LONG-RANGE ICOSOHEDRAL SYMMETRY IN A METALLIC PHASE OBSERVED BY FIELD ION MICROSCOPY

HAL Id: jpa-00225677

https://hal.archives-ouvertes.fr/jpa-00225677

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

LONG-RANGE ICOSOHEDRAL SYMMETRY IN A METALLIC PHASE OBSERVED BY FIELD ION

MICROSCOPY

A. Melmed, R. Klein

To cite this version:

A. Melmed, R. Klein. LONG-RANGE ICOSOHEDRAL SYMMETRY IN A METALLIC PHASE

OBSERVED BY FIELD ION MICROSCOPY. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-

287-C2-290. �10.1051/jphyscol:1986243�. �jpa-00225677�

LONG-RANGE ICOSOHEDRAL SYMMETRY IN A METALLIC PHASE OBSERVED BY FIELD ION MICROSCOPY

A.J. MELMED and R. KLEIN

National Bureau of Standards, Gaithersburg, MD 20899, U.S.A.

Résumé

L' observation directe, par microscopie a champ ionique d'un alliage d'Aluminium et de Manganèse (Mn:12% Atomique) confirme la précédente mise en évidence, par des techniques de diffraction, d'une conformation icosohèdrale a grande échelle. De plus on remarque une grande quantité de desordre cristallin localise' présentant de nombreux défauts de structures "antiphase boundaries". L' existence d

un maclage systématique, qui aurait pu expliquer la symétrie

d'orientation observée, n'a pas pu être démontrée.

Abstract

Direct observations by Field Ion Microscopy of an Al-12 at.? alloy confirm the earlier determination, by diffraction techniques, of icosohedral long range orientational order. Additionally, a large amount of local disprder, replete with defects or antiphase boundaries, is found. There is no evidence for systematic twinning which might account for the observed orientational symmetry.

Recent observations of rapidly-cooled Al-12 at.? Mn by transmission electron microscopy (TEM), electron diffraction in TEM, and x-ray diffraction indicated a structure characterized by long-range icosohedral symmetry and no translational symmetry (1). Considerable theoretical effort ensued and various models have been proposed to account for the diffraction data, including aperiodic, modulated and other structures (2). Multiple twinning in an otherwise normal crystal structure was proposed immediately to account for the observations and, although thoroughly discounted (1) in the first TEM investigation, was given support by a later TEM study (3). We began an FIM investigation of this material to confirm the validity of these conclusions in the real-space atomic structure, and to learn details of the atomic packing, which are not available from the diffraction data.

The alloy was prepared in the form of long ribbons and supplied to us in ribbon-fragments, a few mm in width, 1-2 cm in length and 3-4x10 mm thick.

Slivers, a few millimeters long, partially or completely broken from the edges of the (very brittle) larger fragments, were found in ample quantity for our purposes, and were easily clamped in a bent gold wire attached to the FIM specimen holder.

FIM-sharp tips were made by electropolishing in about 25? HC1 in glycerin, using a few volts A.C. under an optical microscope (t), and finally rinsing in methanol.

Imaging was done in hydrogen at 78 K or in hydrogen or neon at about 30K in an unbaked FIM. Low-temperature neon imaging after initial field evaporation in hydrogen gave the best results.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986243

JOURNAL DE PHYSIQUE

Thus r a r , we have s t u d i e d a b o u t t e n specimens of t h e a s - c o o l e d m a t e r i a l and a few from m a t e r i a l which had been a n n e a l e d . FIM m i c r o g r a p h s i l l u s t r a t i v e of t h e s e r e s u l t s a r e shown i n f i g u r e 1 . The d a r k bands a r e r e g i o n s which f i e l d e v a p o r a t e d r e l a t i v e l y r a p i d l y , s o t h a t t h e y d i d n o t e x p e r i e n c e a s u f f i c i e n t f i e l d s t r e n g t h when t h e v o l t a g e was a d j u s t e d f o r b e s t imaging o f t h e o t h e r r e g i o n . They u n d o u b t l y c o r r e s p o n d t o t h e Ad-rich s h e a t h i n g a r o u n d and p e n e t r a t i n g i n t o t h e i c o s o h e d r a l p h a s e , r e p o r t e d i n t h e e a r l y TEM work ( 1 ) . The r e l a t i v e l y b r i g h t r e g i o n s c o n s t i t u t e t h e a l l o y p h a s e o f i n t e r e s t .

F i g u r e 1. Sequence o f neon f i e l d i o n m i c r o g r a p h s o f A1-12 a t . % M n ( a b o u t 1 7 k V and 30 k ) . D i s t a n c e a c r o s s image c o r r e s p o n d s t o a b o u t 100 nm.

Continued f i e l d e v a p o r a t i o n d u r i n g FIM imaging produced t h e " c o l l a p s i n g r i n g s "

e f f e c t t y p i c a l i n f i e l d e v a p o r a t i o n of a c r y s t a l l i n e specimen. These r i n g

s t r u c t u r e s a r e v i s i b l e i n t h e m i c r o g r a p h s of f i g u r e 1 , photographed d u r i n g s l o w

a t l e a s t 10008, is pentagonal, with 2-,3- and 5-fold r o t a t i o n a l symmetry p o l e s i n l o c a t i o n s which a r e c o n s i s t e n t with t h e long-range icosohedral symmetry determined form t h e d i f f r a c t i o n s t u d i e s ( 1 ) . This can be seen f i g u r e 2. No o t h e r symmetry is c o n s i s t e n t with t h e micrographs. A d d i t i o n a l l y , t h e appearance of r i n g s t r u c t u r e s r e s u l t i n g from f i e l d evaporation implies some degree of o r g a n i z a t i o n oP t h e atoms i n t o planes ( 5 ) .

Figure 2. Superposition of a r e g u l a r pentagon on a t r a c i n g of t h e major pole p o s i t i o n s f o r a sequence of Pour f i e l d i o n micrographs.

Occasionally, during t h e course of f i e l d evaporation, a d d i t i o n a l c o l l a p s i n g r i n g s t r u c t u r e s appeared and then disappeared upon f u r t h e r evaporation of perhaps 20-50 l a y e r s . Sometimes they were along zone l i n e s and sometimes not. I n g e n e r a l , t h e micrographs displayed a r a t h e r poor short-range o r d e r , i n which t h u s f a r we have not found s y s t e m a t i c s t r u c t u r a l f e a t u r e s . Often small loops of atoms, about 3-5 atoms i n diameter were a l s o observed. Close i n s p e c t i o n of t h e micrographs reveals' a p a t c h i n e s s with sub-regions of about 5-10 nm e x t e n t , e x h i b i t i n g some semblance of order within each patch. This patchiness appears t o be c o r r e l a t e d with some of t h e d a r k - f l e l d TEM observations ( 3 ) .

A s seen i n f i g u r e 3 , a d e f e c t would sometimes appear i n t e r s e c t i n g a major pole,

or elsewhere. These may be examples of micro-twinning. I n any c a s e , they p e r s i s t e d

f o r only a few l a y e r s i n depth. No evidence was found f o r m u l t i p l e twinning which

could e x p l a i n t h e observed long-range symmetry. I t should a l s o be noted t h a t t h e

apparent degree of o r d e r a s developed by t h e f i e l d evaporation process and evidenced

by t h e r i n g images was c o n s i s t e n t l y l e s s a t t h e 5-fold poles than a t t h e 2- and

3-fold poles.

JOURNAL DE PHYSIQUE

Figure 3. Neon field ion micrograph of A1-12 at.% Mn (about 16 kV and 30 k).

Oval encloses defective 2-fold pole region, and rectangle encloses a prominent defect. Distance across image corresponds to about 100 nm.

Thus, FIM has confirmed, in real-space, the major conclusions from the early diffraction studies (1) and has also revealed a considerably disturbed

microstructure. Undoubtedly, some of the apparent disorder is artifactitious, due to the binary composition of the alloy, but the main features reported here are not subject to this uncertainty.

Acknowledgement: The authors appreciate the gift of specimen material from and discussion with Dr. J. W. Cahn.

References

1. D. Shechtman, I. Blech, D. Graties and J. W. Cahn, Phys. Rev. Lett. 22 (1984) 1951-1953.

2. D. R. Nelson and B. I. Halperin, submitted to Science.

3. R. D. Field and H. L. Fraser, Mat. Sci. Eng. 68 (1984-1985) L17-L21.

4. A. J. Melmed and J. J; Carroll, J. Vac. Sci. Technol. A2(3) (1984) 1388-1389.

5. E. W. MUller and T. T. Tsong, "Field Ion Microscopy Principles and

Applications," American Elsevier Publishing Co., Inc., New York (1969).