Adaptation of the BaAl₄ Type to Small Cations: The Case of NaGa₄

Single-phase NaGa₄ samples were prepared by annealing stoichiometric element mixtures at 200 °C, 300 °C, and 450 °C in closed tantalum ampoules. No compositional homogeneity range was detected. While single crystals annealed at 200 °C feature a fully ordered crystal structure, a crystal annealed at 300 °C reveals a defect with mutual exchange of Na atoms and Ga₂ dumbbells. The structure motif caused by the violation of translational symmetry resembles the CeMg₂Si₂ type of structure. The NaGa₄ structure constitutes a branch of the BaAl₄ type distinguished by a notably high c/a ratio (space group I4/mmm; a=4.2261(1) Å, c=11.2633(6) Å, c/a=2.67). This distortion enables the adaption of the BaAl₄-type to small cations, when the further shortening of the apical-apical distance d(Ga-Ga) is not possible any more. Therefore, NaGa₄ offers an adaption alternative to the monoclinic distortion observed in CaGa₄ and YbGa₄. Conductivity measurements validate the metallic behavior anticipated by NMR measurements in earlier studies.