Synthesis and Structure of NaGaSn₂. A Zintl Phase with a Helical Framework Structure

A new Zintl phase NaGaSn 2 is obtained pure from stoichiometric reactions of the elements in sealed Ta at ∼450 °C. Its orthorhombic framework structure (space group C 222 1, 6.309(3) × 10.986(4) × 6.162(4) Å) contains one-dimensional helical pores ∼6 Å in diameter along c⃗ (6 atoms/turn) that bind relatively well-ordered sodium countercations at 50% occupancy. The structure can also be described in terms of interlinked puckered double layers normal to b⃗ and parallel to the channels. Gallium is disordered over both tetrahedral framework sites, the gallium-richer and more negative site having closer sodium neighbors. The potassium compound cannot be made. Among analogous phases reported by others, NaInSn 2 has a lower symmetry and ordered cations, while NaGaSn 5 cannot be reproduced.