VOLUME55, NUMBER 15 PHYSICAL REVIEW LETTERS 7 OCTOBER 1985 Amorphous-to-Quasicrystalline Transformation in the Solid State D. A. Lilienfeld, M. Nastasi, H. H. Johnson, D. G. Ast, and J. W. Mayer Department of Materials Science, Cornell University, Ithaca, New York 14853 (Received 25 June 1985) Al 84 Mni6 multilayer films have been amorphized by room-temperature ion-beam irradiation. The amorphous phase was transformed into the quasicrystalline state through two routes: thermal and ion-beam-assisted thermal. The intensity of the quasicrystalline electron diffraction increases continuously with annealing between 270 and 350 °C. Ion irradiation of the amorphous phase pro- duces a more complete set of icosahedral diffraction lines than thermal annealing. PACS numbers: 64.70.Kb, 61.16.Di, 61.50.Cj, 61.80.Jh The quasicrystalline state of matter has received a large amount of study in the last year. The quasicrys- talline state is characterized by long-range icosahedral order. This state was first reported by Shechtman et al. l and has since been the topic of many experi- mental studies. 2 " 7 In all these instances the quasicrys- talline state was prepared by rapid quenching from the liquid state with use of melt spinning 1 "" 7 or a scanning electron beam. 7 In this Letter we report the first observation of the formation of the quasicrystalline state by a solid-state polymorphic transformation from the amorphous phase by either thermal annealing or ion-beam irradia- tion. Although both processing techniques are capable of producing this transformation, only the ion- irradiation-induced transformation generates a nearly complete set of icosahedral powder diffraction lines. These transformations allowed us to probe the ther- modynamic hierarchy of amorphous, quasicrystalline, and crystalline free energies. At 270 °C we confirm the expected thermodynamic hierarchy that the free energy of the amorphous phase is greater than the free energy of the quasicrystalline state, which in turn is greater than the free energy of the equilibrium crystal- line phase. Our results for the amorphous-to- quasicrystalline transformation are consistent with a first- or second-order transformation. We propose a structural model for the amorphous-to-quasicrystalline transformation which can account for the kinetics in both thermal annealing and ion-beam mixing. Multilayer Al-Mn films were deposited on NaCl substrates in a turbomolecular-assisted ion-pumped vacuum system by electron-beam evaporation. Rutherford-backscattering spectrometry determined that the films were 500 A thick and had a composition of Al 84 Mn 16 . The films were floated off of the sub- strates onto transmission-electron-microscopy (TEM) grids and were amorphized by a room-temperature ion-beam irradiation of 8xl0 15 Xe ++ /cm 2 . The amorphous phase was then transformed to the quasi- crystalline state by irradiation (4x 10 15 /cm 2 Xe + + ) at 150 °C. Irradiation experiments performed on coeva- porated amorphous Al 80 Cr 2 o thin films have also pro- duced similar results. 8 For all of the ion irradiations, the ion energy was 600 keV, the beam current was 0.025 juA/cm 2 , and the vacuum in the implant chamber was less than 8xl0~ 7 Torr. Ion ranges for 600-keV Xe are approximately 4 times the Al-Mn film thickness, thus making the incorporation of Xe unlike- ly. Thermal anneals were performed with use of both a turbomolecular-pumped vacuum furnace, with a pressure less than 4x 10" 8 Torr, and in situ annealing in the TEM. Electron diffraction of the multilayer starting ma- terial which was subjected to the room-temperature ir- radiation indicates the presence of an amorphous phase plus crystalline Al. An electron-diffraction pat- tern from the amorphous phase is shown in Fig. 1. Visible is a broad band associated with the amorphous phase and several Al lines. Above this diffraction pat- tern is a microdensitometer scan which shows the structure more clearly. If the amorphous-plus-Al state is further irradiated at 150 °C, a new diffraction pattern is observed (Fig. 2). From a comparison of Figs. 1 and 2, it is clear that the amorphous phase has transformed into a new structure. A comparison of the Al diffraction lines in Figs. 1 and 2 reveals a roughly identical intensity distribution, indicating that the crystalline Al is not involved in the transforma- tion. The reciprocal-lattice spacings (Q) correspond- ing to the diffraction lines in Fig. 2 were determined with use of the Al(lll) reflection as a standard; these results are presented in Table I. The subset labeled Al in Table I is in excellent agreement with the published data for aluminum. 9 However, the remaining Q spac- ings do not correspond to any known equilibrium structures for the Al-Mn system. We therefore inves- tigated the possibility that the remaining set was due to the crystalline state. 1 " 7,10 Several papers 4,11 ' 12 have suggested that the crystal- line diffraction pattern may be indexed through the use of six vectors pointing to the vertices of an icosahedron. With use of the same six vectors as Ban- cel et al. 4 and with their choice of the line at 2.90 A~ I as the (100000) line, it is then possible to index all of the lines seen in the diffraction pattern as icosahedral lines. A comparison between line spacings calculated on the assumption that the (100000) line is the line at © 1985 The American Physical Society 1587