Complex molten chloride salt mixtures of uranium, magnesium,
and
sodium are top candidates for promising nuclear energy technologies
to produce electricity based on molten salt reactors. From a local
structural perspective, LaCl3 is similar to UCl3 and hence a good proxy to study these complex salt mixtures. As
fission products, lanthanide salts and their mixtures are also very
important in their own right. This article describes from an experimental
and theory perspective how very different the structural roles of
MgCl2 and NaCl are in mixtures with LaCl3. We
find that, whereas MgCl2 becomes an integral part of multivalent
ionic networks, NaCl separates them. In a recent article (J. Am. Chem. Soc.
2022, 144, 21751–21762) we have called the disruptive behavior of NaCl
“the spacer salt effect”. Because of the heterogeneous
nature of these salt mixtures, there are multiple structural motifs
in the melt, each with its particular free energetics. Our work identifies
and quantifies these; it also elucidates the mechanisms through which
Cl– ions exchange between Mg2+-rich and
La3+-rich environments.