Quasicrystal approximants with novel compositions and structures
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Quasicrystal approximants with novel compositions and structures
M. Mihalkoviˇc1 and M. Widom, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 1 also at: Institute of Physics, Slovak Academy of Sciences, 84228 Bratislava, Slovakia ABSTRACT
We identify several new quasicrystal approximants in alloy systems in which quasicrystals have not been previously reported. Some occur in alloys with large size contrast between the constituent elements, either containing small Boron atoms, or large Ca/Eu atoms, leading to quasicrystal structures quite different from currently known systems where the size contrast is smaller. Another group of the approximants are layered Frank–Kasper structures, demonstrating competition between decagonal and dodecagonal ordering within this family of structures. INTRODUCTION
Axial quasicrystals are structures that possess one axis of crystallographically forbidden rotational symmetry. The structure is quasiperiodic in the plane perpendicular to this axis, but can be periodic in the direction parallel to the axis. Since they never exactly repeat, their lattice constant can be thought of as infinite within the quasiperiodic plane. Along the periodic direction the lattice constant is finite and might be fairly small. Quasicrystal approximants are ordinary, though complex, crystals whose local structural motifs naturally extend to quasiperiodic structures with the crystallographically forbidden rotational symmetries [1]. Approximants reproduce within their unit cell a portion of an aperiodic structure, certain special fragments of which can be extended periodically while maintaining reasonable local atomic structures. Generally approximants have large lattice parameters in the directions in which the corresponding quasicrystal is quasiperiodic. Structures that possess at least one large lattice parameter are thus candidates for being a quasicrystal approximant. If, in addition they possess a short lattice parameter orthogonal to their large one(s) this direction is a candidate for being an axis of high rotational symmetry in an axial quasicrystal. Experimentally observed axial quasicrystal symmetries are octagonal (8x), decagonal (10x) and dodecagonal (12x). To-date stable decagonal quasicrystals are known in the compounds AlCoNi, AlCoCu, AlMnPd, AlCrNi, AlNiRu, AlCuRh, AlFeNi, ZnMgDy, AlCrFe, GaFeCuSi, AlCuCr and possibly others. In all these cases, the structures are believed to be layered, with a basic 4 ˚ A unit consisting of two atomic layers that are either stacked periodically or with some modulation leading to periodicities of 4, 8, 12 and 16 ˚ A. Perpendicular to the axis the structures are quasiperiodic. Some observed metastable dodecagonal quasicrystals occur in Ta-Te, Ni-Cr, Bi-Mn, Ni-V. In the process of searching for bulk metallic glass-forming compounds [2, 3] we developed a database of intermetallic structures which now contains about 1000 structures drawn mainly from standard references [4, 5, 6, 7]. The data base contains the Pearson symbol of ea
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