Controlling polymorphism during the crystallization of an atomic fluid

We use molecular dynamics simulations to shed light on polymorph selection during the crystallization of the Lennard-Jones fluid. By varying pressure at fixed supercooling, we form large crystallites either of the stable face centered cubic form or of the metastable body centered cubic form and even fine-tune the fractions of stable and metastable polymorphs in the crystallite. We demonstrate that the conditions of crystallization, leading to large bcc crystallites, lie within the occurrence domain of the metastable bcc polymorph. We also find that the predominantly fcc crystallites contain a notable amount of the hexagonal close packed form, due to the cross nucleation of the hcp form on the fcc form. By varying temperature at fixed pressure, we prevent cross nucleation and form pure fcc crystallites. Our results reveal that polymorph selection may take place, and be controlled, during the growth step.     

Martensitic fcc-to-hcp transformation observed in xenon at high pressure

Angle-resolved x-ray diffraction patterns of Xe to 127 GPa indicate that the fcc-to-hcp transition occurs martensitically between 3 and 70 GPa in diamond-anvil cells without an intermediate phase. These data also reveal that the transition occurs by the introduction of stacking disorder in the fcc lattice at low pressure, which grows into hcp domains with increasing pressure. The small energy difference between the hcp and the fcc structures may allow the two phases to coexist over a wide pressure range. Evidence of similar stacking disorder and incipient growth of an hcp phase are also observed in solid Kr.     

Computer simulation of crystal structure formation upon transition from amorphous state

The structure of a solid has been studied by the molecular dynamics technique upon transition from the amorphous state to the crystalline state. The influence of initial conditions in the simulation of an amorphous sample and the temperature of its heating on the resulting structure is examined. It is found that structures of two types can be formed in the sample: single crystals consisting of face-centered cubic (fcc) and hexagonal close-packed (hcp) cells with a small number of pentahedral cells in the boundary region of the sample and block crystals with an ordered pentahedral structure composed of fcc, hcp, pentahedral, and icosahedral cells. Linear chains of vacancies are revealed at the boundaries of blocks.     

Formation and microstructure of carbon encapsulated superparamagnetic Co nanoparticles

Carbon encapsulated magnetic cobalt nanoparticles have been synthesized by the modified arc-discharge method. Both high resolution transmission electron microscopy (HREM) and powder X-ray diffraction (XRD) profiles reveal the presence of 8–15 nm diameter crystallites coated with 1–3 carbon layers. In particular, HREM images indicate that the intimate and contiguous carbon fringe around those Co nanoparticles is good evidence for complete encapsulation by carbon shell layers. The encapsulated phases are identified as hcp α-Co, fcc β-Co and cobalt carbide (Co₃C) nanocrystals using X-ray diffraction (XRD), nano-area electron diffraction (SAED) and energy dispersive X-ray analysis (EDX). However, some fcc β-Co particles with a significant fraction of stacking faults are observed by HREM and confirmed by means of numerical fast Fourier transform (FFT) of HREM lattice images. The carbon encapsulation formation and growth mechanism are also reviewed.     

Microstructural evolution and martensitic transformation mechanisms during solidification processes of liquid metal Pb

The microstructural evolution and the martensitic transformation (bcc–hcp and bcc–fcc) mechanisms during the solidification process of liquid metal Pb were studied by molecular dynamics simulation. Results indicate that, with the decrease of temperature, the system undergoes two phase transitions: from the liquid state into a metastable bcc phase first and then from the bcc phase into a coexisting crystal structure of hcp and fcc phases. Moreover, the complicated martensitic transformation processes are clearly observed by cluster type index method (CTIM) and the tracing method. The two transformation mechanisms are very analogous at the atomic level; the essential difference between them is that, in the bcc–hcp transformation, two adjacent layers shift in opposite directions, whereas in the bcc–fcc transformation, the top layer and bottom layer shift in opposite directions relative to the middle layer. The specific mechanisms for the bcc–hcp and bcc–fcc transformations are confirmed to correspond to the revised Burgers mechanism and Bain mechanism, respectively.     

A New Insight into Cobalt Metal Powder Internal Field 59Co NMR Spectra

The authors present a new approach of internal field ⁵⁹Co NMR spectra assignment leaving apart from “usual” decomposition on “pure” hcp and fcc stackings, and a set of stacking faults (sfs) sf₁–sf₅ with a certain lines position. The authors propose including into consideration not only cobalt structural features as well as its magnetic nature due to the strong ferromagnetism in Co metal. The last fact supposes an existence of different magnetic species such as magnetic domains, domain walls, and single-domain particles, thereby helping to spectral lines assignment according to both structural and magnetic origin. The examined sample contains fcc and hcp resonance peaks in both domains and domain walls giving the hcp to fcc ratio equal to 1.9, as well a significant amount of Co sfs, or Co in loose coordination, up to 10 %. The research exhibits a good agreement of all implemented techniques.     

Roton softening in the solid hydrogens

We present a theory of the roton excitations in high-density J = 0 solid hydrogen. We have calculated dispersion relations and density of states as a function of the density of the solids for both hcp and fcc structures. Increasing the density of the rotationally symmetric fcc solid leads to roton softening. When the X₅ roton energy becomes imaginary symmetry breaking of the ground state occurs, resulting in an orientationally ordered solid.     

Self assembly of inorganic nanocrystals in 3D supra crystals: Intrinsic properties

Here we describe how arrangements of nanocrystals can self-organize in 3D arrays called supra crystals. The 3D arrays can fall into the familiar categories of face centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body centered (bcc) crystals. Intrinsic collective properties of these 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk.We demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supra crystal, analogous to a breathing mode vibration of atoms in a nanocrystal.Comparison between the approaches to saturation of the magnetic curve for supra crystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles either highly crystallized or amorphous.     

New beta(fcc)-cobalt to 210 GPa

We report a new phase transition in cobalt from the magnetic varepsilon(hcp) to a beta(fcc) phase, likely nonmagnetic, at 105 GPa. It occurs martensitically in an extended pressure region between 105 and 150 GPa without any apparent volume change. The fcc phase of Co is in systematic accordance with the 4d and 5d counterparts. The pressure-volume isotherm of beta-Co resembles those of alpha(fcc)-Ni and varepsilon(hcp)-Fe within 1%. The phase diagram of cobalt suggests that the fcc stability increases with increasing occupancy of d-band electrons from Fe to Co to Ni.     

Analysis of the Gorskii-Bragg-Williams model

The state diagrams predicted by the GBW model are analyzed for ordered alloys with fcc and hcp lattices. Features of these state diagrams are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 118–126, January, 1971.     

第一性原理计算Fe从bcc到hcp结构的相变路径及其磁性边界

本文采用基于密度泛函理论的第一性原理方法,计算了压力作用下Fe从bcc到hcp结构相变的势能面、相变路径以及相变过程中的磁性相边界.结果表明:与Burgers路径不同,相变过程中bcc结构(110)bcc面的剪切和相对滑移相互耦合,并伴随有(110)bcc面间距的减小;这一相变机制可以解释Fe的静高压实验中在相变初期观察到的hcp结构异常.因此,并不需要像Wang和Ingalls提出的那样,在相变过程中引入一个亚稳定的fcc相来解释这些实验结果.对相变势能面的计算表明剪切对相变的发生有激活作用.此外,分析表明相变过程中涉及复杂的磁性转变,相变过渡态位置正好位于磁性相边界上,并对原子磁性对结构转变影响的物理机制进行了讨论.     

Three-dimensional strongly coupled plasma crystal under gravity conditions

Experiments were carried out to investigate a three-dimensional (3D) plasma crystal. A method of determining the positions of each individual microparticle has been developed. A crystal volume of about 2x10(4) particles in 19 horizontal planes was analyzed. Direct imaging and the 3D pair correlation function show that "domains" of fcc and hcp lattices coexist in the crystal. Other structures, in particular, the theoretically predicted bcc lattice, were not observed.     

On a New Type of Plastic Deformation Waves in Solids

Plastic-strain localization of fcc, bcc, and hcp single crystalline and polycrystalline materials is investigated in different stages in the loading curve under active uniaxial tension. Localization patterns are found to be of an ordered character and can be classified into four types, each corresponding to a certain stage in the loading curve. The regularities observed are interpreted in terms of self-organization in open systems. It is shown that all types of localization patterns can be treated as autowaves of plastic flow. The wave parameters are analyzed and the fundamental difference from the well-known waves of plasticity under shock loading is demonstrated.     

A 59Co NMR study to observe the effects of ball milling on small ferromagnetic cobalt particles

To demonstrate the potential of nuclear magnetic resonance (NMR) spectroscopy for investigating detailed structural properties in ferromagnetic materials, three different particle sized cobalt (Co) powders have been ball milled for 24 h are accurately characterised by internal-field ⁵⁹Co NMR. The ⁵⁹Co NMR spectra show distinct resonance bands corresponding to the different Co sites, face-centred-cubic (fcc), hexagonal-close-packed (hcp) and stacking faults (sfs), in Co metal powders. The hcp+fcc→hcp phase transition encouraged by ball-milling was observed and quantitative values for each Co environment were obtained.     

Microstructural aspects of the hcp-fcc allotropic phase transformation induced in cobalt by ball milling

A detailed correlation between microstructure evolution and allotropic phase transformations occurring in Co when subjected to ball milling has been carried out. After short-term milling, the starting mixture of hcp + fcc Co develops into an almost pure hcp phase. However, for longer milling times, plastic deformation introduces large amounts of stacking faults, especially of twin type, in the hcp structure. As a consequence, some of the hcp Co is converted back into fcc and the hcp unit cell is progressively anisotropically distorted. After long-term milling, a steady 'pseudo-equilibrium' state is observed, where all microstructural parameters, including the fcc percentage, tend to level off. However, the milling intensity can still be adjusted to increase further the stacking-fault probability and, consequently, the amount of fcc Co in the milled powders. The results imply that the stacking-fault formation, rather than the local temperature rise or crystallite size reduction associated with the milling process, is the main mechanism governing the hcp-fcc transformation.     

合成氨钌催化剂的晶体和形貌敏感性

哈伯-博施法合成氨反应是高温高压的耗能过程,因此降低该过程的能量消耗及开发温和条件下合成氨反应催化剂具有重要意义. 金属钌是合成氨反应中最有前途的催化剂之一,一直备受广泛关注. 确定金属钌催化剂的结构敏感性并提高其比质量活性是多相催化中亟待解决的重要问题. 氮气(N₂)活化是合成氨反应中的关键步骤. 本文通过第一性原理理论计算和微观动力学模拟方法系统研究了具有六方密排和面心立方晶体结构的钌催化剂上N₂活化过程和N₂解离反应速率. 理论计算研究表明,在六方密排Ru形貌中,{2130}晶面具有最高的N₂解离活性,其次是{0001}台阶面,它们比六方密排Ru其他表面上N₂解离反应速率高3个数量级以上;在面心立方Ru形貌中,{211}和{311}表面上N₂解离活性最高. 这些结果都表明台阶面/台阶位对氮气活化至关重要. 虽然六方密排Ru {2130}晶面具有最低的N₂解离能垒,然而由于面心立方Ru上可以暴露更高密度的活性位点,使得面心立方Ru比六方密排Ru具有更高的N₂转化速率. 本研究深入理解了N₂解离过程中,金属Ru 催化剂形貌和晶相结构敏感性,这为设计和优化高活性的合成氨Ru催化剂提供了理论基础.     

Lattice dynamics of solid xenon under pressure

We use density-functional perturbation theory to obtain the phonon spectrum of fcc xenon under pressure. Thermodynamic properties obtained within the quasiharmonic approximation are in fair to good agreement with experiment at zero pressure. The transition pressure from the fcc to hcp phase is predicted to occur at 5 GPa. The fcc structure is found to be dynamically stable up to a pressure of 100 GPa, beyond which the phonon modes at the X and L symmetry points soften. We attribute the observed sluggish kinetics of the fcc-hcp transition to the small energy difference between the phases as well as to the high dynamical stability of the fcc phase.     

Thermal expansion behaviors of hcp and fcc Co nanowire arrays

The lattice parameters of as-prepared and annealed Co nanowires with hcp and fcc structures have been measured using the in situ high-temperature x-ray diffraction method. The hcp and fcc Co nanowires have been fabricated within the porous anodic alumina membranes by a direct-current electrodeposition technique. The results indicate that the variational quantity of the interplanar spacing for hcp Co nanowire arrays is bigger than that for fcc Co nanowire arrays in spite of as-prepared and annealed samples. The structural difference between hcp and fcc Co nanowires results in the different thermal expansion behaviors.     

Structural properties of complex (dusty) plasma upon crystallization and melting

A change in the local order of a bounded complex (dusty) plasma in the process of its crystallization and melting has been examined by molecular dynamics simulations. The dynamics of microparticles is considered in the framework of a Langevin thermostat, the pair interaction between charged particles is described by a screened Coulomb potential (Yukawa potential) with the hard wall potential as a confinement. It has been shown that the beginning of the crystallization of such a system is accompanied by the formation of clusters with the hexagonal close packed (hcp) structure; a noticeable number of these clusters are then transformed to the face centered cubic (fcc) phase. A plasma crystal formed after crystallization consists of the metastable hcp phase, fcc clusters, and a small number of clusters with a body centered cubic (bcc) crystal lattice. Beginning with a certain threshold value of the thermostat temperature, the number of fcc/bcc clusters decreases sharply with increasing temperature, which is an important signature of the beginning of the melting of the plasma crystal.     

Ab initio study of structural and mechanical property of solid molecular hydrogens

Ab initio calculations based on density functional theory (DFT) were performed to investigate the structural and the elastic properties of solid molecular hydrogens (H₂). The influence of molecular axes of H₂ on structural relative stabilities of hexagonal close-packed (hcp) and face-centered cubic (fcc) structured hydrogen molecular crystals were systematically investigated. Our results indicate that for hcp structures, disordered hydrogen molecule structure is more stable, while for fcc structures, Pa3 hydrogen molecular crystal is most stable. The cohesive energy of fcc H₂ crystal was found to be lower than hcp. The mechanical properties of fcc and hcp hydrogen molecular crystals were obtained, with results consistent with previous theoretical calculations. In addition, the effects of zero point energy (ZPE) and van der Waals (vdW) correction on the cohesive energy and the stability of hydrogen molecular crystals were systematically studied and discussed.