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Open AccessArticle

On the New Oxyarsenides Eu5Zn2As5O and Eu5Cd2As5O

Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA
The Charter School of Wilmington, Wilmington, DE 19807, USA
Authors to whom correspondence should be addressed.
Crystals 2020, 10(6), 475;
Received: 20 May 2020 / Revised: 28 May 2020 / Accepted: 1 June 2020 / Published: 3 June 2020
(This article belongs to the Special Issue Compounds with Polar Metallic Bonding Volume II)
The new quaternary phases Eu5Zn2As5O and Eu5Cd2As5O 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 Cmcm (No. 63, Z = 4; Pearson symbol oC52), with unit cell parameters a = 4.3457(11) Å, b = 20.897(5) Å, c = 13.571(3) Å; and a = 4.4597(9) Å, b = 21.112(4) Å, c = 13.848(3) Å, for Eu5Zn2As5O and Eu5Cd2As5O, respectively. The crystal structures include one-dimensional double-strands of corner-shared MAs4 tetrahedra (M = 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 O2– 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 Eu5M2As5O (M = Zn, Cd) reveals that the valence electrons can be counted as follows: 5 × [Eu2+] + 2 × [M2+] + 3 × [As3–] + 2 × [As2–] + O2–, which suggests an electron-deficient configuration. The presumed h+ 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 (Eu5Zn2As5O) or pseudo-gap (Eu5Cd2As5O). In order to achieve such a formal charge balance, and hence, narrow-gap semiconducting behavior in Eu5M2As5O (M = Zn, Cd), europium is theorized to be in a mixed-valent Eu2+/ Eu3+ state. View Full-Text
Keywords: oxypnictide; Zintl phases; europium; mixed valence; zinc; arsenic; oxygen oxypnictide; Zintl phases; europium; mixed valence; zinc; arsenic; oxygen
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MDPI and ACS Style

Darone, G.M.; Baranets, S.A.; Bobev, S. On the New Oxyarsenides Eu5Zn2As5O and Eu5Cd2As5O. Crystals 2020, 10, 475.

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