- Email: [email protected]
Growth of ADP–KDP mixed crystal and its optical, mechanical, dielectric, piezoelectric and laser damage threshold studies
Journal of Crystal Growth 362 (2013) 338–342
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of ADP–KDP mixed crystal and its optical, mechanical, dielectric, piezoelectric and laser damage threshold studies P. Rajesh a,n, P. Ramasamy a, G. Bhagavannarayana b a b
Centre for Crystal Growth, SSN College of Engineering, Kalavakkam 603110, Tamilnadu, India Materials Characterization Division, National Physical Laboratory, Council of Scientific and Industrial Research, New Delhi 110012, India
a r t i c l e i n f o
a b s t r a c t
Available online 31 October 2011
Good quality ADP–KDP mixed crystal (90:10) is grown by slow cooling method. The size of the grown crystal is 80 10 10 mm3. The mounted seed size was 5 10 10 mm3 and the crystal was grown along the ‘c’ axis. HRXRD studies have been done in the near and far regions of the seed crystal. The FWHM of these diffraction curves are 28 and 29 arcsec, which are almost the same. The close values of FWHM of both the specimens indicate that the quality of the crystal remains nearly the same throughout the crystal. 80% of transparency is observed from the UV–vis studies in the entire visible region. Vickers hardness studies indicate that the mixed crystal is mechanically more stable compared to the ADP. Higher piezoelectric coefficient is observed in mixed crystals. Dielectric measurements are carried out. From the laser damage threshold studies, it is observed that higher energy is required to damage the mixed crystal and it indicates that the laser stability of the mixed crystal is high. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. X-ray diffraction A2. Growth from solutions A2. Single crystal growth B2. Dielectric materials
1. Introduction Mixed crystals of ADP (Ammonium dihydrogen phosphateNH4H2PO4) and KDP (Potassium dihydrogen phosphate-KH2PO4) have attracted much attention because of the existence of the interesting spin glass state in a certain intermediate mixing concentration range due to the competing antiferroelectric and ferroelectric interaction. The mixing of ADP and KDP leads to significant change in the properties of the crystals. KDP is wellknown hydrogen-bonded ferroelectrics and ADP is known for its antiferroelectric behavior. Both the materials have attracted extensive attention in the investigation of hydrogen bonding behaviors in crystal and the relationship between crystal structure and their properties [1–5]. In the past decades, many efforts have been made to promote the crystal’s quality and increase the growth rate to meet the requirements of inertial confinement fusion [6]. Mixing of ADP and KDP is being carried out for many years by several researchers [6–9]. Srinivasan et. al., have investigated the different molar ratio concentrations of the mixed crystals [1,2]. However, bulk size crystals have not been grown and there is no report available in the literature on its laser damage, dielectric, piezoelectric and optical properties. In the present work bulk size mixed ADP–KDP (AKDP) crystals have been grown in the ratio of 90:10. The optical, mechanical, laser damage threshold, dielectric and piezoelectric analyses have been
carried out and in order to compare with the present results; pure ADP and KDP crystals have also been grown in identical conditions using the same materials ingredients and compared.
2. Experimental procedure 2.1. Solubility studies The solubilities of pure ADP and KDP were published in many papers [1,2]. The data on mutual solubility between them in water are necessary in order to prepare saturated mixed solutions with different compositions to grow mixed crystals, which are of current interest. The ADP and KDP raw materials with 90:10 ratios have been taken and it was dissolved completely in deionized water. The resistivity of the used deionized water is 18 MO cm. The solution was allowed for complete evaporation. The mixed materials were collected for the solubility. The solubilities of pure ADP, KDP and AKDP in deionized water were assessed as a function of temperature in the range 30–50 1C. The saturated solution was allowed to reach the equilibrium in about 1 day at a chosen temperature and then the solubility was gravimetrically analyzed. The same process was repeated for different temperatures and the solubility curve was obtained. The solubility diagram is given in Fig. 1. 2.2. Crystal growth
n
Corresponding author. Tel.: þ91 9840522490; fax: þ 91 44 27475166. E-mail addresses: [email protected], [email protected] (P. Rajesh), [email protected] (P. Ramasamy). 0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.10.036
AKDP mixed crystals with the ratio of 90:10 were grown from solution by slow cooling along with bidirectional seed rotation
P. Rajesh et al. / Journal of Crystal Growth 362 (2013) 338–342
339
70 AKDP ADP KDP
65 Solubility (g/100 ml)
60 55 50 45 40 35 30 25 20 25
30
35 40 Temperature (°C)
45
50
Fig. 1. Solubility diagram of ADP, KDP and AKDP.
technique. After repeated recrystallization, ADP and KDP materials are used for growth. The materials have been mixed in the molar ratio of 90:10. Srinivasan et al., have shown that the percentage of ADP present in the crystal and in the solution are almost same in the growth condition of our experiment [10]. Pure ADP crystal was mounted as a seed on the center of the platform made up of acrylic material and it is fixed in to the crystallizer. The seed mount platform stirs the solution very well and makes the solution more stable. The uniform rotation of the seed is required so as not to produce stagnant regions or recirculating flows, otherwise inclusions in the crystals will be formed due to inhomogeneous super saturation in the solution. The crystal growth is carried out in a 5000 mL standard crystallizer with 3000 mL solution. The crystallizer temperature is controlled using an external water bath, and the temperature fluctuations are less than 0.01 1C. The saturation temperature was 45 1C. The solution was filtered by Whatman filter paper of pore size 11 mm. After the filtration the solution was overheated to 55 1C for one day. Then the temperature was reduced by 3 1C at 1 1C/h, higher than saturation point and again the temperature was decreased to saturation point at 1 1C/day. 5 10 10 mm3 size pure ADP crystal was fixed in the center of the crystallizer and it was kept inside a constant temperature bath slowly. From the saturation point the temperature was decreased at the rate of 0.5 1C/day and the rotation rate was 35 rpm. After 20 days of growth good quality crystal was harvested. The size of the mixed crystal is 80 10 10 mm3. Pure ADP and KDP crystals are also grown and the crystals are shown in Fig. 2. 2.3. Growth rate The growth rate of AKDP crystal along the ‘‘c’’ direction is approximately 3 times higher than the pure ADP crystal. The crystal frame along the ‘‘a’’-axis is mainly constructed by the H2PO4 anionic groups through the strong hydrogen bonds formed during the crystallization. The constituent cations (K þ and NH4 þ ) enter the interspaces of the zigzagged anionic chains and interconnect the adjacent anionic networks along the c-axis via symmetrical chemical bonds. In a similar way, the crystal structure of AKDP crystal can be regarded as the substitution of NH4 þ ion by K þ ion. Therefore, their growth regulations are analogous: the growth along the a-axis is mainly controlled by the constituent anions, and the growth along the c-axis is the synergetic results of deposition of anions and cations [11,6]. This may be the reason for the higher growth along the ‘‘c’’ axis.
Fig. 2. (a) Pure ADP (b) pure KDP and (c) AKDP crystals grown by slow cooling along with seed rotation method.
3. Characterization The UV–vis spectral studies were carried out using Perkin-Elmer Lambda-35 UV–vis spectrometer in the range 200–1100 nm with slit width 1 nm, scan speed 240 nm/min. The dielectric constant was measured using Agilent 4284-A LCR meter for various temperatures and frequencies. Vickers hardness studies have been carried out using the instrument MITUTOYO model MH 120. Laser-induced surface damage threshold measurements were conducted using a high power Nd:YAG laser operating at 7 ns pulse width. Experiments were performed by keeping the positions of the lens and crystal plate as fixed and
P. Rajesh et al. / Journal of Crystal Growth 362 (2013) 338–342
increasing the laser pulse energy until a visible spot was seen at the surface of the crystal. The crystal was placed at a distance where the beam diameter becomes 1.2 mm at the exit face of the crystal, the beam diameter was measured using knife edge measurements. The piezoelectric studies were made using piezometer system. The a-cut plate of pure ADP crystal and doped crystals were subjected to piezoelectric studies. A precision force generator applied a calibrated force (0.25 N), which generated a charge on the piezoelectric material under test. The output was measured directly from oscilloscope, which gives the d33 coefficient in units of pC/N. Without poling the crystal the piezoelectric measurement was carried out.
Diffracted X-ray intensity [c/s]
340
4. Results and discussion
3000
ADP:KDP=9:1(N) (200) Planes MoKα1
2000 28" 1000
0 -200
4.1. UV–vis spectral studies
Interstitial defect
(+,−,−,+)
-100
0
100
200
100
200
AKDP, ADP and KDP crystal plates cut from various parts of the crystal along its /100S direction with a thickness of about 2 mm, were used for optical measurements, after polishing without any antireflection coating. The higher transmittance has been observed in the entire visible region for all the plates and confirms that the quality of the crystal is good. The large transmission in the entire visible region enables it to be a good candidate for electro-optic applications [12]. The optical transparency of AKDP crystal is higher than pure ADP and lower than pure KDP single crystals. The spectrum is shown in Fig. 3. 4.2. HRXRD analyses
Transmittance ( %)
Curves (a) and (b) in Fig. 4 show the high-resolution diffraction curves (DCs) recorded for the grown ADP and KDP mixed (90:10 ratio) crystal using (200) diffracting planes in the symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoKa1 radiation. The DCs of (a) and (b) are for the specimens cut from the boule close to the seed crystal and far away from the seed crystal, respectively. As seen in the figure, the DCs contain a single and sharp peak and indicate that both the specimens are having good crystalline quality and free from structural grain boundaries. The FWHM (full width at half maximum) of these DCs, respectively, are 28 and 29 arcsec, which are almost same but somewhat more than that expected for an ideally perfect crystal from the plane wave theory of dynamical
95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 200
AKDP ADP KDP
400
600 800 Wavelength (nm)
Fig. 3. UV–vis spectra of the grown crystals.
1000
Diffracted X-ray intensity [c/s]
3000 ADP:KDP=9:1(F) (200) Planes MoKα1 2000
(+,−,−,+)
29" 1000
0 -200
-100 0 Glancing angle [arc s]
Fig. 4. Diffraction curves recorded for mixed crystals of ADP and KDP (AKDP): curves (a) and (b) are, respectively, for specimens cut near and far from the seed crystal.
X-ray diffraction [13], but close to that expected for a nearly perfect real life crystal. The close values of FWHM of both the specimens indicate that the quality of the crystal remains same even if the specimens are taken from the far away regions of the seed crystal, which may be because of the fact that the seed crystal used for the bulk growth is of good quality and the growth conditions were maintained properly throughout the growth process. It is interesting to see the asymmetry of the DC. For a particular angular deviation (Dy) of glancing angle with respect to the peak position, the scattered intensity is much more in the positive direction in comparison to that of the negative direction. This feature clearly indicates that the crystal contains predominantly interstitial type of defects than that of vacancy defects. This can be well understood by the fact that due to interstitial defects (self interstitials or impurities at interstitial sites), which may be due to fast growth and/or impurities present in the raw material, the lattice around these defects undergoes compressive stress [14] and the lattice parameter d (interplanar spacing) decreases and gives more scattered (also known as diffuse X-ray scattering) intensity at slightly higher Bragg angles (yB) as d and sin yB are inversely proportional to each other in the Bragg equation (2d sin yB ¼nl; n and l being the order of reflection and wavelength, respectively, which are fixed). However, the single diffraction curve with reasonably low FWHM indicates that the crystalline perfection is fairly good. The density of such interstitial defects is however very
P. Rajesh et al. / Journal of Crystal Growth 362 (2013) 338–342
17
AKDP ADP KDP
16 15 Dielectric constant
meager and in almost all real crystals including nature gifted crystals, such defects are commonly observed and are many times unavoidable due to thermodynamical conditions and hardly affect the device performance. It is worth mentioning here that the observed scattering due to interstitial defects is of short range order as the strain due to such minute defects is limited to the very defect core and the long order could not be expected and hence the change in the lattice parameter of the crystal is not possible. It may be mentioned here that the minute information like the asymmetry in the DC was possible as in the present sample only because of the high-resolution of the multicrystal X-ray diffractometer used in the present investigation.
341
14 13 12 11 10 9
4.3. Microhardness
8
The hardness number was found to increase with the load. A plot drawn between the hardness value and corresponding loads is shown in Fig. 5. It is observed from the figure that hardness increases with increase in load and the cracks have been observed at 100 g for ADP crystal and it is noted that mixed crystal has higher hardness than pure ADP and lower than pure KDP. This implies that the vacancies present in the ADP are occupied by the KDP molecules and the lattices become strong and leads to the increase of hardness. The obtained results for pure ADP and KDP are in agreement with the reported values [15,16].
7
4.4. Dielectric constant Fig. 6 shows the dielectric constants as a function of temperature at 1 kHz of the grown crystals. It is increasing with increase in temperature. This is the normal dielectric behavior of the crystals [17,18]. More explanations are already given in many papers about the physical phenomena. In the present case the dielectric constant of AKDP is slightly higher than ADP at lower temperatures and merging at 100 1C with pure ADP and beyond the temperature it is equal with pure ADP. 4.5. Piezoelectric studies
Hv (kg/mm2)
The piezoelectric studies indicate that the mixed crystals have higher piezoelectric charge coefficient compared to the pure ADP and KDP crystals. The piezoelectric coefficient of KDP exists only on its Tc, whereas in the case of ADP, at room temperature (30 1C) itself exhibiting significant values. In the present case the value of the mixed crystal is 0.24 pC/N. The values are given in Table 1 in detail. 140 135 130 125 120 115 110 105 100 95 90 85 80 75 70 65 60
40
60
80 100 Temperature (°C)
120
140
Fig. 6. Temperature dependence of dielectric constant of the grown crystals.
Table 1 Piezoelectric charge coefficient values of the grown crystals. S. no.
Crystal (‘‘a’’ axis—slow face)
Piezoelectric charge coefficient (d33) value (pC/N)
1. 2. 3.
ADP KDP ADP:KDP
0.12 0.04 0.24
4.6. Laser damage threshold studies The laser damage threshold studies show that AKDP crystal has high laser stability compared to other crystals. The ADP and KDP crystals could withstand up to 45 and 30 mJ for 30 s, respectively, whereas the mixed crystal withstands slightly lesser than 65 mJ. This indicates that the interstitial positions of the KDP atoms on ADP strengthened its lattices. Similar behavior was reflected on the hardness measurements also. Mutual relation between the results confirms the suitability of the crystal for device fabrications. The detailed data are given in Table 2. The damage patterns observed by scanning electron microscope are given in Fig. 7. The laser damage threshold of KDP type crystals is limited by a number of factors [19,20], most of which are related to the presence of impurities. The effect of impurities on the radiation hardness can be associated either with additional absorption upon substitutional incorporation or with the formation of absorbing second-phase inclusions [21]. The latter process is favored by dislocations. On the other hand, inclusions may, in turn, be responsible for increased dislocation density, which makes it difficult to ascertain, which factor plays a key role in determining the laser damage threshold of the crystal.
5. Conclusions AKDP ADP KDP
0
25
50
75
100
125
150
175
Load (g) Fig. 5. Plot for Vickers hardness for the grown crystals.
200
80 10 10 mm3 AKDP mixed crystal has been grown successfully. Higher transparency in the entire visible region and higher piezoelectric charge coefficient have been observed. HRXRD study indicates that the crystalline perfection is same in the entire crystal. Higher laser stability indicates the suitability of the crystal for device fabrications. It is concluded that the study will be useful for the growth of ADP–KDP mixed crystals with enhanced properties.
342
P. Rajesh et al. / Journal of Crystal Growth 362 (2013) 338–342
Table 2 Laser damage threshold value of AKDP crystals. Energy (mJ)
Time (s)
Effects
Time (s)
Effects
5 10 15–55 ISO-5 56–60 ISO-5 62
30 30 30
No changes observed No changes observed No changes observed
60 60 60
No change No change No change
30
No changes observed
60
No change
30
No changes observed
60
65 75 82
30 30 30
A clear dot is seen Nil A similar dot is seen in the other place of the crystal Nil Heavy damage is seen on the bottom Nil
No change. The crystal became hot – – –
ISO—in steps of.
Fig. 7. SEM micrographs of laser damage patterns of AKDP crystal at 65 mJ (a) 250x resolution (b) 120x resolution with dimensions of damage (c) at 75 mJ and (d) at 82 mJ.
References [1] K. Srinivasan, S. Anbukumar, P. Ramasamy, Journal of Crystal Growth 151 (1995) 226. [2] K. Srinivasan, P. Ramasamy, T. Kar, S.P. Sengupta, Materials Chemistry and Physics 49 (1997) 191. [3] I.M. Pritula, V.I. Salo, M.I. Kolybaeva, Inorganic Materials 37 (2001) 184. ˇ [4] R. Blinc, Sentjuc, Physical Review Letters 19 (1967) 21. [5] R.C. DuVarney, R.P. Kohin, Physical Review Letters 20 (1968) 259. [6] Xiue Ren, Dongli Xu, Dongfeng Xue, Journal of Crystal Growth 310 (2008) 2005. [7] Dongli Xu, Dongfeng Xue, Journal of Crystal Growth 310 (2008) 1385. [8] S. Sen Gupta, S.P. Sen Gupta, Journal of Materials Science Letters 15 (1996) 525. [9] M. Shanmugham, F.D. Gnanam, P. Ramasamy, Journal of Materials Science Letters 5 (1986) 174.
[10] K. Srinivasan, P. Ramasamy, A. Cantoni, G. Bocelli, Materials Science and Engineering B52 (1998) 129. [11] D. Xu, D. Xue, Journal of Crystal Growth 286 (2006) 108. [12] P. Rajesh, P. Ramasamy, Journal of Crystal Growth 311 (2009) 3491. [13] G. Bhagavannarayana, S. Parthiban, Subbiah Meenakshisundaram, Crystal Growth and Design 8 (2008) 446. [14] B.W. Batterman, H. Cole, Reviews of Modern Physics 36 (1964) 681. [15] P. Rajesh, P. Ramasamy, G. Bhagavannarayana, Journal of Crystal Growth 311 (2009) 4069. [16] P. Rajesh, S. Sreedhar, K. Boopathi, S. Venugopal Rao, P. Ramasamy, Current Applied Physics 11 (2011) 1343. [17] P. Rajesh, P. Ramasmay, Materials Letters 63 (2009) 2260. [18] P. Rajesh, P. Ramasamy, Materials Letters 64 (2010) 798. [19] Ken E. Montgomery, Fred P. Milanovich, Journal of Applied Physics 68 (1990) 3979. [20] H. Endert, W. Melle, Physica Status Solidi A 74 (1982) 141. [21] L.V. Atroshchenko, A.B. Blank, L.M. Nikitina, et al., Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy 25 (1989) 1402.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of ADP–KDP mixed crystal and its optical, mechanical, dielectric, piezoelectric and laser damage threshold studies P. Rajesh a,n, P. Ramasamy a, G. Bhagavannarayana b a b
Centre for Crystal Growth, SSN College of Engineering, Kalavakkam 603110, Tamilnadu, India Materials Characterization Division, National Physical Laboratory, Council of Scientific and Industrial Research, New Delhi 110012, India
a r t i c l e i n f o
a b s t r a c t
Available online 31 October 2011
Good quality ADP–KDP mixed crystal (90:10) is grown by slow cooling method. The size of the grown crystal is 80 10 10 mm3. The mounted seed size was 5 10 10 mm3 and the crystal was grown along the ‘c’ axis. HRXRD studies have been done in the near and far regions of the seed crystal. The FWHM of these diffraction curves are 28 and 29 arcsec, which are almost the same. The close values of FWHM of both the specimens indicate that the quality of the crystal remains nearly the same throughout the crystal. 80% of transparency is observed from the UV–vis studies in the entire visible region. Vickers hardness studies indicate that the mixed crystal is mechanically more stable compared to the ADP. Higher piezoelectric coefficient is observed in mixed crystals. Dielectric measurements are carried out. From the laser damage threshold studies, it is observed that higher energy is required to damage the mixed crystal and it indicates that the laser stability of the mixed crystal is high. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. X-ray diffraction A2. Growth from solutions A2. Single crystal growth B2. Dielectric materials
1. Introduction Mixed crystals of ADP (Ammonium dihydrogen phosphateNH4H2PO4) and KDP (Potassium dihydrogen phosphate-KH2PO4) have attracted much attention because of the existence of the interesting spin glass state in a certain intermediate mixing concentration range due to the competing antiferroelectric and ferroelectric interaction. The mixing of ADP and KDP leads to significant change in the properties of the crystals. KDP is wellknown hydrogen-bonded ferroelectrics and ADP is known for its antiferroelectric behavior. Both the materials have attracted extensive attention in the investigation of hydrogen bonding behaviors in crystal and the relationship between crystal structure and their properties [1–5]. In the past decades, many efforts have been made to promote the crystal’s quality and increase the growth rate to meet the requirements of inertial confinement fusion [6]. Mixing of ADP and KDP is being carried out for many years by several researchers [6–9]. Srinivasan et. al., have investigated the different molar ratio concentrations of the mixed crystals [1,2]. However, bulk size crystals have not been grown and there is no report available in the literature on its laser damage, dielectric, piezoelectric and optical properties. In the present work bulk size mixed ADP–KDP (AKDP) crystals have been grown in the ratio of 90:10. The optical, mechanical, laser damage threshold, dielectric and piezoelectric analyses have been
carried out and in order to compare with the present results; pure ADP and KDP crystals have also been grown in identical conditions using the same materials ingredients and compared.
2. Experimental procedure 2.1. Solubility studies The solubilities of pure ADP and KDP were published in many papers [1,2]. The data on mutual solubility between them in water are necessary in order to prepare saturated mixed solutions with different compositions to grow mixed crystals, which are of current interest. The ADP and KDP raw materials with 90:10 ratios have been taken and it was dissolved completely in deionized water. The resistivity of the used deionized water is 18 MO cm. The solution was allowed for complete evaporation. The mixed materials were collected for the solubility. The solubilities of pure ADP, KDP and AKDP in deionized water were assessed as a function of temperature in the range 30–50 1C. The saturated solution was allowed to reach the equilibrium in about 1 day at a chosen temperature and then the solubility was gravimetrically analyzed. The same process was repeated for different temperatures and the solubility curve was obtained. The solubility diagram is given in Fig. 1. 2.2. Crystal growth
n
Corresponding author. Tel.: þ91 9840522490; fax: þ 91 44 27475166. E-mail addresses: [email protected], [email protected] (P. Rajesh), [email protected] (P. Ramasamy). 0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.10.036
AKDP mixed crystals with the ratio of 90:10 were grown from solution by slow cooling along with bidirectional seed rotation
P. Rajesh et al. / Journal of Crystal Growth 362 (2013) 338–342
339
70 AKDP ADP KDP
65 Solubility (g/100 ml)
60 55 50 45 40 35 30 25 20 25
30
35 40 Temperature (°C)
45
50
Fig. 1. Solubility diagram of ADP, KDP and AKDP.
technique. After repeated recrystallization, ADP and KDP materials are used for growth. The materials have been mixed in the molar ratio of 90:10. Srinivasan et al., have shown that the percentage of ADP present in the crystal and in the solution are almost same in the growth condition of our experiment [10]. Pure ADP crystal was mounted as a seed on the center of the platform made up of acrylic material and it is fixed in to the crystallizer. The seed mount platform stirs the solution very well and makes the solution more stable. The uniform rotation of the seed is required so as not to produce stagnant regions or recirculating flows, otherwise inclusions in the crystals will be formed due to inhomogeneous super saturation in the solution. The crystal growth is carried out in a 5000 mL standard crystallizer with 3000 mL solution. The crystallizer temperature is controlled using an external water bath, and the temperature fluctuations are less than 0.01 1C. The saturation temperature was 45 1C. The solution was filtered by Whatman filter paper of pore size 11 mm. After the filtration the solution was overheated to 55 1C for one day. Then the temperature was reduced by 3 1C at 1 1C/h, higher than saturation point and again the temperature was decreased to saturation point at 1 1C/day. 5 10 10 mm3 size pure ADP crystal was fixed in the center of the crystallizer and it was kept inside a constant temperature bath slowly. From the saturation point the temperature was decreased at the rate of 0.5 1C/day and the rotation rate was 35 rpm. After 20 days of growth good quality crystal was harvested. The size of the mixed crystal is 80 10 10 mm3. Pure ADP and KDP crystals are also grown and the crystals are shown in Fig. 2. 2.3. Growth rate The growth rate of AKDP crystal along the ‘‘c’’ direction is approximately 3 times higher than the pure ADP crystal. The crystal frame along the ‘‘a’’-axis is mainly constructed by the H2PO4 anionic groups through the strong hydrogen bonds formed during the crystallization. The constituent cations (K þ and NH4 þ ) enter the interspaces of the zigzagged anionic chains and interconnect the adjacent anionic networks along the c-axis via symmetrical chemical bonds. In a similar way, the crystal structure of AKDP crystal can be regarded as the substitution of NH4 þ ion by K þ ion. Therefore, their growth regulations are analogous: the growth along the a-axis is mainly controlled by the constituent anions, and the growth along the c-axis is the synergetic results of deposition of anions and cations [11,6]. This may be the reason for the higher growth along the ‘‘c’’ axis.
Fig. 2. (a) Pure ADP (b) pure KDP and (c) AKDP crystals grown by slow cooling along with seed rotation method.
3. Characterization The UV–vis spectral studies were carried out using Perkin-Elmer Lambda-35 UV–vis spectrometer in the range 200–1100 nm with slit width 1 nm, scan speed 240 nm/min. The dielectric constant was measured using Agilent 4284-A LCR meter for various temperatures and frequencies. Vickers hardness studies have been carried out using the instrument MITUTOYO model MH 120. Laser-induced surface damage threshold measurements were conducted using a high power Nd:YAG laser operating at 7 ns pulse width. Experiments were performed by keeping the positions of the lens and crystal plate as fixed and
P. Rajesh et al. / Journal of Crystal Growth 362 (2013) 338–342
increasing the laser pulse energy until a visible spot was seen at the surface of the crystal. The crystal was placed at a distance where the beam diameter becomes 1.2 mm at the exit face of the crystal, the beam diameter was measured using knife edge measurements. The piezoelectric studies were made using piezometer system. The a-cut plate of pure ADP crystal and doped crystals were subjected to piezoelectric studies. A precision force generator applied a calibrated force (0.25 N), which generated a charge on the piezoelectric material under test. The output was measured directly from oscilloscope, which gives the d33 coefficient in units of pC/N. Without poling the crystal the piezoelectric measurement was carried out.
Diffracted X-ray intensity [c/s]
340
4. Results and discussion
3000
ADP:KDP=9:1(N) (200) Planes MoKα1
2000 28" 1000
0 -200
4.1. UV–vis spectral studies
Interstitial defect
(+,−,−,+)
-100
0
100
200
100
200
AKDP, ADP and KDP crystal plates cut from various parts of the crystal along its /100S direction with a thickness of about 2 mm, were used for optical measurements, after polishing without any antireflection coating. The higher transmittance has been observed in the entire visible region for all the plates and confirms that the quality of the crystal is good. The large transmission in the entire visible region enables it to be a good candidate for electro-optic applications [12]. The optical transparency of AKDP crystal is higher than pure ADP and lower than pure KDP single crystals. The spectrum is shown in Fig. 3. 4.2. HRXRD analyses
Transmittance ( %)
Curves (a) and (b) in Fig. 4 show the high-resolution diffraction curves (DCs) recorded for the grown ADP and KDP mixed (90:10 ratio) crystal using (200) diffracting planes in the symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoKa1 radiation. The DCs of (a) and (b) are for the specimens cut from the boule close to the seed crystal and far away from the seed crystal, respectively. As seen in the figure, the DCs contain a single and sharp peak and indicate that both the specimens are having good crystalline quality and free from structural grain boundaries. The FWHM (full width at half maximum) of these DCs, respectively, are 28 and 29 arcsec, which are almost same but somewhat more than that expected for an ideally perfect crystal from the plane wave theory of dynamical
95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 200
AKDP ADP KDP
400
600 800 Wavelength (nm)
Fig. 3. UV–vis spectra of the grown crystals.
1000
Diffracted X-ray intensity [c/s]
3000 ADP:KDP=9:1(F) (200) Planes MoKα1 2000
(+,−,−,+)
29" 1000
0 -200
-100 0 Glancing angle [arc s]
Fig. 4. Diffraction curves recorded for mixed crystals of ADP and KDP (AKDP): curves (a) and (b) are, respectively, for specimens cut near and far from the seed crystal.
X-ray diffraction [13], but close to that expected for a nearly perfect real life crystal. The close values of FWHM of both the specimens indicate that the quality of the crystal remains same even if the specimens are taken from the far away regions of the seed crystal, which may be because of the fact that the seed crystal used for the bulk growth is of good quality and the growth conditions were maintained properly throughout the growth process. It is interesting to see the asymmetry of the DC. For a particular angular deviation (Dy) of glancing angle with respect to the peak position, the scattered intensity is much more in the positive direction in comparison to that of the negative direction. This feature clearly indicates that the crystal contains predominantly interstitial type of defects than that of vacancy defects. This can be well understood by the fact that due to interstitial defects (self interstitials or impurities at interstitial sites), which may be due to fast growth and/or impurities present in the raw material, the lattice around these defects undergoes compressive stress [14] and the lattice parameter d (interplanar spacing) decreases and gives more scattered (also known as diffuse X-ray scattering) intensity at slightly higher Bragg angles (yB) as d and sin yB are inversely proportional to each other in the Bragg equation (2d sin yB ¼nl; n and l being the order of reflection and wavelength, respectively, which are fixed). However, the single diffraction curve with reasonably low FWHM indicates that the crystalline perfection is fairly good. The density of such interstitial defects is however very
P. Rajesh et al. / Journal of Crystal Growth 362 (2013) 338–342
17
AKDP ADP KDP
16 15 Dielectric constant
meager and in almost all real crystals including nature gifted crystals, such defects are commonly observed and are many times unavoidable due to thermodynamical conditions and hardly affect the device performance. It is worth mentioning here that the observed scattering due to interstitial defects is of short range order as the strain due to such minute defects is limited to the very defect core and the long order could not be expected and hence the change in the lattice parameter of the crystal is not possible. It may be mentioned here that the minute information like the asymmetry in the DC was possible as in the present sample only because of the high-resolution of the multicrystal X-ray diffractometer used in the present investigation.
341
14 13 12 11 10 9
4.3. Microhardness
8
The hardness number was found to increase with the load. A plot drawn between the hardness value and corresponding loads is shown in Fig. 5. It is observed from the figure that hardness increases with increase in load and the cracks have been observed at 100 g for ADP crystal and it is noted that mixed crystal has higher hardness than pure ADP and lower than pure KDP. This implies that the vacancies present in the ADP are occupied by the KDP molecules and the lattices become strong and leads to the increase of hardness. The obtained results for pure ADP and KDP are in agreement with the reported values [15,16].
7
4.4. Dielectric constant Fig. 6 shows the dielectric constants as a function of temperature at 1 kHz of the grown crystals. It is increasing with increase in temperature. This is the normal dielectric behavior of the crystals [17,18]. More explanations are already given in many papers about the physical phenomena. In the present case the dielectric constant of AKDP is slightly higher than ADP at lower temperatures and merging at 100 1C with pure ADP and beyond the temperature it is equal with pure ADP. 4.5. Piezoelectric studies
Hv (kg/mm2)
The piezoelectric studies indicate that the mixed crystals have higher piezoelectric charge coefficient compared to the pure ADP and KDP crystals. The piezoelectric coefficient of KDP exists only on its Tc, whereas in the case of ADP, at room temperature (30 1C) itself exhibiting significant values. In the present case the value of the mixed crystal is 0.24 pC/N. The values are given in Table 1 in detail. 140 135 130 125 120 115 110 105 100 95 90 85 80 75 70 65 60
40
60
80 100 Temperature (°C)
120
140
Fig. 6. Temperature dependence of dielectric constant of the grown crystals.
Table 1 Piezoelectric charge coefficient values of the grown crystals. S. no.
Crystal (‘‘a’’ axis—slow face)
Piezoelectric charge coefficient (d33) value (pC/N)
1. 2. 3.
ADP KDP ADP:KDP
0.12 0.04 0.24
4.6. Laser damage threshold studies The laser damage threshold studies show that AKDP crystal has high laser stability compared to other crystals. The ADP and KDP crystals could withstand up to 45 and 30 mJ for 30 s, respectively, whereas the mixed crystal withstands slightly lesser than 65 mJ. This indicates that the interstitial positions of the KDP atoms on ADP strengthened its lattices. Similar behavior was reflected on the hardness measurements also. Mutual relation between the results confirms the suitability of the crystal for device fabrications. The detailed data are given in Table 2. The damage patterns observed by scanning electron microscope are given in Fig. 7. The laser damage threshold of KDP type crystals is limited by a number of factors [19,20], most of which are related to the presence of impurities. The effect of impurities on the radiation hardness can be associated either with additional absorption upon substitutional incorporation or with the formation of absorbing second-phase inclusions [21]. The latter process is favored by dislocations. On the other hand, inclusions may, in turn, be responsible for increased dislocation density, which makes it difficult to ascertain, which factor plays a key role in determining the laser damage threshold of the crystal.
5. Conclusions AKDP ADP KDP
0
25
50
75
100
125
150
175
Load (g) Fig. 5. Plot for Vickers hardness for the grown crystals.
200
80 10 10 mm3 AKDP mixed crystal has been grown successfully. Higher transparency in the entire visible region and higher piezoelectric charge coefficient have been observed. HRXRD study indicates that the crystalline perfection is same in the entire crystal. Higher laser stability indicates the suitability of the crystal for device fabrications. It is concluded that the study will be useful for the growth of ADP–KDP mixed crystals with enhanced properties.
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Table 2 Laser damage threshold value of AKDP crystals. Energy (mJ)
Time (s)
Effects
Time (s)
Effects
5 10 15–55 ISO-5 56–60 ISO-5 62
30 30 30
No changes observed No changes observed No changes observed
60 60 60
No change No change No change
30
No changes observed
60
No change
30
No changes observed
60
65 75 82
30 30 30
A clear dot is seen Nil A similar dot is seen in the other place of the crystal Nil Heavy damage is seen on the bottom Nil
No change. The crystal became hot – – –
ISO—in steps of.
Fig. 7. SEM micrographs of laser damage patterns of AKDP crystal at 65 mJ (a) 250x resolution (b) 120x resolution with dimensions of damage (c) at 75 mJ and (d) at 82 mJ.
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