Completing the Ba–As Compositional Space: Synthesis and Characterization of Three New Binary Zintl Arsenides, Ba 3 As 4 , Ba 5 As 4 , and Ba 16 As 11.

Three novel binary barium arsenides, Ba₃As₄, Ba₅As₄, and Ba₁₆As₁₁, were synthesized and their crystal and electronic structures were investigated. Structural data collected via the single-crystal X-ray diffraction method indicate that the anionic substructures of all three novel compounds are composed of structural motifs based on the homoatomic As–As contacts, with [As₂]⁴⁻ dimers found in Ba₅As₄ and Ba₁₆As₁₁, and an [As₄]⁶⁻ tetramer found in Ba₃As₄. Ba₃As₄ and Ba₅As₄ crystallize in the orthorhombic crystal system—with the non-centrosymmetric space group Fdd2 (a = 15.3680(20) Å, b = 18.7550(30) Å, c = 6.2816(10) Å) for the former, and the centrosymmetric space group Cmce (a = 16.8820(30) Å, b = 8.5391(16) Å, and c = 8.6127(16) Å) for the latter—adopting Eu₃As₄ and Eu₅As₄ structure types, respectively. The heavily disordered Ba₁₆As₁₁ structure was solved in the tetragonal crystal system with the space group P 4 ¯ 2 1 m (a = 12.8944(12) Å and c = 11.8141(17) Å). The Zintl concept can be applied to each of these materials as follows: Ba₃As₄ = (Ba²⁺)₃[As₄]⁶⁻, Ba₅As₄ = (Ba²⁺)₅(As³⁻)₂[As₂]⁴⁻, and 2 × Ba₁₆As₁₁ = (Ba²⁺)₃₂(As³⁻) ≈ ₂₀[As₂]⁴⁻ ≈ 1, pointing to the charge-balanced nature of these compounds. Electronic structure calculations indicate narrow bandgap semiconducting behavior, with calculated bandgaps of 0.47 eV for Ba₃As₄, 0.34 eV for Ba₅As₄, and 0.33 eV for Ba₁₆As₁₁. [ABSTRACT FROM AUTHOR]