Exploratory work in the quaternary system of Ca-Eu-Cd-Sb: Synthesis, crystal, and electronic structures of New Zintl Solid Solutions

Abstract

Investigation of the quaternary system, Ca-Eu-Cd-Sb, led to a discovery of the new solid solutions, Ca1-xEuxCd2Sb2, with the CaAl2Si2 structure type (x ≈ 0.3-0.9, hP5, P3m1, a = 4.6632(5)-4.6934(3) Å, c = 7.630(1)-7.7062(7) Å), Ca2-xEuxCdSb2 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 Eu11-xCaxCd6Sb12 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 Ca1-xEuxCd2Sb2 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 Ca2-xEuxCdSb2 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 Ca2-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 Eu11-xCaxCd6Sb12 solid solution with x ≈ 1.