Abstract
A new chelating agent diethylenetriaminepentaacetic acid (DTPA) is employed for the first time to investigate its effect on the metastable zone width, crystal growth and characterization of DAST single crystals. The width of DAST in methanol is measured with and without adding DTPA and EDTA. The presence of DTPA in the DAST solutions enhances the metastable zone width as compared to the EDTA added solutions. The pure and the chelating agents added DAST crystals are grown by the slow cooling method and their crystal size distributions are presented. The addition of the chelating agent DTPA significantly suppresses spurious nucleation and cluster formation in the DAST solution, so that fewer, but larger crystals are easily obtained. The crystalline powder of the grown crystals is examined by X-ray diffraction. The Fourier transform infrared (FTIR) and UV–vis–NIR spectroscopic analyses have been done for the DAST samples. The hardness of the grown pure, DTPA and EDTA added DAST crystals is measured on the (0 0 1) face under different loads.
© 2006 Elsevier B.V. All rights reserved.
PACS: 81.10.Dn; 81.70.Pg
Keywords: Metastable zone width; Nucleation; Solubility; Growth from solution; Recrystallization; Organic compounds; Nonlinear optical materials
1. Introduction
Among the many well known nonlinear optical (NLO) materials, the organic ionic salt crystal 4-N,N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST) is a very promising NLO material, which has been demonstrated to have a very large NLO susceptibility and the largest electro-optic (EO) coefficient of all materials researched up to date[1–5]. It belongs to the monoclinic crystal system with the noncentro symmetric space group Cc and the point group m. It has the following lattice parameters: a = 10.365 ˚A, b = 11.322 ˚A, c = 17.893 ˚A, Z = 4 and
β = 92.24◦[1].
Although DAST crystals have shown the excellent NLO properties in many applications, the growth of high-quality large crystals has been a challenging task. Pan et al. and Sohma et al.
∗Corresponding author. Tel.: +886 2 2363 3917; fax: +886 2 2363 3917.
E-mail address:[email protected](C.W. Lan).
[6,7]have illustrated the feasibility of the growth of large sin-gle crystals more than 16 mm in diameter by seeded growth. Recently, we have grown 2 cm size DAST crystals by the two zone growth technique [8]. Even though high quality DAST crystals were grown by the step nucleation method (SNM), the number of nuclei is still difficult to control. Among the several methods to manipulate crystal nucleation behavior and further controlling crystal growth, the use of a chelating agent, such as ethylenediaminetetraacetic acid (EDTA) could be useful in nucleation control as well. Indeed, several successful exam-ples have been obtained for both organic and inorganic crystals
[9–13].
In search of new chelating agents, we have found that diethylenetriaminepentaacetic acid (DTPA) is a suitable chelat-ing agent to suppress the formation of nuclei and clusters in the case of DAST solution. It is a colorless and white powder. This polycarboxylic multidentate ligand has an excellent chelat-ing ability compared to other hexadentate ligands like EDTA. DTPA binds to a number of metals like Zn2+, Mg2+, Mn2+,
0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.11.004
Fig. 1. Chemical structures of EDTA and DTPA anions.
Co2+, Cu2+, Pb2+, Ca2+and various radio-nuclides. DTPA has five acetate moieties linked by a molecular backbone (Fig. 1) which can tightly complex a metallic ion and forms more sta-ble complexes than that made by other chelating agents like EDTA. It is structurally similar to EDTA except that it has an extra basic unit of NCH2COO−and diethylene in place of
ethy-lene in EDTA. The chemical structures of EDTA and DTPA are shown inFig. 1. Since DTPA is stronger than EDTA, it is often used to lower the effective free metal content of processes. Compared to EDTA, DTPA forms more stable metal complexes, which are more resistant to oxidation and are more compatible in concentrated electrolytes, liquid alkali, etc. As per the above statements, DTPA is expected to be more effective in control-ling the nucleation rate and suppressing the cluster formation by capturing metallic ions in the solution. It has been found that the incorporation of DTPA in the DAST solution enhances the zone width significantly compared to the pure and EDTA added solu-tions. Also it is observed that it reduces the number of spurious nucleation and enhances the growth rate of the crystals.
In this present study, the metastable zone width was measured for pure, DTPA, EDTA added DAST solutions. Their crystal size distributions were analyzed. The grown crystals were char-acterized by the XRD, FTIR and UV–vis–NIR spectroscopic analyses. The hardness of the grown crystals was estimated by the Vickers micro hardness studies.
2. Experimental
DAST was synthesized by the condensation of 4-methyl-N-methyl pyri-dinium tosylate and 4-N,N-dimethylamino-benzaldehyde in the presence of piperidine[8,14,15]. 4-Methyl-N-methyl pyridinium tosylate used in the con-densation reaction was prepared from equimolar quantities of 4-picoline and methyl-p-toluenesulphonate. The entire synthesis process was conducted in dry nitrogen atmosphere to avoid the formation of the orange hydrated form of DAST. The resulting DAST material was further purified by recrystallization from methanol. The recrystallized DAST crystals were used for the nucleation and crystal growth experiments.
For all the experiments, 20, 30 and 40 ppm of EDTA and DTPA were used. The nucleation experiments were carried out in small cells (20 ml) with and without the addition of DTPA and EDTA. The advantages of the small experi-ment have been described in ref.[15]. The small-cell experiments were carried out by putting growth cells into a tray and immersing the tray in a constant-temperature bath (CTB) for nucleation and crystal growth experiments. The accuracy of CTB is within±0.01◦C. In each cell, to avoid the sticking of the
Fig. 2. A clear view of inert fluid-DAST solution interface. crystals on the glass wall, an inert fluid, 3 M Fluorinert electronic liquid FC-77, was used to let the DAST solution sit; 5 ml of preheated FC-77 inert fluid and 10 ml of DAST solution were added for each cell. The clear interface between the inert fluid and the DAST solution was very helpful in the observation of nucleation (Fig. 2). In addition, FC-77 did not wet DAST crystals and hence it helped significantly in harvesting the DAST crystals grown while keeping the solvent from sticking to their surface. To start the nucleation experiments, the solutions were kept at 50◦C for 2 days for homogenization and then cooled to the saturation temperature of 40◦C at a cooling rate of 1◦C h−1. The cool-ing rate was reduced to 1◦C day−1from the saturation temperature. Metastable zone width was measured by the conventional poly-thermal method[16,17]. In this method, the equilibrium saturated solution was cooled from the overheated temperature until a visible crystal was observed.
In order to check the consistency of the results from the nucleation experi-ments, four experiments for each case were carried out under the same condition. By means of a diffractometer (MAC Science Diffractometer MXP-3), X-ray diffraction pattern was recorded for the powdered DAST samples in the range of 5–40◦where the monochromatic wavelength 1.5418 ˚A (Cu K␣) was used. In order to check the incorporation of DTPA or EDTA in the grown DAST crys-tals, a FTIR spectrometer (Perkin-Elmer, Spectrum One) was used to record the spectrum the range of 600–2000 cm−1for the corresponding DAST crystalline powders. Using a UV–vis spectrometer (Model: Jasco 570, Japan), the crystal transmission ranging from the visible to near-IR region was measured. Vickers hardness measurements were done on (0 0 1) face of the grown crystals using a micro-hardness tester (Model HMV-2, Shimadzu, Japan). Five indentations were made on each sample for each load. The diagonal lengths of each indentation were recorded and the averages of the diagonal lengths were used for calcula-tions. The microhardness value of a particular sample was taken by averaging the different values of microhardness at various loads where near constancy in the value of hardness is achieved. At least three crystals from each case were used for the reproducibility.
3. Results and discussion
3.1. Metastable zone width of pure, DTPA and EDTA added DAST solutions
From the solubility studies, no significant change in solubility was observed in the additives added DAST solutions because the
Fig. 3. Metastable zone width of (a) pure and DTPA added DAST solutions and (b) EDTA added DAST solutions.
amount of the chelating agent added was very small (20, 30 and 40 ppm). The solubility of DAST reported in the present paper was consistent with the data given by Wu et al[15]. In ref.[15], the metastable zone widths of the DAST solutions prepared at a saturation temperature of 40◦C were about 4, 6 and 7◦C for the pure (0 ppm EDTA), 10 and 20 ppm EDTA added solutions, respectively. The metastable zone widths (MZW) of pure, DTPA and EDTA added DAST solutions as a function of temperature are shown inFig. 3. From the figure, the metastable zone widths of the DAST solutions saturated at 41.5◦C were about 4.5, 5.8, 2.8 and 2.3◦C for the pure (0 ppm EDTA), 20, 30 and 40 ppm EDTA added solutions, respectively. Only a small variation in the metastable stable zone was observed as compared to the reported values. But in general, the addition of EDTA (up to 20 ppm) enhanced the metastable zone width in both reports. Here, it should be mentioned that the metastable zone width was affected by a number of factors, such as stirring rate, cooling rate of the solution and presence of additional crystals or impurities
[18–21]. In the present study, 40 ppm of DTPA was found to be the threshold concentration in the DAST solutions, at which the
maximum amount of EDTA that could be added for the exper-iments and too much of EDTA added caused the precipitation of EDTA crystals on the DAST crystals leading to poor surface quality. The same situation was again observed in the present work. Hence 20 ppm EDTA added DAST solution showed an increased metastable zone width but as its concentration was increased, the width was found to be decreased abruptly. This trend was not observed in the case of DTPA added DAST solu-tions. If the concentration of DTPA was increased, the zone width also increased. Since DTPA was found to be more sol-uble than EDTA in DAST solution, the precipitation of DTPA crystals was not observed whereas in the case of EDTA added DAST solutions, the precipitation of EDTA crystals was found. Also, it was believed that DTPA at higher concentration lev-els bonded strongly with the metallic ions rather than EDTA due to polycarboxylic multidentate ligands, which lead to an excellent chelating ability compared to other hexadentate lig-ands like EDTA. According the above reasons, the zone width was an erratic way in the EDTA added DAST solutions but it regularly increased in the case of DTPA added DAST solutions. It was believed that the enhancement of metastable zone width in all the DTPA added solutions used in the present work was mainly due to the reduction of chemical activity due to the strong chelating action of DTPA.
3.2. Crystal growth and crystal size distribution
For the crystal growth of pure, DTPA or EDTA added DAST, 20, 30 and 40 ppm of DTPA or EDTA were added in the DAST solutions. The DAST crystals grown at the different concentra-tions of DTPA and EDTA are shown inFig. 4. The harvested crystals were classified according to their sizes, and the size dis-tributions are given inFig. 5. Since there were many tiny crystals in the pure and EDTA (30 and 40 ppm) added DAST solutions, their weights were used rather than number in expressing size distributions. The results of the experiments were consistent and the crystal photographs shown in Fig. 4are from one of the experimental runs, for reference.
From the results of the crystal size distribution, DTPA (20, 30 and 40 ppm) interacted strongly rather than EDTA (up to 20 ppm) with DAST or impurity ions in the solution. The strong interaction between metal ions present in the solutions was dependent on many parameters such as the size of the metal ion, the ability of the chelating agent to fold, the number of
Fig. 4. Grown DAST crystals in different DTPA and EDTA concentrations. (a) DTPA-20 ppm, (b) DTPA-30 ppm, (c) DTPA-40 ppm, (d) 20 ppm, (e) EDTA-30 ppm, (f) EDTA-40 ppm and (g) pure DAST.
interactions between the metal ion and the chelater and several subtle energetic co-ordination. Due to the excellent chelating ability of DTPA, the rate of crystallization was reduced in the DTPA added solutions. Thus, only a few crystals were obtained
Fig. 5. The crystal size distribution of pure, DTPA and EDTA added DAST. (B) DAST + 20 ppm, (C) DAST + 30 ppm, (D) DAST + DTPA-40 ppm, (E) DAST + EDTA-20 ppm, (F) DAST + EDTA-30 ppm, (G) DAST + EDTA-40 ppm and (H) pure DAST.
from 20 ppm of EDTA and DTPA added DAST solutions. This controlled crystallization became more probable with increasing DTPA concentration. But it was improbable with the increas-ing EDTA concentration in the DAST solution. The positive active parts (more in DTPA than EDTA) engaged strongly to capture the metal ions in the DTPA added solutions. Many het-erogeneous nucleations were observed in the EDTA (except 20 ppm) added DAST solutions where a forced induced nucle-ation was prompted due to the precipitnucle-ation of EDTA crystals, which are acting as nucleation sites. Hence many tiny crystals were obtained from the more EDTA added DAST solutions.
With the addition of DTPA, the clustering phenomenon was markedly suppressed. Therefore, in fact, DTPA improved crystal size and decreased the number of crystals. The addition of 30 and 40 ppm of DTPA into the DAST solution reduced clustering to a great extent which in turn yielded bigger size crystals. As shown inFig. 4(c), in 40 ppm DTPA added DAST solution, only two DAST crystals were obtained.
The addition of DTPA reduced the formation of spurious nucleation at 30 and 40 ppm levels, whereas in the case of EDTA at 30 and 40 ppm levels, there were many tiny crystals. The pres-ence of more EDTA in the solutions caused the precipitation of EDTA crystals not only on DAST crystal surface (which leaded to poor surface quality) but also on the inert fluid–DAST solu-tion interface. The obtained two crystals from 40 ppm DTPA added DAST solutions had the size of 6 mm each along a and b
axes. Moreover, 20 ppm EDTA added DAST solution gave good results in terms of crystal growth (Fig. 4(d)) i.e. it controlled the nucleation rate as well.
From the above studies, the number of nucleation decreased in the DTPA added DAST solutions. Moreover, the surface mor-phology of the crystals was considerably improved and the clustering of crystals was significantly reduced. It could be observed from the results that the effect of chelaters in suppress-ing spurious nucleation and clustersuppress-ing was more significant in 30 and 40 ppm of DTPA, and 20 ppm of EDTA added solutions. The crystal growth in 20 ppm of EDTA added DAST solution was consistent with the previous report[15]. The enhancement of metastable regime increased the stability of the solution and hence bigger size crystals could be harvested from the DTPA added DAST solutions at higher concentration levels.
3.3. Characterization studies
From the X-ray powder diffraction patterns of pure, DTPA, EDTA added DAST samples (Fig. 6), the lattice parameter val-ues were calculated using 2θ valval-ues of high intensity peaks corresponding to the h k l planes using the monoclinic crystal-lographic equation. The lattice parameter values of the grown crystals are shown inTable 1. The calculated lattice parameter values of pure, DTPA and EDTA added DAST crystals were comparable with that of pure one and with the values reported by Marder et al.[1]. The small variations in the values of lattice parameter c were observed in the case of more EDTA added DAST crystals. This variation was not observed in the DTPA added DAST crystals.
The FTIR spectra of pure, DTPA and EDTA added DAST samples are shown inFig. 7. From the spectra, there was con-siderable absorption observed in 30 and 40 ppm of EDTA added DAST samples. At 1160 and 1180 cm−1 plane aromatic ring deformations, which are typical for para-substituted benzenes were observed. The CH3 umbrella deformation was assigned
to 1347 cm−1. In-plane stretching frequency of C C C C was found at 1577 cm−1. There was no incorporation of DTPA or EDTA in the DAST molecular structure, which was revealed from the FTIR analysis.
The optical absorption spectra of 40 ppm EDTA and 40 ppm DTPA added DAST crystals are shown inFig. 8. The absorp-tion of pure and EDTA added DAST crystals are reported earlier
[15]. From the observed UV–vis–NIR spectra, it was found that
Fig. 6. X-ray diffraction pattern of (a) pure, (b) 30 ppm DTPA added DAST and (c) 30 ppm EDTA added DAST.
40 ppm DTPA added DAST crystal had about 30 % of absorp-tion, whereas 40 ppm EDTA added DAST crystals exhibited a more absorption, which was about 70%. The difference in the absorptions was mainly due to the following reasons. During crystal growth, an effective suppression of chemical activity of metal ions occurred in the DTPA added DAST solutions due to the formation of complexes with the functional groups of DTPA by its chelating action. Since more metal ions captured by DTPA than EDTA in the solutions, there was a slower nucleation rate and low cluster formation in the DTPA added solutions during
Fig. 7. FTIR spectra of (a) pure, (b) 40 ppm DTPA added DAST and (c) 40 ppm EDTA added DAST.
the crystal growth. Therefore good quality crystals with smooth surface were grown. But a reverse trend was observed in the EDTA added DAST crystals due to low quality and rough sur-face originating from the precipitation of EDTA crystals on the surface of DAST crystals. The absorption edge was observed at about 700 nm, which was 100 nm right shift with the previous report[15]. The change in the absorption edge was observed
Fig. 8. Absorption spectra of as grown (a) 40 ppm DTPA added DAST and (b) 40 ppm EDTA added DAST crystals.
in pure, DTPA and EDTA added DAST crystals. The variation was mainly due to the orientation of the cationic chromophore in DAST crystal lattice with respect to its dielectric axes.
The microhardness was measured on the (0 0 1) face for pure, DTPA and EDTA added DAST crystals under different loads. The average diagonal length of the indented impression was calculated. If the P is the applied load (in kg) and d is the average diagonal length of the indentation impressions (mm), and the angle between the opposite faces of the diamond pyramid is
θ = 136◦, then Hvin kg mm−2is given by
Hv= 1.8544P
d2 (1)
The variation of microhardness (Hv) with the applied loads for
pure, DTPA and EDTA added DAST crystals are shown inFig. 9. From the figure, the decrease of Hvwith the increase of applied
load was observed. It was due to the fact that as energy associated with the indenter was low at lower load (5 g), dislocation at the point of indentation did not allow the indenter to penetrate further. But at higher load, the indenter could easily penetrate owing to the movement of dislocations away from the indenter.
Fig. 9. Variation of microhardness with the different loads for pure, DTPA and EDTA added DAST crystals.
The microhardness values of pure DAST crystals were approximately 24, 18 and 15 kg mm−2 for the loads of 5, 10 and 25 g, respectively. The microhardness number was found in the range of 21–28 kg mm−2 for the DTPA added DAST crystals. In the case of EDTA added DAST crystals, the val-ues were estimated in the range of 13–19 kg mm−2. From the figure, with increasing additive concentration, Hvincreased in
the DTPA added DAST crystals and decreased in the EDTA added DAST crystals. The reasons for the increased hardness values in the DTPA added DAST crystals are explained as fol-lows. DTPA (20, 30 and 40 ppm) interacted strongly than EDTA (except 20 ppm) with DAST or impurity ions in the solution. Since DTPA captured more metal ion than EDTA, an effec-tive suppression of chemical activity of metal ions occurred in the DTPA added DAST solutions. Also, EDTA (except 20 ppm) added DAST crystals possessed the reduced hardness values due to low crystal quality and rough (0 0 1) face resulted from the precipitation of EDTA crystals on the surface of DAST crystals.
4. Conclusions
Diethylenetriaminepentaacetic acid (DTPA) was found to be a new chelating agent to enhance the meta-stability and nucle-ation control in DAST solution. It can be used for the crystal growth of several organic or inorganic materials, that have
lim-or EDTA in the DAST molecular structure. The abslim-orption of the grown crystals was measured. The microhardness studies revealed that the addition of DTPA increased the hardness of the DAST crystals.
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