Advancing Heteroanionicity in Zintl Phases: Crystal Structures, Thermoelectric and Magnetic Properties of Two Quaternary Semiconducting Arsenide Oxides, Eu8Zn2As6O and Eu14Zn5As12O.

Two novel quaternary oxyarsenides, Eu₈Zn₂As₆O and Eu₁₄Zn₅As₁₂O, were synthesized through metal flux reactions, and their crystal structures were established by single-crystal X-ray diffraction methods. Eu₈Zn₂As₆O crystallizes in the orthorhombic space group Pbca, featuring polyanionic ribbons composed of corner-shared triangular [ZnAs₃] units, running along the [100] direction. The structure of Eu₁₄Zn₅As₁₂O crystallizes in the monoclinic space group P2/m and its anionic substructure can be described as an infinite "ribbonlike" chain comprised of [ZnAs₃] trigonal-planar units, although the structural complexity here is greater and also amplified by disorder on multiple crystallographic positions. In both structures, the O^2- anion occupies an octahedral void with six neighboring Eu²⁺ cations. Formal electron counting, electronic structure calculations, and transport properties reveal the charge-balanced semiconducting nature of these heteroanionic Zintl phases. High-temperature thermoelectric transport properties measurements on Eu₁₄Zn₅As₁₂O reveal relatively high resistivity (ρ_500K = 8 Ω·cm) and Seebeck coefficient values (S_500K = 220 μV K^-1), along with a low concentration and mobility of holes as the dominant charge-carriers (n_500K = 8.0 × 10¹⁷ cm^-3, μ_500K = 6.4 cm²/V s). Magnetic studies indicate the presence of divalent Eu²⁺ species in Eu₁₄Zn₅As₁₂O and complex magnetic ordering, with two transitions observed at T₁ = 21.6 K and T₂ = 9 K.