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Growth habit and transparency of sulphate doped KDP crystal
Materials Letters 61 (2007) 2703 – 2706 www.elsevier.com/locate/matlet
Growth habit and transparency of sulphate doped KDP crystal Jianqin Zhang ⁎, Shenglai Wang, ChangShui Fang, Xun Sun, Qingtian Gu, Yiping Li, Bo Wang, Bing Liu, Xiaoming Mu State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Received 29 December 2005; accepted 14 March 2006 Available online 7 November 2006
Abstract KDP crystals were grown from the aqueous solution with different concentrations of sulphate by both the traditional temperature-lowering method and the rapid growth method. Sulphate showed a great effect on the growth and the properties of KDP crystals. With the rise of the dopant concentration, many defects occur such as mother liquid inclusions, parasite crystals and cracks. When the dopant concentration of sulphate reaches a certain value, the ultraviolet transmittance of crystals decreases a lot compared with crystals at low dopant concentration. © 2006 Elsevier B.V. All rights reserved. Keywords: KDP crystal; Sulphate; Crystal growth; Rapid growth; Transmittance
1. Introduction Potassium dihydrogen phosphate (KDP) crystal is an excellent electro-optic and nonlinear material. It is widely applied as laser radiation converters and Q-switches in laser fusion for its high electro-optic and nonlinear coefficient, wide frequency conversion and high damage threshold against high power laser. Now KDP is the only nonlinear material that meets the inertial confined fusion (ICF) [1]. Impurity is regarded as one of the factors that has effects on the habit modification and quality. It is generally thought that the growth and quality of KDP crystal are affected by many factors such as impurity ions, solution supersaturation and pH value [2]. Impurity is a very important factor. It's accepted that the cationic ions of high valency, such as Fe3+, Cr3+ and Al3+, are easily adsorbed on the prismatic faces and inhibit their growth [3,4]; and they are related to high light adsorption in the ultraviolet range [5,6]. Anionic ions with strong H-bond combinability ability, such as oligophosphate [7,8], are easily adsorbed on the pyramidal faces, inhibit their growth and make prismatic faces extended. Organic materials such as glycol and EDTA [9], have significant
⁎ Corresponding author. Tel.: +86 531 88365240; fax: +86 531 88364263. E-mail address: [email protected] (J. Zhang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.161
effects on the crystal growth kinetics and are responsible for low damage threshold values. Sulphate (SO42−) is a common impurity ion in KDP raw materials and it could not be removed easily. So it is necessary to study how sulphate affects the growth habit and optical quality of KDP. In this paper, KDP crystals were grown from the aqueous solution with different concentrations of sulphate and the transmission spectra of these crystals were measured. 2. Experiments The crystals were grown from the aqueous solution with different concentrations of sulphate. The solution was prepared by adding AR grade K2SO4 to the KH2PO4 solution. The KDP raw material is AR grade, among which the concentration of SO4, Cl and N are less than 0.003%, 0.0005% and 0.001% respectively. The solution was overheated at 80 °C for 24 h and filtered through a 0.22 μm membrane. The saturation temperature is 60 ± 2 °C and the growth temperature range is 60–30 °C. The experiments were carried out in a water bath controlled by an automatic temperature apparatus Shimada (FP21) to an accuracy of ± 0.1 °C. The crystal rotates in the mode of “forward–stop–backward” with a speed of 77 rpm. Both the traditional temperature-lowering method and the rapid growth method were used to grow the KDP crystals. The supersaturation temperature of the traditional method and the
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J. Zhang et al. / Materials Letters 61 (2007) 2703–2706
Fig. 3. Transmission spectra of KDP crystals grown by the traditional temperature-lowering method.
Fig. 1. Photograph of KDP crystals grown by the traditional temperature-lowering method, the dopant concentration of sulphate: (a), 100 ppm; (b), 5000 ppm.
rapid growth method is 0.5 °C and 4 °C respectively. In order to obtain reliable experiment results, the experiments were carried out for two times in identical growth conditions. The transmission spectra were measured at room temperature by a HITACHI U-3500 spectrometer in a wave range of 190– 1700 nm. The elements' contents in KDP crystals were measured by ICP-AES analysis method, with accuracy reaching ppm grade. 3. Experiment results 3.1. The effect on the growth habit The effect of K2SO4 on the growth habit is significant. When SO2− 4 dopant concentration is lower than 100 ppm/mol (ppm/mol is the mole ratio of potassium sulphate to potassium dihydrogen phosphate), the crystals were grown transparently without macro defects, as is shown in Fig. 1(a). With the increase of the dopant concentration, the capping process became slow and the crystal grew slow, too. Prism sectors tapered a little. Small regular crystals from spontaneous nucleation were found in the bottom of the growth solution. The higher the dopant concentration, the more the number of the spontaneous crystals. When the dopant concentration is higher than 2500 ppm/mol, many serious cracks appeared near the pyramidal faces of the crystals, indicating great internal stress inside the crystals, as is shown in Fig. 1(b).
Fig. 2. Photograph of crystals grown by the rapid growth method; the dopant concentration of sulphate: (a), 100 ppm; (b), 1000 ppm.
For the crystals grown by the rapid growth method, when the dopant concentration of SO2− 4 is higher than 200 ppm/mol, the mother liquid inclusions were formed near the borders of the pyramidal faces (Fig. 2(b)). The crystals (as is shown in Fig. 2(b)) showed many parasitic crystals in one prismatic face and many cracks when the dopant concentration reaches 1000 ppm/mol. The crystals became opaque when the dopant concentration is more than 2500 ppm/mol. 3.2. The effect on the transparency KDP crystal samples with a thickness of 1.2 cm were polished at face (001) for transmission measurement. Fig. 3 shows the transmission spectra of crystals grown by the traditional temperature-lowering method. The dopant concentration of K2SO4 is 0, 10, 1000 and 2500 ppm/mol. It can be seen from Fig. 3 that transmittances of crystals change a little within the allowable error compared with pure crystal when SO2− 4 dopant concentration is lower than 2500 ppm/mol. When the dopant concentration exceeds 2500 ppm/mol, transmittance decreases a lot in the ultraviolet band. For the crystals grown by rapid growth, the transmission spectra in pyramidal sectors and prismatic sectors are different, as is shown in Figs. 4 and 5. The transmission spectra in pyramidal sectors of these crystals are similar to that of crystals by traditional temperaturelowering method. Only when the dopant concentration exceeds some value (1000 ppm), the transmittance in the ultraviolet band decreases a lot compared with pure crystals. The transmission spectra in prismatic
Fig. 4. Transmission spectra in pyramidal sectors of KDP crystals grown by the rapid growth method.
J. Zhang et al. / Materials Letters 61 (2007) 2703–2706
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Table 1 The element result of KDP crystals grown by the traditional method
Fig. 5. Transmission spectra in prism sectors of KDP crystals grown by rapid growth method.
Fig. 6. Schematic representation of the growth sectors in a KDP crystal by the rapid technique.
sectors decrease rapidly in the ultraviolet band and the cutoff wavelength is about 300 nm. The transmittance of KDP crystals changes very little with the change of the dopant concentration (Fig. 6). 3.3. The element measurement result The element contents were measured by ICP-AES analysis method. We selected the typical elements—Na, Ca, Al and S—for the measurement. The measurement results are shown in Tables 1 and 2. For the crystals using the traditional method, the content of S element that incorporates into the crystals becomes higher with the rise of K2SO4 dopant concentration. When the sulphate dopant concentration arrives at a certain value, the content of S element also increases a lot. For crystals using the rapid growth method, it can be see that the content of S element increases with the increase of the K2SO4 dopant concentration. Meanwhile, the content of S and Al elements in the prismatic sectors are both higher than those in the pyramidal ones.
Sample number
The dopant concentration (ppm)
The measure result by ICP (ppm) Na
Ca
Al
S
1 2 3 4
0 50 500 1500
21.8 20.7 25.0 24.9
8.65 4.29 3.2 6.41
3.04 2.74 3.62 3.34
7.70 8.72 17 44.1
such as mother liquid inclusions, parasite crystals and cracks, which have a close relation to sulphate impurity. From the point of view of crystal structure, the [101] pyramidal faces of KDP crystals are stacked in the mode of two layers of cations over two layers of anionic ions alternatively. Each tetrahedron of [PO4]3− forms four H-bonds with the four nearest [PO4]3− tetrahedron neighbors. The pyramidal face is always positively charged in the growth process. This interfacial character of the growing crystal has already been verified by means of high-resolution X-ray diffraction [10]. The prismatic faces are stacked in a layer mode. Each layer is composed of both cations and anions in the ratio of 1:1. In the usual case, cations are easy to be adsorbed on prismatic faces. When the concentration of anions is high enough, significant anionic adsorption can occur through much weaker H-bond. It is the structure character that makes sulphate easy to access the pyramidal faces and the prismatic faces. SO42− ion has a similar tetrahedron framework as PO43−. The size of SO42− ion is smaller than that of PO43− [11]. The second ionization equation constant (K2) of vitriol (H2SO4) is 1.2 × 10− 2 and that of phosphorus acid (H3PO4) is K2Φ, K2Φ = 6.3 × 10− 8. From the comparability of K2 and K2Φ, it can be seen that the O– H bond of HSO4− is weaker than that of HPO42− and SO42− also has a strong H-bond combinability ability as PO43−. Just because of the configuration similarity of both ions and their strong Hbond combinability ability, sulphate can be easily adsorbed and joined into the crystal lattice. This is also verified by the element result in Tables 1 and 2. The content of S element in crystals increases with the increase of the sulphate dopant concentration. So we concluded that SO42− ions can join into the KDP crystal lattices through H-bond and charge attraction. The mother liquid inclusions and parasite crystals have a close relation to the SO42− ions. At high dopant concentration, more SO42− ions join into crystal lattices and combine with Table 2 The element result of KDP crystals grown by the rapid technique Sample number
4. Discussion 4.1. Discussion on the defects of crystals From the experiments, it can be seen that sulphate had a great effect on the growth habit of KDP crystals. At high dopant concentration of sulphate, KDP crystals showed many defects
1 Pyramidal sector 1 Prismatic sector 2 Pyramidal sector 2 Prismatic sector 3 Pyramidal sector 3 Prismatic sector
The dopant concentration (ppm) 0 200 500
The measure result by ICP (ppm) Na
Ca
Al
S
11.3 11.4 23.1 25.1 26.0 14.6
4.57 4.97 4.63 5.91 6.22 7.05
2.19 5.68 2.01 4.88 3.12 7.48
8.43 10.4 8.29 17.1 19.2 38.0
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[PO4]3−, which can inhibit the movement of some elementary steps. As a result of subsequent movement of macrosteps, the mother solution is involved in the crystal and forms the inclusion. Crystal cracks are mainly caused by SO42− ions in KDP crystals. We can explain this from two aspects. One SO42− carries one negative charge less than one PO43−. After SO42− ions join into the crystal, in order to keep the charge balance, it's necessary to reduce one K+ ion. So the perfection of crystal structure is destroyed, which causes the internal stress of the crystal. The other is the change of the H-bond size and energy. When SO42− ions join into the crystal, they occupy the growing position and combine with [PO4]3− by forming H-bond. The size of SO42− is smaller than that of PO43−, which makes the Hbond to shorten. The O–H bond of HSO4− is weaker than that of H2PO4−, which leads to the decrease of H-bond energy of the crystal. The change of H-bond makes the crystal structure distort to some extent and form great stress and so crystals have great internal stress. When the stress exceeds the regular allowable stacking stress limit of KDP crystal, cracks occur. 4.2. Discussion on transmittance of crystals From the above experiments, it can be concluded that SO42− ions had a great effect on the ultraviolet transmittance of KDP crystals. For KDP crystals grown by the traditional method, when the dopant concentration exceeds a certain value, transmittance decreases a lot in ultraviolet band compared with that of crystals at low dopant concentration. From Table 1, it can be seen that the content of S element of KDP crystals at high dopant concentration is much higher than that at low dopant concentration; and the metal (Al3+) content is also higher than that at low dopant concentration. We also think that SO42− ions can carry much more metal ions of high valency such as Fe3+ and Al3+ into the crystals when they join into the crystal at high dopant concentration. These metal ions have high absorption in ultraviolet band, which causes the decrease of ultraviolet transmittance of KDP crystals. Furthermore, more SO42− ions in the growing positions affect the regular stack of H2PO4− in KDP crystals. So the crystals have minute distortion in microstructure, which can destroy the homogeneity of crystals. The destroyed optical homogeneity might make the transmittance at the Zdirection in the ultraviolet band to decrease a lot. For KDP crystals grown by rapid method, the transmission spectra in pyramidal sectors and prismatic sectors are different. From Table 2, it can be seen that the content of both S and Al elements in the prismatic sectors is higher than that of the pyramidal sectors and the content increases with the increase of the sulphate dopant concentration, which is consistent with the above analysis about the crystal structure. The impurities such as Al, Cr and Fe were reported to incorporate into the KDP crystals in a large amount and the concentration of impurities is many times higher in the prismatic sectors than those in the pyramidal ones [12]. That is to say, metal ions are easily absorbed on the
prismatic faces and SO42− ions can be incorporated into the prismatic faces through the H-bond. Rapid growth rate makes SO42− ions easily carry much more metal ions of high valency into the prismatic faces than the pyramidal faces. Furthermore, the difference of the size and charge between SO42− and PO43− makes the crystal form some vacancy that can adsorb some metal ions on the prismatic faces. So much more metal ions of high valency can be absorbed on the prismatic faces than on the pyramidal faces, which can prove the difference between the transmission spectra in pyramidal sectors and prismatic sectors. The higher ultraviolet absorption caused by metal ions of high valency makes the transmittance of the crystals in prismatic sectors decrease rapidly in the ultraviolet band and the cutoff wavelength is about 300 nm. Therefore, the transmittance in prismatic sectors is affected mostly by metal ions of high valency and transmittance in the pyramidal sectors is affected mostly by SO42− ions. 5. Conclusion • SO42− ions can join into the KDP crystal lattices through Hbond and electron attraction. • SO42− ions have a great effect on the growth habit of KDP crystals at high dopant concentration of sulphate. KDP crystals showed many defects such as mother liquid inclusions, parasite crystals and cracks. • SO42− ions also affect the transparency of the crystal. At high dopant concentration, KDP crystals' transmittance decreases a lot in the ultraviolet band. Acknowledgements This work was supported financially by the State High Technology Program for Inertial Confinement Fusion and Youth Science Foundation of Shandong Province (2004BS04022). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M.A. Rhode, B. Woods, J.J. De Yoreo, et al., Appl. Opt. 34 (1995) 5312. K. Sangwal, Prog. Cryst. Growth Charact. 32 (1996) 3. L.N. Rashkovich, N.V. Kronsky, J. Cryst. Growth 182 (1997) 434. G.M. Loiacono, J.J. Zola, G. Kostecky, J. Cryst. Growth 58 (1982) 495. E. Diequez, A. Cintas, P. Hernandez, J. Cabrera, J. Cryst. Growth 73 (1985) 193. V.I. Bredikhin, N.V. Kiseleva, V.V. Korolikhin, Sov. Nonorg. Mater. 22 (1986) 112. Z.S. Gao, C. Wang, Y.P. Li, et al., J. Synth. Cryst. 23 (1994) 52 (in Chinese). Y.J. Fu, Z.S. Gao, J.M. Liu, et al., J. Cryst. Growth 198–199 (1999) 682. V.A. Kuznetsov, T.M. Okhrimenko, M. Rak, J. Cryst. Growth 193 (1998) 164. S.A. De Vries, E. Vling, P. Goedtkindt, et al., Abstracts of Twelfth Intern. Conf. on Cryst, Jerusalem, Israel, 1998, p. 143. X.Z. Gao, T.Y. Song, X.Q. Wang, Inorganic Chemistry, Higher Education Press, Beijing, 1998. N. Zaitseva, L. Carman, I. Smolsky, et al., J. Cryst. Growth 204 (1999) 512.
Growth habit and transparency of sulphate doped KDP crystal Jianqin Zhang ⁎, Shenglai Wang, ChangShui Fang, Xun Sun, Qingtian Gu, Yiping Li, Bo Wang, Bing Liu, Xiaoming Mu State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Received 29 December 2005; accepted 14 March 2006 Available online 7 November 2006
Abstract KDP crystals were grown from the aqueous solution with different concentrations of sulphate by both the traditional temperature-lowering method and the rapid growth method. Sulphate showed a great effect on the growth and the properties of KDP crystals. With the rise of the dopant concentration, many defects occur such as mother liquid inclusions, parasite crystals and cracks. When the dopant concentration of sulphate reaches a certain value, the ultraviolet transmittance of crystals decreases a lot compared with crystals at low dopant concentration. © 2006 Elsevier B.V. All rights reserved. Keywords: KDP crystal; Sulphate; Crystal growth; Rapid growth; Transmittance
1. Introduction Potassium dihydrogen phosphate (KDP) crystal is an excellent electro-optic and nonlinear material. It is widely applied as laser radiation converters and Q-switches in laser fusion for its high electro-optic and nonlinear coefficient, wide frequency conversion and high damage threshold against high power laser. Now KDP is the only nonlinear material that meets the inertial confined fusion (ICF) [1]. Impurity is regarded as one of the factors that has effects on the habit modification and quality. It is generally thought that the growth and quality of KDP crystal are affected by many factors such as impurity ions, solution supersaturation and pH value [2]. Impurity is a very important factor. It's accepted that the cationic ions of high valency, such as Fe3+, Cr3+ and Al3+, are easily adsorbed on the prismatic faces and inhibit their growth [3,4]; and they are related to high light adsorption in the ultraviolet range [5,6]. Anionic ions with strong H-bond combinability ability, such as oligophosphate [7,8], are easily adsorbed on the pyramidal faces, inhibit their growth and make prismatic faces extended. Organic materials such as glycol and EDTA [9], have significant
⁎ Corresponding author. Tel.: +86 531 88365240; fax: +86 531 88364263. E-mail address: [email protected] (J. Zhang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.161
effects on the crystal growth kinetics and are responsible for low damage threshold values. Sulphate (SO42−) is a common impurity ion in KDP raw materials and it could not be removed easily. So it is necessary to study how sulphate affects the growth habit and optical quality of KDP. In this paper, KDP crystals were grown from the aqueous solution with different concentrations of sulphate and the transmission spectra of these crystals were measured. 2. Experiments The crystals were grown from the aqueous solution with different concentrations of sulphate. The solution was prepared by adding AR grade K2SO4 to the KH2PO4 solution. The KDP raw material is AR grade, among which the concentration of SO4, Cl and N are less than 0.003%, 0.0005% and 0.001% respectively. The solution was overheated at 80 °C for 24 h and filtered through a 0.22 μm membrane. The saturation temperature is 60 ± 2 °C and the growth temperature range is 60–30 °C. The experiments were carried out in a water bath controlled by an automatic temperature apparatus Shimada (FP21) to an accuracy of ± 0.1 °C. The crystal rotates in the mode of “forward–stop–backward” with a speed of 77 rpm. Both the traditional temperature-lowering method and the rapid growth method were used to grow the KDP crystals. The supersaturation temperature of the traditional method and the
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J. Zhang et al. / Materials Letters 61 (2007) 2703–2706
Fig. 3. Transmission spectra of KDP crystals grown by the traditional temperature-lowering method.
Fig. 1. Photograph of KDP crystals grown by the traditional temperature-lowering method, the dopant concentration of sulphate: (a), 100 ppm; (b), 5000 ppm.
rapid growth method is 0.5 °C and 4 °C respectively. In order to obtain reliable experiment results, the experiments were carried out for two times in identical growth conditions. The transmission spectra were measured at room temperature by a HITACHI U-3500 spectrometer in a wave range of 190– 1700 nm. The elements' contents in KDP crystals were measured by ICP-AES analysis method, with accuracy reaching ppm grade. 3. Experiment results 3.1. The effect on the growth habit The effect of K2SO4 on the growth habit is significant. When SO2− 4 dopant concentration is lower than 100 ppm/mol (ppm/mol is the mole ratio of potassium sulphate to potassium dihydrogen phosphate), the crystals were grown transparently without macro defects, as is shown in Fig. 1(a). With the increase of the dopant concentration, the capping process became slow and the crystal grew slow, too. Prism sectors tapered a little. Small regular crystals from spontaneous nucleation were found in the bottom of the growth solution. The higher the dopant concentration, the more the number of the spontaneous crystals. When the dopant concentration is higher than 2500 ppm/mol, many serious cracks appeared near the pyramidal faces of the crystals, indicating great internal stress inside the crystals, as is shown in Fig. 1(b).
Fig. 2. Photograph of crystals grown by the rapid growth method; the dopant concentration of sulphate: (a), 100 ppm; (b), 1000 ppm.
For the crystals grown by the rapid growth method, when the dopant concentration of SO2− 4 is higher than 200 ppm/mol, the mother liquid inclusions were formed near the borders of the pyramidal faces (Fig. 2(b)). The crystals (as is shown in Fig. 2(b)) showed many parasitic crystals in one prismatic face and many cracks when the dopant concentration reaches 1000 ppm/mol. The crystals became opaque when the dopant concentration is more than 2500 ppm/mol. 3.2. The effect on the transparency KDP crystal samples with a thickness of 1.2 cm were polished at face (001) for transmission measurement. Fig. 3 shows the transmission spectra of crystals grown by the traditional temperature-lowering method. The dopant concentration of K2SO4 is 0, 10, 1000 and 2500 ppm/mol. It can be seen from Fig. 3 that transmittances of crystals change a little within the allowable error compared with pure crystal when SO2− 4 dopant concentration is lower than 2500 ppm/mol. When the dopant concentration exceeds 2500 ppm/mol, transmittance decreases a lot in the ultraviolet band. For the crystals grown by rapid growth, the transmission spectra in pyramidal sectors and prismatic sectors are different, as is shown in Figs. 4 and 5. The transmission spectra in pyramidal sectors of these crystals are similar to that of crystals by traditional temperaturelowering method. Only when the dopant concentration exceeds some value (1000 ppm), the transmittance in the ultraviolet band decreases a lot compared with pure crystals. The transmission spectra in prismatic
Fig. 4. Transmission spectra in pyramidal sectors of KDP crystals grown by the rapid growth method.
J. Zhang et al. / Materials Letters 61 (2007) 2703–2706
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Table 1 The element result of KDP crystals grown by the traditional method
Fig. 5. Transmission spectra in prism sectors of KDP crystals grown by rapid growth method.
Fig. 6. Schematic representation of the growth sectors in a KDP crystal by the rapid technique.
sectors decrease rapidly in the ultraviolet band and the cutoff wavelength is about 300 nm. The transmittance of KDP crystals changes very little with the change of the dopant concentration (Fig. 6). 3.3. The element measurement result The element contents were measured by ICP-AES analysis method. We selected the typical elements—Na, Ca, Al and S—for the measurement. The measurement results are shown in Tables 1 and 2. For the crystals using the traditional method, the content of S element that incorporates into the crystals becomes higher with the rise of K2SO4 dopant concentration. When the sulphate dopant concentration arrives at a certain value, the content of S element also increases a lot. For crystals using the rapid growth method, it can be see that the content of S element increases with the increase of the K2SO4 dopant concentration. Meanwhile, the content of S and Al elements in the prismatic sectors are both higher than those in the pyramidal ones.
Sample number
The dopant concentration (ppm)
The measure result by ICP (ppm) Na
Ca
Al
S
1 2 3 4
0 50 500 1500
21.8 20.7 25.0 24.9
8.65 4.29 3.2 6.41
3.04 2.74 3.62 3.34
7.70 8.72 17 44.1
such as mother liquid inclusions, parasite crystals and cracks, which have a close relation to sulphate impurity. From the point of view of crystal structure, the [101] pyramidal faces of KDP crystals are stacked in the mode of two layers of cations over two layers of anionic ions alternatively. Each tetrahedron of [PO4]3− forms four H-bonds with the four nearest [PO4]3− tetrahedron neighbors. The pyramidal face is always positively charged in the growth process. This interfacial character of the growing crystal has already been verified by means of high-resolution X-ray diffraction [10]. The prismatic faces are stacked in a layer mode. Each layer is composed of both cations and anions in the ratio of 1:1. In the usual case, cations are easy to be adsorbed on prismatic faces. When the concentration of anions is high enough, significant anionic adsorption can occur through much weaker H-bond. It is the structure character that makes sulphate easy to access the pyramidal faces and the prismatic faces. SO42− ion has a similar tetrahedron framework as PO43−. The size of SO42− ion is smaller than that of PO43− [11]. The second ionization equation constant (K2) of vitriol (H2SO4) is 1.2 × 10− 2 and that of phosphorus acid (H3PO4) is K2Φ, K2Φ = 6.3 × 10− 8. From the comparability of K2 and K2Φ, it can be seen that the O– H bond of HSO4− is weaker than that of HPO42− and SO42− also has a strong H-bond combinability ability as PO43−. Just because of the configuration similarity of both ions and their strong Hbond combinability ability, sulphate can be easily adsorbed and joined into the crystal lattice. This is also verified by the element result in Tables 1 and 2. The content of S element in crystals increases with the increase of the sulphate dopant concentration. So we concluded that SO42− ions can join into the KDP crystal lattices through H-bond and charge attraction. The mother liquid inclusions and parasite crystals have a close relation to the SO42− ions. At high dopant concentration, more SO42− ions join into crystal lattices and combine with Table 2 The element result of KDP crystals grown by the rapid technique Sample number
4. Discussion 4.1. Discussion on the defects of crystals From the experiments, it can be seen that sulphate had a great effect on the growth habit of KDP crystals. At high dopant concentration of sulphate, KDP crystals showed many defects
1 Pyramidal sector 1 Prismatic sector 2 Pyramidal sector 2 Prismatic sector 3 Pyramidal sector 3 Prismatic sector
The dopant concentration (ppm) 0 200 500
The measure result by ICP (ppm) Na
Ca
Al
S
11.3 11.4 23.1 25.1 26.0 14.6
4.57 4.97 4.63 5.91 6.22 7.05
2.19 5.68 2.01 4.88 3.12 7.48
8.43 10.4 8.29 17.1 19.2 38.0
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J. Zhang et al. / Materials Letters 61 (2007) 2703–2706
[PO4]3−, which can inhibit the movement of some elementary steps. As a result of subsequent movement of macrosteps, the mother solution is involved in the crystal and forms the inclusion. Crystal cracks are mainly caused by SO42− ions in KDP crystals. We can explain this from two aspects. One SO42− carries one negative charge less than one PO43−. After SO42− ions join into the crystal, in order to keep the charge balance, it's necessary to reduce one K+ ion. So the perfection of crystal structure is destroyed, which causes the internal stress of the crystal. The other is the change of the H-bond size and energy. When SO42− ions join into the crystal, they occupy the growing position and combine with [PO4]3− by forming H-bond. The size of SO42− is smaller than that of PO43−, which makes the Hbond to shorten. The O–H bond of HSO4− is weaker than that of H2PO4−, which leads to the decrease of H-bond energy of the crystal. The change of H-bond makes the crystal structure distort to some extent and form great stress and so crystals have great internal stress. When the stress exceeds the regular allowable stacking stress limit of KDP crystal, cracks occur. 4.2. Discussion on transmittance of crystals From the above experiments, it can be concluded that SO42− ions had a great effect on the ultraviolet transmittance of KDP crystals. For KDP crystals grown by the traditional method, when the dopant concentration exceeds a certain value, transmittance decreases a lot in ultraviolet band compared with that of crystals at low dopant concentration. From Table 1, it can be seen that the content of S element of KDP crystals at high dopant concentration is much higher than that at low dopant concentration; and the metal (Al3+) content is also higher than that at low dopant concentration. We also think that SO42− ions can carry much more metal ions of high valency such as Fe3+ and Al3+ into the crystals when they join into the crystal at high dopant concentration. These metal ions have high absorption in ultraviolet band, which causes the decrease of ultraviolet transmittance of KDP crystals. Furthermore, more SO42− ions in the growing positions affect the regular stack of H2PO4− in KDP crystals. So the crystals have minute distortion in microstructure, which can destroy the homogeneity of crystals. The destroyed optical homogeneity might make the transmittance at the Zdirection in the ultraviolet band to decrease a lot. For KDP crystals grown by rapid method, the transmission spectra in pyramidal sectors and prismatic sectors are different. From Table 2, it can be seen that the content of both S and Al elements in the prismatic sectors is higher than that of the pyramidal sectors and the content increases with the increase of the sulphate dopant concentration, which is consistent with the above analysis about the crystal structure. The impurities such as Al, Cr and Fe were reported to incorporate into the KDP crystals in a large amount and the concentration of impurities is many times higher in the prismatic sectors than those in the pyramidal ones [12]. That is to say, metal ions are easily absorbed on the
prismatic faces and SO42− ions can be incorporated into the prismatic faces through the H-bond. Rapid growth rate makes SO42− ions easily carry much more metal ions of high valency into the prismatic faces than the pyramidal faces. Furthermore, the difference of the size and charge between SO42− and PO43− makes the crystal form some vacancy that can adsorb some metal ions on the prismatic faces. So much more metal ions of high valency can be absorbed on the prismatic faces than on the pyramidal faces, which can prove the difference between the transmission spectra in pyramidal sectors and prismatic sectors. The higher ultraviolet absorption caused by metal ions of high valency makes the transmittance of the crystals in prismatic sectors decrease rapidly in the ultraviolet band and the cutoff wavelength is about 300 nm. Therefore, the transmittance in prismatic sectors is affected mostly by metal ions of high valency and transmittance in the pyramidal sectors is affected mostly by SO42− ions. 5. Conclusion • SO42− ions can join into the KDP crystal lattices through Hbond and electron attraction. • SO42− ions have a great effect on the growth habit of KDP crystals at high dopant concentration of sulphate. KDP crystals showed many defects such as mother liquid inclusions, parasite crystals and cracks. • SO42− ions also affect the transparency of the crystal. At high dopant concentration, KDP crystals' transmittance decreases a lot in the ultraviolet band. Acknowledgements This work was supported financially by the State High Technology Program for Inertial Confinement Fusion and Youth Science Foundation of Shandong Province (2004BS04022). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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