Stability diagrams for fourfold coordination of polyvalent metal ions in molten mixtures of halide salts

Abstract

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Elucidating Ionic Correlations Beyond Simple Charge Alternation in Molten MgCl2–KCl Mixtures

“…Whereas those isotopically labeled neutron scattering studies were truly pioneering, the inverse Fourier transformation to generate pair distribution functions was necessarily limited by the maximum q value experimentally accessible. Prior work suggests that truncation may lead to shorter and broader first g ( r ) peaks, which is problematic for the definition of coordination number. , In subsequent studies citing the Biggin and Enderby experiments, the 4.3 ± 0.3 coordination number for chlorides around a magnesium cation was commonly assumed to be 4. ,,,, This supported some early Raman studies implying tetrahedrality in the molten state for this salt. ,,, This was quite exciting at the time, since it implied that the molten state was quite different in terms of coordination number when compared to the crystal where coordination is by six chloride ions.…”

Structure of covalent liquids

“…The structure-sorting approaches have been successfully adapted to classify the ample evidence on the stability of local fourfold or sixfold coordination for polyvalent metal ions in liquid mixtures of their halides with alkali halides [49]. The alkali halide acts primarily as a halogen donor to break the structural connectivity that may be present in the pure polyhalide melt, ultimately allowing the formation of separate local coordination units for the polyvalent metal ions.…”

Studies of the local structures of molten metal halides

“…Akdeniz and Tosi have for instance correlated the entropy and the volume melting in metal halides depending on the valency of the cation 13 and defined a stability diagram for fourfold coordination polyvalent metal ions in mixtures. 14 For most of metal halides, their solid form is a crystal (note that even for glass-forming systems, quenching may alter the liquid structure, so that in situ studies of the liquid remain preferable 15 ). Ergo, it is not possible to quench the liquid and to study its structure at room temperature.…”