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Structure of AlPdMn decagonal quasicrystal studied by high-resolution electron microscopy
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Journal of Non-Crystalline Solids 153&154 (1993) 28-32 North-Holland
L OF
NON-CRYSTALLINESOLIDS
Structure of A1-Pd-Mn decagonal quasicrystal studied by high-resolution electron microscopy Kenji Hiraga Institute for Materials Research, Tohoku University, Katahira, Aoba-Ku, Sendai 980, Japan
Structural features of the Al-Pd-Mn decagonal quasicrystal were studied by high-resolution transmission electron microscopy. The structure of the decagonal quasicrystal may be interpreted as a tiling using three types of tiles, such as a decagon, a star-shaped pentagon and a squashed hexagon. The arrangement of the decagonal atom clusters has long-range correlations, in which they are arrayed on straight lines parallel to the ten-fold directions.
1. Introduction Recently, some stable decagonal quasicrystals have been shown to form in conventionally solidified A l - N i - C o , A I - C u - C o and A I - P d - M n alloys [1-4]. Following the discovery of these quasicrystalline alloys, our group has studied them by high-resolution transmission electron microscopy [5-8]. My intention in this p a p e r is to describe structural features of the A 1 - P d - M n decagonal quasicrystal from high-resolution electron microscopic studies.
2. Basic structure of decagonal quasicrystal The structure of decagonal quasicrystals may be interpreted in terms of the aggregations of atom clusters with decagonal or pentagonal symmetry. In this section, I will describe the basic atom clusters and their local arrangements. Figure 1 shows a high-resolution electron micrograph of the A l - P d - M n decagonal quasicrystal, taken with the incident b e a m parallel to the ten-fold axis. The image was taken from a thin sample less than 10 nm, and so it faithfully reCorrespondence to: Dr K. Hiraga, Institute for Materials Re-
search, Tohoku University, Katahira, Aoba-ku Sendai 980, Japan. Tel: + 81-22 215 2125. Telefax: + 81-22 215 2126.
flects a projected potential. Thus, dark and bright regions correspond to high- and low-potential regions, respectively [5,9]. One notices that the observed image contrast is characterized as an arrangement of small bright ring contrasts. Hereafter, the bright ring contrast is called a ring. In the image, also, one notice a special arrangement of the rings, in which a central ring is surrounded by ten rings. That is the basic atom cluster forming structure of the A 1 - P d - M n decagonal quasicrystal. The strange dark contrasts, which appear at the central rings in the atom clusters, are due to the effect of multiple scatterings. The arrangement of rings in the observed image is schematically drawn in fig. 2. In the drawings, one can clearly see arrangements of the rings and decagonal atom clusters. All the atom clusters are joined with sharing two rings, and so they all have the same orientation, as can be seen in fig. 2(b). Also, it can be clearly seen that gaps leaving in the arrangement of the atom clusters are skillfully filled up by two types of forms of ring arrangements, a star-shaped pentagon and squashed hexagon, as shown in fig. 2(b). Thus, the arrangement of rings forms a tiling using three types of tiles, such as a decagon, star-shaped pentagon and squashed hexagon. It is a filling of space without any overlaps and without gaps. And all the rings are placed at vertices of all tiles and at centers of decagons and star-shaped pentagons,
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
IC Hiraga / Structure of Al-Pd-Mn decagonal crystal studied by HREM
and so they are arrayed on straight lines parallel to the tenfold directions indicated with arrows in fig. 2(a), with bond-orientational order, though there are some displacements of the ring arrays, caused by phason strain. In the observed image of fig. 1, all the rings are surrounded by ten bright dots. Although an atomic arrangement showing the contrast consisting of a bright ring and ten bright dots has not yet been determined, it may be of interest to note that this contrast is similar to that of an icosahe-
29
dral atom cluster, observed in an A l - M n - S i icosahedral quasicrystal [10].
3. Arrangement of atom clusters The atom clusters and their linkage manners, which were mentioned above, are not special features of the decagonal quasicrystals, and they can exist in crystals. The characteristic feature of quasicrystals is the structure with long-range cor-
Fig. 1. High-resolution electron micrograph of a decagonal quasicrystal in the A170Pd13Mni7 alloy annealed at 800°C for 4 days and then quenched in water.
30
K. Hiraga / Structure of AI-Pd-Mn decagonal crystal studied by HREM
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Fig. 2. (a) A distribution of bright rings in the image of fig. 1; (b) an arrangement of atomic clusters enclosed by circles.
relations, which p r o d u c e s sharp diffraction spots. A r r a n g e m e n t s o f a t o m clusters in wide regions can be seen in high-resolution images taken from relatively thick regions, w h e r e contrasts corresponding to the decagonal a t o m clusters are enh a n c e d by the effect of multiple scatterings [5]. Figure 3(a) shows a high-resolution image o f the A I - P d - M n decagonal quasicrystal, taken f r o m a thicker region than the sample of fig. 1, t o g e t h e r with an electron diffraction pattern. F r o m the distribution of bright ring contrasts in the image, an a r r a n g e m e n t of the a t o m clusters was obtained, as shown in fig. 3(c). A tiling of fig. 3(d) was constructed by connecting the a t o m clusters with lines. T h e observed image shows the existence of linear p h a s o n strain; that is, the arrays of a t o m clusters along the p direction m a k e a few steps, whereas those along the q direction show few ones, as shown with white lines in fig. 3(a). Except for the existence of the linear p h a s o n strain, the A 1 - P d - M n decagonal
quasicrystal is considered to be a highly o r d e r e d quasicrystal with long-range correlations, because the a t o m clusters are arrayed on straight lines, as can be seen along the q direction. T h e straight arrays of a t o m clusters along the five-fold directions bring about sharp diffraction patterns with a large n u m b e r of weak spots, as shown in fig. 3(b), t h o u g h positions of the spots are slightly shifted from exact decagonal symmetry positions by the existence of linear p h a s o n strain. A r r a n g e m e n t s of a t o m clusters without linear p h a s o n strain were observed in A I - N i - C o and A 1 - C u - C o decagonal quasicrystals and an atomic a r r a n g e m e n t in the cluster was proposed, in our previous papers [ 5 7]. In this paper, I discussed with an example of the A I - P d - M n decagonal quasicrystal that the structure of the decagonal quasicrystals is formed with the aggregations of atom clusters with decagonal or pentagonal symmetry. This result was confirmed by studies on the A 1 - N i - C o and
Fig. 3. (a) A high-resolution electron micrograph of a decagonal quasicrystal in the AlToPd13Mn17 alloy annealed at 800°C for 4 days and then quenched in water, taken from a relatively thick sample to enhance the bright ring contrasts corresponding to atom clusters. (b) Electron diffraction pattern. (c) A distribution of the bright rings in a part of (a). (d) A tiling constructed with connecting the circles in (c) with lines.
K~ Hiraga / Structure of Al-Pd-Mn decagonal crystal studied by HREM
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K. Hiraga / Structure of Al-Pd-Mn decagonal crystal studied by HREM
A I - C u - C o decagonal quasicrystals [5-7], though atomic arrangements in the atom clusters and linkage manners between the atom clusters in A1-Ni-Co and A1-Cu-Co are different from those of AI-Pd-Mn.
4. Conclusion
From our high-resolution electron microscopic studies of decagonal quasicrystals, it can be concluded that the aggregation of atom clusters with decagonal or pentagonal symmetry, with definite linkages limited by the symmetry of atom clusters, forms the structure of decagonal quasicrystals with long-range correlations.
Referencen [1] A.P. Tsai, A. Inoue and T. Masumoto, Mater. Trans. JIM 30 (1989) 300. [2] A.P. Tsai, A. Inoue and T. Masumoto, Mater. Trans. JIM 30 (1989) 463. [3] L.X. He, Y.K. Wu, X.M. Meng and K.H. Kuo, Philos. Mag. Lett. 61 (1990) 15. [4] C. Beeli, H.-U. Nissen and J. Robadey, Philos. Mag. Lett. 63 (199l) 87. [5] K. Hiraga, F.J. Lincoln and W. Sun, Mater. Trans. JIM 32 (199l) 308. [6] K. Hiraga, W. Sun and F.J. Lincoln, Jpn. J. Appl. Phys. 30 (199l) L302. [7] K. Hiraga, Sci. Rep. RITU A36 (1991) 115. [8] K. Hiraga, W. Sun, F.J. Lincoln, M. Kaneko and Y. Matsuo, Jpn. J. Appl. Phys. 30 (1991) 2028. [9] D. Shindo, K. Hiraga and M. Hirabayashi, Proc. ChinaJapan Seminars Quasicrystals, eds. K.H. Kuo and T. Ninomiya (World Scientific, 1991) p.48. [10] K. Hiraga J. Electron Micros. 40 (1991) 81.
Journal of Non-Crystalline Solids 153&154 (1993) 28-32 North-Holland
L OF
NON-CRYSTALLINESOLIDS
Structure of A1-Pd-Mn decagonal quasicrystal studied by high-resolution electron microscopy Kenji Hiraga Institute for Materials Research, Tohoku University, Katahira, Aoba-Ku, Sendai 980, Japan
Structural features of the Al-Pd-Mn decagonal quasicrystal were studied by high-resolution transmission electron microscopy. The structure of the decagonal quasicrystal may be interpreted as a tiling using three types of tiles, such as a decagon, a star-shaped pentagon and a squashed hexagon. The arrangement of the decagonal atom clusters has long-range correlations, in which they are arrayed on straight lines parallel to the ten-fold directions.
1. Introduction Recently, some stable decagonal quasicrystals have been shown to form in conventionally solidified A l - N i - C o , A I - C u - C o and A I - P d - M n alloys [1-4]. Following the discovery of these quasicrystalline alloys, our group has studied them by high-resolution transmission electron microscopy [5-8]. My intention in this p a p e r is to describe structural features of the A 1 - P d - M n decagonal quasicrystal from high-resolution electron microscopic studies.
2. Basic structure of decagonal quasicrystal The structure of decagonal quasicrystals may be interpreted in terms of the aggregations of atom clusters with decagonal or pentagonal symmetry. In this section, I will describe the basic atom clusters and their local arrangements. Figure 1 shows a high-resolution electron micrograph of the A l - P d - M n decagonal quasicrystal, taken with the incident b e a m parallel to the ten-fold axis. The image was taken from a thin sample less than 10 nm, and so it faithfully reCorrespondence to: Dr K. Hiraga, Institute for Materials Re-
search, Tohoku University, Katahira, Aoba-ku Sendai 980, Japan. Tel: + 81-22 215 2125. Telefax: + 81-22 215 2126.
flects a projected potential. Thus, dark and bright regions correspond to high- and low-potential regions, respectively [5,9]. One notices that the observed image contrast is characterized as an arrangement of small bright ring contrasts. Hereafter, the bright ring contrast is called a ring. In the image, also, one notice a special arrangement of the rings, in which a central ring is surrounded by ten rings. That is the basic atom cluster forming structure of the A 1 - P d - M n decagonal quasicrystal. The strange dark contrasts, which appear at the central rings in the atom clusters, are due to the effect of multiple scatterings. The arrangement of rings in the observed image is schematically drawn in fig. 2. In the drawings, one can clearly see arrangements of the rings and decagonal atom clusters. All the atom clusters are joined with sharing two rings, and so they all have the same orientation, as can be seen in fig. 2(b). Also, it can be clearly seen that gaps leaving in the arrangement of the atom clusters are skillfully filled up by two types of forms of ring arrangements, a star-shaped pentagon and squashed hexagon, as shown in fig. 2(b). Thus, the arrangement of rings forms a tiling using three types of tiles, such as a decagon, star-shaped pentagon and squashed hexagon. It is a filling of space without any overlaps and without gaps. And all the rings are placed at vertices of all tiles and at centers of decagons and star-shaped pentagons,
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
IC Hiraga / Structure of Al-Pd-Mn decagonal crystal studied by HREM
and so they are arrayed on straight lines parallel to the tenfold directions indicated with arrows in fig. 2(a), with bond-orientational order, though there are some displacements of the ring arrays, caused by phason strain. In the observed image of fig. 1, all the rings are surrounded by ten bright dots. Although an atomic arrangement showing the contrast consisting of a bright ring and ten bright dots has not yet been determined, it may be of interest to note that this contrast is similar to that of an icosahe-
29
dral atom cluster, observed in an A l - M n - S i icosahedral quasicrystal [10].
3. Arrangement of atom clusters The atom clusters and their linkage manners, which were mentioned above, are not special features of the decagonal quasicrystals, and they can exist in crystals. The characteristic feature of quasicrystals is the structure with long-range cor-
Fig. 1. High-resolution electron micrograph of a decagonal quasicrystal in the A170Pd13Mni7 alloy annealed at 800°C for 4 days and then quenched in water.
30
K. Hiraga / Structure of AI-Pd-Mn decagonal crystal studied by HREM
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Fig. 2. (a) A distribution of bright rings in the image of fig. 1; (b) an arrangement of atomic clusters enclosed by circles.
relations, which p r o d u c e s sharp diffraction spots. A r r a n g e m e n t s o f a t o m clusters in wide regions can be seen in high-resolution images taken from relatively thick regions, w h e r e contrasts corresponding to the decagonal a t o m clusters are enh a n c e d by the effect of multiple scatterings [5]. Figure 3(a) shows a high-resolution image o f the A I - P d - M n decagonal quasicrystal, taken f r o m a thicker region than the sample of fig. 1, t o g e t h e r with an electron diffraction pattern. F r o m the distribution of bright ring contrasts in the image, an a r r a n g e m e n t of the a t o m clusters was obtained, as shown in fig. 3(c). A tiling of fig. 3(d) was constructed by connecting the a t o m clusters with lines. T h e observed image shows the existence of linear p h a s o n strain; that is, the arrays of a t o m clusters along the p direction m a k e a few steps, whereas those along the q direction show few ones, as shown with white lines in fig. 3(a). Except for the existence of the linear p h a s o n strain, the A 1 - P d - M n decagonal
quasicrystal is considered to be a highly o r d e r e d quasicrystal with long-range correlations, because the a t o m clusters are arrayed on straight lines, as can be seen along the q direction. T h e straight arrays of a t o m clusters along the five-fold directions bring about sharp diffraction patterns with a large n u m b e r of weak spots, as shown in fig. 3(b), t h o u g h positions of the spots are slightly shifted from exact decagonal symmetry positions by the existence of linear p h a s o n strain. A r r a n g e m e n t s of a t o m clusters without linear p h a s o n strain were observed in A I - N i - C o and A 1 - C u - C o decagonal quasicrystals and an atomic a r r a n g e m e n t in the cluster was proposed, in our previous papers [ 5 7]. In this paper, I discussed with an example of the A I - P d - M n decagonal quasicrystal that the structure of the decagonal quasicrystals is formed with the aggregations of atom clusters with decagonal or pentagonal symmetry. This result was confirmed by studies on the A 1 - N i - C o and
Fig. 3. (a) A high-resolution electron micrograph of a decagonal quasicrystal in the AlToPd13Mn17 alloy annealed at 800°C for 4 days and then quenched in water, taken from a relatively thick sample to enhance the bright ring contrasts corresponding to atom clusters. (b) Electron diffraction pattern. (c) A distribution of the bright rings in a part of (a). (d) A tiling constructed with connecting the circles in (c) with lines.
K~ Hiraga / Structure of Al-Pd-Mn decagonal crystal studied by HREM
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K. Hiraga / Structure of Al-Pd-Mn decagonal crystal studied by HREM
A I - C u - C o decagonal quasicrystals [5-7], though atomic arrangements in the atom clusters and linkage manners between the atom clusters in A1-Ni-Co and A1-Cu-Co are different from those of AI-Pd-Mn.
4. Conclusion
From our high-resolution electron microscopic studies of decagonal quasicrystals, it can be concluded that the aggregation of atom clusters with decagonal or pentagonal symmetry, with definite linkages limited by the symmetry of atom clusters, forms the structure of decagonal quasicrystals with long-range correlations.
Referencen [1] A.P. Tsai, A. Inoue and T. Masumoto, Mater. Trans. JIM 30 (1989) 300. [2] A.P. Tsai, A. Inoue and T. Masumoto, Mater. Trans. JIM 30 (1989) 463. [3] L.X. He, Y.K. Wu, X.M. Meng and K.H. Kuo, Philos. Mag. Lett. 61 (1990) 15. [4] C. Beeli, H.-U. Nissen and J. Robadey, Philos. Mag. Lett. 63 (199l) 87. [5] K. Hiraga, F.J. Lincoln and W. Sun, Mater. Trans. JIM 32 (199l) 308. [6] K. Hiraga, W. Sun and F.J. Lincoln, Jpn. J. Appl. Phys. 30 (199l) L302. [7] K. Hiraga, Sci. Rep. RITU A36 (1991) 115. [8] K. Hiraga, W. Sun, F.J. Lincoln, M. Kaneko and Y. Matsuo, Jpn. J. Appl. Phys. 30 (1991) 2028. [9] D. Shindo, K. Hiraga and M. Hirabayashi, Proc. ChinaJapan Seminars Quasicrystals, eds. K.H. Kuo and T. Ninomiya (World Scientific, 1991) p.48. [10] K. Hiraga J. Electron Micros. 40 (1991) 81.