Chemical Twinning in Zintl Phases

In the systems Ba/Mg/(Si, Ge), two new ternary Zintl phases with the compositions BaMg(4)Si(3) and BaMg(4)Ge(3) were found and structurally characterized. Both compounds are isotypic and crystallize with a new structure type [BaMg(4)Si(3), P4/mmm, Z = 1, a = 4.6115(9) Å, c = 8.615(2) Å; BaMg(4)Ge(3), P4/mmm, Z = 1, a = 4.6335(8) Å, c = 8.746(2) Å]. The anionic partial structure is built of X(2)(6)(-) dumbbells and isolated X(4)(-) ions (X = Si, Ge), and the structure can be described according to the Zintl-Klemm-Busmann concept with the formulation (Ba(2+))(Mg(2+))(4)[X(2)(6)(-)][X(4)(-)]. The BaMg(4)X(3) structure can also be described as a result of the fusion of the structures of Mg(2)X and BaMg(2)X(2). In this regard, BaMg(4)Si(3) and BaMg(4)Ge(3) are very striking examples of chemical twinning.