Ternary bromides and iodides of divalent lanthanides and their alkaline-earth analog of the type AMX(3) and AM(2)X(5)

Metallothermic reduction of the tribromides and -iodides MX(3) (M = Sm, Dy, Tm, Yb) with alkali metals as well as with indium and thallium (A = Cs, Rb, K, In, Tl) results in most cases in ternary compounds with the composition AMX(3) and AM(2)X(5), respectively. Analogous compounds with M = Ba, Sr, Ca were synthesized from the binary components. The AMX(3) compounds crystallize with the following types of structure: the perovskite-type and its distorted variants, the NaNbO3-II- and the GdFeO3-type, the NH4CdCl3- and the stuffed PuBr3-type. These structure types differ by a gain of condensation of the [MX(6)] octahedra (three-dimensional connection via corners within the variants of the perovskite-type, double chains of edge- and face-connected octahedra within the NH4CdCl3-type, and layers of corner- and edge-connected octahedra within the stuffed PuBr3-type of structure). This comes along with a reduction of the coordination number of A(+) from 12 (''ideal'' perovskite) to 8 + 2 (GdFeO3-type), 9 (NH4CdCl3-type), and 8 (stuffed PuBr3-type). Thus, the A/[MX(6)] size ratio determines which AMX(3) type of structure is adopted. If the M(2+) ion is large enough, ternary compounds with the composition AM(2)X(5) occur either in addition to the AMX(3) compounds or exclusively. They crystallize with the TlPb2Cl5 type of structure (C.N.(M(2+)) = 7 and 8). All of the AMX(3) and AM(2)X(5) compounds are summarized in a structure field diagram.