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Precipitation of calcium in natural calcium fluoride crystals
Bontinck, W. Dekeyser, W. 1956
Physica X X I I
595-606
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS W. BONTINCK and W. D E K E Y S E R Laboratorium voor Kristalkunde, Rozier 6, Gent, Belgi~
Synopsis Dislocation lines and planar arrangements of dislocations have been made visible by decoration respectively in white fluorite crystals and in purple specimens. Therefore the white crystals were coloured by the Rexer method, the other by heating them in sodium vapor. The dislocations formed very irregular networks situated in {111} planes, the planar arrangements seen end on as lines are identified as the intersection of glide {110} or climb planes with the observation plane. A regular six fold arrangement of dislocation walls has also been found. X-ray analysis and absorption spectra measurements point out that the formed specks consist of calcium particles although sodium was used as colouring agent.
1. Introduction. Dislocation lines could be made visible in silver bromide 1) and in rocksalt 2) 3) b y precipitation around them, of an excess metal obtained respectively b y irradiation, a diffussion process or a chemical reaction. In the case of rocksalt, the geometry of the observed networks has been considered in detail 4) and compared with the models predicted b y F r a n k 5). In order to obtain information about the organisation of dislocations in crystals having another type of lattice, we examined fluorite crystals. The fluorite structure is formed b y three interpenetrating face centered cubic lattices, the relative displacements are (0, 0, 0), (t, ¼, ¼), (~, i, ~). The first is occupied b y the calcium ions, the others b y fluorine. The coordination is 6/3, the shortest and nearest shortest lattice vectors are a/2 [1 I0] and a/2 [121J respectively. Dislocations with Burgersvectors a/2 (110) are b y far the most probable and the expected glide planes are {110}. No definite information about glide elements could be found in the literature. In this paper we report the results obtained on undeformed natural specimens, purple ones from Cornwall (England) and white ones from La Challe (Hautes Alpes, France). 2. Experimental and results. 2.1. P r e p a r a t i o n of specimens. The decoration of the lines was achieved in the white specimens b y the R e x e r method e) as modified b y A m e 1 i n c k x 4). Sodium metal was used for the purpose, it was chosen 595
--
596
W. BONTINCK AND W. DEKEYSER
both for convenience and in order to have a reacting agent different from the elements present in the crystals. As the purple crystals did not resist to such a treatment, t h e y were coloured b y heating them in sodium vapour (R 6g e n e r method)7). In both cases, prolonged heating at 700°C in the absence of oxygen is needed. When the R e x e r method was used, no visible diffusion zone as observed b y A m e l i n c k x in rocksalt could be detected. This made the control of the coloration and decoration very difficult. Cloudy regions, densely packed with colloidal particles resulted ~n most occasions, t h e y hampered observations in a considerable way. The crystals could not be deformed before this treatment. The networks were observed b y ultramicroscopy. To eliminate, scattering by surface features, lavenc~el oil and a cover glass were put on the surface through which the specimens were examined. These were invariably {I 11 } cleavage faces.
a b Fig. 1. a) Pattern of irregular networks as found in fluorite crystals (white specimens France). b) Irregular polygonal meshes.
2.2. 0 b s e r v e d p a t t e r n s. The observed patterns can be classified as following: a) decorated lines forming irregular networks. b) non intersecting walls, forming angles of 60 ° between them. c) similar walls as listed above, but at right angles. d) triangular patterns formed b y the intersection of heavily decorated walls making angles of 60 ° between them. e) precipitation zones, consisting of colloidal particles, sometimes randomly distributed, sometimes situated on~ nearly parallel lines. They differed from those mentioned above b y the spacing of the particles.
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS
597
3. The geometry o] the decorated pattern. 3.1. D e c o r a t e d 1 i n e s. This type of pattern has only be observed in the white specimens. The dislocation lines had no definite direction and formed a great variety of patterns, nearly all situated in {111 } planes. Irregular networks, as well as nearly parallel lines were found; fig. 1, a, b and 3 show such a region. The most remarkable facts are the occurrence of threefold, fourfold and fivefold nodes. The lack of regularity in these patterns, contrasting with what was observed by A m e 1i n c k x in rocksalt is not surprising. Previous to decoration, the rocksalt was deformed and afterwards annealed in order to rearrange the dislocations. Deformation of our specimens was impossible, and what we observe must correspond to a certain extent to the arrangement of the dislocations present in a crystal grown from solution. These patterns are far too irregular to allow any geometrical considerations. A slight rearrangement during the heat treatment needed for their decoration must however have
a
b
Fig. 2. T w o e x a m p l e s of dislocations bended b y t o r q u e due to interaction.
occurred. This can be deduced from the bending of some of the lines resulting from the torque produced by the interaction between dislocations 8). As in rocksalt, m a n y examples were observed, fig. 2 represents one of them. Studies on rocksalt indicated that fourfold nodes were stable, our observations support this view. In calcium fluoride two kinds of such nodes were present. The first type is identical with what was observed by A m el i n c k x ~) ; the node is formed by lines which cut each other at right angles and must have perpendicular Burgersvectors. In the other type of node (fig. 3), the lines are not coplanar. As could be easily deduced from observation, three are in a (111) plane, the fourth not. The most logical explanation is that we are dealing with a
5'98
W. BONTINCK AND W. D E K E Y S E R
dissociation. A possible example is:
a/2[11-~] = a/2[ l OT] + a/2[110] + a/2[ l O1] B o t h terms have equal energies, the reaction is a possible one. Moreover, if all possible Burgersvectors in fluorite are determined in the m a n n e r described by Jaswon and D o v e 9 ) for a d i a m a n t structure, it is found that, only a/2[110] is more probable t h a n a/2[112] (fig. 4 and 5).
A
/
•
/
."
a
.i
'
~
"
b
Fig. 3. a) Nearly parallel dislocation lines and dislocations forming a fourfold node. The angles are different from 90°. b) Detail of the node. A dislocation line (arrow) comes out of the plane. The fivefold node is represented b y fig. 6, we m u s t be dealing here with a dislocation with a large Burgersvector which splits. 3.2. D i s l o c a t i o n walls forming angles o f 60 ° w i t h e a c h o t h e r. The decorated lines, represented on fig. 7, and observed in the purple crystals, remain visible and immobile when the focussing is altered between variable limits. This indicated t h a t t h e y h a d an extension in height some tens of microns as an average. We denote t h e m therefore as walls of dislocations and will speculate later about their nature. Those walls never cut each other, this is not so well visible on photographs as when direhtly observed. The fig. 8, a, b were t a k e n at different depths, it can clearly be seen t h a t the walls which seem to cut each other are at different depths. As one of the edges of the specimen was the intersection of two (111) planes, it was possible to determine the direction of the traces of the walls. T h e y are
P R E C I P I T A T I O N OF CALCIUM IN NATURAL CALCIUM F L U O R I D E CRYSTALS
599
[ 1121, [ 1211 and [211 l, indicating t h a t the wails are situated into { 110) planes• As the expected glide planes in fluorite are precisely those planes, the observed patterns can be explained as resulting from slip• Interaction between the loops in different glide planes has prevented their passage• If so dis2
7
) ~v.~
C
"I
Y~.8)
9 ~ /
t~,o) (~o*)(
A
(:['~.~l,
Fig. 4. Fluorite (1~0) plane showing arrangement of the "three interpenetrating f.c.c. lattices .4, B, C and the . . . 1 2 3 123... stacking, of elevation planes. The two possible Burgersvectors are b ---- a/2[110] and b'= a/2E112~. [ERRATUM: the open cirle (1,1,1) should be a full one] 4A
(o.o.
. "_,o~
=
-
_
Fig. 5. Fluorite (111) plane showing the two possible Burgersvectors b = a!2[10~j and b' = a/21211]. 23 1 2 3 . . . stacking a l o n g b ' a n d . . . 111 ..•stackingalongb • ..1
locations m u s t have piled up in the regions where the lines approach each other and decoration m u s t be more intense in those parts. This effect could not always be observed because of the m u l t i t u d e of lines present in some area, it was however clearly visible when less crowded areas were examined, e.g. in the region represented b y fig. 9.
600
w.
BONTINCK
AND W. DEKEYSER
Single decorated lines cutting the walls are sometimes visible in their vicinity. 3.3Dislocation walls forming angles o f 90 ° . The observed patterns showed a great similarity with those described in the preceding paragraph (fig. 10 a, b). The direction of the lines are now [10T] and [121]. The [121] line was again i¸
+.
.
.
.
.
.
Fig. 6. E x a m p l e of fivevold node. I n a) a n d b) the dislocations split into several dislocation lines.
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.
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.
.:
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j
.
.
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,.'c
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a
~
~'":'~ b
Fig. 7. Intersection of glide planes with the observation planes, with direction [121] [211] [113_].I t c a n b e seen on the photographs t h a t the lines do n o t cut. If t h e y seem to do it, we have a crossing at different depths.
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS
601
the intersection of the E10T] plane with the observation plane. For [10T] several possibilities exist: it can be the intersection of (101), (121) or (l~l)
a b Fig. 8. T w o p h o t o g r a p h s t a k e n at different depths. I t can be clearly seen t h a t t w o crossing intersection lines lie in different planes.
f -4
Fig. 9. T h e effect of piling up : the end part of lines are more d e c o r a t e d and scatter more light.
with the observation plane. (101) has an inclination of 35 ° with respect to the observation plane and a displacement of the line should be visible when the focussing is altered. A very marked change in intensity should also be visible when the microscope stage is rotated, this alters the orientation of the
602
W. BONTINCK AND ~V. DEKEYSER
wall with respect to the incident beam which is perpendicular to the direction of observation. As this is not observed, it follows that the observed wall cannot be situated in a (10 l) plane. Two possibilities remain: t-he wall is situated in a (111) or in a ( 12 l) plane. It has not been possibie to obtain more elements. No observation on another (11 l) plane was possible, and the crystals were too impure to allow an attempt to determine the glide elements~ even at high temperature. This will be tried on synthetic specimens. We can however consider the different possibilities. Glide along (111) and ( 121 ) is not to be expected for the same reasons as in rocksalt.
a
b
Fig. 10. Typical examples of two intersecting dislocation walls forming angles of 90 ° with directions [121] and [10T]. Remark the cloudy region in the midle as a result of rearrangement and the effect of the piling-up on the brilliance.
It is possible to explain the presence o f the walls in a (121) plane in the following way. A source, situated in a (10 f) plane m a y have generated loops b y climb as suggested b y B a r d e e n and H e r r i n g 10). The mechanism is formally identical with the action of a Frar~k and Read generator; b u t the loops are produced b y chmb in a plane perpendicular to a glide plane. As calculated b y B a r d e e n and H e r r i n g, a departure from t h e equilibrium concentration of vacancies is sufficient to activate in the described w a y a dislocation hne of a length of 10.4 crn. The purple crystals, in which these patterns have been observed, are very impure, this ,and the heat
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS
603
treatment m a y have produced locally the conditions necessary for such a process. In the middle of each pattern, a cloudy region was always present, indicating a highly distorted state of the crystals. The ends of the lines are more decorated and reflect consequently more light than the other parts. This again can be interpreted as an effect of piling up of stopped dislocations by unseen obstacles or interaction with those situated in othe~ glide or climb planes. Glide was also observed by A m e 1 i n c k x in rocksalt and the deformation ascribed to unhomogeneous cooling. We are more inclined to believe t h a t the deformation occurred during the heating of the crystals, this is consistent with the heavy decoration of the lines and the presence of the cloudy regions. That impurities have played an essential role in this process m a y be safely concluded because no glide has been detected in the much purer white specimens, only decorated lines forming networks were observed. 3.4. T r i a n g u l a r patterns. Those were only once observed, again, in the purple specimens (fig. 11).
a b Fig. 11. S m a l l a n g l e b o u n d a r i e s f o r m i n g a sixfold a r r a n g e m e n t . N o t e t h e h e a v y decoration at the intersections.
The directions of the lines making the triangles were identical with those observed above. In that particular experiment the decoration was not performed in a nitrogen atmosphere, the treated crystals were milky and mostly nearly opaque. The triangular patterns were found some tens of microns under the surface in one of the very rare places where something could be seen in the interior. We are, here again, dealing with walls. Most remarkable are the intersections, seen in the plane of observation, as very heavily decorated dots at the centers of sixfold stars. The pattern is to be interpreted as a regular arrangement of small angle boundaries. 3.5. P r e c i p i t a t i o n r e g i o n s. These decorated regions (fig. 12) showed great similarity with those observed in sphalerite crystals 11).
604
W. BONTINCK AND W. DEKEYSER
Some d o t s are arranged on parallel lines and form a more or less regular grid. A m e 1 i n~c k x hasalso observed such plane lattice of dots in rocksalt and,explained them as a network from which the nodes were only decorated. In our case, we are more inclined to admit the same mechanism of formation a s in zincsulfide. When the crystals are heated, internal cleavage occurs. The so formed cavities are filled up, during the anneal with calciumfluoride and an excess of calcium.
Fig. 12. Specks in a precipitation zone !yi~.g on nearly parallel lines.
4. T h e decoration process. In his paper on dislocation networks in rocksalt crystals, A m e 1 i n c k x ¢) discusser the nature of the specks decorating the lines. Having proved that they are not formed b y impurities, he concludes that the specks are most probably sodium metal. As we have observed that impurities and an excess of zinc can produce a special kind of precipitation in zinc sulphide, we heated the most impure calcium fluorite crystals for a long period without addition. As no precipitates could b e detected after such a treatment we can also conclude t h a t the impurities have no relation with the specks. In rocksalt, it w a s impossible to Conclude definitely if the specks were formed b y cations of the crystal or if t h e y were formed b y metal which had been p u t i n the cavity. As we used sodium for decorating calcium fluoride, the specks must be or calcium or sodium. The quantities present are t o o small to allow a determination of their nature b y chemical methods. X-ray analysis and absorption spectra made it however possible to conclude that they consist of calcium. The absorption bands of our sodium coloured fluorite were measured and
P R E C I P I T A T I O N OF CALCIUM IN N A T U R A L CALCIUM F L U O R I D E CRYSTALS
605
found to be identical with the c¢and ~ bands observed by M o 1 1 w o 12) and L i i t t i 13) in fluorite crystals treated in calcium vapour.This indicates t h a t it is calcium that precipitates. To see if some diffusion of sodium could occur we made some experiments on rocksalt coloured by divalent metals e.g. Mg and Pb. Next to the F-bands no Z1 band could be detected after irradiation in the F-band. As P i c k 14) has shown, such a band should have been present if divalent ions had diffused into the lattice. This indicates that diffusion is or inexistant or very slow. Similar experiments could not be carried out on calcium fluoride because no such crystals with additions "have been measured yet. It was furthermore observed, after each treatment, that a white powder was formed in the cavity where the sodium metal had been deposited. DebyeScherrer diagrams indicated t h a t it consisted of NaF. This indicates that following reaction occurred in the contact zone: CaF 2 + 2Na = 2NaF + Ca ++ + 2e This also indicates that the observed specks are not formed by sodium. The quoted reaction is a structure sensitive one, it is fully governed by the defects present in the crystal, and it can better be written by using the notation of R e e s 1 5 ) 2F~-/[]~-+2Na~ -~ 2 N a F ~ + 2 [ ] ; - + 2 e 2[7}- + 2e --> 2 e / 0 -
Ca++/[] + = Caion at the surface F ~ - / [ ] - = F ion in the lattice
or
Ca++/[]+ + 2 e -+ Cas/[7 + C a ~ + + / n + + 2 e / D - at jogs ' Ca/disk + 2 []+
Na s = sodium atom at the surface Ca~(sl -- Ca ion in the crystal or ++ / [ ] +-at the surface e / O - = F-center.
The sodium atoms dissociate into an ion and an electron, NaF is formed at surface kink sites. The electron fills a fluorine vacancy and form a colour center. Formally, the reaction can be described as the formation of an excess of metal in the crystals by the migration of electrons from the surface into it and of fluorine from the bulk to the surface. On cooling, a supersaturation results and metal is precipitated at jogs of edge dislocations. As S e i t z le) pointed out, jogs are charged, they are the preferential spots where the electrons will add to calcium ions and form calcium atoms. This mechanism for the speck formation which is comparable to the formation of the latent image in irradiated silver halides is still highly unsatisfactory. We have definite evidence that jogs are essential, but t h a t they are certainly not sufficient. Other factors which we have only partially under control for the moment play an important role. Further investigations are in progress in order to learn more about the mechanism of the speck formation.
606
P R E C I P I T A T I O N OF CALCIUM IN NATURAL CALCIUM F L U O R I D E CRYSTALS
5. Conclusion. It has been proved to be possible to decorate dislocations in natural calcium fluorite crystals. A great difference exists between the rather pure and well transparent white specimens and the coloured ones. In the first, networks of dislocation line are present, in the latter only internal slip lines and small angle boundaries were detected. As no definite information exists about the glide elements, the most logical assumption was made, e.g. that { 110} are the slip planes and a/2 (110) the slip vector. On this basis it is possible to interpret the results in a consistent way. The use of sodium metal for the decoration allowed to obtain some definite information about the nature of the specks and the decoration mechanism.
Acknowledgements. This work is part of a research program (C.E.S.) sponsered b y the "Institut pour l'encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculture" (I.R.S.I.A. Brussels). We also thank Dr R. Gevers for many helpful discussions. Received 13-3-55.
REFERENCES 1) H e d g e s , J . N . and M i t c h e l l , J . W . , P h i l . Mag. 44(1953) 223. 2) A m e l i n c k x , S., V a n d e r V o r s t , W., G e v e r s , R. and D e k e y s e r , W., Phil. Mag. 46 (1955) 450. 3) V a n d e r V o r s t , W. and D e k e y s e r , W., to be published. 4) A m e l i n e k x , S., Phil. Mag. (1956) in the press. 5) F r a n k, F. C., Proc. phys. Soe. (Report of Bristol Conference 1954) (1955) 159. 6) R e x e r , E., Z. Physik 78 (1932) 538. 7) R 6 g e n e r , H., Ann. Physik 29 (1937) 386. 8) R e a d, T. W., Dislocations in Crystals, Mac Graw Hill, New York (1954) 133. 9) J a s w o n, M. A. and D o v e, D. B., Acta Cryst. 8 (1955) 806. 10) B a r d e e n , J. and H e r r i n g , C., Imperfections in nearly perfect Crystals. J o h n Wiley & sons, Inc. New York (1952) 277. 11) B o n t i n e k , W. and D e k e y s e r , W., to be published. 12) M o I 1 w o, E., GSttingen Nachr., math. phys. KI. (1934) 79. 13) L fit y, F., Z. Physik 134 (1953) 596~ 14) P i c k , H., Ann. Physik 35 (1939) 73. 15) R e e s, A. L. G., Chemistry of the defect solid State, Methuen & Co., London (1954) 15. 16) S e i t z, F., Rev. modern Phys. -°3 (1951) 328.
Physica X X I I
595-606
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS W. BONTINCK and W. D E K E Y S E R Laboratorium voor Kristalkunde, Rozier 6, Gent, Belgi~
Synopsis Dislocation lines and planar arrangements of dislocations have been made visible by decoration respectively in white fluorite crystals and in purple specimens. Therefore the white crystals were coloured by the Rexer method, the other by heating them in sodium vapor. The dislocations formed very irregular networks situated in {111} planes, the planar arrangements seen end on as lines are identified as the intersection of glide {110} or climb planes with the observation plane. A regular six fold arrangement of dislocation walls has also been found. X-ray analysis and absorption spectra measurements point out that the formed specks consist of calcium particles although sodium was used as colouring agent.
1. Introduction. Dislocation lines could be made visible in silver bromide 1) and in rocksalt 2) 3) b y precipitation around them, of an excess metal obtained respectively b y irradiation, a diffussion process or a chemical reaction. In the case of rocksalt, the geometry of the observed networks has been considered in detail 4) and compared with the models predicted b y F r a n k 5). In order to obtain information about the organisation of dislocations in crystals having another type of lattice, we examined fluorite crystals. The fluorite structure is formed b y three interpenetrating face centered cubic lattices, the relative displacements are (0, 0, 0), (t, ¼, ¼), (~, i, ~). The first is occupied b y the calcium ions, the others b y fluorine. The coordination is 6/3, the shortest and nearest shortest lattice vectors are a/2 [1 I0] and a/2 [121J respectively. Dislocations with Burgersvectors a/2 (110) are b y far the most probable and the expected glide planes are {110}. No definite information about glide elements could be found in the literature. In this paper we report the results obtained on undeformed natural specimens, purple ones from Cornwall (England) and white ones from La Challe (Hautes Alpes, France). 2. Experimental and results. 2.1. P r e p a r a t i o n of specimens. The decoration of the lines was achieved in the white specimens b y the R e x e r method e) as modified b y A m e 1 i n c k x 4). Sodium metal was used for the purpose, it was chosen 595
--
596
W. BONTINCK AND W. DEKEYSER
both for convenience and in order to have a reacting agent different from the elements present in the crystals. As the purple crystals did not resist to such a treatment, t h e y were coloured b y heating them in sodium vapour (R 6g e n e r method)7). In both cases, prolonged heating at 700°C in the absence of oxygen is needed. When the R e x e r method was used, no visible diffusion zone as observed b y A m e l i n c k x in rocksalt could be detected. This made the control of the coloration and decoration very difficult. Cloudy regions, densely packed with colloidal particles resulted ~n most occasions, t h e y hampered observations in a considerable way. The crystals could not be deformed before this treatment. The networks were observed b y ultramicroscopy. To eliminate, scattering by surface features, lavenc~el oil and a cover glass were put on the surface through which the specimens were examined. These were invariably {I 11 } cleavage faces.
a b Fig. 1. a) Pattern of irregular networks as found in fluorite crystals (white specimens France). b) Irregular polygonal meshes.
2.2. 0 b s e r v e d p a t t e r n s. The observed patterns can be classified as following: a) decorated lines forming irregular networks. b) non intersecting walls, forming angles of 60 ° between them. c) similar walls as listed above, but at right angles. d) triangular patterns formed b y the intersection of heavily decorated walls making angles of 60 ° between them. e) precipitation zones, consisting of colloidal particles, sometimes randomly distributed, sometimes situated on~ nearly parallel lines. They differed from those mentioned above b y the spacing of the particles.
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS
597
3. The geometry o] the decorated pattern. 3.1. D e c o r a t e d 1 i n e s. This type of pattern has only be observed in the white specimens. The dislocation lines had no definite direction and formed a great variety of patterns, nearly all situated in {111 } planes. Irregular networks, as well as nearly parallel lines were found; fig. 1, a, b and 3 show such a region. The most remarkable facts are the occurrence of threefold, fourfold and fivefold nodes. The lack of regularity in these patterns, contrasting with what was observed by A m e 1i n c k x in rocksalt is not surprising. Previous to decoration, the rocksalt was deformed and afterwards annealed in order to rearrange the dislocations. Deformation of our specimens was impossible, and what we observe must correspond to a certain extent to the arrangement of the dislocations present in a crystal grown from solution. These patterns are far too irregular to allow any geometrical considerations. A slight rearrangement during the heat treatment needed for their decoration must however have
a
b
Fig. 2. T w o e x a m p l e s of dislocations bended b y t o r q u e due to interaction.
occurred. This can be deduced from the bending of some of the lines resulting from the torque produced by the interaction between dislocations 8). As in rocksalt, m a n y examples were observed, fig. 2 represents one of them. Studies on rocksalt indicated that fourfold nodes were stable, our observations support this view. In calcium fluoride two kinds of such nodes were present. The first type is identical with what was observed by A m el i n c k x ~) ; the node is formed by lines which cut each other at right angles and must have perpendicular Burgersvectors. In the other type of node (fig. 3), the lines are not coplanar. As could be easily deduced from observation, three are in a (111) plane, the fourth not. The most logical explanation is that we are dealing with a
5'98
W. BONTINCK AND W. D E K E Y S E R
dissociation. A possible example is:
a/2[11-~] = a/2[ l OT] + a/2[110] + a/2[ l O1] B o t h terms have equal energies, the reaction is a possible one. Moreover, if all possible Burgersvectors in fluorite are determined in the m a n n e r described by Jaswon and D o v e 9 ) for a d i a m a n t structure, it is found that, only a/2[110] is more probable t h a n a/2[112] (fig. 4 and 5).
A
/
•
/
."
a
.i
'
~
"
b
Fig. 3. a) Nearly parallel dislocation lines and dislocations forming a fourfold node. The angles are different from 90°. b) Detail of the node. A dislocation line (arrow) comes out of the plane. The fivefold node is represented b y fig. 6, we m u s t be dealing here with a dislocation with a large Burgersvector which splits. 3.2. D i s l o c a t i o n walls forming angles o f 60 ° w i t h e a c h o t h e r. The decorated lines, represented on fig. 7, and observed in the purple crystals, remain visible and immobile when the focussing is altered between variable limits. This indicated t h a t t h e y h a d an extension in height some tens of microns as an average. We denote t h e m therefore as walls of dislocations and will speculate later about their nature. Those walls never cut each other, this is not so well visible on photographs as when direhtly observed. The fig. 8, a, b were t a k e n at different depths, it can clearly be seen t h a t the walls which seem to cut each other are at different depths. As one of the edges of the specimen was the intersection of two (111) planes, it was possible to determine the direction of the traces of the walls. T h e y are
P R E C I P I T A T I O N OF CALCIUM IN NATURAL CALCIUM F L U O R I D E CRYSTALS
599
[ 1121, [ 1211 and [211 l, indicating t h a t the wails are situated into { 110) planes• As the expected glide planes in fluorite are precisely those planes, the observed patterns can be explained as resulting from slip• Interaction between the loops in different glide planes has prevented their passage• If so dis2
7
) ~v.~
C
"I
Y~.8)
9 ~ /
t~,o) (~o*)(
A
(:['~.~l,
Fig. 4. Fluorite (1~0) plane showing arrangement of the "three interpenetrating f.c.c. lattices .4, B, C and the . . . 1 2 3 123... stacking, of elevation planes. The two possible Burgersvectors are b ---- a/2[110] and b'= a/2E112~. [ERRATUM: the open cirle (1,1,1) should be a full one] 4A
(o.o.
. "_,o~
=
-
_
Fig. 5. Fluorite (111) plane showing the two possible Burgersvectors b = a!2[10~j and b' = a/21211]. 23 1 2 3 . . . stacking a l o n g b ' a n d . . . 111 ..•stackingalongb • ..1
locations m u s t have piled up in the regions where the lines approach each other and decoration m u s t be more intense in those parts. This effect could not always be observed because of the m u l t i t u d e of lines present in some area, it was however clearly visible when less crowded areas were examined, e.g. in the region represented b y fig. 9.
600
w.
BONTINCK
AND W. DEKEYSER
Single decorated lines cutting the walls are sometimes visible in their vicinity. 3.3Dislocation walls forming angles o f 90 ° . The observed patterns showed a great similarity with those described in the preceding paragraph (fig. 10 a, b). The direction of the lines are now [10T] and [121]. The [121] line was again i¸
+.
.
.
.
.
.
Fig. 6. E x a m p l e of fivevold node. I n a) a n d b) the dislocations split into several dislocation lines.
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.
O 2\#. •
.
.:
t
j
.
.
"
--,
•
!
/
I
,.'c
J~
a
~
~'":'~ b
Fig. 7. Intersection of glide planes with the observation planes, with direction [121] [211] [113_].I t c a n b e seen on the photographs t h a t the lines do n o t cut. If t h e y seem to do it, we have a crossing at different depths.
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS
601
the intersection of the E10T] plane with the observation plane. For [10T] several possibilities exist: it can be the intersection of (101), (121) or (l~l)
a b Fig. 8. T w o p h o t o g r a p h s t a k e n at different depths. I t can be clearly seen t h a t t w o crossing intersection lines lie in different planes.
f -4
Fig. 9. T h e effect of piling up : the end part of lines are more d e c o r a t e d and scatter more light.
with the observation plane. (101) has an inclination of 35 ° with respect to the observation plane and a displacement of the line should be visible when the focussing is altered. A very marked change in intensity should also be visible when the microscope stage is rotated, this alters the orientation of the
602
W. BONTINCK AND ~V. DEKEYSER
wall with respect to the incident beam which is perpendicular to the direction of observation. As this is not observed, it follows that the observed wall cannot be situated in a (10 l) plane. Two possibilities remain: t-he wall is situated in a (111) or in a ( 12 l) plane. It has not been possibie to obtain more elements. No observation on another (11 l) plane was possible, and the crystals were too impure to allow an attempt to determine the glide elements~ even at high temperature. This will be tried on synthetic specimens. We can however consider the different possibilities. Glide along (111) and ( 121 ) is not to be expected for the same reasons as in rocksalt.
a
b
Fig. 10. Typical examples of two intersecting dislocation walls forming angles of 90 ° with directions [121] and [10T]. Remark the cloudy region in the midle as a result of rearrangement and the effect of the piling-up on the brilliance.
It is possible to explain the presence o f the walls in a (121) plane in the following way. A source, situated in a (10 f) plane m a y have generated loops b y climb as suggested b y B a r d e e n and H e r r i n g 10). The mechanism is formally identical with the action of a Frar~k and Read generator; b u t the loops are produced b y chmb in a plane perpendicular to a glide plane. As calculated b y B a r d e e n and H e r r i n g, a departure from t h e equilibrium concentration of vacancies is sufficient to activate in the described w a y a dislocation hne of a length of 10.4 crn. The purple crystals, in which these patterns have been observed, are very impure, this ,and the heat
PRECIPITATION OF CALCIUM IN NATURAL CALCIUM FLUORIDE CRYSTALS
603
treatment m a y have produced locally the conditions necessary for such a process. In the middle of each pattern, a cloudy region was always present, indicating a highly distorted state of the crystals. The ends of the lines are more decorated and reflect consequently more light than the other parts. This again can be interpreted as an effect of piling up of stopped dislocations by unseen obstacles or interaction with those situated in othe~ glide or climb planes. Glide was also observed by A m e 1 i n c k x in rocksalt and the deformation ascribed to unhomogeneous cooling. We are more inclined to believe t h a t the deformation occurred during the heating of the crystals, this is consistent with the heavy decoration of the lines and the presence of the cloudy regions. That impurities have played an essential role in this process m a y be safely concluded because no glide has been detected in the much purer white specimens, only decorated lines forming networks were observed. 3.4. T r i a n g u l a r patterns. Those were only once observed, again, in the purple specimens (fig. 11).
a b Fig. 11. S m a l l a n g l e b o u n d a r i e s f o r m i n g a sixfold a r r a n g e m e n t . N o t e t h e h e a v y decoration at the intersections.
The directions of the lines making the triangles were identical with those observed above. In that particular experiment the decoration was not performed in a nitrogen atmosphere, the treated crystals were milky and mostly nearly opaque. The triangular patterns were found some tens of microns under the surface in one of the very rare places where something could be seen in the interior. We are, here again, dealing with walls. Most remarkable are the intersections, seen in the plane of observation, as very heavily decorated dots at the centers of sixfold stars. The pattern is to be interpreted as a regular arrangement of small angle boundaries. 3.5. P r e c i p i t a t i o n r e g i o n s. These decorated regions (fig. 12) showed great similarity with those observed in sphalerite crystals 11).
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W. BONTINCK AND W. DEKEYSER
Some d o t s are arranged on parallel lines and form a more or less regular grid. A m e 1 i n~c k x hasalso observed such plane lattice of dots in rocksalt and,explained them as a network from which the nodes were only decorated. In our case, we are more inclined to admit the same mechanism of formation a s in zincsulfide. When the crystals are heated, internal cleavage occurs. The so formed cavities are filled up, during the anneal with calciumfluoride and an excess of calcium.
Fig. 12. Specks in a precipitation zone !yi~.g on nearly parallel lines.
4. T h e decoration process. In his paper on dislocation networks in rocksalt crystals, A m e 1 i n c k x ¢) discusser the nature of the specks decorating the lines. Having proved that they are not formed b y impurities, he concludes that the specks are most probably sodium metal. As we have observed that impurities and an excess of zinc can produce a special kind of precipitation in zinc sulphide, we heated the most impure calcium fluorite crystals for a long period without addition. As no precipitates could b e detected after such a treatment we can also conclude t h a t the impurities have no relation with the specks. In rocksalt, it w a s impossible to Conclude definitely if the specks were formed b y cations of the crystal or if t h e y were formed b y metal which had been p u t i n the cavity. As we used sodium for decorating calcium fluoride, the specks must be or calcium or sodium. The quantities present are t o o small to allow a determination of their nature b y chemical methods. X-ray analysis and absorption spectra made it however possible to conclude that they consist of calcium. The absorption bands of our sodium coloured fluorite were measured and
P R E C I P I T A T I O N OF CALCIUM IN N A T U R A L CALCIUM F L U O R I D E CRYSTALS
605
found to be identical with the c¢and ~ bands observed by M o 1 1 w o 12) and L i i t t i 13) in fluorite crystals treated in calcium vapour.This indicates t h a t it is calcium that precipitates. To see if some diffusion of sodium could occur we made some experiments on rocksalt coloured by divalent metals e.g. Mg and Pb. Next to the F-bands no Z1 band could be detected after irradiation in the F-band. As P i c k 14) has shown, such a band should have been present if divalent ions had diffused into the lattice. This indicates that diffusion is or inexistant or very slow. Similar experiments could not be carried out on calcium fluoride because no such crystals with additions "have been measured yet. It was furthermore observed, after each treatment, that a white powder was formed in the cavity where the sodium metal had been deposited. DebyeScherrer diagrams indicated t h a t it consisted of NaF. This indicates that following reaction occurred in the contact zone: CaF 2 + 2Na = 2NaF + Ca ++ + 2e This also indicates that the observed specks are not formed by sodium. The quoted reaction is a structure sensitive one, it is fully governed by the defects present in the crystal, and it can better be written by using the notation of R e e s 1 5 ) 2F~-/[]~-+2Na~ -~ 2 N a F ~ + 2 [ ] ; - + 2 e 2[7}- + 2e --> 2 e / 0 -
Ca++/[] + = Caion at the surface F ~ - / [ ] - = F ion in the lattice
or
Ca++/[]+ + 2 e -+ Cas/[7 + C a ~ + + / n + + 2 e / D - at jogs ' Ca/disk + 2 []+
Na s = sodium atom at the surface Ca~(sl -- Ca ion in the crystal or ++ / [ ] +-at the surface e / O - = F-center.
The sodium atoms dissociate into an ion and an electron, NaF is formed at surface kink sites. The electron fills a fluorine vacancy and form a colour center. Formally, the reaction can be described as the formation of an excess of metal in the crystals by the migration of electrons from the surface into it and of fluorine from the bulk to the surface. On cooling, a supersaturation results and metal is precipitated at jogs of edge dislocations. As S e i t z le) pointed out, jogs are charged, they are the preferential spots where the electrons will add to calcium ions and form calcium atoms. This mechanism for the speck formation which is comparable to the formation of the latent image in irradiated silver halides is still highly unsatisfactory. We have definite evidence that jogs are essential, but t h a t they are certainly not sufficient. Other factors which we have only partially under control for the moment play an important role. Further investigations are in progress in order to learn more about the mechanism of the speck formation.
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P R E C I P I T A T I O N OF CALCIUM IN NATURAL CALCIUM F L U O R I D E CRYSTALS
5. Conclusion. It has been proved to be possible to decorate dislocations in natural calcium fluorite crystals. A great difference exists between the rather pure and well transparent white specimens and the coloured ones. In the first, networks of dislocation line are present, in the latter only internal slip lines and small angle boundaries were detected. As no definite information exists about the glide elements, the most logical assumption was made, e.g. that { 110} are the slip planes and a/2 (110) the slip vector. On this basis it is possible to interpret the results in a consistent way. The use of sodium metal for the decoration allowed to obtain some definite information about the nature of the specks and the decoration mechanism.
Acknowledgements. This work is part of a research program (C.E.S.) sponsered b y the "Institut pour l'encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculture" (I.R.S.I.A. Brussels). We also thank Dr R. Gevers for many helpful discussions. Received 13-3-55.
REFERENCES 1) H e d g e s , J . N . and M i t c h e l l , J . W . , P h i l . Mag. 44(1953) 223. 2) A m e l i n c k x , S., V a n d e r V o r s t , W., G e v e r s , R. and D e k e y s e r , W., Phil. Mag. 46 (1955) 450. 3) V a n d e r V o r s t , W. and D e k e y s e r , W., to be published. 4) A m e l i n e k x , S., Phil. Mag. (1956) in the press. 5) F r a n k, F. C., Proc. phys. Soe. (Report of Bristol Conference 1954) (1955) 159. 6) R e x e r , E., Z. Physik 78 (1932) 538. 7) R 6 g e n e r , H., Ann. Physik 29 (1937) 386. 8) R e a d, T. W., Dislocations in Crystals, Mac Graw Hill, New York (1954) 133. 9) J a s w o n, M. A. and D o v e, D. B., Acta Cryst. 8 (1955) 806. 10) B a r d e e n , J. and H e r r i n g , C., Imperfections in nearly perfect Crystals. J o h n Wiley & sons, Inc. New York (1952) 277. 11) B o n t i n e k , W. and D e k e y s e r , W., to be published. 12) M o I 1 w o, E., GSttingen Nachr., math. phys. KI. (1934) 79. 13) L fit y, F., Z. Physik 134 (1953) 596~ 14) P i c k , H., Ann. Physik 35 (1939) 73. 15) R e e s, A. L. G., Chemistry of the defect solid State, Methuen & Co., London (1954) 15. 16) S e i t z, F., Rev. modern Phys. -°3 (1951) 328.