EVIDENCE OF A REVERSIBLE MICROCRYSTAL QUASI-CRYSTAL PHASE-TRANSITION IN DECAGONAL AL-CU-CO(-SI)

Experimental evidence of a "first-order" reversible microcrystal-quasicrystal phase transition in decagonal Al63Cu17.5Co17.5Si2 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=b51.5 , c4.13 , 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. © 1995 The American Physical Society.