Diffusion mechanism in liquid MgO under high pressure

In our study, we used molecular dynamics simulation (MD) to study diffusion mechanism in liquid MgO at a temperature of 3800K at a pressure ranging from 0 to 25 GPa. The calculated results and analysis of dynamical properties, spatially heterogeneous dynamics and diffusion mechanisms in MgO liquids will be reported in detail in this work.
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Diffusion mechanism in liquid MgO under high pressure JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2012, Vol. 57, No. 7, pp. 124-133 This paper is available online at http://stdb.hnue.edu.vn DIFFUSION MECHANISM IN LIQUID MgO UNDER HIGH PRESSURE Mai Thi Lan, Nguyen Van Hong and Pham Khac Hung Department of Computational Physics, Institute of Engineering Physics, Ha Noi University of Science & Technology Abstract. The diffusion mechanism in liquid MgO under pressure up to 25 GPa at a temperature of 3800K have been studied using molecular dynamics simulation (MD). The results show that each atom undergoes a series of stages while associated with the unchanged structural unit M gOx or OM gy . The diffusivity strongly depends on the rate of transition in M gOx → M gOx±1 and OM gy → OM gy±1 . Under low pressure, diffusion proceeds due to the transition with x = 3, 4, 5 and y = 3, 4, 5 but mainly x = 4 and y = 4. Under high-pressure, diffusion in the sample proceeds due to a transition with x = 4, 5, 6 and y = 4, 5, 6 but mainly x = 5 and y = 5. Investigating the movement of atoms in liquid MgO shows the spatially heterogeneous dynamics. The diffusion coefficient of Mg and O atoms is also examined through mean square displacement. Structural stability (the life time of basic structural units) is investigated in detail in this work. Keywords: MD simulation, transition, diffusion mechanism, heterogeneous dynamics.1. Introduction In recent years, some research on microstructure and diffusion mechanisms indisordered materials has shown that the spatially heterogeneous dynamics (SHD), i.e.the atom displacement and local structural relaxation, are non-uniformly distributedthrough space. Numerous approaches have been employed to study the dynamics inliquid including the Mode Coupling theory [1] and the Adam-Gibbs theory [2]. However,the atomic mechanism behind those phenomena has not been successfully identifiedin these studies. SHD and the relation between dynamics and diffusion mechanism inliquids and glassy polymers have been shown in [3-5] and it is also predicted that theSHD phenomenon also occurs for non-strong (or fragile) liquids (e.g. liquid MgO).The study of SHD is essential because it provides detailed information on the diffusionReceived August 2, 2012. Accepted September 4, 2012.Physics Subject Classification: 62 44 01 01.Contact Mai Thi Lan, e-mail address: lanmt.iep@gmail.com124 Diffusion mechanism in liquid MgO under high pressuremechanism and phase transitions in disordered material systems. Because it is difficult toperform experimental studies at a high temperature and pressure, the simulation methodis still an effective tool for investigating the structure and dynamics of liquids and glassypolymers [6, 7]. Molecular dynamics simulation is one of the most widely used methods[8]. It can provide more information about physical processes in conditions that othermethods would be impossible or very difficult to apply. While structure and dynamicsof compounds containing Si and Mg in the liquid state (e.g. MgSiO3 and MgSiO4 ) werestudied extensively using both experimental and simulation methods, studies of liquidMgO are very limited. Furthermore, the microstructure, dynamics, diffusion mechanismand relation between microstructure and dynamics as well as diffusion mechanism inliquid MgO is still not clear. In our study, we used molecular dynamics simulation (MD) tostudy diffusion mechanism in liquid MgO at a temperature of 3800K at a pressure rangingfrom 0 to 25 GPa. The calculated results and analysis of dynamical properties, spatiallyheterogeneous dynamics and diffusion mechanisms in MgO liquids will be reported indetail in this work.2. Content2.1. Calculation method The molecular dynamics simulation of liquid MgO was done in a cubic boxcontaining 2000 atoms (1000 Mg and 1000 O) with periodic boundary conditions usingLewis and Catlow potentials. The form of this potential is: qi qj Cij Uij = + Aij exp( − Bij rij ) − 6 rij rijhere qi = +2 and qj = -2 is the ionic charges of Mg and O. rij is the distance between theith and jth atoms. The Mg-Mg, Mg-O and O-O interaction is described by parameters Aij , Bij , Cijwhich can be found in [9]. The initial model is generated by randomly placing 2000 Mgand O atoms in a simulated cubic box with an experimental density ρ = 3, 58g/cm3 andthen heated to 5000K at ambient pressure and relaxed over 100,000 MD steps to removepossible memory effects. This model is then cooled to 3800K at a rate of 0.25 K/ps ...