(Ca/Sr)AuxCd1−x: Stacking variants of the CrB–FeB series

The structural chemistry of binary 1:1 alkaline earth metallides AIIMAIIM (M=p-blockM=p-block or late transition element) is dominated by planar M zig-zag chains, which are stacked in different orientations (CrB (c) to FeB (h) type) and with variable stacking distances (types I and II). As a case study of the electronic influences, the substitution of Au against Cd in the respective Ca and Sr aurides was examined by means of experimental, crystallographic and computational methods. Starting from CaAu, up to 11% of Au can be substituted by Cd without a change in the CrB structure type (orthorhombic, space group Cmcm  , a=398.2(1)a=398.2(1), b=1122.6(6)b=1122.6(6), c=460.9(2)pm, Z=4Z=4, R1=0.0303R1=0.0303). Starting from SrAu (stacking sequence (hc)2(h2c)2(hc)2(h2c)2), depending on the proportion of the Cd substitution a successive change to structures with increased hexagonality is observed: In SrAu0.93Cd0.07SrAu0.93Cd0.07 (monoclinic, space group P21/mP21/m, a=621.3(4)a=621.3(4), b=472.4(2)b=472.4(2), c=1216.1(9)pm, β=96.97(5)∘β=96.97(5)∘, Z=6Z=6, R1=0.0467R1=0.0467) the stacking sequence is h2c, i.e. the hexagonality is 66.67%. A slightly more increased Cd content in SrAu0.78Cd0.22SrAu0.78Cd0.22 (orthorhombic, space group PnmaPnma, a=3243.3(8)a=3243.3(8), b=474.17(8)b=474.17(8), c=626.20(9)pm, Z=16Z=16, R1=0.0682R1=0.0682) drives the hexagonality to 75%, with a (h3c)2(h3c)2 stacking sequence known from several rare earth nickel compounds. Further Cd substitution is not possible. However, in the Cd-rich section of the two series, where the CsCl/β-brassCsCl/β-brass structure type occurs for both alkaline earth elements, a small Au substitution, as determined from powder data by Rietveld refinements, is possible. The substitution limit and the stability ranges of the CsCl and the CrB type can be rationalized from the calculated band structures. Geometrical and electronic criteria are used to compare and discuss the stability ranges in a structural map.

SrAu0.93Cd0.97SrAu0.93Cd0.97, one of the stacking sequences of the CrB/FeB structure type series found in the quasibinary section SrAu–SrCd.Figure optionsDownload as PowerPoint slide