Ordering by cation replacement in the system Na 2- x Li x Ga 7 .

Samples of the pseudo-binary system Na _{2- x} Li _x Ga ₇ ( x ≤ 1) were synthesized from the elements at 300 °C in sealed Ta ampoules or by the reaction of Na ₂ Ga ₇ with LiCl. The peritectic formation temperature decreases with increasing Li content from 501(2) °C ( x = 0) to 489(2) °C ( x = 1). The boundary compositions Na ₂ Ga ₇ and Na ₁ Li ₁ Ga ₇ crystallize with different structure types related by a group-subgroup relation. While the Na-rich compositions ( x ≤ 0.5) represent a substitutional solid solution (space group Pnma ), the Li-rich compositions feature an unconventional replacement mechanism in which Li atoms occupying interstitial positions induce vancancies at the Na positions (space group Cmce ). The crystal structure of Na ₁ Li ₁ Ga ₇ ( a = 8.562(1) Å, b = 14.822(2) Å, c = 11.454(2) Å; Z = 8) was determined from X-ray single-crystal diffraction data, and reveals an anionic framework comprising 12-bonded Ga ₁₂ icosahedra and 4-bonded Ga atoms, with alkali-metal atoms occupying channels and cavities. The arrangement of cations makes NaLiGa ₇ a new structure type within the MgB ₁₂ Si ₂ structure family. Band structure calculations for the composition NaLiGa ₇ predict semiconducting behavior consistent with the balance [Na ⁺ ] ₂ [Li ⁺ ] ₂ [(Ga ₁₂ ) ^2- ][Ga ^- ] ₂ , considering closo Wade clusters [(12b)Ga ₁₂ ] ^2- and Zintl anions [(4b)Ga] ^- . Susceptibility measurements indicate temperature-independent diamagnetic behavior.