VOLUME 82, NUMBER 4 PHYSICAL REVIEW LETTERS 25 JANUARY 1999 Decagonal Epilayers on the Icosahedral Quasicrystal Al 70 Pd 20 Mn 10 B. Bolliger, M. Erbudak, and D. D. Vvedensky* Laboratorium f ür Festkörperphysik, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland A. R. Kortan Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974 (Received 19 October 1998) The pentagonal surface of the icosahedral quasicrystal Al 70 Pd 20 Mn 10 becomes decagonal upon sputtering with Ar 1 ions at elevated temperatures. This decagonal surface has a vastly different chemical composition (Al 22 Pd 56 Mn 22 ) than bulk decagonal quasicrystals (Al 70 Pd 10 Mn 20 ) and is coherent with the icosahedral substrate across the entire macroscopic sample. The transformation of this surface back to the original icosahedral structure and composition during annealing is followed in real space and in real time with secondary-electron imaging. The structural changes during this transformation are discussed in light of current models of decagonal and icosahedral quasicrystals. [S0031-9007(98)08311-2] PACS numbers: 61.44.Br, 61.14. – x, 68.35.Bs The Al-Pd phase diagram contains many complex crys- tal structures [1,2]. The Al-rich alloys, in particular, have attracted considerable attention since the discovery [3] of quasiperiodic structures, especially with the introduction of Mn as a ternary constituent. A stable Al-Pd-Mn icosahe- dral quasicrystal with the composition Al 70 Pd 20 Mn 10 was first reported by Tsai and co-workers [4]. Beeli et al. [5] later found Al 70.5 Pd 13 Mn 16.5 with coexisting icosahedral and decagonal phases after prolonged (1073 K for 7 days) annealing, though, as a single phase, the decagonal struc- ture has the composition Al 70 Pd 10 Mn 20 [6]. The coex- istence of these phases and, in particular, the alignment of the tenfold and pentagonal axes were demonstrated in scanning electron micrographs of the decagonal phase pre- cipitated from a supersaturated icosahedral phase in a solid-state reaction [7]. In all of these studies, the size of the decagonal quasicrystalline samples was submillimeter. In contrast to these studies of the bulk phases of Al-Pd- Mn quasicrystals, structural investigations of their surfaces have been limited to the icosahedral phase [8–12]. But the sensitivity of quasicrystalline surfaces to variations in local chemical composition means that their structures (and properties) can be manipulated either by heat treatment, which causes surface segregation, or by sputtering the surface with noble-gas ions, which preferentially removes particular atoms. In fact, there have been several reports [13–15] of transformations from the icosahedral phase of Al 70 Pd 20 Mn 10 to the B2 CsCl phase (AlPd) induced by Ar 1 bombardment at room temperature (RT). The latter is stable in the bulk only at much higher temperatures, so it is the underlying icosahedral quasicrystal which stabilizes this surface phase at RT. This Letter reports the first observation of a decago- nal Al-Pd-Mn quasicrystal at the surface of icosahedral Al 70 Pd 20 Mn 10 . We have obtained this structure by bom- barding the pentagonal surface of the quasicrystal with Ar 1 ions while maintaining the temperature in the range 500– 700 K. This treatment produces a depletion of Al and an enrichment of both Pd and Mn which results in an ap- proximately 2-nm-thick decagonal epilayer with a vastly different composition (Al 22 Pd 56 Mn 22 ) from bulk decago- nal quasicrystals. Moreover, the decagonal structure ex- tends coherently across the entire 8-mm sample, which is much larger in lateral size than the decagonal Al-Pd- Mn quasicrystals reported to date. Annealing the sample restores both the initial composition and the icosahedral structure of the surface. By monitoring the decagonal-to- icosahedral transition during annealing in real space and in real time with secondary-electron imaging (SEI), we are able to compare and contrast several key structural features of these phases. Secondary-electron imaging is a method for obtaining real-space information about the local geometric arrange- ment of atoms near a surface [16]. This method involves the excitation of the surface with a primary beam of high- energy electrons (2 keV for the work reported here). The quasielastically backscattered (secondary) electrons ema- nating from the surface are then recorded on a hemispheri- cal phosphorescent screen. With every atom effectively acting as an incoherent source of electrons, the forward- focusing effect in electron-atom scattering at energies above a few hundred eV [17] allows the formation of the secondary-electron pattern to be interpreted in terms of direct projections along interatomic directions defined by the source and scatterer atoms. The structural information contained in the secondary-electron pattern originates from a volume whose depth is determined by the escape depth of the secondary electrons (1.5–2 nm [18]) and whose cross section is given by the spot size of the primary-electron beam (0.2 mm). Images can be recorded while the sample is rotated, providing projections along different directions and enabling the differentiation of related structures that 0031-9007 99  82(4)  763(4)$15.00 © 1999 The American Physical Society 763