Topological Analysis of the Electron Localization Function: A Help for Understanding the Complex Structure of Cryolitic Melts

Joubert, Laurent; Picard, Gauthier; Silvi, Bernard; Fuster, Franck

doi:10.1149/1.1391910

Cited by 14 publications

(6 citation statements)

References 25 publications

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“…Recent theoretical studies are relevant. It has been shown in several reports that cations can stabilize both five- and six- coordinated fluoroaluminates. − Joubert and co-workers 48 have calculated the most stable configurations for the cluster of 3 Na + Al + 6 F (i.e., cryolite, Na 3 AlF 6 ). These authors performed their calculations at a very high level using density functional theory (DFT) combined with topological analysis of the electron localization function (ELF) gradient field.…”

Section: Resultsmentioning

confidence: 99%

Raman Study of the Hexafluoroaluminate Ion in Solid and Molten FLINAK

et al. 2000

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Raman spectra have been obtained for matrix-isolated AlF6(3-) in an LiF/NaF/KF (FLINAK) eutectic mixture. Three Raman bands characteristic of the hexafluoroaluminate ion were identified in the solids formed from FLINAK melts which contained small amounts (5-11 mol%) of either AlF3 or Na3AlF6. The three allowed Raman-active bands of the matrix-isolated octahedral complex ion, nu 1(A1g), nu 2(Eg), and nu 5(F2g), were observed at 560.5, 380, and 325 cm-1, respectively, for the solid sample at 25 degrees C. Wavenumbers and relative intensities were similar to those of Na3AlF6 (cryolite), K3AlF6, and K2NaAlF6 (elpasolite) and other crystals known to contain discrete, octahedral AlF6(3-) ions. Peak positions, half-widths, and relative intensities for the bands were measured for samples at temperatures different from room temperature through the melting transition and into the molten state. The transition from high-temperature solid to molten salt at about 455 degrees C occurred gradually without perceptible change in the peak positions, half-widths, or relative intensities. For a sample in molten FLINAK at 455 degrees C, the nu 1(A1g), nu 2(Eg), and nu 5(F2g) modes of the AlF6(3-) ion were observed at 542, 365, and 324 cm-1, respectively. Raman depolarization experiments were consistent with these assignments, and the low value of the depolarization ratio of the nu 1(A1g) mode at 542 cm-1 indicated that the sample was molten above 455 degrees C. Differential thermal analysis also indicated that the FLINAK samples melted at about 455 degrees C. Raman measurements were performed for samples at temperatures from 25 to 600 degrees C in a silver dish, on a hot stage, in an argon-filled atmosphere, under a microscope. Additional Raman experiments were performed on samples at temperatures from 25 to 750 degrees C in a conventional graphite windowless cell, in an argon-filled quartz tube, in a standard furnace. Over the concentration range 4.8-11 mol% AlF3 (CR 23-8.0) in FLINAK, only bands due to the AlF6(3-) ion were detected. There was no evidence to support the presence of other aluminum complexes in these melts.

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Section: Resultsmentioning

confidence: 99%

Raman Study of the Hexafluoroaluminate Ion in Solid and Molten FLINAK

et al. 2000

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“…In many quantum-chemical studies a basic approach is to calculate the parameters of model systems containing an anionic complex with an OS cationic shell (e.g. [14][15][16][17][31][32][33][34]. For similar systems nM + ⋅[NbF 7 ], nM + ⋅[CrCl 6 ] (n = 1n max ) and others, it has been demonstrated that the most thermodynamically stable compositions have some intermediate number of OS cations n < n max , where n max is the maximal number of OS cations held by a given complex.…”

Section: Quantum-chemical Calculationsmentioning

confidence: 99%

Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations

2016

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The standard rate constants of charge transfer (k s) for the Nb(V)/Nb(IV) redox couple in KCl-NbCl5 and CsCl-NbCl5 chloride melts are determined. The Nb(V) electrochemical reduction process in the NaCl-KCl-NbCl5 melt was shown being reversible; hence, the determination of k s appeared impossible. Values of k s in the KCl-NbCl5 melt are higher than those in the CsCl-NbCl5 one. Previously, in a quantum-chemical calculations of nM+·[NbCl6] model systems (M = Na, K, Cs), containing a complex anion with cations outer-sphere (OS) shell (nM+) have been done. It was found that compositions with an intermediate number of OS cations n < n lim, where n lim is the limiting number of OS cations bound to a given complex, are thermodynamically most stable. The aim of this quantum-chemical study is an estimation of the temperature effect on the composition of the stable particles in the extended systems M+∙[NbCl6]+18MCl (M = Na, K, Cs).

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“…As can be seen from Quantum-chemical calculations.-In many quantum-chemical studies, a basic approach is to calculate the parameters of model systems containing an anionic complex with an OS cationic shell (e.g. [14][15][16][17][33][34][35][36]. For similar systems nM + •[NbF 7 ], nM + •[CrCl 6 ] (n = 1n max ) and others, it has been demonstrated that the most thermodynamically stable compositions have some intermediate number of OS cations n < n max , where n max is the maximal number of OS cations held by a given complex.…”

Section: Determination Of the Standard Rate Constants Of Charge-trans...mentioning

confidence: 99%

Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations

Popova

Kremenetsky

Kuznetsov

2017

J. Electrochem. Soc.

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The standard rate constants of charge transfer (k s ) for the Nb(V)/Nb(IV) redox couple in KCl-NbCl 5 and CsCl-NbCl 5 chloride melts have been determined. The redox process in the NaCl-KCl-NbCl 5 melt was shown being reversible; hence, the determination of k s appeared impossible. Values of k s in the KCl-NbCl 5 melt are larger than in the CsCl-NbCl 5 melt. Previously, a quantum-chemical calculations of nM + · [NbCl 6 ] model systems (M = Na, K, Cs), containing a complex anion with cations outer-sphere (OS) shell (nM + ) have been done. It was found that compositions with an intermediate number of OS cations n < n lim , where n lim is the limiting number of OS cations bound to a given complex, are thermodynamically most stable. In this study a quantum-chemical calculations were performed for estimation of the temperature effect on the composition of the stable particles in the extended systems M + · [NbCl 6 ]+18MCl (M = Na, K, Cs Possible schemes of niobium reduction in chloride melts of different composition were discussed at length.1-13 Electrochemical reduction of niobium in LiCl-KCl-NbCl 5 and NaCl-KCl-NbCl 5 melts was also studied 6 and the temperature effect on the niobium electrochemical behavior in LiCl-KCl melt was elucidated. The three cathodic waves observed in the 673-903 K temperature range were interpreted as consecutive transfer of one electron and two electrons in the two consecutive reaction stages. Importantly, the electrochemical reduction in the second stage is accompanied by formation of insoluble in the melt products with the composition ranging from NbCl 2.33 to NbCl 3.13 . At temperatures from 903 to 923 K the lower chlorides become thermodynamically unstable; at 923 K a two-stage electrode process occurred:Later on, data on the niobium cluster compounds formation were confirmed; 7,8 these compounds were found decomposing at 923 K, in full agreement with the previous data. The NaCl-KCl melt has a higher melting than the LiCl-KCl one; therefore, the niobium pentachloride electrochemical reduction is a two-stage process, with a consecutive transfer of one and four electrons according to the Equations 1 and 2. [6][7][8]13 However, there is no data on the standard rate constants of charge-transfer for the Nb(V)/Nb(IV) redox couple.In this work we aimed at the determining of the standard rate constants of charge transfer for the Nb(V)/Nb(IV) redox couple in the following chloride systems: NaCl-KCl (equimolar mixture)-NbCl 5 , KCl-NbCl 5 and CsCl-NbCl 5 at glassy carbon and platinum electrodes.Previously, in Refs. 14-17 quantum- complexes consistent with the change of the experimental standard rate constants of charge transfer in a series of molten salts type (Na-K)Cl eq. -KCl-CsCl. For such systems, the second coordination sphere of the complex (the OS shell) consist of cations Na, K, and Cs, respectively. Estimated value of the E act in this row usually decreases. This is true in the case when the charge transfer mechanism is the same in all systems. In such systems, the change of the reorga...

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Topological Analysis of the Electron Localization Function: A Help for Understanding the Complex Structure of Cryolitic Melts

Cited by 14 publications

References 25 publications

Raman Study of the Hexafluoroaluminate Ion in Solid and Molten FLINAK

Raman Study of the Hexafluoroaluminate Ion in Solid and Molten FLINAK

Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations

Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations

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