Exploratory Work in the Quaternary System of Ca–Eu–Cd–Sb: Synthesis, Crystal, and Electronic Structures of New Zintl Solid Solutions

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}m$1, 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–xEuxCdSb₂ 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.