Synthesis and structure of the Zintl compounds Na2BaTt4 (Tt = Si, Ge)

The isotypic title compounds ate obtained from reactions of stoichiometric mixtures of the elements in welded Ta containers at 700–800 °C. The structure of Na₂BaSi₄ was determined by single-crystal X-ray diffraction (orthorhombic, Ima2 (No. 46), Z = 8, a = 9.524, b = 17.980(3), c = 8.570(3) Å). The compound crystallizes in a novel structure type with Na⁺ and Ba²⁺ cations and two crystallographically-independent isolated Si₄⁴⁻ anions. The anions are nearly regular tetrahedra with bond distances of 2.385(3) to 2.448(4) Å and bond angles between 59.2 and 61.7°. The overall structure may be described as a distorted face-centered-cubic array of Si₄⁴⁻ anions in which Na⁺ and Ba²⁺ atoms occupy all tetrahedral and octahedral holes in an ordered way. The result is closely related to the topology of f.c.c. Li₃Bi type but with ordering of the two cations. Magnetic measurements showed that Na₂BaSi₄ is diamagnetic, and its family can be formulated in terms of oxidation states as Zintl phases (Na⁺)₂Ba²⁺(Tt₄⁴⁻).