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Helical dislocations in GaF2 and NaCl crystals
Amelinckx, S. Bontinck, W. M a e n h o u t - V a n der Vorst, W. 1957.
Physica X X l I I
270-272
L E T T E R TO T H E EDITOR H e l i c a l d i s l o c a t i o n s in CaF 2 a n d N a C I
crystals
Helical dislocations h a v e been observed first in CaF2 1) 2). E v i d e n c e was also presented for their existence in NaCI 31. A possible i n t e r p r e t a t i o n has been discussed with special reference to CaF2 4). The e x p l a n a t i o n was based on climb as a consequence of the precipitation (or generation) of vacancies at .an almost pure screw dislocation. A t t e n t i o n has also been drawn on the possible i m p o r t a n c e of the p h e n o m e n o n for the multiplication of prismatic dislocation loops in crystals grown from the melt, or cooled at a relatively high rate 5).
Fig. 1. E t c h i n g pattern on cleavage face of CaF,, crystal, " a s g r o w n . " (420 × ), Some new results h a v e now been o b t a i n e d for NaC1 and CaF2 which m a y be of some i m p o r t a n c e for a further elucidation of the p h e n o m e n o n . al I t has been possible to prove the occurrence of helices in crystals of CaF2 " a s g r o w n " w i t h o u t a n y further heat t r e a t m e n t . This was a c h i e v e d by the etching of fresh cleavage faces with sulphuric acid. Fig. 1 shows an e x a m p l e of the observed p a t t e r n . It consists of two parallel rows of etchpits. Continued etching causes the centers of the pits to approach one another, t h e n to coalesce and finally to disappear. This proves t h a t the p a t t e r n consists of a sequence of half loops, characteristic for the intersection of a helix with the cleavage plane. Some etching p a t t e r n s show t h a t also sequences of closed loops are present, similar to those observed by decoration 2). b) Silver doped sodiumchloride crystals were annealed in h y d r o g e n at some 500-600°C and slowly cooled in order to " d e c o r a t e " the dislocations with silver particles 8). I t was found t h a t these crystals exhibit helical dislocations of which fig. 2(a) and (b) give examples. The axis of the helices have, within the limits of e x p e r i m e n t a l error, t h e "110] orientation i.e. t h e y are parallel to a possible Burgersvector. This is in a g r e e m e n t with the observations in CaF2 2) 4). W h e n helices were found in the same area. b u t at
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270
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HELICAL DISLOCATIONS IN CaF2 AND NaC1 CRYSTALS
271
different d e p t h in the c r y s t a l a striking p a x t i c u l a r i t y was n o t i c e d systematicall. I m m e d i a t e l y under the surface, which was exposed during the decoration process, a p p a r e n t l y s t r a i g h t lines h a v i n g e x a c t l y the E110] orientations, were observed. F a r t h e r below the surface these lines are visibly sinusoidal. Finally at greater d e p t h (,-~ 30 microns) the helical character, of w h a t first a p p e a r e d as straight lines becomes more and more p r o m i n e n t (fig. 2). This o b s e r v a t i o n illustrates q u i t e convincingly the influence of the n e i g h b o u r h o o d of a large sink (or source) of vacancies as presented by the surface. ~w
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Fig. 2 (a) Helical dislocations in a crystal of NaC1. The axis of the helices are parallel to E110]. Focus was a b o u t 50 microns below the surface. W h i t e bands are o t h e r helices which are out of focus (600 × ). b) H e l i x in NaC1 at higher magnification. N o t e the sharply cut angles of the p r o j e c t e d helix; this suggests a " p o l y g o n a l " shape of the helix, as represented on fig. 3. A somew h a t similar p h e n o m e n o n was observed in CaF2 2). (1200 × ). F r o m the fact t h a t helices are observed in crystals " a s g r o w n " (from a melt) we can conclude t h a t their f o r m a t i o n was v e r y p r o b a b l y due to the condensation of vacancies during cooling. The observations in NaC1 suggest t h a t thelast stage in the history of the helices was a s t r a i g h t e n i n g which was m o s t pronounced for the helices near t h e surface. This process can be i n t e r p r e t e d as a distillation of vacancies from the helix t o w a r d s t h e surface, during the h e a t t r e a t m e n t a c c o m p a n y i n g the decoration process. This allows the helix to reduce its length and to a d o p t finally the configuration of least energy, which according to theoretical calculations 7) appears to be the p u r e screw in t h e NaC1 structure.
272
HELICAL DISLOCATIONS IN C a F 2 AND NaCI CRYSTALS
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(h) Fig. 3. S c h e m a t i c r e p r e s e n t a t i o n of " p o l y g o n a l " helices in NaCI: a) in t w o p r o j e c t i o n s ; b) in space. W e wish t o t h a n k Prof. D r \V. D e k e y s e r for his s t i m u l a t i n g i n t e r e s t . T h i s work is p a r t of a r e s e a r c h p r o g r a m s u p p o r t e d b y I . R . S . I . A . (C.E.S.) (Brussels). S. AMELINCKX, W. BONTINCK, \'V, MAENHOUT-VAN DER VORST, Rozier, 6, Gent, Belgium.
Received 1-2-57. REFERENCES 1) 2) 3) 4) 5) 6) 7)
B o n t i n c k , W. and A m e l i n c k x , S., Phil.Mag. (in tim press). B o n t i n c k , W., Phil. Mag. (in the press). A m e l i n c k x , S., Proceedings of the Lake Placid Conference on Dislocations (1956). A m e l i n c k x , S., B o n t i n c k , W., D e k e y s e r , W. and Seitz, F., Phil. Mag. (in the press). A m e l i n c k x , S. and B o n t i n c k , W., Acta Metallurgica (in the press). Van d e r Vorst, W. and D e k e y s e r , W., Phil. Mag. 1 (1956), 882. H u n t i n g t o n , H. B., Dickey, J. E. and T h o m s o n , R., Phys. Rev. 100 (1955) Ill7.
Physica X X l I I
270-272
L E T T E R TO T H E EDITOR H e l i c a l d i s l o c a t i o n s in CaF 2 a n d N a C I
crystals
Helical dislocations h a v e been observed first in CaF2 1) 2). E v i d e n c e was also presented for their existence in NaCI 31. A possible i n t e r p r e t a t i o n has been discussed with special reference to CaF2 4). The e x p l a n a t i o n was based on climb as a consequence of the precipitation (or generation) of vacancies at .an almost pure screw dislocation. A t t e n t i o n has also been drawn on the possible i m p o r t a n c e of the p h e n o m e n o n for the multiplication of prismatic dislocation loops in crystals grown from the melt, or cooled at a relatively high rate 5).
Fig. 1. E t c h i n g pattern on cleavage face of CaF,, crystal, " a s g r o w n . " (420 × ), Some new results h a v e now been o b t a i n e d for NaC1 and CaF2 which m a y be of some i m p o r t a n c e for a further elucidation of the p h e n o m e n o n . al I t has been possible to prove the occurrence of helices in crystals of CaF2 " a s g r o w n " w i t h o u t a n y further heat t r e a t m e n t . This was a c h i e v e d by the etching of fresh cleavage faces with sulphuric acid. Fig. 1 shows an e x a m p l e of the observed p a t t e r n . It consists of two parallel rows of etchpits. Continued etching causes the centers of the pits to approach one another, t h e n to coalesce and finally to disappear. This proves t h a t the p a t t e r n consists of a sequence of half loops, characteristic for the intersection of a helix with the cleavage plane. Some etching p a t t e r n s show t h a t also sequences of closed loops are present, similar to those observed by decoration 2). b) Silver doped sodiumchloride crystals were annealed in h y d r o g e n at some 500-600°C and slowly cooled in order to " d e c o r a t e " the dislocations with silver particles 8). I t was found t h a t these crystals exhibit helical dislocations of which fig. 2(a) and (b) give examples. The axis of the helices have, within the limits of e x p e r i m e n t a l error, t h e "110] orientation i.e. t h e y are parallel to a possible Burgersvector. This is in a g r e e m e n t with the observations in CaF2 2) 4). W h e n helices were found in the same area. b u t at
-
-
270
-
-
HELICAL DISLOCATIONS IN CaF2 AND NaC1 CRYSTALS
271
different d e p t h in the c r y s t a l a striking p a x t i c u l a r i t y was n o t i c e d systematicall. I m m e d i a t e l y under the surface, which was exposed during the decoration process, a p p a r e n t l y s t r a i g h t lines h a v i n g e x a c t l y the E110] orientations, were observed. F a r t h e r below the surface these lines are visibly sinusoidal. Finally at greater d e p t h (,-~ 30 microns) the helical character, of w h a t first a p p e a r e d as straight lines becomes more and more p r o m i n e n t (fig. 2). This o b s e r v a t i o n illustrates q u i t e convincingly the influence of the n e i g h b o u r h o o d of a large sink (or source) of vacancies as presented by the surface. ~w
L
J
d'
•
•
e"
,g
,
•
,t. " ~ t t
•
o•
t
~o'.
~'
•
",
Fig. 2 (a) Helical dislocations in a crystal of NaC1. The axis of the helices are parallel to E110]. Focus was a b o u t 50 microns below the surface. W h i t e bands are o t h e r helices which are out of focus (600 × ). b) H e l i x in NaC1 at higher magnification. N o t e the sharply cut angles of the p r o j e c t e d helix; this suggests a " p o l y g o n a l " shape of the helix, as represented on fig. 3. A somew h a t similar p h e n o m e n o n was observed in CaF2 2). (1200 × ). F r o m the fact t h a t helices are observed in crystals " a s g r o w n " (from a melt) we can conclude t h a t their f o r m a t i o n was v e r y p r o b a b l y due to the condensation of vacancies during cooling. The observations in NaC1 suggest t h a t thelast stage in the history of the helices was a s t r a i g h t e n i n g which was m o s t pronounced for the helices near t h e surface. This process can be i n t e r p r e t e d as a distillation of vacancies from the helix t o w a r d s t h e surface, during the h e a t t r e a t m e n t a c c o m p a n y i n g the decoration process. This allows the helix to reduce its length and to a d o p t finally the configuration of least energy, which according to theoretical calculations 7) appears to be the p u r e screw in t h e NaC1 structure.
272
HELICAL DISLOCATIONS IN C a F 2 AND NaCI CRYSTALS
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I
;
I1 J/wol I "--d
l
I
t
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(h) Fig. 3. S c h e m a t i c r e p r e s e n t a t i o n of " p o l y g o n a l " helices in NaCI: a) in t w o p r o j e c t i o n s ; b) in space. W e wish t o t h a n k Prof. D r \V. D e k e y s e r for his s t i m u l a t i n g i n t e r e s t . T h i s work is p a r t of a r e s e a r c h p r o g r a m s u p p o r t e d b y I . R . S . I . A . (C.E.S.) (Brussels). S. AMELINCKX, W. BONTINCK, \'V, MAENHOUT-VAN DER VORST, Rozier, 6, Gent, Belgium.
Received 1-2-57. REFERENCES 1) 2) 3) 4) 5) 6) 7)
B o n t i n c k , W. and A m e l i n c k x , S., Phil.Mag. (in tim press). B o n t i n c k , W., Phil. Mag. (in the press). A m e l i n c k x , S., Proceedings of the Lake Placid Conference on Dislocations (1956). A m e l i n c k x , S., B o n t i n c k , W., D e k e y s e r , W. and Seitz, F., Phil. Mag. (in the press). A m e l i n c k x , S. and B o n t i n c k , W., Acta Metallurgica (in the press). Van d e r Vorst, W. and D e k e y s e r , W., Phil. Mag. 1 (1956), 882. H u n t i n g t o n , H. B., Dickey, J. E. and T h o m s o n , R., Phys. Rev. 100 (1955) Ill7.