Diffusion mechanism and dependence of diffusion on sodium silicate compositions

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THE EUROPEAN PHYSICAL JOURNAL B

Regular Article

Diffusion mechanism and dependence of diffusion on sodium silicate compositions Nguyen Thi Thanh Ha a , Nguyen Thi Trang, Hoang Viet Hung, Tran Thuy Duong, and Pham Khac Hung Hanoi University of Science and Technology, No. 1, Dai Co Viet, Hanoi, Vietnam

Received 6 March 2020 / Received in final form 1 June 2020 Published online 27 July 2020 c EDP Sciences / Societ`

a Italiana di Fisica / Springer-Verlag GmbH Germany, part of Springer Nature, 2020 Abstract. The distribution of sodium and diffusion mechanism in sodium-silicate melt with various compositions are investigated via molecular dynamics simulation. The microstructure and dynamical characteristic have been studied with the help of Voronoi polyhedron, simplex and Si-O subnet, oxygen-cluster. The simulation results reveal that Na atoms tend to be in the O polyhedrons and not in Si ones. Moreover, the Na atoms are mainly located in non-bridging oxygen (NBO) and free oxygen (FO) polyhedrons. The Voronoi volume of bridging oxygen (BO) or NBO weakly depends on the number of Na located in polyhedron which indicates the strong Si-O bond compared to Na-O bond. The structure of sodium silicate melt consists of two separate regions: the Na-poor regions of Si-BO subnets and a Na-rich region of NBO-FO cluster. The density of sodium in the NBO-FO cluster is by 3–6 times larger than the one of Si-O subnets. This largest NBO-FO cluster represents a diffusion channel for sodium in sodium-silicate. Furthermore, we find that each BO polyhedron contains 0 or 1 Na atom meanwhile each NBO polyhedron contains 1 or 2 Na atoms. Thereby, the BO and NBO polyhedron contains 1 and 2 sites, respectively. The energy for Na atom located in BO site is larger than that in NBO site. The transition energy for Na atom moving from present site to neighboring NBO site is smaller than that to neighboring BO site. So, Na atoms move frequently to neighboring polyhedrons of NBO, and rarely to the BO ones.

1 Introduction Sodium-silicate is a prototype of a network forming system with network modifiers and it has a significant role in scientific and technologic development [1–3]. The structure of sodium-silicate often concentrates on their cationic constituents, distribution of alkali atoms as well as influence of alkali ions on structure network. These characteristics directly impact the physical properties of material system. Therefore, sodium-silicates have been intensively studied by experimental techniques such as X-ray diffraction, photoelectron spectroscopy, in-situ Raman spectroscopy, elastic neutron scattering [2,4–6], and simulations [7–9]. Namely, in works [5,6], the result of X-ray photoelectron spectroscopy research has shown that the continuous random network of oxygen-shared SiO4 tetrahedrons is altered by sodium ions. In which, Si and O atoms act as network former meanwhile Na atoms play a role as network modifier. X-ray and neutron diffraction studies in works [2,10] shown that Si-O bond distance increases with sodium content. The O-

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