J. Phys. Chem. 1995,99, 7130-7136
7130
Growth and Perfection of Organic Nonlinear Optical Materials. Kinetics and Mechanism of the Growth of N-(4-Nitrophenyl)prolinol (NPP) Crystals from Methanol and Toluene Solution Boris Yu Shekunov,?Evelyn E. A. Shepherd, John N. Sherwood," and Graham S. Simpson Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow GI IXL, Scotland, U.K. Received: November 17, 1994@
Organic nonlinear optical materials are difficult to prepare in single-crystalline form due to the inhibition of growth by the solvent. This yields poor growth characteristics which result in the development of strain and inclusions in the grown crystal. A study has been made of the growth mechanism and morphology of N-(4nitropheny1)prolinol using both a polar and nonpolar solvent. The results are used to define conditions for the preparation of large crystals up to 2 x 0.8 x 0.8 cm3 of exceptionally high optical and structural perfection.
1. Introduction
N-(4-Nitrophenyl)prolinol (NPP) belongs to the new generation of efficient nonlinear optical materials developed by the molecular engineering of organic compounds. ls2 The commercial use of such crystals is still very limited due to the considerable problems experienced in their growth to large sizes. NPP crystals of high quality are impossible to grow using vapor or melt growth techniques because of the low vapor pressure of the material and its progressive decomposition near and above the melting temperature.2 Attempts have been made to grow this crystal from solutions by both temperature lowering and gel growth technique^.^,^ In both cases strong solvent-solute interactions cause the blocking of the growth faces, formation of liquid inclusions, and morphological instability. This is the principal limitation to the growth of large crystal^.^ Attempts to overcome this problem by, for example, increasing the supersaturation lead to the development of optical scattering centers such as solvent inclusions and twin formation. These problems arise as a consequence of the nature of this type of organic material with its highly polar molecular and crystallographic structure and relatively weak intermolecular bonds. For these reasons the growth of such crystals is very sensitive to variations in temperature, solute concentration, and type of solvent. The present examination was initiated to investigate the mechanism of NPP growth in solvents of widely different polarity in an attempt to optimize growth parameters and develop methods for the growth of large, optically more perfect crystals. 2. Experimental Section 2.1. Growth Rate Experiments. Growth rate studies were carried out using a laser interferometry technique6 in which the crystal face under investigation is the reflecting surface in one of the arms of a Michelson interferometer. This produces an interference pattern of the topology of the growing crystal surface. The number and development of growth centers which appear as hillocks on the surface can be monitored in-situ during growth and specific growth rates and growth mechanisms defined. The particular measurements which are made from the interference patterns are the slope p of the growth hillocks and the velocity v at which these hillocks expand laterally. These Present address: Department of Pharmaceutical Sciences, De Montfort University, Leicester LE1 9BH, England. Abstract published in Advance ACS Abstracts, April 1, 1995. @
0022-365419512099-7130$09.0010
parameters are obtained from the development of the interference patterns with time. The growth rate of the hillock normal to the surface is then R = pv. The crystallization system was designed to enable the study of seed crystals using only small volumes (-100 mL) of methanol solution at temperatures up to 50 "C in a flow-through optical cell. The seeds with a chosen natural face were cut from crystals grown in large crystallizers. The area of each face was around 4 mm2. For crystallizationfrom toluene solution, the high temperature necessary and the large viscosity of the solution prohibited measurements in the solution flow system. Qualitative observations of crystal habit variations were made in a stirred thermostated vessel? The crystallographic indices of the faces of NPP crystals were determined by optical goniometry using relatively large (2 cm) NPP crystals grown under well-defined conditions in a standard constant temperature solvent evaporation growth system. Solutions were prepared using recrystallized NPP synthesized according to previously described methods.' Solvents used were of analytical grade (BDH Anal&). The supersaturation u was calculated assuming ideal behavior u = ln(c/c,) (c and co are the actual and saturated solution concentrations). In the experiments with methanol, o was altered by varying the solution temperature t. co was calculated from measured solubility curves (Figure 1). 2.2. X-ray Section Topography. X-ray section topographs7 were taken using synchrotron radiation collimated using 50 pm slits. Possible radiation damage was eliminated by inserting an 0.8 mm aluminum filter in the incident beam. The diffraction conditions were selected to give the best compromise between geometric distortion of the image and absorption of the X-rays: 080 reflection (Fog0= 40.5) at 1.54 8, and O B = 24.4'. The possibility of harmonic contamination was minimal. At this wavelength the product of the mass absorption coefficient p and the crystal thickness t was p t = 1.9. The topographs were recorded on Ilford L4 nuclear plates.*
3. Results 3.1. Growth from Methanol Solution. 3.1.1. Crystal Habit and Development. NPP belongs to the crystal symmetry group P21 in which the 2-fold axis is the coordinate b axis. The basic forms of NPP crystals are (OOl}, (OiO), {101}, (01 1). (1 lo), (Oil), and (170) (Figure 2). The indices of faces have been chosen to agree with the discrimination between the
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 18, 1995 7131
Growth of Organic Nonlinear Optical Materials
-
*
-
a
0 26
-
a
-
-
*
34
b w
w
-
42
t°C
m
Figure 1. Solubility curves of NPP in (a) methanol and (b) toluene.
J l ? Figure 3. Interferometry pictures of the (001) face during growth from
a
Figure 2. Habit of NPP crystal (a) and a crystal grown in methanol at o = 0.08 and t = 32 "C from elongated seed.
polar (010)and (070) faces by solvent attachment following crystal molecular structure considerations (see section 4.1). Under normal circumstances, large NPP crystals cannot be grown. Small prismatic seeds produced by the direct nucleation of methanol solutions have completely blocked (OiO), (01l), (110),(Oil),and (170) faces. The result of this is that attempts to continue growth lead to the diminishment and loss of the more rapidly growing initially major ( 101) and (001) faces and eventually to the cessation of growth. Thus the crystal seeds can never be developed to large crystals. The development of each face is characterized by a critical supersaturation, a,, at which growth starts at a given temperature. The a, of the (011) and ( 1 lo} faces is around 0.20 and closely approaches the limit of the metastable zone of the solution (a 0.25 at t = 25 "C) which defines the point at which bulk nucleation can occur. The a, of the (Oil), (liO ), and (OiO) face was greater than the metastable zone width. The (001} and ( 101) faces had significantly smaller critical supersaturations: a,((OOl}) = 0.064 and a, ({ 101)) = 0.074. In methanol solution, NPP crystals also have an almost zero growth rate along the b axis. As will be shown from the kinetic data, the normal growth rates R of the ( 101) faces are always higher than that of the (001) faces if both are growing simultaneously. Within the interval 0.064 < a < 0.074, the { 101) growth sectors are dominant. The (001) growth sectors dominated at higher supersaturations. 3.1.2. (001) Faces. For the full range of supersaturation a < 0.25, growth on the (001) and (101) faces occurs by a dislocation-controlled mechanism as evidenced by surface topography (Figure 3). Figure 3 demonstrates a basic peculiarity of the (001)growth surface (which is also typical for the ( 101) faces). At supersaturations a little higher than a,, a large number of dislocation hillocks with distorted shapes could be seen (Figure 3a). The hillock form, slope, and normal growth rate were different for different hillocks and also changed with time for a particular hillock. Sometimes a particular hillock disap-
-
methanol solution: (a) o = 0.08 and (b) o = 0.24, t = 30 "C.
peared but later was reactivated by the emission of growth steps from neighboring hillocks. Usually there were around 10 sources over the 4 mm2 surface. Although the growth rate of individual hillocks oscillated with a period of ca. 5-10 min, the average normal growth rate of the whole face remained almost constant. Each hillock had too short a lifetime to cover all the face and dominate the growth process. As the supersaturation was increased, the oscillations in growth rate became smaller. At the highest a only a few hillocks remained, each retaining a similar elliptical form (Figure 3b), and the oscillations ceased. The relationship between the slope of the hillocks p and supersaturation a is shown in Figure 4. In the early stage of growth, the mean p increased with a. The dispersion of p at low a reflects the different activity of the hillocks within the low range of supersaturation. Finally, at high values of a the oscillations stopped and the slopes of the hillocks were almost constant. This was the region of "stable" growth. The above changes are reflected in variations of the tangential velocity of the vicinal surface v (velocity of growth steps) with a (Figure 4). The v(a)function is characterized by a curvature (I) just above a, and becomes linear at the largest supersaturations (111). The interval I1 is a transitional region. A comparison of the behavior of v and p confirms that the velocity decreases with interstep distance. This defines the dispersion of v at constant a within the supersaturation range I and, to a lesser extent, in range 11. The growth of the face which occurs within the supersaturation range I (Figure 4) is unstable because of spontaneous fluctuations of step velocity and, consequently, the normal growth rate. Area I11 corresponds to values of the supersaturation where the influence of solvent on the steps becomes linear. Under these conditions the kinetic coefficient of step movement can be calculated from the f ~ r m u l a p: ~= (dv/da)dc* where 6vh3a is the slope of the linear part of the curve v(a),d = 1.39 g/cm3 is the crystal density, and c* = 0.051 g/cm3 is the concentration of the solution at the given temperature. From Figure 4b (dashed line): /3 = 0.15 cm/s at t = 30 "C. The formation of a plateau in the function p(a) (Figure 4a) is consistent with the interpretation that hillocks originate at the emergent sites of wide dislocation bunche~.~ Figure 6a shows that the normal growth rate, R = pv, is almost linearly dependent on a (the spread of the points reflects the growth rate oscillations). This arises from the combination of the limiting behavior of p at high supersaturation and nonlinear behavior of v at low supersaturation (due to solvent effect); thus R(a) is linear.
Shekunov et al.
7132 J. Phys. Chem., Vol. 99, No. 18, I995
4
4'
O
1
0
0
2l
-3~m/s
I
4 /
4'
I I
0,10-*
1 5
1-0
20
Figure 4. Dependence on supersaturation of the step velocity (v) and steepness (p) of dislocation hillocks on the (001) face. Methanol solution, t = 30 "C.
3.1.3. (101) Faces. Growth steps on the (101) faces appeared to interact with methanol in a similar manner to those on the (001) faces. However, for all of the measured supersaturation ranges, v(u) was nonlinear (Figure 5). Taking into account that ~ ~ ( ( 1 0 1>) )ao((OO1)), one can speculate that adsorption of the solvent is somewhat stronger on the (101) face. The functionp(u) does form a limiting plateau similar to that on the (001) faces. The major difference between the growth of these faces is that the growth rate for the (101) faces is almost 1 order of magnitude greater than that for the (001) faces at higher supersaturations (Figure 6 ) . As a consequence, growth will be more strongly influenced by the speed of solution flow across the growing surface. Curves 1 and 2 in Figure 5 show how v(u) changes with solution flow speed for the hillocks in the center of the face. It was also noticed that the velocity of the growth steps near the crystal edges was as much as 2 times larger than in the face center. All of these factors are consistent with the existence of a strong, nonuniform boundary layer. Such a nonuniform distribution of surface supersaturation promotes the formation of macrosteps and hence inclusion capture during growth at high supersaturations. Two kinds of macrosteps were observed. The first had the character of kinematic waves moving rapidly. The macrosteps of the second type were slow moving, high, and most dangerous because of their tendency to trap inclusions. The risers of these macrosteps are, in fact, so high that they reflect the geometry of adjoining faces. Such morphological changes are very typical for organic crystal^.^^^ Under poor hydrodynamical conditions, the macrosteps formed readily on the (101) face. The best environment for the growth was found to be with the direction of solution flow perpendicular to the b axis of the crystal. The
10
15
Figure 5. Dependence on supersaturation 0 of the step velocity (v) and steepness (p) of dislocation hillocks on the (101) face. Velocity of solution flow: curve 1,30 c d s ; curve 2 , 6 c d s ; methanol solution, t = 30 "C.
equal development of the (001) and { 101) growth sectors was observed in the range of supersaturation 0.07-0.08. 3.1.4. (011) Faces. Growth of the (011) faces was zero for all supersaturations lower than u, = 0.204. The blocked faces were smooth, and no dislocation hillocks were observed. At higher supersaturations two changes took place. First, very steep dislocation hillocks appeared on the face. Second, the remaining smooth surface between the dislocation hillocks showed very slow growth. The last phenomenon is probably connected with the initiation of two-dimensional nucleation on these faces simultaneously with the vicinal growth. The growth rate dependencies of the vicinal and smooth surfaces are shown in Figure 7. Eventually, with development of the dislocation hillocks, the (01 1) surfaces became rough. Massive inclusion formation occurred in these growth sectors on further growth. 3.2. Growth from Toluene. The solubility of NPP is 10 times smaller in toluene than in methanol at room temperature (Figure 1). All faces are entirely blocked in toluene solution at temperatures up to 50 "C over the whole metastable zone (u % 0.27). Crystals can only be grown from this solvent at relatively high temperatures 70-90 "C where growth has specific additional peculiarities. At these temperatures the (01l), (1lo), and (070) faces (see Figure 2) became rough and grew by a continuous growth mechanism as indicated by their curved form. The faces (001) and ( 101) remained flat and presumably propagated by a dislocation mechanism. They rapidly grew out. For geometrical reasons, the relatively slow-growing (01 1) and (110) faces were suppressed by the slowest growing (OiO) face and, consequently, were not presented in the final habit. Figure 8a shows the result; such crystals have a quasiconic morphologically unstable surface (note the macroscopic roughness in the figure). The effect of that instability which
Growth of Organic Nonlinear Optical Materials
J. Phys. Chem., Vol. 99, No. 18, 1995 7133 b
~,10-5 b 34
I
0.5 cm
2-
a
T
1.
b 1
Figure 8. NPP crystal grown in toluene at CJ = 0.05 and t = 85.5 "C
.
(a) and t
R , 1 O-%m/s
a-
a
. 4-
,
a,1 0 - 2
* 1-0
As temperature decreased, the initiation of both the faceblocking mechanism and the roughening transition occurred at different temperatures for the different faces. This resulted in significant changes to the morphology of the crystal. Figure 8b shows the elongated (parallel to the b axis) habit of the crystal which developed during temperature decrease from 90 to 60 "C. Under these conditions, the smooth { lOl} faces were blocked at some temperatures while the other faces resumed growth by the continuous mechanism. The lowest transition temperature was observed for the (OiO), (Oil), and (170) faces (in Figure 8b these faces have rounded form). The (001) and { lOl} faces dominate. As a consequence of structural imperfections and solvent incorporation, the crystals growing at high temperature were deeper in color and less transparent than those prepared from methanol at low temperature.
2-0
Figure 6. Normal growth rate of dislocation hillocks on the (a) (001) and (b) (101) faces. Methanol solution, t = 30 "C.
I
* 60 "C (b).
R, 10 -6 cm/s
19 20 21 u, 10-2 Figure 7. Dependence on supersaturation o of the normal growth rate
(R) of the dislocation hillocks (a) and flat surfaces between dislocation hillocks (b) on the (011) face.
increased with supersaturation and crystal size probably reflects an increase in thickness of the diffusion layer around the crystal. The habit change implies that toluene influences most strongly the growth of those faces which intersect the b axis and, most of all, the (070) face. This was also observed in methanol. However, the a, determined for all faces was small: a, .e 0.01 at t = 85 "C. This is possibly a consequence of the high temperature and the increased concentration of the solution. The latter is probably the more important factor, since for toluene co(t)is very nonlinear and ~ ~ ( 8 6 "C)/c,(30 .5 "C) % 45.
4. Discussion 4.1. Interaction between the Solvent and Growing Crystal Faces. The kinetic data define that the growth and perfection of NPP crystals are controlled by solvent-crystal face interactions, solubility, and growth mechanism. The last is consequent upon whether or not a crystal face is thermodynamically rough yielding continuous growth or smooth yielding dislocation growth. All of these factors are closely related to the crystallographic structure of the crystal faces. Figure 9 shows the two most representative projections of the crystal structure on the (100) and (010) planes, respectively, drawn by means of the computer program ATOMS.I0 NPP molecules are known to form zigzag chains along the b axis with the mean plane of the molecules lying approximately in the (101) plane. They are held head to tail by hydrogen bonds between nitro and hydroxyl groups in adjacent molecules. The configuration of a particular face can be found by minimizing the mean surface energy (W) for all possible planes comprising the face. This means that the completed growth layer must have a minimum surface (and adsorption) energy: n
m
where €0 is energy of interaction i a n d j molecules calculated by means of the program HABIT" using 6-12 interatomic potentials12 and ground state atomic charges computed using the CNDO method.I3 n is a number of all potential sites for adsorption. m implies the number of interrupted bonds on a flat surface. The results are given in Table 1.
Shekunov et al.
7134 J. Phys.
Figure 9. (100) (a) and (010) (b) projections of the NPP crystal structure. The dashed lines show the structure of the most important faces. TABLE 1: Mean Surface Energies W, Slice Energies E, and Face Factor = W/E for the Most Important Crystal Faces of NPP type and density faces W.kcdmol E, kcdmol of H-bonds 0.289 none 24.267 7.013 (001) 0.415 none 22.102 (100) 9.178 0.426 none 21.926 9.354 (101) ~~~
(010) (010)
{llO} 11101 {Oll} {Oll}
8.602 8.602 9.534 9.534 8.068 8.068
22.678 22.678 21.746 21.746 23.212 23.212
(++I (--I
0.379 0.379 0.438 0.438 0.347 0.347
(+)
(-1 (+I (-1
The last column shows presence of interrupted hydrogen bonds. The assignment (+) means the positive direction of hydrogen bonding formed by the -OH groups, (-) is the negative direction of the bonds comprimising the -NO* group, and (++) and (--) imply double density of these bonds. LI
The face factor 6 = W/E defines the surface transition temperature (the greater 6, the lower the surface transition temperature). The calculated lattice energy is (W E) = 3 1.28 kcaumol. The dipole moment of NPP was found to be 7.447 D. The qualitative picture shown by the table is insensitive to changes in interatomic potential^^^*'^ and variations in atomic charges found by means of the MNDO method.16 The dashed lines in Figure 9 show different types of faces. The (OOl), { 1011, and (100) faces formed by the zigzag chains have no interrupted hydrogen bonds. The (101) faces have the maximum reticular density of atoms. However, they are not the most energetically favorable faces (see Table 1). This results from the very strong van der Waals interaction between NPP molecules, especially along the a axis. Each molecule on the (010) and (070) faces has interrupted (normal to the face) hydrogen bonds. The former face comprises -OH groups, the latter consists of -NO2 groups. As Figure 9a shows, one of two molecules on the (011) and (Oil) faces possesses a normal hydrogen bond formed by :OH and -NO2 groups, respectively. The same can be found on the (110) and (170) faces. The magnitude of the surface energy for the (h01) faces are consistent with the observed growth dependencies. The { 101) and (100) faces have greater W than the (001) faces and, correspondingly, are faster growing faces. Adsorption of methanol should be stronger on the (101) faces which is in
+
agreement with experiment (see 3.1.2 and 3.1.3). However, for the faces intersecting hydrogen bonds, the values of W (and 6) itself cannot explain observed variations in habit and the transitions from smooth to rough faces. The peculiarities of solvent-face interaction must be taken into account. For example, according to the calculations, the painvise sets of crystal faces (010) and (OiO), (110) and (011) and (Oil) have the same surface energies and, therefore, should show the same growth rate if proportionality between surface energies in vacuum and solution is assumed.” In fact, the faces oriented toward the positive direction of the b axis have a much weaker interaction with methanol and toluene (smaller magnitude of go) than the negative faces. It is very likely that this behavior arises as a consequence of the formation of a strongly adsorbed layer of solvent molecules on those faces. To remove such a layer during motion, the steps have to overcome an activation barrier which corresponds to the adsorption energy of solvent molecules. Hydroxyl groups of methanol molecules would form strong hydrogen bonds with the nitro groups of NPP molecules in the negative direction of the b axis which would be similar to the bonding between NPP molecules and themselves. The calculated strength of these bonds is about 3 kcal/mol. In contrast, the “positive” crystal faces with -OHtails would form relatively weak hydrogen bonds similar to those between methanol molecules (0.5 kcal/moll*). The (h01) faces would not form specific hydrogen bonds at all. The (070) face has double the number of N02-OH bonds, and therefore, growth would be decelerated to a greater extent than for the other faces. There is no hydrogen bonding possible between NPP and toluene molecules. However, the local dipole moment of the nitro group (-1.23 D) is much greater than that for the hydroxyl group (-0.17 D). Correspondingly, the dipole-dipole interactions are potentially strongest for the (OiO), (lie), and (Oil) faces and weakest for the (001) and (101) faces. As a result, the last faces should grow much faster. The low solubility of NPP in toluene means a high concentration of the solvent on all crystal faces19 which would obstruct growth at low temperatures. The stronger the adsorption of solvent on a crystal face, the lower the surface energy and the lower the surface transition temperature. l9 Consequently, the negative faces (OTO), (Oil), and (170) all should remain more rough than the (h01) faces at lower temperatures. It should be noted that impurities in solution could yield similar effects. However, the purity of
Growth of Organic Nonlinear Optical Materials b
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J. Phys. Chem., Vol. 99, No. IS, 1995 7135
a
0.4cm
Figure 10. Photograph of an optically perfect NPP crystal.
b
the material used coupled with consideration of the behavior in both methanol and toluene leads us to conclude that the solvent effect dominates.
5. Preferable Growth Conditions for Large Crystals Although preliminary experiments show that crystals prepared from solution in methanol show the best perfection, the fact that growth from this solution is inhibited along the [OlO] and [070] directions places a major limitation on its use for the preparation of large crystals. The consequences of this restriction are that small seeds prepared from this solution show only limited imperfect growth as stubby needles elongated along the [ 1001 direction. Samples of sufficient size for optical assessment cannot be prepared. In contrast, growth from solutions in toluene does yield growth along [OlO], but the product is not as perfect as from methanol. Adjustment of the temperature and supersaturation can however yield at t 60 "C a habit much elongated along the [OlO] direction and with well-developed but small growth sectors in the lateral directions (Figure 8b). Due to the low solubility of NPP in toluene, further growth is slow and only very thin needle-like crystals result even after lengthy growth periods. Such crystals form ideal seeds for use for continued growth from methanol solution since, in this solvent, growth of high-quality material can be encouraged in the lateral directions ((001) and (101)) of the needle-like seed. The higher solubility of the material in this solvent encourages more rapid growth. This strategy was attempted. Selected needle-like crystals carefully grown from toluene solution up to 2 cm in length were suspended in saturated methanol solution in a standard temperature-lowering growth system. The crystals were suspended vertically and rotated around the [OlO] axis with accelerated rotation and reversal to ensure that the hydrodynamic conditions noted above (section 3.1.3) were observed. Following some minor dissolution of the surface to remove extraneous nuclei which might yield dislocations and macrosteps, the surfaces were refaceted at low supersaturation. This completed, the supersaturation was increased to 0 = 0.07-0.08 (section 3.1.3) and the crystal allowed to grow. This supersaturation should allow the best development of the ( 101) faces while not restricting development on (001). Choice of the more ideal (higher supersaturation) conditions for the growth of the (001) sectors would undoubtedly lead to inclusions in ( 101) sectors and unusable crystals. The result was the formation of crystals of long dimension ([OlO]) equal to that of the original seed and 1-2 cm in the lateral dimension. Figure 10 shows a photograph of such a crystal prepared from a seed 2 cm long. The optical perfection is apparent. The crystal contains no visible striations or inclusions such as are commonly found in crystals of this material.
C
-
Figure 11. (a) Section topograph from the central portion of a NPP crystal. (b) Schematic representation of the topogragh shown in'panel a. (c) Section topograph from the lower portion of the same NPP crystal with additional (1 10) sectors.
6. Structural Characterization
6.1. X-ray Topographic Studies of Crystal Perfection. Figure 1la shows a synchrotron radiation section topograph recorded from the central portion of the NPP crystal. Figure 11b shows a schematic representation of the position of the main features in Figure lla. The topograph exhibits all the major defects which are likely to be associated with solution grown crystals, namely, seed position ( S ) , growth sector boundaries (B), growth striations (G), and growth dislocations (D). The needle seed crystal ( S ) is visible in the center of the image. The interface between the seed and the crystal is not as highly strained as that exhibited by MBANP.' This is probably due to the extremely small size of the seed and the careful control of the initial growth onto the seed. Growth sector boundaries between the (001) and (101) sectors are clearly visible in the topograph. The position and nature of the growth sector boundaries give a clear insight into the growth history of the crystal. Two of the four growth sector boundaries (between (001) and (101) and (007) and (107) are almost perfectly straight, indicating that the relative growth rate of the two sectors was uniform. These straight growth sector boundaries are imaged as dark lines, indicating that they arise mainly from kinematical contrast. The other pair of growth
7136 J. Phys. Chem., Vol. 99, No. 18, 1995 sector boundaries follows a less regular path indicating fluctuations in the relative growth rates of the sectors. They are not so highly contrasted. Growth striations (G) which are visible in both (001) sectors arise from the incorporation of either solvent or other impurities into the growing crystal face. Despite this structural imperfection, the sectors were optically transparent. This imperfection contrasts markedly with the situation in the two { 101) sectors which show almost uniform contrast indicating little or no such incorporation. In general, the ( 101) sectors show fewer defects. This confirms the observation that low supersaturation under which the growth occurred was more favorable for perfect growth on the (101) faces (sections 3.1.3 and 5 ) . All the dislocations present in the topograph can be grouped under the general description of growth dislocations. These dislocations tend to be nucleated at seedcrystal interfaces a n d or inclusions. The dislocations also lie near parallel to the growth normal for each particular sector. In the (001) sectors a bundle of dislocations (Dl) has nucleated at the seed interface and propagated through the sector to intersect the free surface at an angle of close to 90”. In the (101) sectors the only dislocations visible in this particular reflection are again nucleated at the seedcrystal interface (D2). Figure l l c shows a corresponding topograph which was recorded closer to the “capped” end of the crystal. In this case additional faces, { 1lo}, are imaged but the overall picture in terms of the defect content and position is very similar to that shown in Figure 1la. Both topographs highlight the extremely high quality of the sample crystal. It is interesting to note that the NPP crystals grown from solution in methanol exhibit a morphology dominated by {OOl} and { 101) faces whereas those crystals grown from gel4 show (010) as the major morphological feature. In addition we have found no evidence of twinning in any of the NPP crystals grown from solution in methanol which is in contrast to results obtained by Andreazza et aL4 from NPP crystals grown in gels.
7. Conclusions This study confirms that investigations of growth mechanism and solvent-interface interaction can be used as a method to define the optimum growth parameters for a particular crystallization system. It has been shown that optical quality NPP crystals can be obtained from both methanol and toluene solutions by varying growth temperature and seed forms. For
Shekunov et al. methanol solutions this can be achieved by developing the (001) and ( 101) crystal sectors of needie-like seeds (prepared from toluene solution) within an optimum supersaturation range and under properly selected hydrodynamical conditions.
Acknowledgment. This research has been supported by a grant from the US.Naval Research Laboratory. We gratefully acknowledge the help and advice of Dr. A. Israel in computer calculations. The synchrotron radiation experiments were carried out at DRAL’s Daresbury Laboratory. The assistance of the director and his staff in performing these experiments is gratefully acknowledged. References and Notes (1) Zyss, J.; Nicoud, J. F.; Coquillay, M. J. Chem. Phys. 1984, 81, 4160. (2) Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: New York, 1987; Vol. 1, p 270. (3) Youping, H.; Genbo, S.; Feng, P.; Bochang, W.; Rihong, J. J.Cryst. Growth 1991, 113, 157. (4)Andreazza, P. Josse, D.; Lefaucheux, F.; Robert, M. C.; Zyss, J . Phys. Rev. B. 1992, 45, 7640. ( 5 ) Shepherd, E. E. A.; Sherwood, I. N.; Simpson, G. S.; Yoon, C. S. J. Cryst. Growth 1991, 113, 360. (6) Rashcovich, L. N.; Shekunov, B. Yu. J. Cryst. Growth 1991, 112, 183. (7) Halfpenny, P. I.; Sherwood, J. N. Philos. Mag. Lett. 1990, 62, 1. (8) Lang, A. R. In Diffraction and Imaging Techniques in Materials Science; Amelinckx, S., Gevers, R., Van Landuyt, J., Eds.; North-Holland: Amsterdam, 1978; p 623. (9) McEwan, A.; Shekunov, B. Yu; Shepherd, E. E. A.; Sherwood, J. N. Manuscript in preparation. (10) Dowty, E. Am. Mineral. 1980, 65, 465. (11) Clydesdale, G.; Docherty, R.; Roberts, K. J. Comp. Phys. Commun. 1991, 64, 311. (12) Lisson, S.; Hagler, S. T.; Dauber, P. J. Am. Chem. SOC.1979,101, 5111. (13) Morley, J. 0.; Pavlides, P.; Pugh, D. Int. J. Qwnt. Chem. 1992, 43, 7. (14) Momany, F. A.; Cmthers, L. M.; Maguire, R. F.; Scheraga, H. A. J. Phys. Chem. 1974, 78, 1595. (15) Nemethy, G.; Pottle, M. S.; Scheraga, H. A. J. Phys. Chem. 1983, 87, 1883. (16) Docherty, R.; Roberts, K. J.; Dowty, E. Comp. Phys. Commun. 1988, 51, 423, (17) Bennema, P.; Van der Eerden, J. P. In Morphology of Crystals; Sunagawa, I., Ed.; Terra Scientific Publishing Co.: Tokyo, 1987. (18) Del Bene, J. E. J . Chem. Phys. 1971, 55, 4633. (19) Voronkov, V. V.; Chemov, A. A. Sov. Phys.-Crystallogr. (Engl. Transl.) 1967, I 1 57 1. ~
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