Pd 0.213 Cd 0.787 and Pd 0.235 Cd 0.765 Structures: Their Long c Axis and Composite Crystals, Chemical Twinning, and Atomic Site Preferences

We present single‐crystal studies of Pd 0.213 Cd 0.787 and Pd 0.235 Cd 0.765 , synchrotron powder studies of Pd 1− x Cd x , 0.755≥ x ≥0.800, and LDA‐DFT and extended Hückel (eH) calculations on these or related phases. The two single‐crystal structures have a , b , and c axis lengths of 9.9013(7), 14.0033(10), 37.063(24) and 9.9251(3), 14.0212(7), 60.181(3) Å, respectively and they crystallize in the space groups Ccme and F 2 mm , respectively (solved as (3+1)‐dimensional crystals their most convenient superspace group is Xmmm (00γ) s 00). The structures have two different structural components each with their own separate axis parameters. Powder data shows that the ratio of these separate axes ( S / L ) varies from 1.615 to 1.64, values near the golden mean (1.618). For Pd 0.213 Cd 0.787 , different Pd and Cd site occupancies lead to variation in the R factor from 2.6–3.6 %. The site occupancy pattern with the lowest R factor (among the 26 820 variants studied) is the exact site occupancy pattern predicted by LDA‐DFT parameterized eH Mulliken charge populations. The phases can be understood through a chemical twinning principle found in γ‐brass, the parent structure for the above phases (a relation with the MgCu 2 Laves phase is also noted). This twinning principle can be used to account for Cd and Pd site preferences. At the same time there is a clean separation among the Cd and Pd atoms for the two separate chain types at height b =0 and 1/2. These results indicate that Cd:Pd stoichiometry plays a role in phase stability.