Synthesis and characterization of lithium-carbon compounds for hydrogen storage

Three carbon materials were prepared for the synthesis of Li-C compounds, such as Li intercalated graphite. The materials were as-received high purity polycrystalline graphite (G), graphite milled under a hydrogen atmosphere (HG), and graphite milled an argon atmosphere (AG). With respect to the difference for them, HG preserved a better crystalline structure than AG. Each material was milled with Li, where the products are denoted as Li-G, Li-HG, and Li-AG. In XRD patterns of Li-G and Li-HG, the peaks corresponding to LiC₆ and LiC₁₂ were revealed, while no peaks were observed in the case of Li-AG. However, the formation of lithium carbide Li₂C₂ was suggested for Li-AG by a thermal analysis under an inert gas. After the hydrogenation, LiH was formed for all the compounds, and graphite was recovered for Li-G and Li-HG. Each hydrogenated compound desorbed H₂ with different profile by heating up to 500 °C. As a reaction product, Li₂C₂ was formed for the hydrogenated Li-HG and Li-AG. In the case of the hydrogenated Li-G with better crystalline structure, Li intercalated graphite were formed after the dehydrogenation. Therefore, it is concluded that the hydrogen absorption and desorption process of Li intercalated graphite was different from those of Li₂C₂.     

Structure and phase stability of hydrogenated first-stage alkali- and alkaline-earth metal–graphite intercalation compounds

We report the hydrogenation temperature and pressure dependence of hydrogen absorption capacity in CaC₆. In addition, the structure and phase stability of hydrogenated CaC₆, LiC₆ and KC₈ has been investigated using X-ray diffraction and Raman spectroscopy. It is found that the hydrogen chemisorption in both CaC₆ and LiC₆ leads to either metastable or higher stage intermediate compounds but eventually leaves completely deintercalated graphite by forming stable metal hydrides. In contrast hydrogenation of KC₈ generates a restaged ternary hydride compound. The phase stability of hydrogenated compounds is discussed in the text based on the observed experimental results and correlated to the thermodynamic and kinetic aspects of intercalated metal.     

Nb2Al (100)表面的电子结构和相对稳定性

采用基于密度泛函理论的第一性原理计算,研究了Nb₂Al(100)表面不同终止端的电子结构、表面能和热力学性质。计算结果表明,由于表面弛豫和表面态的形成,所有终止端表面的电子结构均表现出增强的金属性质和减弱的共价性质。根据不同终止端表面的表面能计算,分析了非化学计量比表面的稳定性。在富含Nb和Al的条件下,C终止端表面(Nb₂₂Al₁₂)是热力学最稳定的表面。此外,计算了Nb₂Al(100)表面的功函数,表明该表面获得和失去电子的能力与形成前的纯元素表面相似。     

Discrete–variational Xα calculations of graphite and alkali–metal graphite intercalation compounds

First-principles molecular orbital calculations using discrete–variational (DV)-Xα method are carried out on the model clusters of graphite and alkali–metal graphite intercalation compounds MC_x(M=Li, Na, K, Rb, and Cs). Results of the calculations are used so as to simulate the experimentally observed near-edge X-ray-absorption fine-structures (XANES) and ultraviolet photoelectron spectra. For the clusters of graphite and KC_x, the calculated partial electronic densities of states (PDOSs) are in good agreement with the experimental C K-edge and K K-edge XANES spectra, respectively. Furthermore, the accordance between the calculated DOSs and the observed UPS spectra of graphite and RbC_x is also satisfactory. It is shown that the Fermi level is pushed up into the conduction band of graphite by doping alkali metals.     

Electronic and magnetic structures and bonding properties of Ce2T2X (T = nd element; X = Mg,Cd, Pb or Sn) intermetallics from first principles

The electronic structure and chemical bonding properties of four families of Ce₂T₂X (T = nd element; X = Mg, Cd, Pb or Sn) intermetallics crystallizing in an ordered U₃Si₂ type structure are shown from density functional theory (DFT) calculations to present electronic and magnetic structure properties arising from their peculiar valence electron count (VEC). Trends of the magnetism are discussed in terms of the characteristics of the Ce(4f) states as well as the energetic position of the transition metal element d states.     

Electronic structure of the new manganese ternary hydride Mg3MnH7

We have investigated the electronic structure of the new manganese hydride Mg₃MnH₇ by calculating ab initio the energy bands, density of states and the partial wave analysis of the density of states at each atomic site. We found that this hydride is an insulator with a large indirect energy gap of 2.56 eV. Partial wave analysis of the density of states shows that the electronic properties of this compound are strongly dominated by the bonding features of the MnH₆ and Mg₂H units.     

Crystal structure of the mirror symmetry 10H-type long-period stacking order phase in Mg-Y-Zn alloy

The crystal structure of the 10H-type long-period stacking order structure in Mg-Y-Zn alloy was investigated by first-principle calculations. The calculated results show that the accurate positions and distinctive arrangement of Zn and Y atoms in the most stable 10H-type LPSO phase exhibit mirror symmetry with respect to the atomic layer C₆, which agrees well with the experimental observations. Theoretical calculations still indicate that the mirror symmetry 10H-type ABACBCBCAB phase is not distorted, the lattice distortion of other LPSO phases may originate from the asymmetry of Zn element in the chemical order and stacking order. The obtained electronic density of states (DOS) reveals the underlying mechanism for mirror symmetry of 10H-LPSO phase.     

Theoretical study of structural, electronic, lattice dynamical and dielectric properties of SrAl2O4

We investigate the structural, electronic, lattice dynamical, and dielectric properties of SrAl₂O₄ within density-function theory. The crystal structure is fully relaxed, and the structural parameters are found to be well consistent with the experimental data. The first pressure derivatives of the bulk modulus are predicted to be 2.5 and 4.3 for local density approximation (LDA) and generalized gradient approximation (GGA), respectively. The electronic band structure shows that the valence band maximum is comprised of O 2p states and a small amount of Al 3s and 3p states, and the conduction band minimum is comprised of Sr 5s and a small amount of O 2p, Al 3s and Al 3p states. The phonon frequencies at the center of the Brillouin zone and the dielectric permittivity tensors are calculated using density-function perturbation theory. The electronic (?_∞) and static (?₀) dielectric permittivity tensors are theoretically predicted by the calculations with both LDA and GGA formalisms. The results show that the electronic dielectric permittivity is isotropic, while the static dielectric permittivity exhibits to be somewhat anisotropic due to the dominant ionic contributions in static dielectric permittivity.     

X-ray photoelectron spectroscopy studies of SbF5 intercalated graphite

Two samples of graphite intercalated with SbF₅ have been studied by X-ray photoelectron spectroscopy (x.p.s.). In each case two fluorine (1s) peaks were observed, with binding energies 685.0 eV and 688.0 eV. The peak at 685.0 eV is attributed largely to volatile, quasi-liquid SbF₅ present in the interlayer. The peak at 688 eV is assigned to involatile SbF₆^? ions located at fixed sites in the interlayer, the fluorine atoms undergoing strong polarization interaction with the graphite lattice. Changes observed in carbon (1s) and antimony (3d_{$\text{5}\text{2}$}) spectra are consistent with these assignments.     

First principles investigation of the substitutional doping of Mn in Mg2Ni phase and the electronic structure of Mg3MnNi2 phase

The substitutional doping of Mn in Mg₂Ni phase and the electronic structure of Mg₃MnNi₂ phase have been investigated by first principles density functional theory calculations. The calculation of enthalpy of formation shows that among the four different lattice sites of Mg(6f), Mg(6i), Ni(3b) and Ni(3d) in Mg₂Ni unit cell, the most preferable site of substitution of Mn in Mg₂Ni lattice has been confirmed to be Mg(6i) lattice site. The constructed Mg₉Mn_3Mg(6i)Ni₆ structure by replacing 3 Mg atoms at Mg(6i) lattice sites with 3 Mn atoms in the Mg₂Ni unit cell is less stable. In contrast, the cubic Mg₃MnNi₂ phase that has the same composition as that of Mg₉Mn_3Mg(6i)Ni₆ structure possesses good stability. Analysis of density of states (DOS) indicates that there is a strong hybridization between Mg s, Mg p and Ni d electrons, which is dominant in controlling the structural stability of pure and Mn-doped Mg₂Ni phases. The Mn-substitution in Mg₂Ni unit cell weakens the interaction between Mg s, Mg p and Ni d electrons, especially for Mg₉Mn_3Mg(6i)Ni₆ phase. The cubic Mg₃MnNi₂ phase possesses a strong hybridization between Mn and Mg, Ni atomic orbits under simultaneously retaining the strong bonding among Mg s, Mg p and Ni d electrons. Based on the calculated results, the stability of phases gradually decreases along the sequence pure Mg₂Ni phase > Mg₃MnNi₂ phase > Mn-substitution doped Mg₂Ni phase.     

Mössbauer study of IF5-intercalated graphite

Graphite intercalation compounds (GIC) with IF₅ were investigated at 4.2 K by Mössbauer spectroscopy with the 57.6-keV gamma resonance of ¹²⁷I. The identical hyperfine parameters (isomer shift, axially-symmetric electric field gradient tensor) observed for three differently prepared samples (powdered and highly-oriented pyrolytic graphite (HOPG)) reveal the presence of square-pyramidal IF₅ intercalated molecules, as in the pure molecular solid. Apart from absorber thickness effects, no changes in the intrinsic spectral shapes of the resonance spectra were found for different orientations of the γ-ray direction relative to the graphite $\text{c}$ axis. This means that the four-fold molecular axis of IF₅ must be tilted by an angle close to 54.7° relative to $\text{c}$; such is the case if one of the triangular bases of IF₅, formed by three fluorine atoms, is oriented parallel to the carbon planes. The observed anisotropy of the recoil-free fraction shows that the IF₅ molecules are much more strongly bound in the $\text{c}$ direction as compared to the direction parallel to the graphite planes.     

Nanoscale Studies of the Early Stages of Oxidation of a TiAl-Base Alloy

The strategy to perform nanoscale studies of the initial stages of oxidation of TiAl involved first gaining some information on the electronic structure of pure TiO₂ surfaces and then on TiAl surfaces before and after oxidation both in low- and high-oxygen potentials. Both materials were studied in atomically-cleaned states generated by repeated sputtering and heating. It was found that the oxygen vacancies created additional defect states in the band gap of stoichiometric TiO₂. The results obtained on TiO₂ were used as fingerprints to study the oxide nucleation. The results on the initial stages of oxidation of TiAl confirm the nucleation of Ti₂O₃ islands of nanometer size and monolayer height in a low-oxygen-pressure environment, whilst a TiO₂ layer developed in an atmospheric environment. The ledges on atomically-cleaned surfaces usually acted as nucleation sites.     

Electronic structure and thermal properties of doped CaMnO3 systems

The electronic and thermal properties of hole (Na) and electron (Ga) doped CaMnO₃ systems are investigated based on the first principle density functional theory calculations using plane wave basis and pseudo-potential method. A semiconductor-to-conductor transition and a distorted band structure are found for the doped systems; enhanced density of states near Fermi level is observed. The phonon transfer speed and the phonon mean free path are lowered; meanwhile, the phonon specific heat is heightened in comparison with that of the undoped CaMnO₃ system, resulting in enhanced phonon thermal conduction. The calculation results indicate that the doped systems should have improved thermoelectric performance.     

First-principles studies of defects,mechanical properties and electronic structure of Cr-based Laves phases

First-principles calculations have been performed to investigate the defects, mechanical properties and electronic structure of MCr₂ (M = Nb, Ti and Zr) Laves phases. The results show that the ternary additive of V preferentially substitutes the Cr site and Zr the Nb site in NbCr₂. W has a weak site preference on the Cr site in NbCr₂. The ternary site substitutions are related to the electronic structure of NbCr₂. Further calculations of the densities of states and charge density distribution of TiCr₂ show that a strong covalent bonding between the small atoms (Cr) along the Kagome net forms a tetrahedral electronic network, which is a common feature for the electronic structure in Laves phases. Mechanical properties, such as elastic constants, elastic moduli and stacking fault energies of ZrCr₂, were calculated additionally. The intrinsic and extrinsic stacking fault energies of ZrCr₂ are found to be 112 mJ m⁻² and 98 mJ m⁻², respectively.     

Density functional theory study of water-gas shift reaction on TM@Cu<Subscript>12</Subscript> core-shell nanoclusters

The mechanism of water-gas shift reaction on the transition metal of Co, Ni, Cu (from the 3d row), Rh, Pd, Ag (from the 4d row), Ir, Pt, and Au (from the 5d row) @Cu₁₂ bimetallic clusters have been studied using density functional theory (DFT) calculations. Three reaction mechanisms including redox, carboxyl, and formate mechanisms, which are equal to CO* + O* → CO₂ (g), CO* + OH* → COOH* → CO₂ (g) + H*, and CO* + H* + O* → CHO* + O* → HCOO** → CO₂ (g) + H*, respectively, have been studied. The result revealed that the WGSR prefer to follow the carboxyl mechanism on the TM@Cu₁₂ surfaces. The rate-controlling step of WGS reaction is H₂O dissociation into OH and H or COOH decomposition into CO and OH. The transition metal additive in Cu cluster could enhance the activity of water dissociation, which is beneficial for WGS reaction. Especially, doping Ni has the largest promotion effect in reducing the active barrier, the reason is electronic effect. The calculation indicates that Ni@Cu₁₂ is thus the promising candidates for improved WGSR catalysts. In addition, The TOF values are studied to estimate effectively activity of the TM@Cu₁₂ cluster. To get insight into conclusion, reaction mechanism and structure of cluster was elucidated by the relative energy profiles and detailed electronic local density of states (LDOS).     

First-principles calculations and X-ray spectroscopy studies of the electronic structure of CuWO4

First-principles self-consistent band-structure calculations of copper tungstate, CuWO₄, have been made using the full potential linearized augmented plane wave (FP-LAPW) method. Total and partial densities of states of the constituent atoms of CuWO₄ have been derived. The results obtained reveal that the valence band of CuWO₄ is dominated by contributions of the O 2p-like states, while the W 5d-like states are the main contributors into the conduction band of the tungstate. Additionally, the FP-LAPW calculations render that the W 5d- and Cu 3d-like states contribute mainly at the bottom and at the top of the valence band of CuWO₄, respectively. In the present work, the X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) methods were also employed to investigate experimentally the electronic structure of copper tungstate. For the mentioned compound, the XES bands reflecting the energy distribution of mainly the W 5d-, Cu 3d- and O 2p-like states were derived and compared on a common energy scale with the X-ray photoelectron valence-band spectrum. A rather good agreement of the experimental and theoretical data concerning electronic properties of CuWO₄ has been obtained in the present paper. Measurements of the energy shift of the XAS W L_III edge of CuWO₄ clearly demonstrate that tungsten atoms in copper tungstate are in the formal valence state +6.     

Modulation of electronic density in waved graphite layers

We present a theoretical investigation of electronic density distribution in waved graphene and graphite with layers repeatedly deformed along the zigzag or armchair directions. Effect of deformation on the band structure of graphene and graphite was examined using density functional theory. It was found that the density of states depends sensitively on the position of carbon atom on the curved surface of graphene. Strong localization of electrons on the top and bottom parts of armchair-edged deformed graphene produces conduction channels along the wave crest. Zigzag-edged waved graphene can possess metallic or semiconducting behavior. Interlayer interaction in waved graphite results in smoothing of anisotropy of local density of states. Hence, optical and electrical properties of corrugated graphite materials can be controlled by changing the bending direction, out-of plane bending height and interlayer separation.     

Lamellar graphite compounds with potassium and benzene: structure and transformations

The thermal, oxidation and hydrolytic stabilities of the ternary lamellar graphite compound C₂₄K(C₆H₆)₃ have been investigated. The results of the study have suggested a model C₂₄K(C₆H₆)₃ structure according to which benzene penetrates both the interlayer space containing potassium and the unfilled interlayer graphite space, the benzene molecules being arranged at right angles to the carbon layers of graphite.     

Cu6Sn5和Ni3Sn4结构性能的第一原理计算

基于密度泛函理论的第一原理,计算了锡基无铅焊点界面常见的金属间化合物Cu6Sn5和Ni3Sn4的平衡晶格常数、合金形成焓以及弹性常数,分析了结构稳定的电子机制.结果表明,Cu6Sn5较Ni3Sn4合金形成能负,因此Cu6Sn5在热力学上更稳定,其合金化能力也较强.在力学性能方面,两相均属脆性相,表现出弹性各向异性,而Ni3Sn4的键合作用较强,弹性模量、剪切模量均大于Cu6Sn5,但Cu6Sn5表现出更好的塑性.从电子结构的角度,Cu6Sn5的成键主要来自于Cu原子d,p轨道与Sn原子p杂化,而Ni原子d轨道与Sn原子p轨道的强烈杂化作用是Ni3Sn4成键的主要原因.     

First principles calculations on elasticity, electronic structure and bonding properties of antiperovskites ANTi3 (A = Al, In and Tl)

We use an ab initio pseudopotential plane wave (PP-PW) method within the generalized gradient approximation (GGA) and the local density approximation (LDA) to study the structural, elastic and electronic properties of the unexplored antiperovskite ANTi₃ compounds. The elastic constants C₁₁, C₁₂, C₄₄ and their pressure dependence are calculated. We derived the bulk, shear and Young's moduli for ideal monocrystalline and for polycrystalline ANTi₃ aggregates which we have classified as ductile in nature. Band structures reveal that these compounds are conductors. The covalent ionic bands nature is due to the strong hybridization between Ti 3d and N 2p states. The Ti 3d states play dominant roles near the Fermi levels for all these compounds. The energy difference between spin polarized calculations and the nonspin polarized calculations indicate that ANTi₃ compounds exhibit magnetism at their equilibrium lattice constants.