Synthesis, Structure, and Properties of the Complex Zintl Phase Eu9Zn4.5As9: A Candidate Topological Insulator and Thermoelectric Material

Reported are the synthesis and the detailed analysis of the crystal and electronic structure of the novel Zintl phase Eu9Zn4.5As9. This material was identified in the densely populated Eu–Zn–As compositional space. For structure determination and for property measurements, suitable single crystals of this compound were grown from either Sn- or Pb-flux. Single-crystal X-ray diffraction methods indicate that Eu9Zn4.5As9 crystallizes in the orthorhombic crystal system with the space group Pnma (a = 12.1953(7) Å, b = 4.3730(2) Å, c = 42.674(2) Å) and is formally isostructural to Ca9Mn4+xSb9, the less common “9–4–9” type. The structure is heavily disordered, with multiple partially occupied sites, yet, according to the Zintl-Klemm formalism, a charge-balanced composition (Eu2+)9(Zn2+)4.5(As3−)9 is attained. Electronic structure calculations for a model, disorder-free structure indicate no energy gap between the valence and the conduction bands, and suggest a (semi)metallic behavior. Preliminary susceptibility measurements confirm the expected divalent nature of Eu2+ ([Xe] 4f7 ground state).