Zink-reiche Erdalkalimetall-Verbindungen AZn5 und AZn11: Kristallstrukturen und chemische Bindung / Zinc-rich Alkaline Earth Compounds AZn5 and AZn11: Crystal Structures and Chemical Bonding

Abstract

The zinc-rich compounds in the binary systems with the heavier alkaline earth elements CaZn11 and AZn5 (A = Ca, Sr, Ba) have been synthesized from melts of the elements. Their crystal structures, which were in principle known from very early powder or single crystal film experiments, have been refined on the basis of modern single crystal X-ray data. CaZn5 (hexagonal, space group P6/mmm, a = 538.99(2), c = 424.56(1) pm, Z = 1, R1 = 0.0144) crystallizes with the CaCu5 structure type and exhibits a small but distinct phase width Ca1−xZn5+2x up to a composition of Ca0.87Zn5.26 (a = 533.38(1), c = 430.04(1) pm, R1 = 0.0170) achieved through a gradual substitution of Ca by Zn2 dumbbells. The high-temperature form of SrZn5, which was prepared by quenching of melted samples, also adopts the ideal CaCu5 structure type (a = 554.1(2), c = 428.2(2) pm, R1 = 0.0314). The room temperature modification of SrZn5 (orthorhombic, space group Pnma, a = 1313.3(3), b = 529.91(10), c = 669.72(13) pm, Z = 4, R1 = 0.0349) forms a singular structure, whereas the Ba compound of corresponding composition (orthorhombic, space group Cmcm, a = 1078.3(7), b = 839.8(5), c = 532.0(3) pm, Z = 4, R1 = 0.0281) crystallizes with a third, also rather uncommon structure. For the compound CaZn11 (BaCd11 structure type; tetragonal, space group I41/amd, a = 1068.11(10), c = 682.81(7) pm, Z = 4, R1 = 0.0299) the originally proposed structure type was confirmed and also refined using single crystal data. The chemical bonding in all title compounds is analyzed using FP-LAPWband structure methods. Together with geometrical criteria and observed valence electron numbers of isotypic compounds, the results are used to compare and discuss the stability of the different structures of the intermetallic phases in the systems AZn5 and AM11 (M = Zn, Cd, Hg).