Five new ternary indium-arsenides discovered. Synthesis and structural characterization of the Zintl phases Sr3In2As4, Ba3In2As4, Eu3In2As4, Sr5In2As6 and Eu5In2As6

Five new ternary indium aresenides, AE(3)In(2)As(4) (AE = Sr, Ba, Eu), and AE(5)In(2)As(6) (AE = Sr, Eu), have been synthesized using molten metal In, Pb, and Sn fluxes. Structure elucidation was aided by powder X-ray and single kcrystal X-ray diffraction. This revealed Sr3In2As4 and Eu3In2As4 are orthorhombic and isostructural to Sr3In2P4 (space group Pnnm, Z = 2), while Ba3In2As4 is monoclinic and isostructural to Ca3Al2As4 (space group C2/c, Z = 4). In addition, Sr5In2As6 and Eu5In2As6 crystallize orthorhombically and adopt the Ca5Ga2As6 structure type (space group Pbam, Z = 2). The valence electron count for all structures conform to the Zintl-Klemm concept, and as such can be rationalized as divalent Sr, Ba, Eu cations and polyanionic In-As fragments of different dimensionality. In Sr3In2As4 and Eu3In2As4, one-dimensional chains [In2As4](6-) running along the crystallographic c-axis exist, and they are made up of edge- and corner-shared tetrahedra InAs4. The [In2As4](6-) two-dimensional sheets in the structure of Ba3In2As4 are composed of a pair of edge-shared InAs4 tetrahedra, that are further connected by comer-sharing. In Sr5In2As6 and Eu5In2As6, the InAs4 tetrahedra are only corner-shared to make infinite [MAs(2)Aa(2/2)](6-) chains running along the a-axis, which are dimerized via As-As bonds into [In2As6](10-) ribbons. Electronic structure calculations affirm these assignments and suggest intrinsic, narrow-gap seminconducting behavior.