The Alkali-Poor Part of the Pseudoternary Triangle AX/BX2/MX3: Crystal Structures, Properties, and Potentials of (Alkali)/Alkaline-Earth/Rare-Earth Chloride Materials

Abstract

Halides of mono-, di-, and trivalent metals of the general formulas AX, BX2, and MX3 may react with each other, forming a large variety of ternary and even quaternary phases in the solid state. Aside from the alkali-poor quaternary chlorides of the ABMX6 type, which are derivatives of the UCl3 type of structure, the alkali cation free pseudoternary chlorides that form in the systems BCl2/MCl3, especially with B = Ba, are reviewed. Examples of ternary chlorides are Ba2EuCl7, with isolated [EuCl7] monocapped trigonal prisms; Ba2Cl[ScCl6], with isolated octahedra [ScCl6] and lonesome Cl-; Ba8[{(Sm6)Z}Cl32], and Ba9[{(Sm6)Z}Cl34], with Z being presumably an oxide ion residing in the large hole of the [Sm6Cl36] polyhedral clusters that occur in these fluorite type superstructures; and, finally, BaGdCl5, with corrugated layers of edge-connected [GdCl8] square antiprisms. These halides may be cationic conductors (AgSrSmCl6), insulating mixed-valence halides such as NaNd2Cl6, or semiconductors such as Pr2Br5 with...