Electronic structure of layered compounds

The electronic structure of the intercalated graphite compounds XC₆ (X = Ca, Sr, Ba, Yb, and La) has been studied using the linearized augmented plane-wave method. It has been found that the electronic structure of the carbon layers in these compounds is qualitatively different from a two-dimensional graphite structure. A lower critical superconducting-transition temperature in YbC₆, as compared with that in CaC₆, at a higher electron density in the carbon layers can be explained by the strong hybridization of the p states of carbon and the d states of ytterbium near the Fermi level. An increase in the critical temperature would be expected in the compounds XC₆ with Group III metals, for example, in LaC₆.     

应变对层状锰系锂离子电池正极材料输出电压的影响

利用第一性原理方法系统研究了不同应 变模式对LiMnO₂和Li₂MnO₃输出电压的影响, 建立了输出电压与弹性常数及应变之间的关系. 发现所有应变对输出电压都是降低的, 且应变效应是各向异性的. 大部分的单轴应变5%时对输出电压的降低都小于0.1 V. 由于层状的电极材料层间的键合作用较弱, 且受脱锂后形成的锂空位影响较大, 当从锂层脱出锂时, 垂直于层方向的应变对输出电压影响较大; 而对Li₂MnO₃系统从过渡金属层中脱锂时, 平行于层的应变对输出电压影响更大. Li₂MnO₃骨架支撑的层状固溶体系中, 应变使高电压充电阶段的电压维持在截断电压之下, 并打开过渡金属层中锂的迁移通道, 产生较为持久的充电而可能获得较大的充电容量.     

Structural and electrochemical study of Cr-substituted layered manganese oxide cathode materials

A lithiated layered Mn–Cr compound, Li[Cr_0.29Li_0.24Mn_0.47]O₂ was synthesized by a solution method with subsequent quenching. The crystal structure was investigated by X-ray diffraction (Rietveld refinement) and Electron diffraction showing co-existence of rhombohedral and monoclinic structures. According to the co-indexed electron diffraction patterns and HRTEM images, Li[Cr_0.29Li_0.24Mn_0.47]O₂ electrode was composed of nano-scale domains indexed in monoclinic and hexagonal structures, simultaneously. The nano-composite cathode successfully prevents spinel-like structural transformation during cycling and delivered a good reversible capacity of about 195 mAh/g between 2.4 and 4.7 V.     

Synthesis and characterization of SWNT-heavy alkali metal intercalation compounds,effect of host SWNTs materials

Singlewall carbon nanotubes (SWNTs) produced by electric-arc and laser ablation methods were characterized by X-ray diffraction before and after the reaction with alkali metals (M=K, Rb, and Cs). Reaction with annealed SWNTs gave MC₈ composition at saturation. The alkali metal lattice showed short range order incommensurate with graphene cylinders of SWNTs. X-ray diffractogram simulations have enabled the study of the influence of SWNTs structure on that of intercalation compounds. Chemically-purified bundles, constituted of open SWNTs, can be intercalated inside and between the tubes forming disordered structures. Annealed or pristine bundles were intercalated only between the tubes leading to short or long range ordered structure depending on host crystallinity and alkali metal (K, Rb or Cs). The expansion of the 2D SWNTs lattice after intercalation is comparable to graphite intercalation compounds. Some 2D arrangements of SWNTs and K atoms are proposed and discussed to reproduce XRD results. ¹³C NMR and ESR studies of annealed doped SWNTs emphasize the fact that the intercalation compounds of SWNTs are metallic.     

The influence,on lithium electrochemical intercalation,of bond ionicity in layered chalcogenophosphates of transition metals

In the course of lithium electrochemical intercalation in the host structure of layered M∥PX₃ phases (M = V, Mn, Fe, Co, Ni, X = S, Se), it was shown that the best energy yield was obtained from low ionicity bond materials. The absorption edge energy, along with the free energy of the intercalation reactions have been correlated in a satisfactory way to the ionicity fi of the M-X bonds. These diagrams indicate which phases have to be looked at to obtain the maximum electrochemical yields.     

The effect of intercalation on the exction spectrum and structure of layered single crystals

The excitonic absorption spectra of intercalated Ba_xGaSe crystals (0xGaSe (x=0.05, 0.1) were calculated. X-ray structure analysis has revealed the first evidence of additional scattering in the form of broadened maxima in the x-ray diffraction patterns of Ba_xGaSe crystals, which does not occur in the source material GaSe. The appearance of broadened maxima in the intercalated crystal is due to the fact that incorporated ions of Ba at some critical concentration (x>0.3) generate local regions of intercalant enrichment.Institute for Materials Science, Ukrainian Academy of Sciences, Chernovtsi Branch. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 51–56, October, 1992.     

Self-assembled nanofold network formation on layered crystal surfaces during metal intercalation

We study the formation of planar network nanostructures, which develop during metal deposition on initially smooth surfaces of layered compounds. Using in situ low-energy electron microscopy for dynamic observation and high-resolution transmission electron microscopy for structure analysis, we have observed the rapid formation of hexagonal networks of linear "nanofolds" with prismatic cavities on top of layered VSe2 crystals. Their formation results from relaxation of compressive strains which build up during Cu intercalation into a thin surface layer.     

Confinement of electrons in layered metals

We analyze the out of plane hopping in models of layered systems where the in-plane properties deviate from Landau's theory of a Fermi liquid. We show that the hopping term acquires a nontrivial energy dependence, due to the coupling to in-plane excitations, and the resulting state, at low temperatures, can be either conducting or insulating in the third direction. The latter is always the case if the Fermi level lies close to a saddle point in the dispersion relation.     

Electronic structure, magnetism and superconductivity of layered iron compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe (Se, Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.     

Electronic structure of alkali halide surfaces upon ion irradiation

By a careful analysis of the electron loss spectra of some alkali halide crystals after ion irradiation we have demonstrated that defect generation depends on ion energy nonmonotonously. This effect is connected with inelastic interaction between ion and surface atoms.     

Electronic structure of layered uranium compounds from photoemission spectroscopy

We present the photoemission results of two layered tetragonal compounds, the anti-ferromagnet UAsSe and ferromagnet USb₂. We observed intriguing electronic structure for both UAsSe and USb₂, in which relatively dispersive and narrow 5f bands are present. In the vicinity of the Fermi edge we found a very sharp photoemission peak with dispersion of several meV along the Γ to Z direction of the Brillouin zone. We also found a broader, hybridized f-character band with dispersion of several hundred meV along the Γ to X direction. Narrow and dispersive bands in these U-based magnetic materials are reminiscent of band magnetism as previously found in some transition metals.     

Transport properties of alkali metal-graphite intercalation compounds

Basal resistivity ?_a has been measured $\text{in situ}$ on MC₈ compounds (M=K, Rb, Cs) from 5–300K. We find ?_a(T) = A+BT+CT², in agreement with Suematsu et. al., except our value of A is ～200 times smaller implying fewer defects. In MC₈ compounds R_H at 5K is positive and increases linearly with magnetic field, suggesting a complex Fermi surface. On the other hand, R_H for RbC₂₄ is negative and field-independent but the magnitude is inconsistent with a simple one-carrier model (assuming one free electron per Rb).     

Electronic structure of zintl compounds (NaTl) and alkali pnictides

The presence of both like and unlike atom nearest neighbours in the NaTl lattice is used to test the dependence of tight-binding interactions on bond type. The atoms are found to be almost neutral, and little evidence of the classic Na⁺Tl^- bonding models is found. The bands of alkali pnictides such as Li₃Sb are calculated by scaling interactions from LiAl. The Li salts are found to have almost neutral sites whereas the Cs salts are strongly ionic. We classify the bonding generally as charge transfer, not covalent.     

Electronic structure of carbon nanotubes modified by alkali metal atoms

The electronic structure and parameters of the energy band structure of (n, 0)-type nanotubes modified by alkali metal atoms (Li, Na) and intercalated by potassium atoms are studied. The quantum-chemical semiempirical MNDO method and a model of the covalent cyclic cluster built in via ionic bonding are used to model infinitely long nanotubes. The electronic density of states of modified nanotubes is found. It is shown that semiconductor-metal transitions can occur in semiconductor nanotubes and that semimetal nanotubes can undergo metal-metal transitions.     

Electronic structure of quantum dots with two electrons in tilted magnetic fields

The energy levels of a spheroidal quantum dot in tilted magnetic fields are calculated and compared with those for a disk-like dot. The magneto-optical spectrum and the ground state show considerable difference between the two quantum dots. We find that this feature can be understood by considering the magneto-electric hybridization and electron-electron interaction effects on the energy spectrum.     

Thin graphite overlayers: Graphene and alkali metal intercalation

Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the π states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage 1 compounds C₈K or C₈Rb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 × 2) ordered alkali metal layer adjacent to the uppermost carbon layer.     

Elastic properties of nanostructured materials with layered grain boundary structure

Atomistic calculations of the elastic constants for a bulk nanostructured material that consists of a layered structure where alternating layers meet along high angle grain boundaries and where atoms interact via a Lennard-Jones potential are presented. The calculations of the elastic constants were performed in the frame of homogeneous deformations for a wide range of layer widths ranging from 2.24 up to 74.62 nm. The results showed that the relaxation of the atomic structure affects the elastic constants for the cases where more than 5% of atoms are located in the GB region. Also it was found that the way that external stresses are applied on the system affects the values of the obtained elastic properties, with the elastic constants related to the characteristic directions of the grain boundary being the most affected ones. The findings of this work are of interest for the fabrication methods of nanostructured materials, the measurement methods of their elastic properties as well as multiscale modeling schemes of nanostructured materials.     

Electronic structure of adsorbed alkali atoms for comparison with UPS data

We have generalised our earlier work calculating the induced density of states in alkali chemisorption to include the valence np level in addition to the s level. This allows the theory to take account of the possibility of sp hybridization. Hybridization is said to be weak is the sp level separation ΔE_sP exceeds twice the width of the s or p_z resonance. In the converse case hybridization is strong and the induced density of states no longer reflects distinguishable s and p_z resonances. We argue that ΔE_sP for the chemisorbed alkali exceeds considerably that for the free alkali atom. In the case of Na on r_s = 2 jellium (considered typical of the smaller alkali on r_s = 2?3 substrates) hybridization is strong for the atomic value of ΔE_sp but weak when a somewhat larger value is taken, making the conclusions a little indefinite. For Cs on Cu(111), we find a clear situation of weak hybridization with the atomic ΔE_sP and a fortiori with a more realistic value. There should thus be a well defined 6s resonance in this system. Our estimate of the width of the 6s resonance is in good agreement with photoemission data.