Systematical study of dimer diffusion on metal fcc(0 0 1) surfaces

We study systematically the dimer diffusion on a series of metal fcc(0 0 1) surfaces. The atomic interactions are modeled by the realistic model potentials including embedded-atom method potential, surface-embedded-atom method potential, and Rosato-Guillopé-Legrand potential. Based on the results of the static calculations and the molecular dynamics simulations, three different kinds of fcc(0 0 1) surfaces can be distinguished named hard, middle, and soft. On the different kind of surfaces, not only the dominant diffusion mechanism but also the physical model for exchange mechanism is different. In addition, besides the conventional hopping and exchange mechanisms, some other interesting diffusion mechanisms for dimers are observed in our molecular dynamics simulations such as exchange rotation mechanism, cooperative hopping mechanism, and cooperative exchange mechanism.     

STM-mediated atom motion: a Co atom and mixed CoCu<Subscript>n</Subscript> chains on a Cu(111) surface

Performing atomic scale simulations, we study the effect of the scanning tunneling microscopy tip on atom motion on a metal surface at zero bias voltage. We concentrate on a Co atom and mixed CoCu _n (n ? 68) chains on a Cu(111) surface. It is revealed that the atom motion can be tuned by adjusting the tip-substrate distance. The change in the potential landscape induced by the tip is found to depend on the tip height. In the presence of the tip, the Co atom can freely jump from the fcc site to the hcp site or vice versa when putting the tip above the adatom at a certain height. For the mixed CoCu _n chains on the Cu(111) surface, the diffusion barrier of the end Co atom from the fcc site to the nearby hcp site increases with the increasing chain length and reaches the limit when the chain length is beyond CoCu₇ without the tip. Especially, the short chains can perform a collective motion with the help of the tip. The importance of the relaxation induced by the tip-adatom interaction is demonstrated.     

Strain Rate Sensitivities of Face-Centred-Cubic Metals Using Molecular Dynamics Simulation

We use dislocation theory and molecular dynamics （MD） simulations to investigate the effect of atom properties on the macroscopic strain rate sensitivity of f cc metals. A method to analyse such effect is proposed. The stress dependence of dislocation velocity is identified as the key of such study and is obtained via 2-D MD simulations on the motion of an individual dislocation in an fcc metal. Combining the simulation results with Orowan＇s relationship, it is concluded that strain rate sensitivities of fcc metals are mainly dependent on their atomic mass rather than the interatomic potential. The order of strain rate sensitivities of five fcc metals obtained by analysing is consistent with the experimental results available.     

THEORETICAL STRENGTH AND PHASE STABILITY OF Cu AND Ni UNDER [100] UNIAXIAL LOADING

Based on Born's criteria we studied phase stability and theoretical strength of fcc crystals of copper and nickel under [100] uniaxial loading. The calculation was carried out using a simple and completely analytical embedded atom method(EAM) potential proposed by the present authors. For Cu, the calculated value of its theoretical strength (0.33×10¹¹ dyn·cm^-2) agrees well with the experimental value (0.30×10¹¹ dyn·cm^-2), while the calculated strain (9.76%) is somewhat larger than the experimental one (2.8%). For Ni, its theoretical strength and strain predicted using the EAM potential are found smaller than those predicted using a pair potential. It is worthy to note that unlike previous calculations, in which pair potentials were used and three unstressed fcc, bcc, and fct structures included (for Ni only fcc state is found stable, while for Cu both fcc and bcc states are predicted stable), in present calculations using EAM potential the [100] primary loading path passes through only two zeroes (a stable unstressed fcc structure and an unstable stress-free bcc structure) either for Cu or for Ni.     

On accuracy of different cluster models used in describing ordering phase transitions in fcc alloys

A general formulation of cluster methods applied to calculations of thermodynamic quantities of alloys in terms of renormalizing fields describing interaction between a cluster and its environment is given. We have shown that the well-known cluster variation method and the cluster field method, which was suggested earlier, are special cases of our approach. These methods have been used in calculations of phase diagrams of fcc alloys with L1₂ and L1₀ ordering transitions with several realistic interaction models. It turns out that, for all these models, the simple tetrahedron version of the cluster field method suggested in this paper describes the phase diagrams almost as accurately as more complicated cluster variation techniques. Possible applications of the tetrahedron version of the cluster field method to inhomogeneous states and kinetics of phase transitions in fcc alloys are discussed. Zh. éksp. Teor. Fiz. 115, 158–179 (January 1999)     

First-principles calculation of the phonon frequencies in γ Fe

The total energy, the equilibrium lattice constant, and the bulk modulus of the fcc phase of iron have been calculated by the full-potential LMTO method. The use of a generalized gradient approximation in the calculation of the electronic structure and lattice properties of γ-Fe is discussed. Next, the transverse phonon frequency at the W point of the Brillouin zone of a fcc lattice is calculated by the “frozen-phonon” method in the harmonic approximation. A local minimum has been found in the curve of the variation of the total energy of the system as a function of the amplitude of the atomic displacements corresponding to the chosen normal mode. To take account of anharmonic effects, a pseudoharmonic approximation is used and an effective potential that approximates the curve of the variation of the total energy of the system and depends on the temperature via the correlation function of the mean-square displacement of atoms from their equilibrium positions is constructed. The theoretical temperature dependence of the effective frequency of the phonon mode responsible for the structural phase transition corresponds qualitatively to the experimentally observed dependence. A new interpretation is given for the structural phase transition as a transition of the corresponding phonon mode from the excited to the ground state. Fiz. Tverd. Tela (St. Petersburg) 39, 171–175 (January 1997)     

Atomistic simulation of fccben bcc phase transition in single crystal Al under uniform compression

By molecular dynamics simulations employing an embedded atom model potential, we investigate the fcc-to-bcc phase transition in single crystal Al, caused by uniform compression. Results show that the fcc structure is unstable when the pressure is over 250 GPa, in reasonable agreement with the calculated value through the density functional theory. The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail. The bcc (011) planes are transited from the fcc (111) plane and the (111) plane. We suggest that the transition mechanism consists mainly of compression, shear, slid and rotation of the lattice. In addition, our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.     

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.     

Adsorption and interfacial phenomena of a Lennard-Jones fluid adsorbed in slit pores: DFT and GCMC simulations

ABSTRACT

Confinement of fluids in porous media leads to the presence of solid–fluid (SF) interfaces that play a key role in many different fields. The experimental characterisation of SF interfacial properties, in particular the surface tension, is challenging or not accessible. In this work, we apply mean-field density functional theory (DFT) to determine the surface tension and also density profile of a Lennard-Jones fluid in slit-shaped pores for realistic amounts of adsorbed molecules. We consider the pore walls to interact with fluid molecules through the well-known 10-4-3 Steele potential. The results are compared with those obtained from Monte Carlo simulations in the Grand Canonical Ensemble (GCMC) using the test-area method. We analyse the effect on the adsorption and interfacial phenomena of volume and energy factors, in particular, the pore diameter and the ratio between SF and fluid–fluid dispersive energy parameters, respectively. Results from DFT and GCMC simulations were found to be comparable, which points to their reliability.     

Molecular dynamics simulations of point defects in plutonium grain boundaries

A modified analytic embedded atom method (MAEAM) potential is constructed for fcc updelta-Pu. Molecular dynamics (MD) simulations with the potential are performed to investigate the interactions between two symmetrical tilt grain boundaries (GBs) and point defects such as He atom, vacancy and self-interstitial atom (SIA) in Pu. The calculated results show that point defect formation energies are on average lower than those in the lattice but variations from site to site along the GBs are very remarkable. Both substitutional and interstitial He atoms are trapped at GBs. Interstitial He atom is more strongly bound at the GB core than the substitutional He atom. The binding energy of SIA at GB core is higher than those of He atom and vacancy. GB core can bind many He atoms and SIAs due mainly to the fact that it contains many vacancies. Compared with He atom and SIA, the vacancy far from GB core is difficult to diffuse into the core. The GBs can act as sinks and sources of He atoms and SIAs, which may be a reason for the swelling of Pu after a period of self-irradiation because of the higher concentration of vacancy in the bulk.     

Performance of different types of detectors in a high-pressure X-ray study of phase transition

Abstract

Phase transitions in praseodymium and lanthanum under pressure have been studied using a synchrotron powder X-ray diffraction technique. A structure refinement of the distorted fcc phase of Pr using diffraction data collected with an imaging plate (IP) detector demonstrate that among some possible structures the rhombohedral structure with space group R3m best reproduces the observed diffraction pattern. The distorted fcc-fcc phase transition in La was observed as a function of the temperature at 23 GPa using a CCD-based detector. A five-minute exposure sufficiently long to measure the intensities of very weak superlattice reflections from the distorted fcc phase, which has been found to transform to the fcc phase at 550 K. The performance of the IP and a CCD-based detector are compared and their future developments discussed.     

铂纳米颗粒生长和表面结构的理论预测

本文应用最近所建立的凝结势模型[2009物理学报583293;2009J.Chem.Phys.130164711]来确定铂纳米颗粒的表面结构,利用分子动力学模拟验证了该模型的可靠性.基于该模型所进行的第一性原理计算表明,各种形状的铂颗粒表面都以fcc的(111)面为主(约80%),(100)面形成的概率约10%,该结果与已有实验观测相符合.由于凝结势计算简单,该模型应是一种从理论上确定纳米颗粒表面结构的简便方法.     

Real-time icosahedral to fcc structure transition during CoPt nanoparticles formation

Nucleation and growth of supported CoPt nanoparticles were studied in situ and in real time by combined grazing incidence small-angle x-ray scattering (GISAXS) and grazing incidence x-ray diffraction (GIXD). GISAXS provides morphological features of nanoparticles as a function of size, shape and correlation distance between particles, while GIXD allows the determination of the atomic structure. We focus on the formation of ultrasmall CoPt nanoparticles, in the 1–4 nm size range at 500^○C. The structural analysis method based on the Debye equation is coupled with cluster model calculations performed by Monte Carlo simulations using a semi-empirical tight-binding potential to interpret diffraction spectra and structural transitions. Our results show that the cluster structure evolution during the growth is size-dependent and composition-dependent, yielding an icosahedral to fcc structure transition.     

Equilibrium concentration of point defects in crystalline4He at O K

We calculate the concentrations of vacancies and intersitials in the ground state of a Bose solid which models⁴He. Because ground-state boson wave functions are nodeless, their probability densities correspond to classical Boltzmann factors, and properties of Bose solids, such as the concentration of vacancies and interstitials, can be calculated using classical statistical mechanics. We model the ground-state wave function of⁴He with the product (Jastrow) form that corresponds to a classical 1/r ^b pair potential, and use a quasiharmonic approximation to calculate the concentrations of vacancies and interstitials in an fcc lattice with this potential. We find that the fractional concentration of vacancies at the melting point is 1.60×10^–5 for 1/r ⁹ and 6.36×10^–6 for 1/r ⁶, while the interstitial fractional concentrations are 1.32×10^–3 and 1.08×10^–5, respectively; the defect concentrations decrease by 7–16 orders of magnitude when the crystal density increases by 50%. At the same density, and with the same 1/r ⁹ potential, the concentration of vacancies in an hcp lattice is essentially the same as in an fcc lattice, but the interstitial concentration is much lower, apparently because the fcc lattice contains a more favorable split-interstitial site than does hcp. Therefore, our fcc vacancy results should be directly relevant for (hcp)⁴He, providing what we think is a lower bound on the vacancy concentration, while the interstitial concentration in⁴He is probably much lower than our results.     

Diffraction peak profiles of surface relaxed spherical nanocrystals

Abstract

A model is proposed for surface relaxation of spherical nanocrystals. Besides reproducing the primary effect of changing the average unit cell parameter, the model accounts for the inhomogeneous atomic displacement caused by surface relaxation and its effect on the diffraction line profiles. Based on three parameters with clear physical meanings - extension of the sub-coordination effect, maximum radial displacement due to sub-coordination, and effective hydrostatic pressure - the model also considers elastic anisotropy and provides parametric expressions of the diffraction line profiles directly applicable in data analysis. The model was tested on spherical nanocrystals of several fcc metals, matching atomic positions with those provided by Molecular Dynamics (MD) simulations based on embedded atom potentials. Agreement was also verified between powder diffraction patterns generated by the Debye scattering equation, using atomic positions from MD and the proposed model.     

Phase transition of samarium under high pressure

Abstract

The DAC X-ray power photograph method was employed for studing the phase transition of samarium up to 26.3 GPa. The experimental results show that the dhcp and fcc high pressure phase of Sm appeared at about 4.0 and 12.5 GPa and room temperature respectively. The dhcp phase was kept until 19.6 GPa. A model for Sm-type -? dhcp -? fcc phase transition is provided in this paper.     

Shear viscosity of pseudo hard-spheres

We present molecular dynamics simulations of pseudo hard sphere fluid (generalized WCA potential with exponents (50, 49) proposed by Jover et al. [J. Chem. Phys 137, (2012)] using GROMACS package. The equation of state and radial distribution functions at contact are obtained from simulations and compared to the available theory of true hard spheres (HS) and available data on pseudo hard spheres. The comparison shows agreements with data by Jover et al. and the Carnahan–Starling equation of HS. The shear viscosity is obtained from the simulations and compared to the Enskog expression and previous HS simulations. It is demonstrated that the PHS potential reproduces the HS shear viscosity accurately.     

Thorium under strong compression—a test case for the evaluation of EOS data by different forms and procedures

Abstract

X-ray diffraction on Thorium under pressures to 300 GPa at ambient temperature provides new EOS data for both the (low pressure) fcc phase as well as for the (high pressure) bct phase. A detailed evaluation of these data and a comparison with previous results shows systematic differences in the fitted parameters resulting from the use of different EOS forms and from the correlations in the parameters used in all these fitting procedures. The absolute uncertainties in these parameters are elucidated and special attention is given to the compatibility of different EOS forms for phases which are related by second order transitions.     

Magnetic and structural characterizations on nanoparticles of FePt,FeRh and their composites

The various compositions of FePt and FeRh nanoparticles, and their composite particles have been fabricated by the solution-phase chemical method and their magnetic properties characterized. High-resolution transmission electron microscopic observations indicate that mono-dispersed FeRh and FePt/FeRh nanoparticles are fabricated with the average size of 3–5 nm. However, larger size particles are distributed in the annealed state. From X-ray diffraction results, the as-deposited FeRh nanoparticles reveal a chemically disordered fcc structure which can be transformed into CsCl-type structure through thermal annealing. Similarly, the annealed FePt nanoparticles show the L1₀-phase fct structure although the fcc structure is apparent in the as-deposited state. It is also found that the first time in the exchange bias effect in the composite of ferromagnetic (FePt) and anti-ferromagnetic (FeRh) nanoparticles; result in a shift of the hysteresis loop after field cooling process.