Long-Period Ordered Structures of the Au-Rich Au-Mn Alloys
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LONG-PERIOD ORDERED STRUCTURES OF THE Au-RICH Au-Mn ALLOYS
D. Watanabe and 0. Terasaki Department of Physics, Faculty of Science, Sendai 980, Japan
Tohoku University,
ABSTRACT Changes in the ordered structures of the Au-Mn system, existing at temperatures below 400*C, with composition have been studied in the range of 20-28 at.% Mn by high-resolution electron microscopy. The superstructures, Au 4 Mn, Au 2 2 Mn6 , Au 3 1 Mn9 and 2d-APS(I), exist in the range of 20-23 at.% Mn. The Au 2 2 Mn6 and Au 3 1 Mn9 are the one-dimensional (id) and two-dimensional (2d) antiphase structure (APS), respectively, based on the Au 4 Mn, and the 2d-APS(I) is based on the D0 2 2 structure and consists of parallelogram and lozenge shaped domains. When Mn content increases, the Mn-Mn nearest-neighbour pairs are formed across the antiphase boundaries of the 2d-APS(I), and the structure changes to orthorhombic 2d-Au 3 Mn at about 24 at.%Mn. When Mn content increases further, the Mn-Mn pairs align in the direction and the structure transforms continuously to the monoclinic Au 5 Mn2 . INTRODUCTION The crystal structures of Au-Mn alloys in the composition range of 2028 at.% Mn have been studied by many investigators using X-ray and electron diffraction techniques. The well-known structures Au 4 Mn [11 and Au 5 Mn2 [2] exist in the stable ordered state at both ends of the composition range, and the existence of many different superstructures have been proposed in between, e.g. D0 2 2 structure (3], two-dimensional antiphase structure (2dAPS) Au 3 Mn [4], a series of structures derived from the D0 2 2 type with longperiod stacking order, e.g. 5H, 3R and 6H [3], AullMn 4 [51, etc. The appearance of these structures is known to depend quite sensitively upon composition and heat treatment, and the diffraction patterns often show a variety of incommensurate characters. However, the formation condition of the structures is still not clear and the phase diagram cannot be drawn definitely in this composition range. In the last few years, high-resolution electron microscopy experiments have been carried out extensively and systematically by the present authors as well as by a group of Amelinckx, for the purpose of solving these problems. In these studies, the ordered arrangement of Mn atoms in the superstructures was determined directly from the high-resolution superstructure images in an atomic scale. The principal features of the change in crystal structure of the stable phase existing at relatively low temperatures with composition in the range of 20-28 at.% Mn are described in the present paper.
EXPERIMENTAL Bulk alloys were prepared by melting appropriate amounts of Au and Mn with 99.99 purity in evacuated silica tubes. After heat treatment for homogenization (at 880'C) and ordering, the specimens for electron microscopy were prepared by jet electropolishing [5]. High-resolution Mat. Res. Soc. Symp. Proc. Vol. 21 (1984) QElsevier Science Publishing Co., Inc.
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superstructure images were taken with the 1000 kV electron microscope (JEM1000
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