Ordering by cation replacement in the system Na2-xLixGa7

Samples of the pseudo-binary system Na2-xLixGa7 (x <= 1) were synthesized from the elements at 300 degrees C in sealed Ta ampoules or by the reaction of Na2Ga7 with LiCl. The peritectic formation temperature decreases with increasing Li content from 501(2) degrees C (x = 0) to 489(2) degrees C (x = 1). The boundary compositions Na2Ga7 and Na1Li1Ga7 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 Na1Li1Ga7 (a = 8.562(1) & Aring;, b = 14.822(2) & Aring;, c = 11.454(2) & Aring;; 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 NaLiGa7 a new structure type within the MgB12Si2 structure family. Band structure calculations for the composition NaLiGa7 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.