Tetrahedral Framework Structures: Polymorphic Phase Transition with Reorientation of Hexagonal Helical Channels in the Zintl Compound Na2ZnSn5 and Its Relation to Na5Zn2+xSn10–x

Two modifications of the new Zintl compound Na2ZnSn5 were synthesized by direct reactions of the elements. hP-Na2ZnSn5, which is metastable under standard conditions, is obtained by fast cooling of a melt of stoichiometric composition. Slow cooling of such a melt or tempering of hP-Na2ZnSn5 (e.g., at 300 °C) leads to the thermodynamically stable tI-Na2ZnSn5. Both phases show an open framework structure of four-bonded Zn and Sn atoms exhibiting hexagonal helical channels in which the Na atoms are situated with disorder. Whereas the Zn–Sn network of hP-Na2ZnSn5 is analogous to known Tr–Sn networks (Tr = Ga, In), tI-Na2ZnSn5 features a closely related novel framework with a different channel structure. In the structure model for hP-Na2ZnSn5 there is only one, Zn/Sn mixed occupied, site for the framework atoms, whereas Zn and Sn are fully ordered on three sites in the case of tI-Na2ZnSn5. The phase transition from hP-Na2ZnSn5 to tI-Na2ZnSn5 was studied using high-temperature powder and single-crystal X-ray di...