In Situ Experimental Evidence for a Nonmonotonous Structural Evolution with Composition in the Molten LiF−ZrF₄ System

Abstract

We propose in this paper an original approach to study the structure of the molten LiF-ZrF(4) system up to 50 mol % ZrF(4), combining high-temperature nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure (EXAFS) experiments with molecular dynamics (MD) calculations. (91)Zr high-temperature NMR experiments give an average coordination of 7 for the zirconium ion on all domains of composition. MD simulations, in agreement with EXAFS experiments at the K-edge of Zr, provide evidence for the coexistence of three different Zr-based complexes, [ZrF(6)](2-), [ZrF(7)](3-), and [ZrF(8)](4-), in the melt; the evolution of the concentration of these species upon addition of ZrF(4) is quantified. Smooth variations are observed, apart from a given composition at 35 mol % ZrF(4), for which an anomalous point is observed. Concerning the anion coordination, we observe a predominance of free fluorides at low concentrations in ZrF(4), and an increase of the number of bridging fluoride ions between complexes with addition of ZrF(4).