F-AlCoPdGe Alloy with Three Types of Pseudo-Mackay Clusters

The structure of an F-AlCoPdGe alloy was determined by single-crystal X-ray diffraction analysis: space group Pa3̅ (No. 205), Pearson symbol cP1128-142.4, a = 24.4338(4) Å; R1 = 0.0526 for the observed 5106 reflections with $F_0$ > 4.0σ$(F_0)$. The F-AlCoPdGe alloy exhibited three types of pseudo-Mackay clusters (pMCs). The first shell of the Co(13)-pMC at 24d indicated a coordination polyhedron typically found in Al-Co binary approximants; the second shell was a combination of an M icosahedron and an Al icosidodecahedron, where M is a mixed site of Pd and/or Co. The first shell of the Co(14)-pMC at 4b consisted of 20 partially occupied Al positions arranged in a dodecahedral fashion; its second shell was also composed of an M icosahedron and an Al icosidodecahedron. The first shell of the Al(18)-pMC at 4a consisted of a combination of an M cube and an Al octahedron, resulting in the rhombic dodecahedral arrangement; its outer shell structure was similar to those of the other pMCs. The structure of the F-AlCoPdGe phase comprised an arrangement of these three types of pMCs together with interstitially placed Al icosahedra around the Pd/Al(1) and Pd/Al(2) sites.