Synchrotron x-ray powder diffraction studies on an icosahedral Cd-Yb quasicrystal (iCd-Yb), its crystalline approximant of Cd6Yb (cCd-Yb) and the related Cd25Eu4 (cCd-Eu) material were performed under pressures of up to about 40 GPa at room temperature. The icosahedral lattice of the atomic cluster in iCd-Yb and the bcc lattices in cCd-Yb and cCd-Eu were found to be stable under pressure. Although the Tsai-type atomic cluster can possibly be distorted in various ways, low-symmetry distortion modes do not seem to develop even at high pressure. It is considered that the cluster structure of centrosymmetric dense atomic packing prevented low-symmetry distortions and consequently provided the cluster lattice stability. The bulk moduli were also determined to be B0(iCd-Yb) 49.2(3) GPa, B 0(cCd-Yb) 46.1(7) GPa and B0(cCd-Eu) 49.6(3) GPa, which were much lower than the typical reported values for other quasicrystalline alloys.
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