Evidence of a reversible microcrystal-quasicrystal phase transition in decagonal Al-Cu-Co(-Si)

Experimental evidence of a ``first-order'' reversible microcrystal-quasicrystal phase transition in decagonal Al₆₃{Cu}17.5Co17.5{Si}₂$ is presented. High-resolution single-crystal x-ray diffraction studies clearly show that (i) the low-temperature phase is microcrystalline and it is formed of coherent twins of the (5,7) crystalline approximant with cell parameters (a=b{}51.5 {}, c{}4.13 {}, and {γ}=108^∘{}), and (ii) the high-temperature phase is the true quasicrystalline phase. Our results are in favor of a random-tiling description of the quasicrystalline state, i.e., for a phase stabilized by entropy effects.