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−x)Eu(x)Cd(2)Sb(2), with the CaAl(2)Si(2) structure type (x ≈ 0.3–0.9, hP5, P [Formula: see text] m1, a = 4.6632(5)–4.6934(3) Å, c = 7.630(1)–7.7062(7) Å), Ca(2−x)Eu(x)CdSb(2) with the Yb(2)CdSb(2) type (x ≈ 0.6, oS20, Cmc2(1), a = 4.646(2) Å, b = 17.733(7) Å, c = 7.283(3) Å), and Eu(11−x)Ca(x)Cd(6)Sb(12) with the Sr(11)Cd(6)Sb(12) 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−x)Eu(x)Cd(2)Sb(2) series indicated expansion of the unit cell upon an increase in the Eu content, in accordance with a larger ionic radius of Eu(2+) vs. Ca(2+). The Ca(2−x)Eu(x)CdSb(2) composition with x ≈ 0.6 adopts the non-centrosymmetric space group, Cmc2(1), although the parent ternary phase, Ca(2)CdSb(2), crystallizes in the centrosymmetric space group, Pnma. Two non-equivalent Ca sites in the layered crystal structure of Ca(2−x)Eu(x)CdSb(2) 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−x)Ca(x)Cd(6)Sb(12) solid solution with x ≈ 1.