Ordering by cation replacement in the system Na2−xLixGa7

Abstract

Samples of the pseudo-binary system Na_2−xLi_xGa₇ (x ≤ 1) were synthesized from the elements at 300 °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) °C (x = 0) to 489(2) °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) 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 Ga₁₂ 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[(Ga₁₂)²⁻][Ga^–]₂, considering closo Wade clusters [(12b)Ga₁₂]²⁻ and Zintl anions [(4b)Ga]⁻. Susceptibility measurements indicate temperature-independent diamagnetic behavior.