Molecular dynamics studies of complexing in binary molten salts. II. Molten M3AX6 and MA3X1

Molecular dynamics calculations were performed on two liquid ionic solutions, M3AX6 and MA3X10, containing a total of about 500 ions. Four different sets of radii for the M+, A3+, and X− ions were considered in a simplified Tosi–Fumi potential; the cation–anion distances were kept constant but the cation–anion radius ratios varied. These sets generated configurations in which the coordination of A3+ by X− ranged from octahedral to tetrahedral. Our goal was to investigate typical complexing molten salt solutions such as fluoroyttriates and chloroaluminates. In all the melts studied, A3+ tended to have preferential even coordination numbers, 4 or 6, and to form independent AX3−nn moieties in very dilute solutions. As the concentration of AX3 increased, these moieties became interconnected by close to linear A–X–A bridges to form larger species. In the octahedrally coordinated melts, this led to a three-dimensional network with intermediate range order and to the formation of bridged species containing A3+ equilateral triplets, even in M3AX6. In the tetrahedrally coordinated M3AX6 melts, the value for the equilibrium constant of 2AX−4 ⇄A2X−7 +X− was found to be 4×10−2 which is close to the value obtained previously for MAX4. Consequently, the A–X–A bridges were most stable in the octahedrally coordinated melts. For three of the four sets of ionic radii investigated, a composition dependent distribution of the coordination numbers of A3+ cations by X− was observed.