Decagonal quasicrystal and related crystalline phases in Mn-Ga alloys with 52 to 63 a/o Ga
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I.
INTRODUCTION
BOTH Mn and Ga have several allotropic forms (Ga under high pressure) and some of them also have complex structures. The Mn-Ga system is known to have 12 binary phases,[1–5] and the equilibrium diagram from 30 to 80 at. pct Ga is shown in Figure 1 (Mn5Ga7 here was misprinted as Mn3Ga7 in the original phase diagram).[4] The structures of the Mn-rich Mn3Ga2 and the Ga-rich Mn2Ga5 are relatively simple:[5] a-Mn3Ga2 (AuCu type), g-Mn3Ga2 (g-brass type), and Mn2Ga5 (tetragonal Mn2Hg5 type, a 5 0.8803 nm and c 5 0.2694 nm). However, the crystal lattices of the four phases occurring in the composition range of 52 to 63 at. pct Ga, designated Mn3Ga5, Mn5Ga7, Mn6Ga7, and MnGa in the Binary Alloy Phase Diagram (Figure 1),[4] are still unknown. These phases are formed by a series of peritectic reactions and their compositions are so close to each other that it is not easy to produce single-phase samples, saying nothing of growing single crystals. Their X-ray powder photographs are very complicated and so far have not been indexed.[1,2,3] Therefore, it is of interest to study these structures by means of selected area electron diffraction in an electron microscope and to ascertain their lattice types and parameters. Whenever a sample with a predominant phase was available, a magnetic measurement was made. It was found that only the tetragonal Mn3Ga5 is ferromagnetic at room temperature. J.S. WU, Graduate Student, and K.H. KUO, Professor, are with the Department of Materials Engineering, Dalian University of Technology, 116024 Dalian, China. J.S. WU, Research Staff Member, and K.H. KUO, Professor, are also associated with the Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, 100080 Beijing, China. Manuscript submitted August 23, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Both the first icosahedral quasicrystal (IQC) and the first decagonal quasicrystal (DQC) were found in very rapidly solidified Al-Mn alloys. The Al-Mn IQC was found in Al alloys with 10 to 14 at. pct Mn,[6] whereas the DQC was found in alloys with 10 to 20 at. pct Mn.[7,8] Since Ga (once called eka-Al) belongs to the same group of elements as Al in the Periodic Table and, moreover, the icosahedral cluster Ga12 occurs abundantly in the structures of binary and ternary Ga intermetallic compounds (for a review, see Reference 9), we might expect to find these quasicrystals in binary Mn-Ga alloys. Indeed, an IQC has been reported recently by Tartas and Knystautas[10] in a multilayered film with an average composition of Ga85Mn15 prepared by vapor deposition followed by ion mixing and studied by means of electron diffraction ring patterns. Inspired by the well-known (Al,Zn)49Mg32 IQC, Spaepen et al.[11] have found a stable IQC in the Ga20Mg37Zn43 alloy with a pentagonal dodecagonal solidification morphology. Based on the similarity of the corresponding electron diffraction patterns (EDPs), a number of Al-Mn intermetallic compounds were suggested to be structurally closely related to the Al-Mn IQC and DQC, such as the hexag
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