Modeling Ionic Transport and Manifestation of Mixed Alkali Effect in Glass and Glass-Forming Melts of Lithium and Sodium
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deling Ionic Transport and Manifestation of Mixed Alkali Effect in Glass and Glass-Forming Melts of Lithium and Sodium Niobium Phosphates M. S. Salamatova, *, I. A. Sokolova, b, A. V. Petrovc, and I. V. Murinc aSt.
Petersburg Polytechnic University, St. Petersburg, 195251 Russia Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, 199034 Russia c Saint Petersburg State University, St. Petersburg, 199034 Russia *e-mail: [email protected]
b
Received February 25, 2020; revised April 17, 2020; accepted June 5, 2020
Abstract—The diffusion coefficients of Li+ and Na+ ions in glass and glass-forming melts of lithium and sodium niobium phosphates, which are in satisfactory agreement with the experimental data available in the literature, are obtained by the molecular dynamics method. The nonlinear change in the migration parameters (manifestation of the mixed alkali effect) is confirmed at the equivalent replacement of Li+ ions by Na+ ions in this system. The analysis of the radial pair correlation functions demonstrates an increased formation of lithium-sodium ionic pairs in bimetallic compositions, responsible for manifestation of the mixed alkali effect. Keywords: molecular dynamics, lithium and sodium niobium phosphates, glass, glass-forming melt, mixed alkali effect, diffusion coefficient, radial distribution function DOI: 10.1134/S1087659620050065
INTRODUCTION It is known from the literature that alkaline (in particular, lithium) niobium phosphate glass has quite high values of ionic conductivity, in comparison, for example, with silicate systems [1–3], which makes it possible in principle to use them as solid electrolytes for lithium-ion batteries and other solid-state electrochemical devices [4, 5]. It is also known that with the use of relatively low-power femtosecond laser radiation in these glassy systems, it is possible to create gradient optical structures due to the laser-stimulated diffusion of alkaline ions from the focal point to the edges of the laser’s region of action without noticeable destruction of the glass grid as a whole, which also argues for the strong migration characteristics of alkaline ions in glass of this system [6, 7]. At the same time, the calculations indicate that the temperature at the focal point of the laser beam reaches several thousand Kelvin [8], which undoubtedly arouses interest in migration processes in the corresponding melts. The presence of Nb2O5 in the composition of alkaline phosphate glass increases the chemical stability of these types of glass [9] due to the organic incorporation of niobium oxide into the phosphate component of the structure and formation of Nb–O–P bonds [10]. However, despite the fact that there is no information in the literature about the presence of iono-
genic niobium-containing fragments in the composition of alkaline niobium phosphate glass, the introduction of niobium oxide (up to 20 mol %) is accompanied by a slight increase in the migration characteristics of alkaline ions (in particular, Li+ io
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