Structural and dynamical investigation of Mg 2 SiO 4 liquid
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ORIGINAL PAPER
Structural and dynamical investigation of Mg2SiO4 liquid L T Mai* and N T Nguyen Department of Computational Physics, School of Engineering Physics, Hanoi University of Science and Technology, No. 1, Dai Co Viet Street, Hanoi 10000, Vietnam Received: 18 November 2019 / Accepted: 6 March 2020
Abstract: We have numerically studied structure and dynamics in Mg2SiO4 liquid. Eight models at pressure up to 40 GPa and at a temperature of 3500 K have been constructed by molecular dynamics simulation. The structural characteristics such as short-, intermediate-range order (denoted as SRO, IRO, respectively) structure and network structure are analyzed via the radial distribution functions, coordination units, bond angle, bond length as well as the number of corner-, edge- and face-sharing bonds and characteristics of all type OTy linkages. To clarify the dynamical properties, the models at different pressures are relaxed in NVE ensemble for a long time. The diffusivity of the individual atomic species is determined through the dependence of the mean-squared displacement on number of MD steps. Under compression, the local environment around Si and Mg atoms as well as the IRO structure in Mg2SiO4 changes significantly. The degree of polymerization of the Si–O network increases with pressure. The Si–O bonds are broken; the Mg atoms tend to incorporate into the Si–O network via both bridging oxygen (BO) and non-bridging oxygen (NBO) to form the –Si–O–Mg– network in Mg2SiO4 liquid. In the considered pressure range, the Mg atom always diffuses faster than the O atom and the O atom diffuses faster than the Si atom. The diffusivity of Si, Mg and O atom decreases with increase in pressure. Keywords: Magnesium silicate; Molecular dynamics simulation; Structure; Polymerization; Diffusion
1. Introduction Magnesium silicate (MgO–SiO2) is one of the major planetary materials. Thus, the understanding of its properties under high temperature and pressure conditions is important to understand the interior structure of Earth. Furthermore, magnesium silicate is also an important material in many high technology applications such as porous ceramic membranes for catalytic reactors, biomedical glass and refractory brick. Hence, the knowledge of structure and properties of MgO–SiO2 in both glass and liquid is necessary to our understanding about the physical and thermal evolution of the Earth as well as controlling the process of material fabrication technology. Therefore, MgO–SiO2 system has been widely studied by different experiments and simulation methods during the last several years [1–15]. However, experiment is usually difficult to do due to the high melting temperature of MgO– SiO2 (2163 K ± 25 K [7, 13]). Available experimental results are usually limited to the ambient or low pressure
and temperature. Using neutron and X-ray diffraction data with a reverse Monter-Carlo modeling [11–13], authors indicated that the mean coordination number (CN) of Mg is 4.5 ± 0.1 and Mg–O bond length (BL) is about ˚ and 2.21 ± 0.01 A ˚
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