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Electron microscopy of near-icosahedral stable AlCuLi quasicrystals
} O U R N A L OF
Journal of Non-Crystalline Solids 153&154 (1993)68-71 North-Holland
NON-CRYffALLINESOLIDS
Electron microscopy of near-icosahedral stable A1-Cu-Li quasicrystals H . - U . N i s s e n a n d C. B e e l i 1 Laboratorium fiir Festk6rperphysik, ETH H6nggerberg, CH-8093 Ziirich, Switzerland
In order to clarify inconsistencies in previous electron microscopic and X-ray results on the structure of icosahedral AI-Cu-Li quasicrystals, a reinvestigation of this material has been made using selected area electron diffraction techniques, high resolution electron microscopy and image contrast calculations based on the models of Elswijk et al. and De Boissieu et al. A good fit for the first of these models and a somewhat less perfect fit for the latter in the projection normal to a two-fold axis are obtained. The high resolution micrographs show evidence for a contrast modulation with a 10 A period normal to five-fold symmetry axes indicating a domain structure which may be connected with a chemical modulation.
1. Introduction T h e i c o s a h e d r a l A I - C u - L i T 2 p h a s e was disc o v e r e d in 1985 by S a i n f o r t a n d D u b o s t [1,2] a n d d e s c r i b e d as t h e first stable quasicrystal phase. X - r a y p r e c e s s i o n a n d e l e c t r o n d i f f r a c t i o n d a t a in part revealed deviations from icosahedral symmetry [3,4], while o t h e r s i n d i c a t e d p e r f e c t icosahed r a l s y m m e t r y [5]. A n i m p o r t a n t s t e p to solve its a t o m i c s t r u c t u r e was an X - r a y r e d e t e r m i n a t i o n o f t h e a v e r a g e s t r u c t u r e [6] o f t h e r e l a t e d cubic R p h a s e (m156Cu12Li32) , t h e c o m p o s i t i o n o f which is n e a r t h a t o f t h e T 2 p h a s e (m157Cu10.8Li32.2) [2]. By c o m b i n i n g X - r a y d i f f r a c t i o n with n e u t r o n p o w d e r d i f f r a c t i o n data, D e Boissieu et al. [7] obtained a structure description of the T 2 phase, using a s i x - d i m e n s i o n a l c r y s t a l l o g r a p h y a p p r o a c h , with r e s i d u a l factors o f t h e o r d e r o f R = 0.15. Elswijk et al. p r o p o s e d a t h r e e - d i m e n s i o n a l P e n rose tiling for t h e T 2 p h a s e , i n t r o d u c i n g an ' u n physical' A I / C u - L i d i s o r d e r . K n o w l e s [8] was a b l e to index t h e S A D p a t t e r n s o f t h e T 2 p h a s e
Correspondence to: Dr H.-U. Nissen, Laboratorium fiir Festk6rperphysik, ETH H6nggerberg, CH-8093 Ziirich, Switzerland. Tel: +41-1 3772262. Telefax: +41-1 3715989. 1 Present address: NIRIM, l-1 Namiki, Tsukuba, Ibaraki 305, Japan.
on the basis o f a large cell a p p r o x i m a n t crystal structure, which gave e v i d e n c e of systematic deviations f r o m i c o s a h e d r a l s y m m e t r y in the T 2 phase, as d i d a c o m p a r i s o n o f X - r a y a n d S A D d a t a [3-51. In a study using conv'etgent b e a m e l e c t r o n diffraction the T 2 p h a s e was c l a i m e d to be a d e c a g o n a l l y t w i n n e d cubic crystal s t r u c t u r e [9]. T h e s e o b s e r v a t i o n s can b e e x p l a i n e d by c h a n g e s in t h e s t r u c t u r e d u e to h e a t i n g d u r i n g s p e c i m e n p r e p a r a t i o n by ion e t c h i n g which p/'obably causes loss o f lithium [10-12].
2. Electron diffraction study A f r a g m e n t from the i n t e r i o r of an A I - C u - L i alloy has b e e n c r u s h e d in an a g a t e m o r t a r . T h e p o w d e r f r a g m e n t s w e r e t r a n s f e r r e d in an alcohol s u s p e n s i o n o n t o c a r b o n holey foils. U s i n g a 300 k V t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e , m a n y pairs o f s e l e c t e d a r e a diffraction ( S A D ) p a t t e r n s w e r e m a d e of t h e s a m e A 1 - C u - L i quasicrystal specim e n region, using a p e r t u r e sizes c o r r e s p o n d i n g to a d i a m e t e r o f 0.52 a n d 2.5 txm, respectively. In all cases a s y m m e t r y b r e a k was m o r e clearly e v i d e n t in the p a t t e r n s m a d e with the s m a l l e r o f these two a p e r t u r e s , i n d i c a t i n g that the s t r u c t u r e is
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights rese~ed
H.-U. Nissen, C Beeli / Electron microscopy of near-icosahedral stable AI-Cu-Li quasicrystals
Fig. 1. Portion of a SAD pattern normal to a two-fold axis of A1-Cu-Li quasicrystal with symmetry axes indicated by arrows.
locally inhomogeneous, the homogeneous domains having a diameter of approximately 100 to 300 nm. In SAD patterns taken with the beam parallel to a two-fold symmetry axis (fig. 1) groups of sharp additional intensities arranged in triplets a n d / o r hexagons around one strong spot were observed. A second feature peculiar to the T 2 phase s p e c i m e n investigated is shown in fig. 1. Along directions parallel to a five-fold symmetry axis the connection line for groups of adjacent reflections is seen to deviate by approximately 2 ° and parallel to a three-fold axis up to approximately 8 ° from the orientation of the respective symmetry axis.
3. Results from HRTEM images and image simulation Transmission electron micrographs (throughfocus series) of a T 2 specimen were made using a 300 kV Philips CM 30 instrument and compared to dynamical contrast calculations based on the model of Elswijk et al. [6] for planes normal to the two-fold as well as the three-fold and five-fold symmetry axes. The thickness of the specimen can be assumed to be between 20 and 80 A, because the thinnest edge of a powder fragment has been used to obtain the images of the
69
through-focus series. The contrast simulations showed that the contrast is very similar for thicknesses between 20 and 64 A. Only for thicknesses more than 100 ,~ a strongly different contrast has been noted. Figure 2 shows an-example of a comparison of high resolution micrographs with this contrast calculation for the plane normal to the two-fold axis. The figure shows three different defocus values; the one in the center corresponds to the Scherzer defocus value. The assumed specimen thickness for the calculation was 64 .~. A similar calculation was made for the same set of micrographs on the basis of the model of De Boissieu et al. [7]. This calculation is also shown in fig. 3 for the plane normal to the two-fold axis and for the same three defocus values as fig. 2, but for a thickness of 60 ,~. It is obvious that the fit is inferior to the fit for the calculation based on the model of Elswijk et al. This is most evident for the image corresponding to the Scherzer defocus value of 57 nm in figs. 2 and 3, respectively. For details of the image calculations, see ref. [12].
4. Discussion The good fit of the electron micrographs with the calculations based on the model of Elswijk et al. is astonishing in view of the averaged nature of this structure model, and it is also remarkable, that, at least for the direction normal to a two-fold axis of icosahedral symmetry, the model of De Boissieu et al. with its much more detailed assumptions regarding the atomic decoration, does not improve the fit with the observed images. The direct comparison of our simulations based on the model of Elswijk et al. with the excellent image in the plane normal to a five-fold symmetry axis by Hiraga [13,14] shows that this image simulation is in perfect agreement for this projection direction also. The high resolution electron micrographs reveal a contrast modulation in the A l - C u - L i structure with a 10 .A period normal to five-fold symmetry axes. The inhomogeneities in this modulation define a domain texture in this material with domain sizes in the order of 300 to 500 ,~.
70
H.-U. Nissen, C. Beeli / Electron microscopy of near-icosahedral stable AI-Cu-Li quasicrystals
Since the m o d e l of D e Boissieu et al. has Li-atoms only or AI- a n d C u - a t o m s only o n atomic p l a n e s n o r m a l to the five-fold as well as to the three-fold symmetry axes, the structural m o d u l a tion may be r e l a t e d to a chemical m o d u l a t i o n
with d o m a i n s of the same d i a m e t e r [12]. Since single-quasicrystal X-ray diffraction data corres p o n d to an average over approximately 10 9 to 1012 d o m a i n s , only the s h o r t - r a n g e c o n t r i b u t i o n of the chemical m o d u l a t i o n will be recognizable
Fig. 2. Observed high resolution electron rnicrographs of AI-Cu-Li quasicrystal (a)-(c) together with contrast simulations (d)-(f) for an assumed thickness of 64 ,~,. Defocus values 27 nm for (a) and (d), 57 nm for (b) and (e), 87 nm for (c) and (f). Notice that the scale of the micrographs is different from that of the calculations. The width of the simulated images corresponds to 36.3 ,~.
H.- U. Nissen, C. Beeli / Electron microscopy of near-icosahedral stable A l - C u - L i quasicrystals
. . . . .
~
~
71
in t h e X - r a y d a t a a n d r e c o v e r e d in a s t r u c t u r e determination.
5. Conclusions The present investigation suggests that A1C u - L i q u a s i c r y s t a l s h a v e a v e r y c o m p l e x struct u r e w h i c h d o e s n o t c o r r e s p o n d to a n i d e a l q u a sicrystal w i t h i c o s a h e d r a l s y m m e t r y . T h e c o n t r a s t m o d u l a t i o n s o b s e r v e d in h i g h r e s o l u t i o n e l e c t r o n m i c r o g r a p h s n o r m a l t o a t w o - f o l d s y m m e t r y axis probably result from a chemical modulation wave o w i t h a p e r i o d o f a p p r o x i m a t e l y 10 A , t h e o c c u r r e n c e o f w h i c h is a s p e c i a l f e a t u r e a m o n g t h e stable icosahedral quasicrystals.
References [1] P. Sainfort, B. Dubost and A. Dubus, C.R. Akad. Sc. Paris 301 (1985) 689. [2] P. Sainfort and B. Dubost, J. de Phys. 47 (1986) C3-321. [3] N. Yu, R. Portier, K. Yu-Zang and J. Bigot, Phil. Mag. Lett. 57 (1988) 35. [4] M. Cantoni, H.-U. Nissen and C. Beeli, Proc. 3rd Colloq. sur les Quasicristaux, Nancy (1989) p. C1. [5] F. Denoyer, G. Herger, M. Lambert, J.M. Lang and P. Sainfort, J. de Phys. 48 (1987) 1357. [6] H.B. Elswijk, J.T.M. De Hosson, S. van Smaalen and J.L. de Boer, Phys. Rev. B38 (1988) 1681. [7] M. de Boissieu, Ch. Janot, J.-M. Dubois, M. Audier and B. Dubost, J. Phys.: Condens. Matter 3 (1991) 1. [8] K.M. Knowles, in: Quasicrystalline Materials, eds. Ch. Janot and J.-M. Dubois (World Scientific, Singapore, 1988) p. 158. [9] K.S. Vecchio and D.B. Williams, Phil. Mag. B57 (1988) 535. [10l M. Cantoni, Diploma-Thesis, ETH Zuerich, unpublished (1988). [11] K. Sadananda, A.K. Singh and M.A. Imam, Phil. Mag. Lett. 58 (1988) 25. [12] C. Beeli, Ph.D. Thesis, Dissertation ETH Zuerich, No. 9801 (1992). [13] K. Hiraga, Springer Series in Solid State Sciences, Vol. 93 (1990) p. 68. [14] K. Hiraga, J. Electron Microsc. 40 (1991) 81. Fig. 3. Simulated images normal to the two-fold axis, based on the model of De Boissieu et al. for an assumed thickness of 60 ,&. Defocus values: 27 nm for (a), 57 nm for (b) and 87 nm for (c). The image corresponds to a square of 60 ,~.
Journal of Non-Crystalline Solids 153&154 (1993)68-71 North-Holland
NON-CRYffALLINESOLIDS
Electron microscopy of near-icosahedral stable A1-Cu-Li quasicrystals H . - U . N i s s e n a n d C. B e e l i 1 Laboratorium fiir Festk6rperphysik, ETH H6nggerberg, CH-8093 Ziirich, Switzerland
In order to clarify inconsistencies in previous electron microscopic and X-ray results on the structure of icosahedral AI-Cu-Li quasicrystals, a reinvestigation of this material has been made using selected area electron diffraction techniques, high resolution electron microscopy and image contrast calculations based on the models of Elswijk et al. and De Boissieu et al. A good fit for the first of these models and a somewhat less perfect fit for the latter in the projection normal to a two-fold axis are obtained. The high resolution micrographs show evidence for a contrast modulation with a 10 A period normal to five-fold symmetry axes indicating a domain structure which may be connected with a chemical modulation.
1. Introduction T h e i c o s a h e d r a l A I - C u - L i T 2 p h a s e was disc o v e r e d in 1985 by S a i n f o r t a n d D u b o s t [1,2] a n d d e s c r i b e d as t h e first stable quasicrystal phase. X - r a y p r e c e s s i o n a n d e l e c t r o n d i f f r a c t i o n d a t a in part revealed deviations from icosahedral symmetry [3,4], while o t h e r s i n d i c a t e d p e r f e c t icosahed r a l s y m m e t r y [5]. A n i m p o r t a n t s t e p to solve its a t o m i c s t r u c t u r e was an X - r a y r e d e t e r m i n a t i o n o f t h e a v e r a g e s t r u c t u r e [6] o f t h e r e l a t e d cubic R p h a s e (m156Cu12Li32) , t h e c o m p o s i t i o n o f which is n e a r t h a t o f t h e T 2 p h a s e (m157Cu10.8Li32.2) [2]. By c o m b i n i n g X - r a y d i f f r a c t i o n with n e u t r o n p o w d e r d i f f r a c t i o n data, D e Boissieu et al. [7] obtained a structure description of the T 2 phase, using a s i x - d i m e n s i o n a l c r y s t a l l o g r a p h y a p p r o a c h , with r e s i d u a l factors o f t h e o r d e r o f R = 0.15. Elswijk et al. p r o p o s e d a t h r e e - d i m e n s i o n a l P e n rose tiling for t h e T 2 p h a s e , i n t r o d u c i n g an ' u n physical' A I / C u - L i d i s o r d e r . K n o w l e s [8] was a b l e to index t h e S A D p a t t e r n s o f t h e T 2 p h a s e
Correspondence to: Dr H.-U. Nissen, Laboratorium fiir Festk6rperphysik, ETH H6nggerberg, CH-8093 Ziirich, Switzerland. Tel: +41-1 3772262. Telefax: +41-1 3715989. 1 Present address: NIRIM, l-1 Namiki, Tsukuba, Ibaraki 305, Japan.
on the basis o f a large cell a p p r o x i m a n t crystal structure, which gave e v i d e n c e of systematic deviations f r o m i c o s a h e d r a l s y m m e t r y in the T 2 phase, as d i d a c o m p a r i s o n o f X - r a y a n d S A D d a t a [3-51. In a study using conv'etgent b e a m e l e c t r o n diffraction the T 2 p h a s e was c l a i m e d to be a d e c a g o n a l l y t w i n n e d cubic crystal s t r u c t u r e [9]. T h e s e o b s e r v a t i o n s can b e e x p l a i n e d by c h a n g e s in t h e s t r u c t u r e d u e to h e a t i n g d u r i n g s p e c i m e n p r e p a r a t i o n by ion e t c h i n g which p/'obably causes loss o f lithium [10-12].
2. Electron diffraction study A f r a g m e n t from the i n t e r i o r of an A I - C u - L i alloy has b e e n c r u s h e d in an a g a t e m o r t a r . T h e p o w d e r f r a g m e n t s w e r e t r a n s f e r r e d in an alcohol s u s p e n s i o n o n t o c a r b o n holey foils. U s i n g a 300 k V t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e , m a n y pairs o f s e l e c t e d a r e a diffraction ( S A D ) p a t t e r n s w e r e m a d e of t h e s a m e A 1 - C u - L i quasicrystal specim e n region, using a p e r t u r e sizes c o r r e s p o n d i n g to a d i a m e t e r o f 0.52 a n d 2.5 txm, respectively. In all cases a s y m m e t r y b r e a k was m o r e clearly e v i d e n t in the p a t t e r n s m a d e with the s m a l l e r o f these two a p e r t u r e s , i n d i c a t i n g that the s t r u c t u r e is
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights rese~ed
H.-U. Nissen, C Beeli / Electron microscopy of near-icosahedral stable AI-Cu-Li quasicrystals
Fig. 1. Portion of a SAD pattern normal to a two-fold axis of A1-Cu-Li quasicrystal with symmetry axes indicated by arrows.
locally inhomogeneous, the homogeneous domains having a diameter of approximately 100 to 300 nm. In SAD patterns taken with the beam parallel to a two-fold symmetry axis (fig. 1) groups of sharp additional intensities arranged in triplets a n d / o r hexagons around one strong spot were observed. A second feature peculiar to the T 2 phase s p e c i m e n investigated is shown in fig. 1. Along directions parallel to a five-fold symmetry axis the connection line for groups of adjacent reflections is seen to deviate by approximately 2 ° and parallel to a three-fold axis up to approximately 8 ° from the orientation of the respective symmetry axis.
3. Results from HRTEM images and image simulation Transmission electron micrographs (throughfocus series) of a T 2 specimen were made using a 300 kV Philips CM 30 instrument and compared to dynamical contrast calculations based on the model of Elswijk et al. [6] for planes normal to the two-fold as well as the three-fold and five-fold symmetry axes. The thickness of the specimen can be assumed to be between 20 and 80 A, because the thinnest edge of a powder fragment has been used to obtain the images of the
69
through-focus series. The contrast simulations showed that the contrast is very similar for thicknesses between 20 and 64 A. Only for thicknesses more than 100 ,~ a strongly different contrast has been noted. Figure 2 shows an-example of a comparison of high resolution micrographs with this contrast calculation for the plane normal to the two-fold axis. The figure shows three different defocus values; the one in the center corresponds to the Scherzer defocus value. The assumed specimen thickness for the calculation was 64 .~. A similar calculation was made for the same set of micrographs on the basis of the model of De Boissieu et al. [7]. This calculation is also shown in fig. 3 for the plane normal to the two-fold axis and for the same three defocus values as fig. 2, but for a thickness of 60 ,~. It is obvious that the fit is inferior to the fit for the calculation based on the model of Elswijk et al. This is most evident for the image corresponding to the Scherzer defocus value of 57 nm in figs. 2 and 3, respectively. For details of the image calculations, see ref. [12].
4. Discussion The good fit of the electron micrographs with the calculations based on the model of Elswijk et al. is astonishing in view of the averaged nature of this structure model, and it is also remarkable, that, at least for the direction normal to a two-fold axis of icosahedral symmetry, the model of De Boissieu et al. with its much more detailed assumptions regarding the atomic decoration, does not improve the fit with the observed images. The direct comparison of our simulations based on the model of Elswijk et al. with the excellent image in the plane normal to a five-fold symmetry axis by Hiraga [13,14] shows that this image simulation is in perfect agreement for this projection direction also. The high resolution electron micrographs reveal a contrast modulation in the A l - C u - L i structure with a 10 .A period normal to five-fold symmetry axes. The inhomogeneities in this modulation define a domain texture in this material with domain sizes in the order of 300 to 500 ,~.
70
H.-U. Nissen, C. Beeli / Electron microscopy of near-icosahedral stable AI-Cu-Li quasicrystals
Since the m o d e l of D e Boissieu et al. has Li-atoms only or AI- a n d C u - a t o m s only o n atomic p l a n e s n o r m a l to the five-fold as well as to the three-fold symmetry axes, the structural m o d u l a tion may be r e l a t e d to a chemical m o d u l a t i o n
with d o m a i n s of the same d i a m e t e r [12]. Since single-quasicrystal X-ray diffraction data corres p o n d to an average over approximately 10 9 to 1012 d o m a i n s , only the s h o r t - r a n g e c o n t r i b u t i o n of the chemical m o d u l a t i o n will be recognizable
Fig. 2. Observed high resolution electron rnicrographs of AI-Cu-Li quasicrystal (a)-(c) together with contrast simulations (d)-(f) for an assumed thickness of 64 ,~,. Defocus values 27 nm for (a) and (d), 57 nm for (b) and (e), 87 nm for (c) and (f). Notice that the scale of the micrographs is different from that of the calculations. The width of the simulated images corresponds to 36.3 ,~.
H.- U. Nissen, C. Beeli / Electron microscopy of near-icosahedral stable A l - C u - L i quasicrystals
. . . . .
~
~
71
in t h e X - r a y d a t a a n d r e c o v e r e d in a s t r u c t u r e determination.
5. Conclusions The present investigation suggests that A1C u - L i q u a s i c r y s t a l s h a v e a v e r y c o m p l e x struct u r e w h i c h d o e s n o t c o r r e s p o n d to a n i d e a l q u a sicrystal w i t h i c o s a h e d r a l s y m m e t r y . T h e c o n t r a s t m o d u l a t i o n s o b s e r v e d in h i g h r e s o l u t i o n e l e c t r o n m i c r o g r a p h s n o r m a l t o a t w o - f o l d s y m m e t r y axis probably result from a chemical modulation wave o w i t h a p e r i o d o f a p p r o x i m a t e l y 10 A , t h e o c c u r r e n c e o f w h i c h is a s p e c i a l f e a t u r e a m o n g t h e stable icosahedral quasicrystals.
References [1] P. Sainfort, B. Dubost and A. Dubus, C.R. Akad. Sc. Paris 301 (1985) 689. [2] P. Sainfort and B. Dubost, J. de Phys. 47 (1986) C3-321. [3] N. Yu, R. Portier, K. Yu-Zang and J. Bigot, Phil. Mag. Lett. 57 (1988) 35. [4] M. Cantoni, H.-U. Nissen and C. Beeli, Proc. 3rd Colloq. sur les Quasicristaux, Nancy (1989) p. C1. [5] F. Denoyer, G. Herger, M. Lambert, J.M. Lang and P. Sainfort, J. de Phys. 48 (1987) 1357. [6] H.B. Elswijk, J.T.M. De Hosson, S. van Smaalen and J.L. de Boer, Phys. Rev. B38 (1988) 1681. [7] M. de Boissieu, Ch. Janot, J.-M. Dubois, M. Audier and B. Dubost, J. Phys.: Condens. Matter 3 (1991) 1. [8] K.M. Knowles, in: Quasicrystalline Materials, eds. Ch. Janot and J.-M. Dubois (World Scientific, Singapore, 1988) p. 158. [9] K.S. Vecchio and D.B. Williams, Phil. Mag. B57 (1988) 535. [10l M. Cantoni, Diploma-Thesis, ETH Zuerich, unpublished (1988). [11] K. Sadananda, A.K. Singh and M.A. Imam, Phil. Mag. Lett. 58 (1988) 25. [12] C. Beeli, Ph.D. Thesis, Dissertation ETH Zuerich, No. 9801 (1992). [13] K. Hiraga, Springer Series in Solid State Sciences, Vol. 93 (1990) p. 68. [14] K. Hiraga, J. Electron Microsc. 40 (1991) 81. Fig. 3. Simulated images normal to the two-fold axis, based on the model of De Boissieu et al. for an assumed thickness of 60 ,&. Defocus values: 27 nm for (a), 57 nm for (b) and 87 nm for (c). The image corresponds to a square of 60 ,~.