REVEALING AND DETERMINING THE FORMS OF COMPONENTS IN THE PRODUCT OF INTERACTION OF EUROPIUM (III) FLUORIDE WITH MELT OF NaCl-KCl

Abstract

As part of the study of the solubility of lanthanide fluorides in salt melts, a study of the EuF3-NaCl-KCl system (upper and bottom parts) was carried out.  This system is of particular interest due to the fact  that Europium has two oxidation states (+2 and +3),  and chloride ions are weak reducing agents. The studies were carried out by chemical and nondestructive spectroscopic methods. As for the  latter, solid-phase luminescence (SPL), diffusereflectance spectroscopy (DRS), and X‑ray diffraction phase analysis (XRD) were used. The total content of lanthanides in the upper and bottom parts of the  samples was determined spectrophotometrically and complexonometrically, respectively, and the content of Eu2+ was determined spectrophotometrically by the weakening of the KMnO4 color using redox reactions between Eu2+–V(V) and V(IV) (the content is equivalent to Eu2+) – KMnO4. The system (upper part) LaF3-NaCl-KCl was also studied (for Lanthanum the only oxidation state is +3) for the comparison. It was shown that during the dissolution of Europium fluoride in the NaCl-KCl melt, a partial reduction of  Eu3+ to Eu2+ occurs. The data of SFL, DRS and chemical analysis showed that Eu2+ is the dominant form of europium in the upper part of the sample, and the content of EuF2 found by the chemical method (2.54% wt.) is close to the sum of the contents of EuF3 and EuF2 found by  quantitative XRD (2,5% wt.). At the same time, the data of chemical and X‑ray diffraction phase analysis agree satisfactorily for the LaF3-NaCl-KCl sample. It has been suggested that the EuF3 phase detected by XRD could appear because of the oxidation of  europium during the storage of the sample, and, possibly, due to the effect of ionizing radiation on the system at measuring. It has been established by spectroscopic methods that both valence forms of Europium are present in the bottom part of the sample, and the chemical analysis data (namely, the found content of fluoride ions) indicate the presence of other (except of  Europium fluorides) fluorine-containing phases in this part. To identify the anionic forms of Eu3+, we used the dependence of the position of the diffuse reflection bands in the DR spectra change of variousinorganic compounds of trivalent europium form the reduced electronic polarizability of the ligand anions. According to the position of the reflection bands in the spectrum of the studied sample, it was found that the dominant form of Eu(III) in the bottom part is EuOCl∙25EuF3. Thus, using a combination of various physical and chemical methods, the presence of different valence and anionic forms of Europium in the EuF3-NaCl-KCl sample was shown and the dominant forms of Eu were established in the upper and bottom parts of the studied system.