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

Halides of mono-, di-, and trivalent metals of the general formulas AX, BX 2, and MX 3 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 ABMX 6 type, which are derivatives of the UCl 3 type of structure, the alkali cation free pseudoternary chlorides that form in the systems BCl 2 /MCl 3, especially with B = Ba, are reviewed. Examples of ternary chlorides are Ba 2 EuCl 7, with isolated [EuCl 7 ] monocapped trigonal prisms; Ba 2 Cl[ScCl 6 ], with isolated octahedra [ScCl 6 ] and lonesome Cl -; Ba 8 [{(Sm 6 )Z}Cl 32 ], and Ba 9 [{(Sm 6 )Z}Cl 34 ], with Z being presumably an oxide ion residing in the large hole of the [Sm 6 Cl 36 ] polyhedral clusters that occur in these fluorite type superstructures; and, finally, BaGdCl 5, with corrugated layers of edge-connected [GdCl 8 ] square antiprisms. These halides may be cationic conductors (AgSrSmCl 6 ), insulating mixed-valence halides such as NaNd 2 Cl 6, or semiconductors such as Pr 2 Br 5 with interesting magnetic ordering at lower temperatures, or they may serve as hosts for luminescent ions. Ba 2 ErCl 7, for example, is an efficient near-infrared to near-ultraviolet upconversion material.