Structural Variability versus Structural Flexibility. A Case Study of Eu9Cd4+xSb9 and Ca9Mn4+xSb9 (x ≈ 1/2)

The focus of this article is on the synthesis and structural characterization of the new ternary antimonides Eu9Cd4+xSb9 and Ca9Mn4+xSb9 (x ≈ 1/2). Although these compounds have analogous chemical makeup and formulas, which may suggest isotypism, they actually belong to two different structure types. Eu9Cd4.45(1)Sb9 is isostructural with the previously reported Eu9Zn4.5Sb9 (Pbam), and its structure has unit cell parameters a = 12.9178(11) Å, b = 23.025(2) Å, and c = 4.7767(4) Å. Ca9Mn4.41(1)Sb9 crystallizes in the orthorhombic space group Pnma with unit cell dimensions a = 12.490(2) Å, b = 4.6292(8) Å, and c = 44.197(8) Å and constitutes a new structure type. The two structures are compared and contrasted, and the structural relationships are discussed. Exploratory work aimed at the arsenic-based analogues of either type led to the identification of Ca9Zn4.46(1)As9, forming with the latter structure [a = 11.855(2) Å, b = 4.2747(8) Å, and c = 41.440(8) Å]. Differential thermal analysis and electrical resistivity measurements, performed on single crystals of Ca9Zn4+xAs9, indicate high thermal stability and semiconducting behavior. Magnetic susceptibility measurements on Eu9Cd4+xSb9 samples confirm the expected Eu2+ ([Xe]­4f7) ground state.