Extended X-ray absorption fine-structure study of the local structure of quasicrystalline Al₆₅Cu₂₀Co₁₅ in the decagonal phase

Abstract

The structure of quasicrystalline Al₆₅Cu₂₀Co₁₅ in the decagonal phase has been investigated using the extended X-ray absorption fine-structure (EXAFS) technique. The EXAFS measurements were made on Co and Cu K edge absorptions. The results show that the near-neighbour structures around both Co and Cu consist of 9±2 Al atoms at distances of 2·43 ± 0·03 and 2·47 ± 0·03 Å respectively. This indicates that the atomic environment around Co is essentially the same as that around Cu, and both of them are very similar to that of Co in crystalline Al₁₃CO₄, implying that the local environment of quasicrystalline Al₆₅Cu₂₀CO₁₅ is indeed closely related to that of crystalline Al₁₃CO₄. The results also indicate that there are Co-Al, Co-(Co, Cu) and Cu-Al coordination shells at about 4 Å which may reflect the structural unit cage in quasicrystalline Al₆₅Cu₂₀Co₁₅ because the periodicity along the tenfold axis is about 4 Å as confirmed by electron diffraction. The decrease in near-neighbour coordination distances of both Co and Cu in quasicrystalline Al₆₅Cu₂₀Co₁₅, compared with that of Co in crystalline Al₁₃Co₄ is consistent with asymmetry or large disorder in the partial radial distribution functions. This asymmetric feature implies that structural models based on a hard-sphere approximation are not appropriate to the structure of quasicrystalline Al₆₅Cu₂₀Co₁₅.