Electron diffraction study of octagonal-cubic phase transitions in Mn-Si-Al

Abstract

We report the observation of two kinds of octagonal-cubic transition to either the ␤-Mn-type structure or the Mn 3 Si crystal following annealing Mn-Si-Al octagonal quasicrystal at different heating speeds. A theoretical analysis has been given and the simulated diffraction patterns match the experimental results well. It is confirmed that there exist two kinds of phason strain fields, which transform the octagonal phase into the two cubic crystalline phases, respectively. It is conjectured that the octagonal-cubic transitions feature the formation of local periodic structures.Since the experimental discovery of icosahedral quasicrystals, 1 a number of different quasicrystals have been prepared and studied. Furthermore, many periodic structures closely related to quasicrystals have been discovered. 2 Because they have local atomic structures and chemical composition similar to the related quasicrystals, these kinds of materials are named the ''approximants'' of quasicrystals. In experiments, quasicrystalline phases often coexist with their approximant phases, and reversible transformations between quasicrystals and their approximants have been observed. For instance, it has been reported that the AlCuCoSi decagonal phase transforms to a microcrystalline state of a ͑5, 7͒ approximant at lower temperature, and vice versa. 3 In this case, there is not a smooth shift of diffraction peak positions between those of the approximant and those of the decagonal quasicrystal, but instead, there is a coexistence of the two sets of peaks with changes in relative intensities through the phase transition. On the other hand, in rapidly solidified AlMnSi and AlCrSi alloys intermediate structures during the transformation of a decagonal quasicrystal to tenfold twins of a crystalline structure have been observed. 4 Experimental electron diffraction patterns shows the continuous shift of spots from the quasicrystal to the B-centered orthorhombic phase through the intermediate stages.Approximant structures can be described by the so-called ''strip-rotation'' projection scheme, 5 the projected strip in the higher-dimensional space is rotated away from the original one generating the quasiperiodic tiling so that the rotated strip is rational with respect to the higher-dimensional lattice. In general, the quasicrystal structure can be considered as the limit of sequence of periodic structures with the increasing unit cells of the approximants. Moreover, this kind of distortion of the system is induced by the phason strain field, the phason degrees of freedom associated with translations in the perpendicular space. Therefore the transformation from the quasicrystal to the approximant can be explained to be of phason-type. 6,7 The two-dimensional octagonal quasicrystal is a metastable phase which was found in Cr-Ni-Si, V-Ni-Si, Mn-͑Si, Al͒ alloy systems. [8][9][10] In a previous study, the symmetry of an octagonal quasicrystal was determined to be 8/mmm or 8/m. 11 Transmission electron microscopic ͑TEM͒ observations showed that 45°microt...

Twenty years of structure research on quasicrystals. Part I. Pentagonal, octagonal, decagonal and dodecagonal quasicrystals

“…a one-parameter rotation in the nD description) for this transition was published by Baake, Joseph, Kramer (1991). A theoretical analysis of the transformation of o-Mn--Si--Al to the b-Mn structure and to the Mn 3 Si structure was performed by Xu, Wang, Lee, Fung (2000). The first transition takes place if the metastable octagonal phase is heated rapidly, and the other if it is heated slowly.…”

Structural model for octagonal quasicrystals derived from octagonal symmetry elements arising inβ-Mncrystallization of a simple monatomic liquid

“…Prolonged heating, the most likely path to the growth of such crystals, inevitably leads to decomposition of the quasicrystal, usually into a ␤-Mn-type phase, 7,8,10 but the formation of a AuCu 3 -type phase has also been observed. 11 Because of this metastability, the techniques for investigating these phases experimentally have been limited mainly to high-resolution electron microscopy ͑HREM͒ and electron diffraction.…”