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 Eu Cd₂Sb₂, with the CaAl₂Si₂ structure type ( ≈ 0.3⁻0.9, 5, 3 ¯ 1, = 4.6632(5)⁻4.6934(3) Å, = 7.630(1)⁻7.7062(7) Å), Ca Eu CdSb₂ with the Yb₂CdSb₂ type ( ≈ 0.6, 20, 2₁, = 4.646(2) Å, = 17.733(7) Å, = 7.283(3) Å), and Eu Ca Cd₆Sb with the Sr Cd₆Sb type ( ≈ 1, 58, 2/ , = 32.407(4) Å, = 4.7248(5) Å, = 12.377(1) Å, = 109.96(1)°). Systematic crystallographic studies of the Ca Eu Cd₂Sb₂ series indicated expansion of the unit cell upon an increase in the Eu content, in accordance with a larger ionic radius of Eu vs. Ca . The Ca Eu CdSb₂ composition with ≈ 0.6 adopts the non-centrosymmetric space group, 2₁, although the parent ternary phase, Ca₂CdSb₂, crystallizes in the centrosymmetric space group, . Two non-equivalent Ca sites in the layered crystal structure of Ca Eu CdSb₂ 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 Ca Cd₆Sb solid solution with ≈ 1.