are discussed using simple model calculations based on the tight-binding method and the jellium approach: (1) The 4d-4d interactions in Ag and Pd, being repulsive at the beginning in the 4d series and attractive around the middle, can be understood by considering both the changes of the d bond and the repulsive energies between different atomic rearrangement of isolated impurities and impurity pairs. For 3d impurity pairs with the large local moments (Cr to Co) in Cu and Ni, magnetic effects also become important. (2) The repulsive interactions of 4sp-4sp in Cu as well as of 5sp-5sp in Ag can be understood by the electrostatic interaction between the excess ionic charges of the impurities, being screened by the conduction electrons of the host, while the stronger repulsive interactions of 4sp-4sp in Ni as well as of 5sp-5sp in Pd mainly arise from the breakup of the sp-d bonds between the sp impurities and the transition-metal hosts.

Magnetism and electronic structure of 3d and 4d ions in metals

The magnetism and electronic structure of 3d and 4d ions in metals exhibit qualitative differences in sp metal hosts compared to hosts with d band electrons. 3d and 4d ions in sp metal hosts, in particular in alkali metals, reflect the phenomena of well-defined ionic ground states, large orbital magnetism, mixed valence, and crystal field splittings smaller than the LS coupling. Magnetic 3d and 4d states in sp metals cannot be described by one-electron approaches based on Anderson type models but require an analysis based on ionic configurations where the tendencies towards reduced magnetism can be well reproduced by increasing spin fluctuation rates. The behavior of d ions in sp metals exhibit basic common features to the physics of stable and unstable f states in metals. In contrast, the local moment formation of Fe, Co and Ni ions in transition metal hosts, in Cu, Ag, Au, and even in Hg is governed by inter-atomic interactions of the magnetic d states with host d band electrons. As the leading contribution to magnetic stability an effectively ferromagnetic interaction between impurity d states and host d band electrons has been suggested, which in certain hosts successfully suppress spin fluctuations arising from antiferromagnetic d-sp exchanges. All host dependent trends of moment stability of 3d and also of 4d ions in metals are consistent with this proposal.