Quaternary Zintl Antimonide Na5YbMg4Sb5

Reported are the synthesis, and the crystal and electronic structure of Na6-xYbxMg4Sb5 (x ≈ 1), a newly discovered quaternary antimonide with unusual disorder between Na and Yb atoms. The synthesis was achieved through solid-state reactions in sealed containers, and the structural characterization was carried out using single-crystal X-ray diffraction methods. The new compound crystallizes in an orthorhombic crystal system with the space group Pnma (no. 62; Pearson symbol oP60) with unit cell parameters a = 15.6169(13) Å, b = 4.6743(4) Å, and c = 22.4985(18) Å and V = 1642.3(2) Å3, constituting a new structure type. The unique and complex arrangement of three types of cations and a singular type of anions represented by the simplified chemical formula Na5YbMg4Sb5 can be rationalized by the Zintl-Klemm concept as (Na+)5(Yb2+)(Mg2+)4(Sb3-)5. Electronic structure calculations confirm this reasoning and indicate a direct band gap (0.55 eV) semiconducting behavior. From a fundamental point of view, the identification of Na5YbMg4Sb5 underscores the need for more exploratory work in this largely uncharted phase space of alkali/alkaline-earth/rare-earth metal Zintl phases.