Direct imaging of icosahedral clusters in a cubic Cd6Ca approximant crystal

The structure of a cubic Cd 6 Ca crystal, which is constructed by a bcc packing of multiple-shell icosahedral atomic clusters and regarded as a 1/1 approximant to a binary Cd 5.7 Ca icosahedral quasicrystal, has been investigated using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM). Direct imaging of the projected icosahedral clusters reveals no significant ADF-contrast around the cluster center, suggesting that the Cd atoms at the core of the cluster are not located at definite position such as the cubic atomic sites proposed by X-ray analysis, but distributed in a significantly disordered manner.