Probing the limits of the Zintl concept: Structure and bonding in rare-earth and alkaline-earth zinc-antimonides Yb₉Zn _4+xSb₉ and Ca₉Zn_4.5Sb₉

A new transition metal Zintl phase, Yb₉Zn_4+xb ₉, was prepared by high-temperature flux syntheses as large single crystals, or by direct fusion of the corresponding elements in polycrystalline form. Its crystal structure was determined by single-crystal X-ray diffraction. Its Ca-counterpart, hitherto known as Ca₉Zn₄Sb ₉, and the presence of nonstoichiometry in it were also studied. Yb₉Zn_4+xb₉ was found to exist in a narrow homogeneity range, as suggested from the crystallographic data at 90(3) K (orthorhombic, space group Pbam (No. 55), Z = 2): (1) a = 21.677(2) Å, b = 12.3223(10) Å, c = 4.5259(4) Å, R1 = 3.09%, wR2 = 7.18% for Yb ₉Zn_4.23(2)Sb₉; (2) a = 21.706(2) Å, b = 12.3381(13) Å, c = 4.5297(5) Å, R1 = 2.98%, wR2 = 5.63% for Yb ₉Zn_4.380(12)Sb₉; and (3) a = 21.700(2) Å, b = 12.3400(9) Å, c = 4.5339(4) Å, R1 = 2.75%, wR2 = 5.65% for Yb₉Zn_4.384(14)Sb₉. The isostructural Ca ₉Zn_4.478(8)Sb₉ has unit cell parameters a = 21.830(2) Å, b = 12.4476(9) Å, and c = 4.5414(3) Å (R1 = 3.33%, wR2 = 5.83%). The structure type in which these compounds crystallize is related to the Ca₉Mn₄Bi₉ type, and can be considered an interstitially stabilized variant. Formal electron count suggests that the Yb or Ca cations are in the +2 oxidation state. This is supported by the virtually temperature-independent magnetization for Yb₉Zn _4.5Sb₉. Electrical resistivity data show that Yb ₉Zn_4.5Sb₉ and Ca₉Zn _4.5Sb₉ are poor metals with room-temperature resistivity of 10.2 and 19.6 mΩ·cm, respectively.