Structural complexity in gallium under high pressure: relation to alkali elements

Ga-II, the stable phase of Ga between 2 and 10 GPa at room temperature, is shown to have a complex 104-atom orthorhombic structure. A new phase, Ga-V, is found between 10 and 14 GPa, with a rhombohedral hR6 structure. Ga-II has a modulated layer structure like those recently reported for Rb-III and Cs-III, with similar 8- and 10-atom a-b layers stacked along the c axis in the sequence 8-10-8-8-10-8-8-10-8-8-10-8. The cI16 structure of Li and Na can be understood as a stacking of very similar 8-atom layers. It is suggested that a Hume-Rothery mechanism contributes to the occurrence of these complex structures in such different metals.     

Simulation of silicon nanoparticles stabilized by hydrogen at high temperatures

The stability of different silicon nanoparticles are investigated at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a “coat” from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen “coat” has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si₆₀ cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si₆₀ cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen “coat” and containing 60 hydrogen atoms in the interior space has a higher stability. Such cluster has smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270-280 K.     

Predicted novel high-pressure phases of lithium

Under high pressure, "simple" lithium (Li) exhibits complex structural behavior, and even experiences an unusual metal-to-semiconductor transition, leading to topics of interest in the structural polymorphs of dense Li. We here report two unexpected orthorhombic high-pressure structures Aba2-40 (40 atoms/cell, stable at 60-80 GPa) and Cmca-56 (56 atoms/cell, stable at 185-269 GPa), by using a newly developed particle swarm optimization technique on crystal structure prediction. The Aba2-40 having complex 4- and 8-atom layers stacked along the b axis is a semiconductor with a pronounced band gap >0.8 eV at 70 GPa originating from the core expulsion and localization of valence electrons in the voids of a crystal. We predict that a local trigonal planar structural motif adopted by Cmca-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.     

Molecular dynamics study of hydrogenated silicon clusters at high temperatures

This paper reports on a study of the stability of silicon clusters of intermediate size at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a ‘coat’ from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen ‘coat’ has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si₆₀ cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si₆₀ cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen ‘coat’ and containing 60 hydrogen atoms in the interior space has a higher stability. Such clusters have smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270–280 K.     

Atomic structure of icosahedral B4C boron carbide from a first principles analysis of NMR spectra

Density functional theory is demonstrated to reproduce the 13C and 11B NMR chemical shifts of icosahedral boron carbides with sufficient accuracy to extract previously unresolved structural information from experimental NMR spectra. B4C can be viewed as an arrangement of 3-atom linear chains and 12-atom icosahedra. According to our results, all the chains have a CBC structure. Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few percent have a B (12) structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites.     

Magnetic neutron scattering magnetic behaviour of a low-carrier density Kondo-lattice system ceas

Abstract

We have determined the magnetic structure of a low-carrier Kondo-lattice system CeAs, and have observed a softening of the crystalline electric field excitations. Despite the prediction of a recent magnetic polaron model in which CeAs and CeP are expected to show a stacking order of T₇ and T₈ layers, CeAs does not show such a stacking structure under pressure. The ordering in the intermediate phase is a regular ferromagnetic order and that of the low-temperature phase is a canted type-I AF.     

并五苯同质异相体中分子间势能与能带计算

采用Born-Mayer-Haggins对势模型，分析了并五苯分子间势能及其相互作用. 用紧束缚模型计算了两种并五苯同质异相体结构的能带宽度. 计算带宽随温度升高减小8%—14%. 关键词： 并五苯 同质异相体 分子间势能 能带计算     

Structures and Electronic Properties ofCuN (N≤ 13) Clusters

A systematic study on the structures and electronic properties of copper clusters has been performed using the density functional theory. In the calculation, there are many isomers near the ground state for small copper clusters. Our results show that the three-dimensional isomers of copper clusters start from Cu₇ cluster and then show a tendency to form more compact structures. The results of the formation energy and the second derivative of binding energy with cluster size show that besides N = 8, N=11is also a magic number. Furthermore, it is the first time to find that the ground state of 11-atom clusters is a biplanar structure as same as the 13-atom cluster. The clear odd-even alternation as cluster size for the formation energy indicates the stability of electronic close shell existed in the range studied.     

The ground-state structure and physical properties of RuC: first-principles calculations

We have extensively explored the ground-state structure of RuC using the particle swarm optimization algorithm for crystal structural prediction. A hexagonal R-3m structure has been proposed as the best candidate, which is energetically more favorable than the previously proposed zinc blend structure. The R-3m-RuC possesses alternative stacking of double hexagonal close-packed Ru atom layers and C atom layers, and it is dynamically stable evidenced by the calculation of phonon dispersion. The calculated large bulk modulus, shear modulus, and elastic constant C 44 reveal that it is an ultra-incompressible and hard material. The evidence of strong covalent bonding of Ru-C, which plays an important role to form a hard material, is manifested by the partial densities of states analysis.     

First principles simulation of a ceramic /Metal interface with misfit

The relaxed atomic structure of a model ceramic/metal interface, 222MgO/Cu, is simulated, including lattice constant mismatch, using first principles local-density functional theory plane wave pseudopotential methods. The 399-atom computational unit cell contains 36 O and 49 Cu atoms per layer in accordance with the 7/6 ratio of MgO to Cu lattice constants. The atomic layers on both sides of the interface warp to optimize the local bonding. The interface adhesive energy is calculated. The interface electronic structure is found to vary appreciably with the local environment.     

Localization of π‐electron density in twisted bilayer graphene

In bilayer graphene, mutual rotation of layers has strong effect on the electronic structure. We theoretically study the distribution of electron density in twisted bilayer graphene with the rotation angle of 21.8° and find that regions with AA‐like and AB‐like stacking patterns separately contribute to the interlayer low‐energy van Hove singularities. In order to investigate the peculiarities of interlayer coupling, the charge density map between the layers is examined. The presented results reveal localization of π‐electrons between carbon atoms belonging to different graphene layers when they have AA‐like stacking environment, while the interlayer coupling is stronger within AB‐stacked regions.

Charge density map for bilayer graphene with a layer twist of 21.8° (interlayer region).     

Epitaxial regrowth and defect formation during pulsed nanosecond annealing of amorphous layers. Part 1: Experimental results

The results of structure investigations of antimony ion-implanted silicon layers after the pulsed nanosecond annealing by ruby laser have been presented. Formation of micro-twins and tetrahedrons of stacking faults has been found to take place on the initial amorphous/crystalline Si interface for various degrees of liquid Si overcooling.     

Strain interactions and defect formation in stacked InGaAs quantum dot and dot-in-well structures

The growth of high-quality stacked quantum dot (QD) structures represents one of the key challenges for future device applications. Electronic coupling between QDs requires closely separated electronic levels and thin barrier layers, requiring near identical composition and shape, despite strong strain interactions. This paper presents a detailed characterization study of stacked InGaAs QD and InAs/InGaAs dot-in-well (DWELL) structures using cross-sectional transmission electron microscopy. For In_.5Ga_.5As/GaAs QD structures we have observed optimized stacking using a barrier thickness 12 nm.We also report studies of stacking in DWELL laser structures. Despite reports of very low threshold currents in such lasers, designed for 1.3 μm emission, performance is limited by gain saturation and thermal excitation effects. We have explored solutions to these problems by stacking multiple DWELL layers of three, five and 10 repeats. Initial attempts at stacked multilayer structures, particularly samples with a large number of repeats, produced variable results, with a number of the final devices characterized by poor emission and electrical characteristics. Analysis by transmission electron microscopy has identified the presence of large defective regions arising from the complex interaction of dots on several planes and propagating threading dislocations into the cladding layers. The origin of this defect is identified as the coalescence of QDs at very high density and the resulting dislocation propagating to higher dot planes. An effective modified method to reduce the defect density by growing the barrier layer at higher temperature will be discussed. Finally, we report the growth of a stacked 10-layer structure using relatively thin barriers, grown using this technique.     

Isothermal-isobaric computer simulations of melting and crystallization of a Lennard-Jones system

Constant-pressure, constant-temperature molecular dynamics simulations are carried out to study the behaviour of the microscopic atomic structure via the melting and crystallization processes of a model system composed of 864 Lennard-Jones (LJ) particles with periodic boundary conditions. On heating an fcc crystal of LJ particles, it is ascertained that melting takes place. On the other hand, a LJ liquid, when quenched slowly, crystallizes into a stacking of layers with stacking faults where each layer forms a close-packed structure with occasional point defects. A large hysteresis in the volume-temperature curve is observed.     

Dependence of the probability of formation of laves phases of various crystal structures on the density of stacking faults

A model is proposed for the arrangement of close packed layers in the fcc structure, which makes it possible to find the dependence of the probability of formation of various close packed structures on the density of stacking faults in the initial structure. We derive a criterion (_m), which determines the sequence of structural transitions as the density of stacking faults increases. The sequence of structural transitions of Laves phases established on the basis of this criterion is in full agreement with the existing experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 69–75, August, 1971.     

LEED crystallographic analysis for the structure formed by 2 ML of O at the Zr(0001) surface

A tensor LEED analysis is reported for the Zr(0001)-(1 × 1)-O surface which involves oxygen at a total coverage of 2 monolayers. The structure is indicated to have two layers of O: one forms an overlayer in which the O atoms bond to hollow sites of three-fold coordination on the regular metal surface, while the other layer has the O atoms in tetrahedral hole sites between the first and second metal layers. The stacking sequence, designated as (C)B(A)AB... corresponds to the first three layers of anion-terminated cubic ZrO₂, although some lateral compression is needed for superposition on the regular hcp Zr structure. The absorption of O in the tetrahedral holes results in a significant expansion in the first-to-second Zr---Zr interlayer spacing to about 3.44 Å from the bulk vaue of 2.57 Å. The O---Zr bond lengths are estimated to equal 2.07 Å for the overlayer O atoms, and 2.21 Å for the O atoms in tetrahedral hole sites. Comparisons are made with the structures of the corresponding 0.5 and 1 ML surfaces formed by the O/Zr(0001) system.     

High-pressure synthesis, structural and Raman studies of a two-dimensional polymer crystal of

Two-dimensional polymerisation of a C₆₀ single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C₆₀-C₆₀ dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P4₂/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C₆₀ is discussed. Received 8 October 1999     

自适应免疫优化算法研究大尺寸Cu-Au合金团簇稳定结构

合金团簇所具备的催化和光学等方面特性与团簇的尺寸、元素组成和元素序列密切关联,因而确定其稳定结构是研究纳米团簇合金性质的首要任务.本文利用基于内核构建的自适应免疫优化算法研究了完整元素组成的CunAum(n+m=61及79)二元合金团簇的稳定结构.应用多体Gupta势函数描述Cu-Au团簇原子间的相互作用.研究结果表明:对于CunAum(n+m=61)团簇,除了当n=12-15时为由三个双二十面体面面相连组成的环状结构外,其余均为二十面体结构.原子总数为79的Cu-Au合金团簇包括堆积缺陷的面心立方结构、双面心立方结构、二十面体、十面体和由四个双二十面体面面相连组成的环状结构.且当Au原子比例高和低时其主要构型分别为二十面体和十面体.此外,还分析了Cu-Au合金团簇结构势能量的分布情况及团簇的相对稳定性.原子分布规律显示Cu原子趋于占据内层,而Au原子趋向于分布在外层.     

碱土-铈氟碳酸盐矿物晶体结构中的无序堆垛和交生

用晶格象技术研究了同时含有钙和钡的铈氟碳酸盐矿物,发现这类矿物的晶体是由不同组分的钙-铈氟碳酸盐矿物和不同组分的钡-铈氟碳酸盐矿物单层无序堆垛而成,在百埃数量级的范围内,观察到不同组分钙-铈氟碳酸盐矿物的交生现象,由此提出伦琴钙铈矿可能有一种新的多型体。研究结果显示了晶格象技术直接观察这类矿物晶体结构的优越性。 关键词：     

Dislocation mechanism for transformation between cubic ice Ic and hexagonal ice Ih

Cubic ice I_c is metastable, yet can form by the freezing of supercooled water, vapour deposition at low temperatures and by depressurizing high-pressure forms of ice. Its structure differs from that of common hexagonal ice I_h in the order its molecular layers are stacked. This stacking order, however, typically has considerable disorder; that is, not purely cubic, but alternating in hexagonal and cubic layers. In time, stacking-disordered ice gradually decreases in cubicity (fraction having cubic structure), transforming to hexagonal ice. But, how does this disorder originate and how does it transform to hexagonal ice? Here we use numerical data on dislocations in hexagonal ice I_h to show that (1) stacking-disordered ice (or I_c) can be viewed as fine-grained polycrystalline ice with a high density of extended dislocations, each a widely extended stacking fault bounded by partial dislocations, and (2) the transformation from ice I_c to I_h is caused by the reaction and motion of these partial dislocations. Moreover, the stacking disorder may be in either a higher stored energy state consisting of a sub-boundary network arrangement of partial dislocations bounding stacking faults, or a lower stored energy state consisting of a grain structure with a high density of stacking faults, but without bounding partial dislocations. Each state transforms to I_h differently, with a duration to fully transform that strongly depends on temperature and crystal grain size. The results are consistent with the observed transformation rates, transformation temperatures and wide range in heat of transformation.