Ion specific effects on the structure of molten AF-ZrF4 systems (A+ = Li+, Na+, and K+)

The structure of AF-ZrF(4) system (A(+) = Li(+), Na(+), K(+)) compounds in the liquid state is studied using an approach combining EXAFS spectroscopy with molecular dynamics simulations. A very good agreement is observed between the two techniques, which allows us to propose a quantitative description of the liquids. From the Zr(4+) solvation shell point of view, we observe a progressive stabilization of the 7-fold and then of the 6-fold coordinated complexes when passing from Li(+) to Na(+) and K(+) as a "counterion". Particular attention is given to the systems consisting of 35 mol % of ZrF(4). At that particular composition, the ZrF(6)(2-) complex predominates largely whatever the nature of the alkali. The calculated vibrational properties of this complex are in excellent agreement with a previous Raman spectroscopy experiment on molten KF-ZrF(4). The most important differences are observed for the lifetime of these octahedral units, which increases importantly with the size of the monovalent cation. On a larger scale, an intense first sharp diffraction peak is observed for the Zr(4+)-Zr(4+) partial structure factor, which can be attributed to the correlations between the octahedral units formed.