Structural Variability versus Structural Flexibility. A Case Study of Eu₉Cd_4+xSb₉ and Ca₉Mn_4+xSb₉ (x ≈ ¹/₂)

The focus of this article is on the synthesis and structural characterization of the new ternary antimonides Eu₉Cd_4+xSb₉ and Ca₉Mn_4+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. Eu₉Cd_4.45(1)Sb₉ is isostructural with the previously reported Eu₉Zn_4.5Sb₉ (Pbam), and its structure has unit cell parameters a = 12.9178(11) Å, b = 23.025(2) Å, and c = 4.7767(4) Å. Ca₉Mn_4.41(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 Ca₉Zn_4.46(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 Ca₉Zn_4+xAs₉, indicate high thermal stability and semiconducting behavior. Magnetic susceptibility measurements on Eu₉Cd_4+xSb₉ samples confirm the expected Eu²⁺ ([Xe]­4f⁷) ground state.