Nonicosahedral Equilibrium Overlayers of Icosahedral Quasicrystals

Abstract

We demonstrate that icosahedral Al-Pd-Mn quasicrystals can have nonicosahedrally ordered thermodynamic equilibrium overlayers. The formation of orthorhombic or decagonal equilibrium surface structures is determined by the phase equilibrium of the ternary alloy at given composition and temperature as well as by the surface acting as nucleation site. Nonequilibrium steady-state orthorhombic and hexagonal structures can also be derived with the same methodology when taking preferential evaporation into account. The results describe consistently all presently observed surface structures. DOI: 10.1103/PhysRevLett.95.256105 PACS numbers: 68.35.ÿp, 61.44.Br, 64.75.+g One of the scientifically most intriguing questions in the field of nonperiodic solids is the thermodynamic stability of their surfaces. The stability of the surfaces is closely connected to the formation mechanism and a major factor in the growth of nonperiodic solids. Therefore, surfaces of quasicrystals, i.e., solids with a well defined but nonperiodic crystallography, were intensively investigated in the last decade. However, against all expectations, not only one surface structure but rather a large number of different surface structures were observed, e.g., on icosahedral AlPd-Mn quasicrystals [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. These different surface structures differ even in their basic symmetry, which ranges from cubic via orthorhombic and decagonal to icosahedral. To date it is unclear if these surface structures are equilibrium surfaces or not. In fact, for all observed nonicosahedral structures it is typically assumed that they were obtained kinetically due to preferential sputtering, preferential desorption, and/or segregation during the cleaning procedures [5,7,9,12]. In contrast, the icosahedrally ordered surface structure is implicitly considered to be the equilibrium surface of an icosahedral quasicrystal, although no experimental proof has ever been published. So far only the facets of grown-in voids were identified as equilibrium surfaces, but except their composition, nothing is known about the atomic structure [11,15]. As a consequence, at present the structure of the equilibrium surface(s) of icosahedral quasicrystals is not known.In this Letter we determine the thermodynamically stable structures at surfaces of icosahedral Al-Pd-Mn quasicrystals, using extended annealing experiments and a precision determination of the relevant parts of the ternary Al-Pd-Mn phase diagram. On this basis we distinguish between equilibrium and kinetically reached steady-state surface structures and extract their formation mechanisms. We show that in all cases the driving force for the formation of crystallographically different overlayers is the motion of the system toward its phase equilibrium, modulated by preferential evaporation, if present. These results allow us to demonstrate that both nonicosahedral (orthorhombic and decagonal) and icosahedral structures can exist in thermodynamic equilibrium at the surfaces, depending on the i...

A study of the high-Al part of the Al–Pd–Re phase diagram

“…At 1000 • C the compositional range of the I-phase is between about Al 71.9 Pd 17.7 Re 10.4 and Al 68.5 Pd 22.8 Re 8.7 , and at 790 • C between about Al 71.6 Pd 18.8 Re 9.6 and Al 68.6 Pd 23.1 Re 8.3 . As a consequence, the precipitation of secondary phases may occur in the icosahedral matrix upon cooling similarly to that observed in Al-Pd-Mn (see [14] and references therein).…”

A Distinctive Feature of the Surface Structure of Quasicrystals: Intrinsic and Extrinsic Heterogeneity

“…(While some conditions of surface preparation can produce periodic overlayers, the parameters that produce periodic vs. quasiperiodic surfaces are now fairly well‐defined and well‐understood 19. 21–26)…”