Cadmium-Auride AIICdxAu2– x (AII = Ca, Sr) Synthese, Kristallstruktur, chemische Bindung/ Cadmium-Aurides AIICdxAu2−x (AII = Ca, Sr) – Synthesis, Crystal Structure, Chemical Bonding

Abstract

Following the observation that the binary dicadmides and diaurides of calcium and strontium (A) both form the KHg2 structure type, the two sections AIICdxAu2−x have been studied systematically by means of synthetic, X-ray structural and theoretical investigations. The binary border compound CaCd2 is dimorphic forming the KHg2 structure at elevated temperatures (orthorhombic, space group Imma, a = 488.63(9), b = 754.1(2), c = 851.3(2) pm, Z = 4, R1 = 0.0498) and the MgZn2-type Laves phase at ambient conditions (hexagonal, space group P63/mmc, a = 599.71(9), c = 962.7(2) pm, Z = 4, R1 = 0.0309). Starting from the known binary calcium auride CaAu2 only a very small amount of Au can be replaced by Cd. Around the 1 : 1 ratio of Au and Cd the TiNiSi structure type (orthorhombic, space group Pnma), an ordered variant of the KHg2 type, has a small homogeneity range (CaCdxAu2−x with x = 1/0.76(2): a = 735.0(1)/731.7(1), b = 433.66(6)/431.43(7), c = 873.7(2)/869.9(2) pm, Z = 4, R1 = 0.0482/0.0539). The analogous structure type is also observed in the Sr compounds with the difference that in this case a continuous transition from the KHg2 type of SrAu2 (i. e. x = 0) towards the distorted TiNiSi structure type (up to x = 0.86) is observed in the series SrCdxAu2−x (for x = 0.86(1)/0.45(1): a = 764.0(1)/758.4(1), b = 458.07(7)/474.6(1), c = 872.16(12)/829.2(2) pm, Z = 4, R1 = 0.0446/0.0410). Attempts to prepare the Ca compounds of intermediate composition around a Cd content of x ≈ 0.5 resulted in the formation of the Aurich phase Ca5Cd2Au10 crystallizing with the Zr7Ni10 structure type (orthorhombic, space group Cmca, a = 1390.6(4), b = 1015.7(3), c = 1025.6(2) pm, Z = 4, R1 = 0.0657). In this compound, Cd and Ca occupy common crystallographic sites, which are occupied by In in the isotypic ternary compound Ca4In3Au10. Similarly, at the Cd-rich parts of the sections AIICdxAu2−x no simple phase width of the KHg2 structure type exists. In the case of the calcium series the new compound Ca11Cd18Au4, which shows only a very small phase width, is formed instead. This compound crystallizes with a new structure type (Ca11Cd18+xAu4−x with x = 0.6/0: tetragonal, space group I41/amd, a = 1030.83(6)/1029.39(6), c = 3062.5(3)/3051.0(3) pm, Z = 4, R1 = 0.0475/0.0379) exhibiting a complicated Cd/Au polyanion with four-, five- and six-bonded Cd/Au atoms. The results of FPLAPWband structure calculations are used to explain the electronic stability of the compounds. The calculated Bader charges of cadmium and gold atoms (and In and Au atoms for comparison) are used to discuss the transition between Cd-rich cadmides (like CaCd2 and Ca11Cd18Au4), auridocadmates (like CaCdAu) and the Cd-poor cadmium aurides (like Ca5Cd2Au10).