On the New Oxyarsenides Eu5Zn2As5O and Eu5Cd2As5O

Abstract

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.

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

“…However, the exact source of oxygen remains unidentified. In prior works, the serendipitous formation of pnictide oxides has been observed, for example in the cases of AE 5 M 2 As 5 O (AE = Ba, Eu; M = Zn, Cd; Pn = As, Sb), 1,34,53,54 process with the rare-earth metal. 57 However, experiments conducted in Nb and BN crucibles under identical conditions also yielded title oxypnictides, dismissing the possibility that alumina crucibles act as the oxygen source.…”

“…However, the exact source of oxygen remains unidentified. In prior works, the serendipitous formation of pnictide oxides has been observed, for example in the cases of AE 5 M 2 As 5 O ( AE = Ba, Eu; M = Zn, Cd; Pn = As, Sb), ,,, U 2 Cu 2 As 3 O, and Ba 5 Cd 2 Sb 4 O 2 , to name a few. Our initial hypothesis was based on the reported studies, where alumina crucibles are the oxygen source via a reduction process with the rare-earth metal .…”

“…Inspired by the abundant structural and physical properties of the heteroanionic materials, we have discovered two novel quaternary oxypnictides, Eu 8 Zn 2 As 6 O and Eu 14 Zn 5 As 12 O. These compounds enrich the quaternary Eu–Zn–As–O compositional diagram (Figure b), which previously contained a single known compound, Eu 5 Zn 2 As 5 O (Ba 5 Cd 2 Sb 5 F structure type) . In this study, we discuss synthetic challenges, offer a comprehensive structural analysis, and elucidate the electronic structure of these newly discovered complex, heavily disordered heteroanionic Zintl compounds.…”

“…These compounds enrich the quaternary Eu−Zn−As−O compositional diagram (Figure 1b), which previously contained a single known compound, Eu 5 Zn 2 As 5 O (Ba 5 Cd 2 Sb 5 F structure type). 34 In this study, we discuss synthetic challenges, offer a comprehensive structural analysis, and elucidate the electronic structure of these newly discovered complex, heavily disordered heteroanionic Zintl compounds. Our ability to synthesize large single crystals of Eu 14 Zn 5 As 12 O allowed for a comprehensive characterization of its physical properties.…”

On the Effects of Aliovalent Substitutions in Thermoelectric Zintl Pnictides. Varied Polyanionic Dimensionality and Complex Structural Transformations─The Case of Sr3ZnP3 vs Sr3AlxZn1–xP3

“…Phases listed in the compositional diagrams were the most common side products, although several novel compounds were also identified. For instance, the recently reported Eu 5 Zn 2 As 5 O phase was received as a minor product in the reaction, targeted to synthesize the title Eu 3 ZnAs 3 . In fact, an inadvertent formation of multinary oxypnictides is often observed during the synthesis of Zintl phases .…”