The Effect of the Second Coordination Sphere on Vanadium Speciation in Molten Chlorides
Authors: Polovov Ilya B. , Chernyshov Mikhail V. , Volkovich Vladimir A. , Vasin Boris D. , Griffiths Trevor R. 
Published: February 2020

Molten salts have a wide range of unique properties that make them particularly useful in many areas of modern technology. From the fundamental point of view, analysis of speciation of vanadium in chloride melts leads to deeper understanding the nature of complex ion formation in ionic media.
In the present work formation of vanadium complex ions in 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl mixtures at 450-750 oC was studied employing high temperature electronic absorption spectroscopy. The electronic absorption spectra (EAS) were recorded in the course of dissolution of vanadium(II) and vanadium(III) chlorides, anodic dissolution of metallic vanadium, and chlorination of various vanadium oxides (V2O3, V2O4, V2O5).
Dissolution of VCl3 in fused alkali chlorides leads to the formation of V(III) species. Average oxidation state of vanadium in the quenched melt samples was 2.9-3.1. Spectral curves were resolved into two individual bands attributed to the 3T1g→3T1g(P) and 3T1g→3T2g electronic transitions in the octahedral VCl6
3-complex. The positions of the band maxima shift towards longer wavelengths with increasing average radius of alkali metal cation, in agreement with the expected decrease of the polarizing effect of the second coordination sphere.
Vanadium dichloride added to the alkali chloride melts partly disproportionated forming V(III) species and vanadium metal. The resulting spectrum represented a superposition of the absorption bands of V2+ and V3+ions. The average oxidation state of vanadium in the quenched melts was higher than two and the concentration of vanadium was noticeably lower than expected.
The best method for preparing vanadium(II) containing melts is the anodic dissolution of the metal. Position of the maximum in the EAS of V(II) in 3LiCl-2KCl at 450 oC (18770 cm-1) is very close to that of V(III), being shifted to higher energies only by about 900 cm-1 (Figure). This band corresponds to 4A2g→4T1g (P) electronic transition in the octahedral VCl6
4- complex (d3-configuration). Second band, with the maximum at 11890 cm-1 is attributed to 4A2g→4T1g electron transition in the same ion. Increasing temperature and the mean radius of the alkali metal cation on the solvent melt resulted in certain changes of EAS of vanadium(II)-containing melts, i.e., the intensity of the second peak decreased and a new, third, band appeared around 13300 cm-1 (Figure). The position of this third peak did not change with temperature and was the same in different melts. This band is attributed to 4T1→4T1 (P) electronic transition in the tetrahedral VCl4
2- ion. Therefore, increasing temperature and the mean radius of the salt-solvent cation shifts the VCl6
4-–VCl4
2-equilibrium towards the tetrahedral species.
The results of in situ spectroscopy measurements conducted during the reactions of various vanadium oxides with hydrogen chloride in alkali chloride melts showed that the nature of the products formed does not depend on the type oxide and the flow rate of HCl. Oxidation state of vanadium in all the resulting melts was close to four. The EAS corresponded to the absorption of vanadyl-ions. Oxygen-containing vanadium(IV) species exhibited somewhat anomalous behavior in NaCl-2CsCl melt as the temperature was varied. Increasing temperature resulted in decreasing intensity of the band at 13000-14000 сm-1. This is explained by the coordination environment of vanadyl species in NaCl-2CsCl melts, different from the melts containing smaller cations. This conclusion was confirmed by XRD and Raman spectroscopy analysis of the quenched melts.
Figure. Resolution into Gaussian bands of EAS recorded after anodic dissolution of vanadium metal in 3LiCl-2KCl (a) and NaCl-2CsCl (b) eutectic melts at 450 and 750 oC, respectively.

Figure 1