Self-assembly of a colloidal interstitial solid with tunable sublattice doping

We determine the phase diagram of a binary mixture of small and large hard spheres with a size ratio of 0.3 using free-energy calculations in Monte Carlo simulations. We find a stable binary fluid phase, a pure face-centered-cubic (fcc) crystal phase of the small spheres, and binary crystal structures with LS and LS(6) stoichiometries. Surprisingly, we demonstrate theoretically and experimentally the stability of a novel interstitial solid solution in binary hard-sphere mixtures, which is constructed by filling the octahedral holes of an fcc crystal of large spheres with small spheres. We find that the fraction of octahedral holes filled with a small sphere can be completely tuned from 0 to 1. Additionally, we study the hopping of the small spheres between neighboring octahedral holes, and interestingly, we find that the diffusion increases upon increasing the density of small spheres.     

Topology of the spin-polarized charge density in bcc and fcc iron

We report the first investigation of the topology of spin-polarized charge density, specifically in bcc and fcc iron. While the total spin-density is found to possess the topology of the non-magnetic prototypical structures, the spin-polarized charge densities of bcc and high-spin fcc iron are atypical. In these cases, the two spin densities are correlated: the spin-minority electrons have directional bond paths and deep minima, while the spin-majority electrons fill these holes, reducing bond directionality. The presence of distinct spin topologies allows us to show that the two phase changes seen in fcc iron (paramagnetic to low-spin and low-spin to high-spin) are different. The former follows the Landau symmetry-breaking paradigm and proceeds without a topological transformation, while the latter involves a topological catastrophe.     

高压下ZnS的电子结构和性质

运用密度泛函理论体系下的平面波赝势(PWP)和广义梯度近似(GGA)方法，计算研究了闪锌矿结构的ZnS晶体在不同的外界压强下的电子结构. 通过分析发现，随着外界压强的增大，晶格常数和键长在不断缩小，从S原子向Zn原子转移的电荷越来越少，Zn—S键的共价性逐渐增强，Zn原子和S原子的态密度都有不同程度的变化，而且还有向低能量移动的趋势. 当外界压强达到24GPA时，ZnS从直接带隙半导体变成间接带隙半导体，而且随着压强的增大，间接带隙逐渐变小，直接带隙逐渐增大. 关键词： 闪锌矿结构 态密度 能带结构 密度泛函理论     

H,O,N,He原子掺杂Al原子的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法,对Al中分别加入H,O,N和He原子后的晶体状态进行了研究.通过晶体结构和形成能分析比较了杂质原子占据不同位置的难易程度及对晶体稳定性的影响,并从态密度、电荷密度和电荷布居的角度,分析了其电子结构.结果表明:H、O和N原子占据金属Al的四面体间隙最稳定,而He原子主要占据金属Al的八面体间隙. O和N原子与Al原子具有强烈的共价作用,H原子与Al原子存在共价作用但相对较弱,而He原子与Al原子的相互作用以范德华力为主.进一步揭示了四种原子在金属Al中不同行为的电子机制.     

Pressure Raman effect in TCNQ salts

We have investigated the pressure dependence of the Raman active intramolecular modes in anthracene-TCNQ, pyrene-TCNQ, and alkali-TCNQ salts in the pressure range between 0 kbar abd 30 kbar. The measured value of the fractional frequency variation with pressure, dlnν₄/dP, for the ν₄ mode (C=C wing stretching) of TCNQ^- molecules is approximately twice as large as that of TCNQ°. This observation is interpreted as due to a change in the electronic charge distribution on the C=C wing bond region of the TCNQ^- molecules at high pressures.     

双层h-BN/Graphene结构稳定性及其掺杂特性的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法研究了双层h-BN/Graphene的稳定性及其掺杂特性.研究发现,双层h-BN/Graphene能带结构在K点处有一个小的带隙,在费米能处有类Graphene的线性色散关系.通过施加应变和掺杂来调节带隙,发现掺杂后费米能级附近引入的新能级,主要是N原子的贡献,掺杂后的Na原子和N,C之间存在电荷转移,材料转变为金属性.电荷的转移、载流子密度的增加,在电子元器件中有重要的应用前景.     

Intermolecular overlap geometry gives two classes of fulleride superconductor: electronic structure of 38 K Tc Cs3C60

Superconductivity emerges for the A15 polymorph of the fulleride Cs3C60 upon compression to a pressure of approximately 4 kbar. Using density functional theory we study the bonding in the A15 phase as a function of unit cell volume comparing it to that in the fcc polymorph. We find that, despite its smaller packing density, the bcc-derived A15 phase has both a substantially wider bandwidth for the partially occupied t1u band and a higher density of states at the Fermi level. This result can be traced to the striking differences in the nature of the interanion Tc--the two sphere packings (body centered versus face centered) observed experimentally produce two electronically distinct classes of fulleride superconductors.     

Structural, Electronic Properties and Chemical Bonding of Borate Li4CaB2O6 under High Pressure： an Ab Initio Investigation

We calculate structural, electronic properties and chemical bonding of borate Li4CaB2O6 under high pressure by means of the local density-functional pseudopotential approach. The equilibrium lattice constants, density of states, Mulliken population, bond lengths, bond angles as well as the pressure dependence of the band gap are presented. Analysis of the simulated high pressure band structure suggests that borate Li4CaB2O6 can be used as the semi-conductor optical material. Based on the Mulliken population analysis, it is found that the electron transfer of the Li atom is very different from that of other atoms in the studied range of high pressures. The charge populations of the Li atom decrease with the pressure up to 60 GPa, then increase with the pressure.     

Thermodynamics and defect structures of silver sulfide

Published measurements of sulfur vapor pressure and silver electromotive force were used to determine thermodynamic properties of silver sulfide above 379 K. They were Gibbs-Duhem integrated to estimate the formation properties of stoichiometric Ag₂S of fcc, bcc, and monoclinic crystal structures. Statistical thermodynamics was applied to estimate free energies and find possible atom arrangements in off-stoichiometric silver sulfide. Theoretical calculations show that silver vacancies and atoms may be in quasi-chemical equilibrium between tetrahedral and octahedral sites in the fcc structure and between two states of atoms within tetrahedral sites in the bcc structure and within octahedral sites in the monoclinic structure. A strong indication is that vacancy clusters should predominate, each containing four-atom vacancies in the fcc phase and three-atom vacancies in the bcc phase.     

Cooperative effects between tetrel bond and other σ–hole bond interactions: a comparative investigation

Covalently bonded atoms of Groups IV–VII tend to have anisotropic charge distributions, the electronic densities being less on the extensions of the bonds (σ–holes) than in the intervening regions. These σ–holes often give rise to positive electrostatic potentials through which the atom can interact attractively and highly directionally with negative sites. In this work, cooperative effects between tetrel bond and halogen/chalcogen/pnicogen bond interactions are studied in multi-component YH₃M···NCX···NH₃ complexes, where Y = F, CN; M = C, Si and X = Cl, SH and PH₂. These effects are analysed in detail in terms of the structural, energetic, charge-transfer and electron density properties of the complexes. The nature of the σ–hole bonds is unveiled by quantum theory of atoms in molecules and natural bond orbital theory. A favourable cooperativity is found with values that range between ?0.34 and ?1.15 kcal/mol. Many-body decomposition of interaction energies indicate that two-body energy term is the most important source of the attraction, which its contribution accounts for 87%–96% of the total interaction energy.     

Electric field gradient for interstitial positive muons in fcc metals: An AB Initio calculation of size effect

The electric field gradients (EFGs) resulting from interstitial point defects in fcc metals have been investigated. The defect induced charge density, used to evaluate the valence effect EFG, is calculated self-consistently in the density functional formalism. An ab initio calculation of the size effect EFG is carried out for a positive muon at an octahedral site in the fcc lattice in the elastic continuum model. The components of the strain field tensor are evaluated assuming the lattice of dressed point ions interacting through the screened Coulomb potential. No adjustable parameter has been used. The theoretical results are in good agreement with the experimental values within experimental uncertainties. It is emphasized that both the strain and conduction electron contributions are equally important in the estimation of the electric field gradient.     

Predicted formation of superconducting platinum-hydride crystals under pressure in the presence of molecular hydrogen

Noble metals adopt close-packed structures at ambient pressure and rarely undergo structural transformation at high pressures. Platinum (Pt) is normally considered to be unreactive and is therefore not expected to form hydrides under pressure. We predict that platinum hydride (PtH) has a lower enthalpy than its constituents solid Pt and molecular hydrogen at pressures above 21.5?GPa. PtH transforms to a hexagonal close-packed or face-centered cubic (fcc) structure between 70 and 80 GPa. Linear response calculations indicate that PtH is a superconductor at these pressures with a critical temperature of about 10-25?K. These findings help to shed light on recent observations of pressure-induced metallization and superconductivity in hydrogen-rich materials. We show that the formation of fcc noble metal hydrides under pressure is common and examine the possibility of superconductivity in these materials.     

Resistivity of positive-J spin glasses: Study ofLaGd under hydrostatic pressure

Results of a new calculation of the resistivity in the noise model of spin glasses, based on the –J S interaction forpositive J, are compared with measurements of the resistivity in an fcc La-8at% Gd alloy under hydrostatic pressures up to 12 kbar, the first positive-J resistivity investigations under pressure. A positive value ofJ that decreases with increasing pressure, in accord with other experiments, leads to quantitative agreement between theory and experiment if one uses a large electronic density of states at the Fermi energy, in accord with specific heat measurements and electronic band structure calculations in fcc La. Impurity potential scattering is found to contribute significantly to the resistivity. Its size and pressure variation is determined.This work is supported in part by the Deutsche Forschungs-gemeinschaft.     

Effect of pressure on the M?ssbauer isomer shift and quadrupole splitting in Au(I)-cyanides

The effect of pressure on the Mössbauer isomer shift and quadrupole splitting was studied with the 77 keV resonance of¹⁹⁷Au in the Au(I)-cyanides AuCN and KAu(CN)₂ at 4.2 K under pressures up to 80 kbar. We observe an increase of the electron density and a decrease of the absolute value of the electric field gradient at the Au nuclei in both compounds with pressure. This variation is different from the correlation that has been established for these parameters within the chemical series of Au(I) compounds at ambient pressure. It indicates that the-bonding increases strongly under pressure. X-ray diffraction studies on AuCN under pressures up to 115 kbar together with investigations of the Goldanskii-Karyagin-effect on AuCN at ambient pressure support the previous assumption that the sign of the electric field gradient at the Au(I) sites is negative.     

Excitation-induced atomic desorption and structural instability of III–V compound semiconductor surfaces

We discuss the laser-induced structural instability of III–V compound semiconductor surfaces. The electronic instability is characterized by local bond rupture of both metallic and nonmetallic atoms at intrinsic surface sites, with the bond-rupture rate super linearly dependent on the excitation density. Spectroscopic studies show that bulk-valence excitation triggers surface bond rupture, and that valence holes are the responsible species. From our results, we propose a mechanistic model based on the two-hole localization induced by a high density of non-equilibrated valence holes, and demonstrate that the model describes all the important features quantitatively and consistently. Additional evidence that further supports the validity of the two-hole mechanism is presented.     

Effect of pressure on the Mössbauer isomer shift and quadrupole splitting in Au(I)-cyanides

The effect of pressure on the Mössbauer isomer shift and quadrupole splitting was studied with the 77 keV resonance of¹⁹⁷Au in the Au(I)-cyanides AuCN and KAu(CN)₂ at 4.2 K under pressures up to 80 kbar. We observe an increase of the electron density and a decrease of the absolute value of the electric field gradient at the Au nuclei in both compounds with pressure. This variation is different from the correlation that has been established for these parameters within the chemical series of Au(I) compounds at ambient pressure. It indicates that the-bonding increases strongly under pressure. X-ray diffraction studies on AuCN under pressures up to 115 kbar together with investigations of the Goldanskii-Karyagin-effect on AuCN at ambient pressure support the previous assumption that the sign of the electric field gradient at the Au(I) sites is negative.     

Hf-C体系的高压结构预测及电子性质第一性原理模拟

本论文中, 采用晶体结构预测软件USPEX结合第一性原理方法全面地搜索了Hf-C体系在高压下的晶体结构, 预测得到了两种新的化合物及HfC在高压下的相变路径. 压力低于100 GPa 时, 除了常压下的结构HfC, Hf₃C₂, Hf₆C₅, 并没有得到新的热力学稳定结构. 在200 GPa时, 预测得到了一种新化合物——Hf₂C, 空间群为I4/m; 且HfC的结构发生了相变, 空间群由Fm3m变为C2/m. 在300 GPa时, 预测得到了另一种新化合物——HfC₂, 空间群为Immm. 而在400 GPa时, HfC的结构再次发生相变, 空间群为Pnma. 通过能量计算, 得到了Hf-C体系的组分-压力相图: 在压力分别低于15.5 GPa和37.7 GPa时, Hf₃C₂和Hf₆C₅是稳定的; 压力分别大于102.5 GPa和215.5 GPa时, Hf₂C和HfC₂变成稳定化合物; HfC的相变路径为Fm3m→C2/m→Pnma, 相变压力分别为185.5 GPa 和322 GPa. 经结构优化后, 得到了这四种高压新结构的晶体学数据, 如晶格常数、原子位置等, 并分析了其结构特点. 对于Hf-C 体系中的高压热力学稳定结构, 分别计算了其弹性性质和声子谱曲线, 证明是力学稳定和晶格动力学稳定的. 采用第一性原理软件VASP模拟高压结构的能带结构、态密度、电子局域函数和Bader 电荷分析, 发现HfC(C2/m, Pnma结构), Hf₂C 和HfC₂ 中Hf-C 键具有强共价性、弱金属性和离子性, 且C-C 间存在共价作用.     

高x值混晶Hg1-xCdxTe光吸收边的压力效应

采用金刚石对顶砧高压装置,在0—36kbar流体静压力范围和室温条件下测量了高x值的P型碲镉汞混晶Hg_0.3Cd_0.7Te光吸收边及其随压力的变化。300K下Hg_0.3Cd_0.7Te光学能隙的实验值与用经验公式的计算值一致。用最小二乘法拟合不同压力下的实验结果,得到Hg_0.3Cd_0.7Te能隙的一阶压力系数α=8.7×10^-11eV/Pa,与用化学键介电 关键词：     

Microscopic theory of desorption of neutral atoms due to high excitonic density at the surfaces of III–V compounds

A microscopic theory of desorption of neutral atoms from the surface of crystalline GaP is presented. Derived results suggest that bonds are broken at the surface due to high excitonic density, so that a pair of excited holes can get localised on the same bond because such an excited state has much lower energy than that of a free exciton state. Any bond with a pair of holes, instead of covalent electrons, will be broken. Strong exciton-lattice interaction is assumed. It is argued that the mechanism of atomic desorption from surfaces is analogous to that of polymer ablation; and the desorption of neutral atoms increases linearly to super linearly with the increase in laser fluence. This agrees well with experiments.     

LCAO studies of hydrogen chemisorption on nickel: I. Tight-binding calculations for adsorption on periodic surfaces

The model we have used to study hydrogen chemisorption on nickel surfaces is a tightbinding Extended Hückel method applied to finite (periodic) crystals up to about 250 atoms, the non-orthogonal basis set comprising five 3d orbitals, one 4s orbital and three 4p orbitals per atom. After calculating the band structure of fcc nickel, we have examined, by this model, the effect of the (100), (110) and (111) surfaces on the local density of states and the charge distribution. The results agree closely with moment calculations of the density of states in semi-infinite crystals and with experimental (XPS, UPS and INS) spectra. Extensive studies have been made of the influence of adsorption on the (partial) densities of states in order to illuminate the nature of the chemisorption bond. Particularly, we have concluded that both the 3d electrons and the conduction electrons take part in this bond. Equilibrium positions for adsorption on various sites have been determined and the adsorption energy has been computed and compared with experimental data. We find that the stability of adsorption decreases in the order (110) > (100) > (111) and Atop > Bridge > Centred.