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Twin boundary shift in (110) twins of n-octacosane crystals
Journal of Crystal Growth 60(1982) 175—178 North-Holland Publishing Company
175
LEYITER TO THE EDITORS TWIN BOUNDARY SHIFT IN (110) TWINS OF n-OCTACOSANE CRYSTALS P.B.V. PRASAD Department of Physics, Government College, Sultanabad 505-185, India
and I.V.K. BHAGAVAN RAJU Department of Physics, Kakativa University, Warangal 506-009, India
Received 26 June 1982
A characteristic shift of the twin boundary in normal octacosane crystals is reported. This shift is always found to be towards the ~a1ientangle side of the twinned crystal and never towards the reentrant angle side. These results are explained considering the growth •ate anisotropy of the twinned layer based on a monomolecular layer growth model.
Normal long chain hydrocarbons are known to ~rysta1Iizeas thin rhombic platelets and the occur•ence of growth twins is not uncommon. The (110) win law was observed for n-C34H70 (Amelinckx 1,2]), n-C16H74 (Keller [3]), n-C94H190 (Khoury
~
41) and n-C1~H202(Dawson [51).The (110) twins
____
ire observed by the present authors for an orthohombic polymorph of n-C28 H58 crystals, which :ontain neighbouring homologues. A characteristic hift of twin boundary is sometimes observed in he twins of n-octacosane crystals and this pheomenon is discussed in this report. Crystals of n-octacosane are grown by slow vaporation of dilute toluene solutions at room ~mperature. Microscopic observations are made nder transmitted and polarized light. These studs revealed the presence of (110) twins in addition the normal crystals (fig. 1). The twinned crystals ~e observed to be elongated along the twin Dundary. In some of the cases, the shape of the vinned crystal is such that the reentrant angle one is present, while the salient angle is absent ig. 2a). In fig. 2, one observes a gradual shift of Le twin boundary towards one side of the crystal revealed by the intensity profiles (fig. 2b). The •
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D22-0248/82/0000—0000/$02.75
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________
______
_______
-______
-
-
-
(~‘
~ ,
‘
.
-.
-
.
Fig. I. Photomicrograph obtained under polarized light showing normal and twinned platelets of n-C28H58 crystals. Both the reentrant and salient angles are present in the twinned crystal. Magnification 450X.
1982 North-Holland
I 7t~
PB. V Prasad et al.
/
Twin boundary shift in (110) twins of n-octacosane
r’• Fig. 2. Platelet of n-C,
5H~5shos4.ing a characteristic shift of the twin boundary towards the salient obtained under (a) transmitted light and (b) under polarized light. Magnification 450 X.
__________ ___________
_______
.4
~ _______ _______
‘7’
~
-~
angle
side. Photomicrograph
shift of the twin boundary (indicated by the arrows, fig. 3) can also be ascertained from the etch pit patterns, where only the reentrant angle is present. Twins with such boundary shifts that cause the disappearance of reentrant angles and thus contam only salient angles, however, are not observed. According to the observations of Frank [6] and Stranski [7] in the case of different crystalline species, the occurrence of reentrant angles is followed by an increase in growth kinetics. Boistelle and Aquilano [8] studied the effect of reentrant and salient angles on growth kinetics. According to them, the position of a reentrant angle is a permanent kink, from which the filling up of the
Fig, 3. Twinned crystal showing only the reentrant angle and a shift of the twin boundary towards the salient angle side. The twinned region of the platelet can be identified from the morphology of the etch patterns. Magnification 450 X.
/
PB. V Prasad et al.
Twin boundary shift in (110) twins of n-octacosane
without any one- or two-dimensional nucleation, This enhanced growth could be the reason for the observed elongation of the crystals along the twin boundary. The growth of the faces forming the salient angle cannot start at the top of the salient angle, because of the very high probability of desorption of an isolated molecule adsorbed in Salient angle
~,
a ~‘~eentrant
~“J
--
-
-
-
~j
-
1
-
-
-
.~r -
C Fig. 4. Schematic representation of twinned platelets. (a) Twinned platelet showing both the salient and reentrant angles, the growth rates of rows A and B are nearly equal. The twinned boundary is a straight line. (b) The growth rate of row B is greater than that of A, which leads to a shift of the twin boundary. (c) A mono-molecular layer of a twinned crystal with a characteristic shift of the twin boundary (indicated by arrows) towards the salient angle side, which leads to the disappearance of the angle. However, the reentrant angle ~s present (see fig. 3).
177
this position [8]. Therefore a molecule can be adsorbed at the top of the salient angle only by a preexisting row of molecules and this angle by no means can generate a new layer. Under these circumstances, new rows can be generated on the faces forming the salient angle either by two-dimensional nucleation or by continuous layer growth depending upon the saturation levels. The twin boundary shift is explained on the basis of an idealized mono-molecular layer growth model. In such a model, the filling up of the faces is reduced to the filling up of rows. A (110) twin is considered (figs. 4a and 4b) where the two faces in contact have the same extension along the twin boundary and the pair of the faces constituting the reentrant and salient angle have equal dimensions. It is assumed that the two rows of molecules (A and B in fig. 4b) are growing simultaneously on the two faces forming the salient angle and that the the in rapidity rows with or the which ratemolecules of growthare of incorporated two rows is unequal. As a consequence, one row (row B) adangle. Row B can also advance further, while vances faster, and reaches the top of the salient crossing the top, for two reasons: (i) row A (fig. 4b) growing from the opposite side fails to offer any resistence to the movement or to the onward growth of row B, as its growth rate is relatively low and hence cannot reach the twin boundary at the same time at which row B reaches the twin boundary; (ii) the direction of advancement of row B, after it crosses the top of the salient angle, always lies away from the direction of advancement of row A. Therefore, no repulsion arises, as the close approach of the molecular rows is avoided. Growth of the subsequent rows occurs on the modified rows and evidently the dimensions of the two layers thus resulting from the growth of such rows are different, even though the two initial layers in twin position are of equal dimensions. Clearly a boundary shift is associated with the change in the dimensions of the growth layers (fig. 4c) The cause of occurrence of a difference in the growth rates of the two corresponding faces forming a salient angle of a twin can be understood by taking into account the changes in local energy
178
P.R. V. Prasad et al
/
Twin boundary shift in (110) twins of n-octacosane
which alter the growth rate of a face and thus make it difficult to relate the growth rate to the supersaturation of the environment [9]. Konak [10] states that the growth rate of not only different faces of a crystal, but also the same face of different crystals is not same, when grown under identical conditions, and that the discrepancies are due to the internal structure of crystals, such as density and distribution of dislocations etc. Konak [101 argues that each crystal is an entity with its own growth characteristics. Therefore it seems that factors such as local energy changes and nature of the defect substructure may induce an anisotropy in the growth rates of rows (faces) A and B (fig. 4). Further it is observed that the shift of the twin boundary does not occur on the reentrant angle side. The position of the reentrant angle is a permanent kink [8], from which the filling up of the Kll0)~ rows of the crystal faces in a twin configuration starts spontaneously. Therefore: (i) the molecule that initiates growth on either side always lands at the kink or correct positton; (it) growth due to two-dimensional nucleatton originate at any other point on the faces constitut-
..
ing the reentrant angle, and advance towards the junction, which thus causing a boundary shift is highly improbable, because such nucleation is less preferred in the presence of an energetically favoured kink position. Because of these two reasons, twin boundary shift does not occur on the reentrant angle side, causing the disappearance of the reentrant angle. The authors thank Professor D.B. Sirdeshmukh for his helpful suggestions and encouragement.
References [I] S. Amelinckx, Cryst. 99 (1956) 16. [2] S. Amelinckx, Acta Acta Cryst. (1956) 217. [3} A. Keller, Phil. Mag. 6 (1961) 329
[4) F. Khoury, J. AppI. Phys. 34 (1963) 73. [5] I.M. Dawson, Proc. Roy. Soc. (London) 214 (1952) 72.
[6] F.C. Frank, Disc. Faraday Soc. 5 (1949) 48. [7] Stranski, Disc. Faraday Soc. (1949) A34 69. (1978) 406. [8] IN. R. Boisielle and D. Aquilano, Ada5 Cryst. [9] C.W. Bunn and H. Emmett, Disc. Faraday Soc. 5 (1949)
119. [10] AR. Konak, J. Crystal Growth 22 (1974) 67.
175
LEYITER TO THE EDITORS TWIN BOUNDARY SHIFT IN (110) TWINS OF n-OCTACOSANE CRYSTALS P.B.V. PRASAD Department of Physics, Government College, Sultanabad 505-185, India
and I.V.K. BHAGAVAN RAJU Department of Physics, Kakativa University, Warangal 506-009, India
Received 26 June 1982
A characteristic shift of the twin boundary in normal octacosane crystals is reported. This shift is always found to be towards the ~a1ientangle side of the twinned crystal and never towards the reentrant angle side. These results are explained considering the growth •ate anisotropy of the twinned layer based on a monomolecular layer growth model.
Normal long chain hydrocarbons are known to ~rysta1Iizeas thin rhombic platelets and the occur•ence of growth twins is not uncommon. The (110) win law was observed for n-C34H70 (Amelinckx 1,2]), n-C16H74 (Keller [3]), n-C94H190 (Khoury
~
41) and n-C1~H202(Dawson [51).The (110) twins
____
ire observed by the present authors for an orthohombic polymorph of n-C28 H58 crystals, which :ontain neighbouring homologues. A characteristic hift of twin boundary is sometimes observed in he twins of n-octacosane crystals and this pheomenon is discussed in this report. Crystals of n-octacosane are grown by slow vaporation of dilute toluene solutions at room ~mperature. Microscopic observations are made nder transmitted and polarized light. These studs revealed the presence of (110) twins in addition the normal crystals (fig. 1). The twinned crystals ~e observed to be elongated along the twin Dundary. In some of the cases, the shape of the vinned crystal is such that the reentrant angle one is present, while the salient angle is absent ig. 2a). In fig. 2, one observes a gradual shift of Le twin boundary towards one side of the crystal revealed by the intensity profiles (fig. 2b). The •
.
D22-0248/82/0000—0000/$02.75
•
©
________
______
_______
-______
-
-
-
(~‘
~ ,
‘
.
-.
-
.
Fig. I. Photomicrograph obtained under polarized light showing normal and twinned platelets of n-C28H58 crystals. Both the reentrant and salient angles are present in the twinned crystal. Magnification 450X.
1982 North-Holland
I 7t~
PB. V Prasad et al.
/
Twin boundary shift in (110) twins of n-octacosane
r’• Fig. 2. Platelet of n-C,
5H~5shos4.ing a characteristic shift of the twin boundary towards the salient obtained under (a) transmitted light and (b) under polarized light. Magnification 450 X.
__________ ___________
_______
.4
~ _______ _______
‘7’
~
-~
angle
side. Photomicrograph
shift of the twin boundary (indicated by the arrows, fig. 3) can also be ascertained from the etch pit patterns, where only the reentrant angle is present. Twins with such boundary shifts that cause the disappearance of reentrant angles and thus contam only salient angles, however, are not observed. According to the observations of Frank [6] and Stranski [7] in the case of different crystalline species, the occurrence of reentrant angles is followed by an increase in growth kinetics. Boistelle and Aquilano [8] studied the effect of reentrant and salient angles on growth kinetics. According to them, the position of a reentrant angle is a permanent kink, from which the filling up of the
Fig, 3. Twinned crystal showing only the reentrant angle and a shift of the twin boundary towards the salient angle side. The twinned region of the platelet can be identified from the morphology of the etch patterns. Magnification 450 X.
/
PB. V Prasad et al.
Twin boundary shift in (110) twins of n-octacosane
without any one- or two-dimensional nucleation, This enhanced growth could be the reason for the observed elongation of the crystals along the twin boundary. The growth of the faces forming the salient angle cannot start at the top of the salient angle, because of the very high probability of desorption of an isolated molecule adsorbed in Salient angle
~,
a ~‘~eentrant
~“J
--
-
-
-
~j
-
1
-
-
-
.~r -
C Fig. 4. Schematic representation of twinned platelets. (a) Twinned platelet showing both the salient and reentrant angles, the growth rates of rows A and B are nearly equal. The twinned boundary is a straight line. (b) The growth rate of row B is greater than that of A, which leads to a shift of the twin boundary. (c) A mono-molecular layer of a twinned crystal with a characteristic shift of the twin boundary (indicated by arrows) towards the salient angle side, which leads to the disappearance of the angle. However, the reentrant angle ~s present (see fig. 3).
177
this position [8]. Therefore a molecule can be adsorbed at the top of the salient angle only by a preexisting row of molecules and this angle by no means can generate a new layer. Under these circumstances, new rows can be generated on the faces forming the salient angle either by two-dimensional nucleation or by continuous layer growth depending upon the saturation levels. The twin boundary shift is explained on the basis of an idealized mono-molecular layer growth model. In such a model, the filling up of the faces is reduced to the filling up of rows. A (110) twin is considered (figs. 4a and 4b) where the two faces in contact have the same extension along the twin boundary and the pair of the faces constituting the reentrant and salient angle have equal dimensions. It is assumed that the two rows of molecules (A and B in fig. 4b) are growing simultaneously on the two faces forming the salient angle and that the the in rapidity rows with or the which ratemolecules of growthare of incorporated two rows is unequal. As a consequence, one row (row B) adangle. Row B can also advance further, while vances faster, and reaches the top of the salient crossing the top, for two reasons: (i) row A (fig. 4b) growing from the opposite side fails to offer any resistence to the movement or to the onward growth of row B, as its growth rate is relatively low and hence cannot reach the twin boundary at the same time at which row B reaches the twin boundary; (ii) the direction of advancement of row B, after it crosses the top of the salient angle, always lies away from the direction of advancement of row A. Therefore, no repulsion arises, as the close approach of the molecular rows is avoided. Growth of the subsequent rows occurs on the modified rows and evidently the dimensions of the two layers thus resulting from the growth of such rows are different, even though the two initial layers in twin position are of equal dimensions. Clearly a boundary shift is associated with the change in the dimensions of the growth layers (fig. 4c) The cause of occurrence of a difference in the growth rates of the two corresponding faces forming a salient angle of a twin can be understood by taking into account the changes in local energy
178
P.R. V. Prasad et al
/
Twin boundary shift in (110) twins of n-octacosane
which alter the growth rate of a face and thus make it difficult to relate the growth rate to the supersaturation of the environment [9]. Konak [10] states that the growth rate of not only different faces of a crystal, but also the same face of different crystals is not same, when grown under identical conditions, and that the discrepancies are due to the internal structure of crystals, such as density and distribution of dislocations etc. Konak [101 argues that each crystal is an entity with its own growth characteristics. Therefore it seems that factors such as local energy changes and nature of the defect substructure may induce an anisotropy in the growth rates of rows (faces) A and B (fig. 4). Further it is observed that the shift of the twin boundary does not occur on the reentrant angle side. The position of the reentrant angle is a permanent kink [8], from which the filling up of the Kll0)~ rows of the crystal faces in a twin configuration starts spontaneously. Therefore: (i) the molecule that initiates growth on either side always lands at the kink or correct positton; (it) growth due to two-dimensional nucleatton originate at any other point on the faces constitut-
..
ing the reentrant angle, and advance towards the junction, which thus causing a boundary shift is highly improbable, because such nucleation is less preferred in the presence of an energetically favoured kink position. Because of these two reasons, twin boundary shift does not occur on the reentrant angle side, causing the disappearance of the reentrant angle. The authors thank Professor D.B. Sirdeshmukh for his helpful suggestions and encouragement.
References [I] S. Amelinckx, Cryst. 99 (1956) 16. [2] S. Amelinckx, Acta Acta Cryst. (1956) 217. [3} A. Keller, Phil. Mag. 6 (1961) 329
[4) F. Khoury, J. AppI. Phys. 34 (1963) 73. [5] I.M. Dawson, Proc. Roy. Soc. (London) 214 (1952) 72.
[6] F.C. Frank, Disc. Faraday Soc. 5 (1949) 48. [7] Stranski, Disc. Faraday Soc. (1949) A34 69. (1978) 406. [8] IN. R. Boisielle and D. Aquilano, Ada5 Cryst. [9] C.W. Bunn and H. Emmett, Disc. Faraday Soc. 5 (1949)
119. [10] AR. Konak, J. Crystal Growth 22 (1974) 67.