| Summary: | The new quaternary phases Eu<sub>5</sub>Zn<sub>2</sub>As<sub>5</sub>O and Eu<sub>5</sub>Cd<sub>2</sub>As<sub>5</sub>O have been synthesized by metal flux reactions and their structures have been established through single-crystal X-ray diffraction. Both compounds crystallize in the centrosymmetric space group <i>Cmcm</i> (No. 63, <i>Z</i> = 4; Pearson symbol <i>oC</i>52), with unit cell parameters <i>a</i> = 4.3457(11) Å, <i>b</i> = 20.897(5) Å, <i>c</i> = 13.571(3) Å; and <i>a</i> = 4.4597(9) Å, <i>b</i> = 21.112(4) Å, <i>c</i> = 13.848(3) Å, for Eu<sub>5</sub>Zn<sub>2</sub>As<sub>5</sub>O and Eu<sub>5</sub>Cd<sub>2</sub>As<sub>5</sub>O, respectively. The crystal structures include one-dimensional double-strands of corner-shared <i>M</i>As<sub>4</sub> tetrahedra (<i>M</i> = Zn, Cd) and As–As bonds that connect the tetrahedra to form pentagonal channels. Four of the five Eu atoms fill the space between the pentagonal channels and one Eu atom is contained within the channels. An isolated oxide anion O<sup>2–</sup> is located in a tetrahedral hole formed by four Eu cations. Applying the valence rules and the Zintl concept to rationalize the chemical bonding in Eu<sub>5</sub><i>M</i><sub>2</sub>As<sub>5</sub>O (<i>M</i> = Zn, Cd) reveals that the valence electrons can be counted as follows: 5 × [Eu<sup>2+</sup>] + 2 × [<i>M</i><sup>2+</sup>] + 3 × [As<sup>3–</sup>] + 2 × [As<sup>2–</sup>] + O<sup>2–</sup>, which suggests an electron-deficient configuration. The presumed <i>h</i><sup>+</sup> hole is confirmed by electronic band structure calculations, where a fully optimized bonding will be attained if an additional valence electron is added to move the Fermi level up to a narrow band gap (Eu<sub>5</sub>Zn<sub>2</sub>As<sub>5</sub>O) or pseudo-gap (Eu<sub>5</sub>Cd<sub>2</sub>As<sub>5</sub>O). In order to achieve such a formal charge balance, and hence, narrow-gap semiconducting behavior in Eu<sub>5</sub><i>M</i><sub>2</sub>As<sub>5</sub>O (<i>M</i> = Zn, Cd), europium is theorized to be in a mixed-valent Eu<sup>2+</sup>/ Eu<sup>3+</sup> state.
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