Studying diffusion mechanism and dynamics slowdown in iron liquid

The dynamic properties of iron liquid (Fe) are studied by molecular dynamics (MD) simulation. We trace the evolution of local density fluctuations (LDFs) in Fe liquid over the simulation time and in the 300-2300 K temperature range. The result simulation reveals that atomic diffusion is realized through the LDFs and the high localization LDFs at low temperature in the iron liquid is the cause of the anomalous dynamics slowdown. We find that the diffusion depends on both rate of LDFs and the averaged square displacement of particles Fe as one LDF occurs. As the temperature decreases, both quantities reduce.
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Studying diffusion mechanism and dynamics slowdown in iron liquidNguyễn Thị Thanh Hà và ĐtgTạp chí KHOA HỌC & CÔNG NGHỆ135(05): 167 - 172STUDYING DIFFUSION MECHANISM AND DYNAMICS SLOWDOWN INIRON LIQUIDNguyen Thi Thanh Ha*, Le Van Vinh, Pham Khac HungHanoi University of Science and TechnologySUMMARYThe dynamic properties of iron liquid (Fe) are studied by molecular dynamics (MD) simulation.We trace the evolution of local density fluctuations (LDFs) in Fe liquid over the simulation timeand in the 300-2300 K temperature range. The result simulation reveals that atomic diffusion isrealized through the LDFs and the high localization LDFs at low temperature in the iron liquid isthe cause of the anomalous dynamics slowdown. We find that the diffusion depends on both rateof LDFs and the averaged square displacement of particles Fe as one LDF occurs. As thetemperature decreases, both quantities reduce.Keywords: Molecular dynamics simulation, iron liquid, dynamics slowdown, diffusion, localdensity fluctuations.INTRODUCTION*This transition to a disordered solid known asthe glass transition is accompanied with thedrastic increase in the viscosity and a subtlechange in the structure. Understanding themicroscopic mechanism governing glasstransitions is one of the most importantproblems in statistical physics [1-3]. To tacklethis problem, several working hypotheseshave been proposed. The studies from refs.[48] focus on the dynamics heterogeneity, thepercolation in real space and properties ofenergy landscapes. They found the existenceof mobile and immobile regions whichmigrate in the space over time. Authors in [910] put forward the mechanism by which thesmall modification of statistic densitycorrelations can produce an extremely largedynamical change. The essential result in thisdirection is the mode coupling theory [9] thatpredicts a freezing of dynamics from the nonlinear feedback effect. The theoretical andexperimental investigations on universalmechanisms controlling slow dynamics havebeen done for long time, however as mentionin [11] many open questions are stillremained.Iron is an important element and has manyindustrial applications. Therefore, knowledge*Tel: 0983 012387, Email: ha.nguyenthithanh1@hust.edu.vnabout their microstructure and dynamicalproperties would be essential to understandthis material [12-14]. In this paper, MDsimulation is conducted to examine thedynamics in iron liquid. Our purpose is toclarify the diffusion mechanism and the causeof slowdown in the iron liquid near glasstemperature.CALCULATION PROCEDUREMD simulation is conducted for 104 atommodels with periodic boundary conditionsusing Pak–Doyama potential [15]. Tointegrate the equation of motion Verletalgorithm is used with MD step of 0.67 fs.Initial configuration is obtained by randomlyplacing all atoms in a simulation box. Thenthis sample is equilibrated at temperature of6000 K and cooled down to desiredtemperature. Next, a long relaxation has beendone in ensemble NPT (constant temperatureand pressure) by 105 MD steps to obtain theequilibrium sample. We prepare six models(M1, M2... M6) have been constructed atambient pressure and at temperature of 300 K,800 K, 1200 K, 1500 K, 1800K and 2300 K. Tostudy dynamical properties the obtainedsamples are relaxed in ensemble NVE (constantvolume and energy) over 5x106 steps.167Nitro PDF Software100 Portable Document LaneWonderlandTạp chí KHOA HỌC & CÔNG NGHỆNguyễn Thị Thanh Hà và ĐtgObviously, the diffusivity in system isimpossible if no exchanging the coordinatedFe occurs. Therefore, we trace the evolutionof local density fluctuations (LDF) in Feliquid over the simulation time. To calculatethe coordination number we use the cutoffdistance RO=3.35 Å chosen as a minimumafter first peak of PRDF. The local densityaround ith particle can be quantified as:ni  OiVO135(05): 167 - 172sphere. If the number nOi changes, then thelocal density around ith particle varies. Itmeans that the change of nOi at some momentsrepresents the local density fluctuation (LDF)act. The existence of non-mobile and mobileregions is originated from the densityfluctuation in the liquid.RESULTS AND DISCUSSIONTo test the validity of MD model one usuallydetermines the pair radial distributionfunctions (PRDF). They are very close tosimulation result reported in ref. [14, 16] andin good agreement with experimental data.(1)where VO= 4RO3/3; nOi is the number ofparticles in a coordination sphere of ithparticle; RO is the radius of the coordination3BSimulationExperiment [16]2g(r)104ASimulationExperiment [14]3210024681012r, ÅFig 1. The pair radial distribution functions for amorphous solid iron at300K (A) and liquid iron at 1500K (B)Fig.2 The schematic illustration of local density fluctuations for selected particle168Nitro PDF ...