Molecular Models of the REM-Ion-Based Complexes in Fluoride Salt Melts

Abstract

Fluoride salt melts are used in many metallurgical processes and technologies. Even very small additives of rare earth metals (REMs) are known to significantly change their structure-sensitive properties. Possible mechanisms of this influence and various models of the participation of REM ions in the structuring of fluoride melts are discussed. The possibility of formation of complexes of the composition LnF₆M₁₄ (Ln = La, Ce, Pr, Nd, Sm, Eu; M = Li, Na, K, Cs) by REM ions in fluoride salt melts has been confirmed by molecular mechanics and quantum chemistry methods. A similar composition was postulated in the literature due to the interpretation of spectral data and the hypothesis of the proximity of the structures of fluoride salts in liquid and solid states at high temperatures. The data obtained in calculations with the MM⁺ force field on the geometric structure of molecular models of such complexes in the region of the minima of the potential energy surface are presented. Three types of structures of different geometric structure are identified. The possibility of forming a self-consistent electronic subsystem in optimized-geometry models is shown. The densities of electronic states (DOSs) calculated in the valance and Sparkle approximations are analyzed. The electronic levels of the alkali metal atoms coordinated by REM ions are shown to significantly affect the DOS of the base of the complex, which indicates the prospects of the developed models for the subsequent study of the spectral properties. The proposed approach, which begins with the construction of molecular models for REM-based complexes ends with the calculation of the spectral characteristics associated with the electronic structure and geometry of these complexes, is an alternative to the traditional path from properties to a structure.