Search Results (4)

Reported are the synthesis and structural characterization of a series of quaternary lithium-alkaline earth metal alumo-silicides and alumo-germanides with the base formula A₂LiAlTt₂ (A = Ca, Sr, Ba; Tt = Si, Ge). To synthesize each compound, a mixture of the elements with the desired stoichiometric ratio was loaded into a niobium tube, arc welded shut, enclosed in a silica tube under vacuum, and heated in a tube furnace. Each sample was analyzed by powder and single-crystal X-ray diffraction, and the crystal structure of each compound was confirmed and refined from single-crystal X-ray diffraction data. The structures, despite the identical chemical formulae, are different, largely dependent on the nature of the alkaline earth metal. The differing cation determines the structure type—the calcium compounds are part of the TiNiSi family with the Pnma space group, the strontium compounds are isostructural with Na₂LiAlP₂ with the Cmce space group, and the barium compounds crystallize with the PbFCl structure type in the P4/nmm space group. The anion (silicon or germanium) only impacts the size of the unit cell, with the silicides having smaller unit cell volumes than the germanides. Full article

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 O^2– 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 × [As^3–] + 2 × [As^2–] + O^2–, 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. Full article

Investigation of the quaternary system, Ca–Eu–Cd–Sb, led to a discovery of the new solid solutions, Ca_1−xEu_xCd₂Sb₂, with the CaAl₂Si₂ structure type (x ≈ 0.3–0.9, hP5, P $\overline{3}$ m1, a = 4.6632(5)–4.6934(3) Å, c = 7.630(1)–7.7062(7) Å), Ca_2−xEu_xCdSb₂ with the Yb₂CdSb₂ type (x ≈ 0.6, oS20, Cmc2₁, a = 4.646(2) Å, b = 17.733(7) Å, c = 7.283(3) Å), and Eu_11−xCa_xCd₆Sb₁₂ with the Sr₁₁Cd₆Sb₁₂ type (x ≈ 1, mS58, C2/m, a = 32.407(4) Å, b = 4.7248(5) Å, c = 12.377(1) Å, β = 109.96(1)°). Systematic crystallographic studies of the Ca_1−xEu_xCd₂Sb₂ series indicated expansion of the unit cell upon an increase in the Eu content, in accordance with a larger ionic radius of Eu²⁺ vs. Ca²⁺. The Ca_2−xEu_xCdSb₂ composition with x ≈ 0.6 adopts the non-centrosymmetric space group, Cmc2₁, although the parent ternary phase, Ca₂CdSb₂, crystallizes in the centrosymmetric space group, Pnma. Two non-equivalent Ca sites in the layered crystal structure of Ca_2−xEu_xCdSb₂ get unevenly occupied by Eu, with a preference for the interlayer position, which offers a larger available volume. Similar size-driven preferred occupation is observed in the Eu_11−xCa_xCd₆Sb₁₂ solid solution with x ≈ 1. Full article

Synthesis and single-crystal X-ray structure determination of the new antimonide oxide, Ba₅Cd₂Sb₄O₂ are reported. Ba₅Cd₂Sb₄O₂ crystallizes in the monoclinic space group C2/m (No. 12) with unit cell parameters: a = 17.247(7) Å, b = 4.9279(18) Å, c = 12.240(5) Å, and β = 132.558(4)°; Z = 2. Its crystal structure can be described as a polyanionic [Cd₂Sb₄]^6– sub-lattice made up of fused CdSb₄ tetrahedra, stacked between puckered slabs of oxo-anions, O^2–, and Ba²⁺ cations. This structure can also be described as a “double-salt”, i.e., a structure composed of fragments from the Zintl phase Ba₃Cd₂Sb₄ intercalated by two BaO-like moieties. The topological similarities between the structures of these compounds are discussed. Full article