Crystal structure and pseudo-Mackay clusters of R-AlPdCo

The structure of the rhombohedral (R-) AlPdCo phase was determined by single-crystal X-ray diffraction: space group R3̄ (No. 148), a = 2.91019(8)nm and c = 1.31854(4) nm, V = 9.6709(5)nm³, atoms/cell = 660, F(000) = 12820, R = 0.0481 for the observed 3354 reflections measured by Mo Kα radiation (λ = 0.071069 nm). R-AlPdCo has two types of pseudo-Mackay clusters (pMCs) around Co(12) at 18f and Al/Pd(14) at 3a. The first shell of Co(12)-pMC consists of nine Al sites. The second shell of the Co(12)-pMC is a combination of (M, Co)- icosahedron (ICO) and Al-icosidodecahedron (IDO), where M is a mixed site of Pd, Co or Al. On the other hand, the first shell of the other pMC around Al/Pd(14) consists of two sub-shells: an M-cube and Al-octahedron. The outer shell of the Al/Pd(14)-pMC consists of an M-ICO and Al-IDO. These pMCs interpenetrate each other by sharing the edges of the second ICO shells and their interstitial space is filled by Al-ICOs around M(1) and M(2). These structural features suggest that the structure of R-AlPdCo is realized by linking the Co(12)- and Al/Pd(14)-pMCs together with two types of smaller Al-ICO around the M(1) and M(2) sites.