Elsevier

Journal of Alloys and Compounds

Volume 786, 25 May 2019, Pages 953-959
Journal of Alloys and Compounds

Study of the NaF-ScF3 system as a molten bath for production of Sc alloys: A combination of NMR and molecular dynamics simulations

https://doi.org/10.1016/j.jallcom.2019.02.057Get rights and content

Highlights

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    NaF-ScF3 molten system was characterized by combination of HT NMR and Molecular Dynamics calculations.

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    The agreement between the experimental NMR and the calculated data was observed.

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    New interatomic potential for the molten NaF-ScF3 system were developed.

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    Network formation in the melt was established.

Abstract

In situ high temperature NMR spectroscopy was used to characterize the NaF-ScF3 melt over a wide range of compositions. 19F, 23Na, and 45Sc NMR spectra were acquired in NaF-ScF3 melts of up to 70 mol% of ScF3. The interpretation of all experimental results obtained in situ in the melt is significantly enhanced by the contribution of Molecular Dynamics (MD) calculations. A new interatomic potential for the molten NaF-ScF3 system was developed by using a Polarizable Ion Model (PIM). The potential parameters were obtained by force-fitting to density functional theory (DFT) reference data. MD simulations were combined with further DFT calculations to determine NMR chemical shifts for 19F, 23Na, and 45Sc. The agreement between the experimental NMR data and the corresponding calculated data from our applied computational protocol indicated the polymerization and network formation in the melt. Additionally, the density and the electrical conductivity in the molten state were calculated from the statistical analysis of ionic trajectories obtained through MD simulations.

Introduction

One of the main fields of application of scandium is the development of lightweight, high-strength aluminum alloys [1]. Scandium in aluminum-based alloys is introduced during their manufacture as an aluminum-scandium master alloy where scandium content is varied from 1.5 to 5 wt%. The structure of the Al-Sc master alloy represents a solid solution of scandium in aluminum with dispersed particles of the Al3Sc intermetallic compound [2]. The maximal solubility of scandium in aluminum-based solid solutions does not exceed 0.8 wt% [3]. Thus, А13Sс is formed during both initial and eutectic crystallization, and as result of decomposition of a supersaturated solid solution. This intermetallic phase, with a face centered cubic lattice, has very close geometrical parameters to the aluminum lattice that results in the unique influence of scandium on structure and properties of aluminum alloys [4].

The main manufacturers of Al-Sc master alloys use the following methods for their synthesis: direct alloying of scandium metal with aluminum, electrowinning from molten salts containing scandium salts or oxide, and aluminothermic reduction of scandium salts or Sc2O3 [5]. The last technique allows technological use of scandium concentrate based on NaScF4 that was obtained from waste solutions arising from uranium underground leaching according to the original method [6].

Therefore, the study and comprehension of the ionic structure of the electrolyte, consisting of NaF and ScF3, represents an important contribution to understanding the mechanism of the formation of Al3Sc. Chemical and physical properties of this melt strongly depend on scandium speciation.

The information concerning scandium speciation in halide systems is relatively poor. Moreover, most authors have used limited number of techniques in their works. Several electrochemical studies indicated that complex Sc3+ ions are the only soluble scandium species in chloride and chloride-fluoride molten salts [1,7,8], however their structure in the melts is unknown. CsI-ScI3 and CsCl-ScCl3 mixtures were studied in detail in both molten and solid state by Raman spectroscopy over the entire composition range at temperatures up to 1000 °C [[9], [10], [11]]. In the molten CsI-ScI3 system with scandium iodide mole fractions below 0.6, the presence of ScI63− and ScI4− species in the equilibrium was proposed. In the CsCl-ScCl3 system, a different ionic species, i.e. ScCl74−, ScCl63−, Sc2Cl93−, and ScCl4−, were established. A cluster-like model for the structure of pure molten ScCl3 was also proposed, where fragments of scandium octahedra bridged by chlorides were terminated with scandium tetrahedra having terminal chlorides.

The aim of this contribution is to investigate the structure of molten NaF-ScF3 binary systems by a multinuclear (19F, 45Sc, and 23Na) in situ NMR spectroscopy at high temperature and by computations combining Molecular Dynamics (MD) simulations and density functional theory (DFT) calculations. To perform MD simulations over a large dynamic time interval (1–5 ns), a classical interaction atomic potential for NaF-ScF3 melts has been specially developed. The interatomic potential was validated over a wide range of compositions, by computing the NMR chemical shifts of each nucleus in the framework of the density functional theory. DFT calculations were performed on configurations generated along the MD simulations without further optimization. Statistical analysis of ion trajectory from MD simulations allowed to describe accurately the structure of the melts and to establish the relation between the physicochemical and structural properties.

Section snippets

Materials

For the preparation of the samples, the following chemicals were used: NaF (Fluka, min. 99.9%; dried in vacuum at 773 K for 4 h), ScF3 (Sigma-Aldrich, 99.9%), without further purification. All fluorides were stored in a dry box under argon atmosphere maintained below 0.3 ppm of moisture and 0.1–0.5 ppm of oxygen. Different compositions of ScF3 and NaF were prepared by mixing and grinding the corresponding fluorides. For high temperature (HT) NMR measurements, approximately 60 mg of the mixtures

HT NMR in NaF–ScF3 melts

The NMR spectra acquired in the molten state consist in a single Lorentzian line, characteristic of a rapid exchange at the timescale of NMR between the different atomic configurations around the observed nucleus. The measured chemical shift is thus the average of the individual chemical shifts of the different species.

The 19F spectra obtained in NaF–ScF3 melts at high temperature, over the whole range of compositions are presented in Fig. 1. 19F chemical shifts obtained in our system ranged

Conclusions

The local structure of scandium and fluoride ions was investigated in the molten NaF- ScF3 system by combining HT NMR measurements and molecular dynamics simulations. From 45Sc high-temperature NMR spectra an average coordination number of 6 has been determined for the scandium on all domains of composition of 5–70 mol%. The 19F chemical shift evolution coincides with the existence of at least three types of fluorines: free at infinite dilution, fluorines connected with one scandium, and

Acknowledgment

This work was supported by the Ministry of Education and Science of the Russian Federation No. 02.G25.31.0210 of 27.04.2016. For the calculations, we thank the “Centre de Calcul Scientifique en region Centre” (Orleans, France). We thank also Dr. M. Salanne and Dr. M. Pitcher for useful discussions.

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