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Growth and characterization of single crystal of pentachloropyridine
ARTICLE IN PRESS
Journal of Crystal Growth 285 (2005) 111–116 www.elsevier.com/locate/jcrysgro
Growth and characterization of single crystal of pentachloropyridine R.N. Raia,, K.B.R. Varmab a
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan, India b Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India Received 6 January 2005; received in revised form 11 June 2005; accepted 2 August 2005 Available online 3 October 2005 Communicated by R.S. Feigelson
Abstract Bulk single crystals of pentachloropyridine (PCP) were grown by the Bridgman–Stockbarger method. The single crystalline nature of the grown crystals was confirmed using powder X-ray diffraction (XRD) techniques. These were transparent in the 315–2000 nm range. The second harmonic generation (SHG) efficiency of polycrystalline materials using Kurtz powder technique was found to be twice that of the well-known organic nonlinear optical (NLO) material, urea. The mixing behavior of PCP with succinonitrile (SCN) and its effect on SHG and micro-hardness of PCP, along with the physicochemical properties of different compositions of PCP and SCN system were studied in detail. Bulk single crystals of the PCP solid solution containing 0.02 mole fraction of SCN have also been grown using the Bridgman–Stockbarger technique. r 2005 Elsevier B.V. All rights reserved. Keywords: A1. Crystal growth; A1. Micro-hardness; A1. Optical properties; A1. Second harmonic generation; A1. Temperature profile; A2. Bridgman–-Stockbarger
1. Introduction The increasing demand on organic materials for technological applications, which includes optical frequency doublers, ultra-fast modulators, amplifiers and switches, has prompted researchers to look for newer promising materials [1–5]. One of Corresponding author. Tel.: +91 1596 245073 x 276/454;
fax: +91 1596 244183. E-mail address: [email protected] (R.N. Rai).
the main criteria for an organic nonlinear optical (NLO) material is that its crystal structure lacks a center of symmetry. In molecular crystals, symmetry depends mainly on the polarizability of the electrons in the p-bonding orbital. This is in contrast with inorganic materials where p-electron delocalization is absent and lattice vibrations play a dominant role. Hence, the structural studies related to bonding properties of the atoms in the molecules as well as the molecules in the crystal help in speculating on the properties of material.
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.08.012
ARTICLE IN PRESS R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
The pentachloropyridine (PCP) molecule consists of five atoms of chlorine attached to a pyridine ring containing a nitrogen atom. Therefore, the molecular structure of PCP predicts that it could crystallize in non-centrosymmetric space group. Furthermore, binary organic materials are known to exhibit better optical properties than their parent components [6–8] and, hence, PCP and succinonitrile (SCN) were chosen for the detail study of physicochemical properties and the SHG property. In this article, we report details concerning the growth of bulk single crystals of PCP and PCP doped with SCN from their melts, along with their micro-hardness, optical and nonlinear optical properties. The comparative properties of PCP crystals to that of PCP doped with SCN, and the phase diagram study of the PCP and SCN system are also reported.
250 L Tc 200 Temperature (°C)
112
L1 + L 2 150 Tm
L1
100
L2 PCP(S1) + L2
50
E PCP(S1) + SCN(S2) 0.0
2. Experimental procedure 2.1. Materials and purification The purification of SCN (Aldrich, Germany) was done by repeated distillation under reduced pressure, while PCP (Aldrich, Germany) was purified by recrystallization from carbon tetrachloride. The purity of PCP and SCN were checked by their melting points, which were found to be 125 and 56.5 1C, respectively. 2.2. Phase diagram The phase diagram of the PCP–SCN system was established, using the method reported [9,10], in the form of temperature–composition curve (Fig. 1). The mixtures of two components were taken, covering the entire range of compositions. The melting/miscibility temperatures of each composition were recorded using a melting point apparatus attached with a precision thermometer associated with an accuracy of 70.5 1C. 2.3. Growth of bulk single crystals To grow the single crystals from their melts, the Bridgman–Stockbarger method was used. Suitable
Mh
M
PCP
0.2
0.4
0.6
Mole fraction of SCN
0.8
1.0 SCN
Fig. 1. Phase diagram of pentachloropyridine–succinonitrile system; —melting/miscibility temperature.
temperature gradients were established in a vertical two zones furnace, having a ceramic baffle that separates the zones, and the temperature of each zone was maintained using two different temperature controllers. The top zone of the furnace was maintained at a higher temperature (130 1C, i.e. 5 1C higher than that of the melting point of PCP) than the bottom zone (110 1C). The optimized temperature profile for growing PCP and PCP doped with SCN crystals is given in Fig. 2. For growing PCP crystals the ampoule-lowering rate was maintained at 4.0 mm/day. When the crystal growth run was completed, the temperature of the furnace was cooled down to room temperature at a rate of 0.5 1C/h. 2.4. Micro-hardness measurement A Shimadzu Micro Hardness Tester (HMV-2) with a mini load machine and a diamond pyramid indenter was used for the measurement of Vickers hardness and the values were obtained from the following equation, by taking the test load and the
ARTICLE IN PRESS
Temperature (°C)
R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
113
130
2000 nm. The thicknesses of the crystals were around 2.0 mm.
120
2.7. Second harmonic generation (SHG) measurement
110
100
90
Temperature (°C) -5
0 5 10 15 20 25 30 35 40 Length of the furnace (bottom to top, in cm)
Fig. 2. Temperature profile of furnace used for the single crystals growth of PCP and doped PCP.
To evaluate the SHG efficiencies, powders of different crystallite sizes were prepared using different sizes of mesh and Kurtz powder technique was adopted [11]. The thickness of the materials was also maintained the same by making a sandwich between two glass plates. The materials were exposed to the laser beam using DCR-11, Nd:YAG laser with a pulse width of 10 ns and at the repetition rate of 10 Hz. The OPHIR power meter was used to measure the laser output power in the transmission mode. A comparison was made with that of urea powders.
3. Results and discussion mean of the diagonal length of indentation into account: F HV ¼ 0:1892 2 , d where HV, F and d are the Vickers hardness, test load (N) and mean of the indentation diagonal length (mm), respectively. 2.5. XRD characterization The single crystalline nature of the PCP crystal was confirmed using powder XRD. Besides the phase diagram study, the powder XRD was also extended to confirm the nature of the mixture (0.98 mole fraction PCP and 0.02 mole fraction SCN), whether it is a mechanical mixture or a solid solution. 2.6. Optical characterization The optical transmittance spectrum of PCP and PCP doped with SCN were done on polished crystals using a Hitachi U-3000, UV–Vis. spectrometer whereas, to study the range of transparency, UV/Vis/NIR (JASCO model V-570) spectrometer was used in the absorption mode from 190 to
3.1. Phase diagram The solid–liquid and liquid–liquid equilibrium data is given in Fig. 1. This temperature–composition diagram shows the formation of a monotectic and a eutectic at SCN mole fractions of 0.0456 and 0.9658, respectively. The upper critical temperature being 99.0 1C above the monotectic temperature (M h ). The melting temperature of PCP decreases to a small extent with the addition of the second component (SCN). Beyond this composition a slight addition of the second component (SCN) gives an immiscibility region (L1+L2). This phase equilibria study found that there are three isothermal solidification reactions of interest. The first reaction is L ! L1 þL2 . The phase separation kinetics here is complicated. The various possibilities of phase separation along with thermal properties have been discussed earlier [12]. The second reaction is the monotectic reaction L1 ðrich in PCPÞ ! S1 ðrich in PCPÞ þ L2 ðrich in SCNÞ:
ARTICLE IN PRESS R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
114
The third reaction is the eutectic reaction L2 ðrich in SCNÞ
! S1 ðrich in PCPÞ þ S2 ðrich in SCNÞ: The monotectic, eutectic and critical solution temperatures are 121.0, 54.0 and 220.0 1C, respectively. To assess the comparative micro-hardness, optical and NLO properties of pure PCP crystals to that of SCN-doped PCP crystals, the compositional region from PCP up to the monotectic composition was used for growing crystals of doped PCP. 3.2. Studies on PCP, and PCP and SCN solid solution crystals The temperature profile that was employed for crystal growth was recorded after stabilizing it for 24 h. The top zone of the furnace was maintained at a higher temperature and the ampoule containing the melt was allowed to pass from the top to the bottom. The thermal gradient inside the furnace (Fig. 2) was not uniform, and for the portion of interest it could be computed from the graph. However, along the most probable portion of crystal growth, between 124.0 and 115.0 1C, it was found to be 1.41 1C/cm. The rate of ampoule lowering was found suitable when it was in the range of 3.0–5.0 mm/day. A section of the grown PCP crystal is depicted in Fig. 3. It was observed
that the bulk crystal grown was transparent and without cracks inside, whereas crystal cracking was observed on the outer surface, particularly at the crystal—glass wall interface. These cracks might be the consequence of strain (Fig. 3) as it is significant in the neck portion of the ampoule where the radius of the ampoule has a sharp point for facilitating the nucleation. The tetragonal structure [13] of the PCP crystals was confirmed by X-ray powder diffraction, which was also used to study the compositions of PCPand SCN-doped samples up to the monotectic composition. Only peaks associated with PCP were found, indicating that PCP–SCN mixtures up to a monotectic composition are solid solutions. Based on this study, a crystal of PCP doped with SCN (0.02 mole fraction) was grown. The optical spectrum (Fig. 4) was taken on polished plates of the crystal of about 2.0 mm thicknesses. The optical transmittance for PCP and doped PCP crystals was over 70%. The percent optical transmission of SCN-doped PCP crystals was better than that of pure PCP, while the cut-off wavelengths for both the crystals were almost the same (315 nm). The study of transparency range of PCP crystal, using UV/Vis/NIR spectrometer, (inset of Fig. 4) inferred that the crystal is transparent from 2000 to 315 nm, which is quite impressive because it is difficult to find crystals with this wide range of transparency and
1.0 PCP doped with SCN
% Transmittance
0.8 PCP
0.6
0.4
0.2
0.0 100
Fig. 3. Photograph of PCP crystal.
200
300
400 500 600 Wavelength (nm)
700
800
900
Fig. 4. Optical transmittance spectrum of PCP and doped PCP with SCN crystals.
ARTICLE IN PRESS R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
transmission [1,4]. Better NLO materials might be synthesized by increasing the conjugated length and by substituting donor and acceptor groups but there is no control on its consequence, i.e. the red shift of the cut-off wavelengths. The powder SHG measurements showed an intense emission of green light for pure PCP as well as solid solutions of SCN-doped crystals. A comparison was made with the SHG emission of known organic material, urea, maintaining the thickness and crystallite size the same. The intensities recorded for PCP and doped PCP crystals were 2.0 and 2.2 times, respectively, that of urea. The powder SHG measurements are not very accurate as the intensity depends on the crystallite size as well as the coherence length [14]. Therefore, the present SHG data obtained in polycrystalline materials mainly confirms the existence of optical nonlinearities and non-centrosymmetric space groups. The micro-hardness of the crystals was measured on the (0 0 1) plane. At a particular load of 98.07 mN for a dwell time of 10 s, the crystal hardness of PCP and PCP doped with SCN were found to be 13.5 and 11.0 HV, respectively. Alloying behavior has shown increase [15] as well as decrease [16] of micro-hardness values as it depends on interactions between the components involved. The slight decrease in hardness value for the doped PCP crystal may be due to the loose packing of the solid solution of PCP and SCN. Generally for miscibility gap systems the interac-
1.2x104
Intensity (a.u.)
1.0x104 8.0x103 6.0x103
115
tion between like molecules becomes more than unlike molecules [12,17]. Furthermore, in present system SCN is a soft material than PCP; therefore, in the doped PCP crystal the presence of SCN molecules in between the lattices of PCP would have reduced the actual interactions between PCP–PCP molecules. The powder XRD was recorded on (0 0 1) polished plane of PCP crystal from 101 to 701 of 2y values and is shown in (Fig. 5). Only one intense peak was observed that infers the crystalline and single orientation of the grown PCP crystal.
4. Conclusions The binary PCP–SCN phase diagram was determined and shows the formation of a monotectic and a eutectic, with a very high miscibility gap between 0.0456 and 0.9658 mole fraction of SCN. We have demonstrated that large (30.0 mm long and 12 mm diameter), high-quality single crystals can be grown from both PCP- and SCNdoped PCP melts. Both crystals, PCP and PCP doped with SCN were found to be transparent from 2000 to 315 nm. Optical transmittance spectrum of PCP and doped PCP showed more than 70% and 80% transmittance, respectively. The measured mechanical harness value of PCP crystals was more than that of PCP doped with SCN crystals. On the other hand, the SHG efficiency of doped PCP was found better than that of pure PCP. The SHG intensity showed by PCP was twice that of the well-known organic NLO material, urea. Thus, the optical and nonlinear optical properties along with micro-hardness of PCP crystals suggest it as a potential crystal to be considered for frequency doubler applications.
4.0x103
Acknowledgements
2.0x103 0.0 10
20
30
40
50
60
2θ
Fig. 5. Powder XRD pattern of PCP crystal.
70
R.N. Rai sincerely thanks the Department of Science and Technology, New Delhi for financial support under the Young Scientist Award. Thanks also to Prof. P.K. Das, Department of Inorganic
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and Physical Chemistry, I.I.Sc., Bangalore for the SHG assessment. References [1] P. Gunter, Nonlinear Optical Effects and Materials, Springer, Berlin, 2000. [2] R.W. Munn, C.N. Ironside, Principles and Applications of Nonlinear Optical Materials, Chapman & Hall, London, 1993. [3] J. Thompson, R.I.R. Blyth, M. Mazzeo, M. Anni, G. Gigli, R. Cingoloni, Appl. Phys. Lett. 79 (2001) 560. [4] X.T. Tao, D.R. Yuan, N. Zang, M.H. Jiang, Z.S. Shao, J. Appl. Phys. Lett. 60 (12) (1992) 1415. [5] S Miyata, X.T. Tao, Synthetic Met. 81 (1996) 99. [6] J. Choi, M.D. Aggarwal, W.S. Wang, B. G. Penn, D.O. Frazier, Proceedings of SPIE—The International Society for Optical Engineering 3793(Operational Characteristics and Crystal Growth of Nonlinear Optical Materials) (1999) 55–63.
[7] N.B. Singh, T. Henningsen, R.H. Hopkins, R. Mazelsky, R.D. Hamacher, E.P. Supertzi, F.K. Hopkins, D.E. Zelmon, O.P. Singh, J. Crystal Growth 128 (1993) 976. [8] R.N. Rai, C.W. Lan, J. Mater. Res. 17 (7) (2002) 1857. [9] U.S. Rai, R.N. Rai, Chem. Mater. 11 (1999) 3031. [10] U.S. Rai, Pinky Pandey, R.N. Rai, J. Crystal Growth 220 (2000) 610. [11] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798. [12] R.N. Rai, J. Mater. Res. 19 (2004) 1348. [13] H.J. Rossel, H.G. Scott, J. Cryst. Mol. Struct. 3 (1973). [14] S.R. Marder, J.W. Perry, C.P. Yakymyshyn, Chem. Mater. 6 (1994) 1137. [15] J.M. Chang, A.K. Batra, R.B. Lal, J. Crystal Growth 158 (1996) 284. [16] S. Kalainath, M. Beatrice Margaret, T. Irusan, Cryst. Eng. 5 (2002) 71. [17] H. Moore, N.D. Spencer, Encyclopaedia of Chemical Physics and Physical Chemistry, IOP Publishing Ltd., 2001.
Journal of Crystal Growth 285 (2005) 111–116 www.elsevier.com/locate/jcrysgro
Growth and characterization of single crystal of pentachloropyridine R.N. Raia,, K.B.R. Varmab a
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan, India b Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India Received 6 January 2005; received in revised form 11 June 2005; accepted 2 August 2005 Available online 3 October 2005 Communicated by R.S. Feigelson
Abstract Bulk single crystals of pentachloropyridine (PCP) were grown by the Bridgman–Stockbarger method. The single crystalline nature of the grown crystals was confirmed using powder X-ray diffraction (XRD) techniques. These were transparent in the 315–2000 nm range. The second harmonic generation (SHG) efficiency of polycrystalline materials using Kurtz powder technique was found to be twice that of the well-known organic nonlinear optical (NLO) material, urea. The mixing behavior of PCP with succinonitrile (SCN) and its effect on SHG and micro-hardness of PCP, along with the physicochemical properties of different compositions of PCP and SCN system were studied in detail. Bulk single crystals of the PCP solid solution containing 0.02 mole fraction of SCN have also been grown using the Bridgman–Stockbarger technique. r 2005 Elsevier B.V. All rights reserved. Keywords: A1. Crystal growth; A1. Micro-hardness; A1. Optical properties; A1. Second harmonic generation; A1. Temperature profile; A2. Bridgman–-Stockbarger
1. Introduction The increasing demand on organic materials for technological applications, which includes optical frequency doublers, ultra-fast modulators, amplifiers and switches, has prompted researchers to look for newer promising materials [1–5]. One of Corresponding author. Tel.: +91 1596 245073 x 276/454;
fax: +91 1596 244183. E-mail address: [email protected] (R.N. Rai).
the main criteria for an organic nonlinear optical (NLO) material is that its crystal structure lacks a center of symmetry. In molecular crystals, symmetry depends mainly on the polarizability of the electrons in the p-bonding orbital. This is in contrast with inorganic materials where p-electron delocalization is absent and lattice vibrations play a dominant role. Hence, the structural studies related to bonding properties of the atoms in the molecules as well as the molecules in the crystal help in speculating on the properties of material.
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.08.012
ARTICLE IN PRESS R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
The pentachloropyridine (PCP) molecule consists of five atoms of chlorine attached to a pyridine ring containing a nitrogen atom. Therefore, the molecular structure of PCP predicts that it could crystallize in non-centrosymmetric space group. Furthermore, binary organic materials are known to exhibit better optical properties than their parent components [6–8] and, hence, PCP and succinonitrile (SCN) were chosen for the detail study of physicochemical properties and the SHG property. In this article, we report details concerning the growth of bulk single crystals of PCP and PCP doped with SCN from their melts, along with their micro-hardness, optical and nonlinear optical properties. The comparative properties of PCP crystals to that of PCP doped with SCN, and the phase diagram study of the PCP and SCN system are also reported.
250 L Tc 200 Temperature (°C)
112
L1 + L 2 150 Tm
L1
100
L2 PCP(S1) + L2
50
E PCP(S1) + SCN(S2) 0.0
2. Experimental procedure 2.1. Materials and purification The purification of SCN (Aldrich, Germany) was done by repeated distillation under reduced pressure, while PCP (Aldrich, Germany) was purified by recrystallization from carbon tetrachloride. The purity of PCP and SCN were checked by their melting points, which were found to be 125 and 56.5 1C, respectively. 2.2. Phase diagram The phase diagram of the PCP–SCN system was established, using the method reported [9,10], in the form of temperature–composition curve (Fig. 1). The mixtures of two components were taken, covering the entire range of compositions. The melting/miscibility temperatures of each composition were recorded using a melting point apparatus attached with a precision thermometer associated with an accuracy of 70.5 1C. 2.3. Growth of bulk single crystals To grow the single crystals from their melts, the Bridgman–Stockbarger method was used. Suitable
Mh
M
PCP
0.2
0.4
0.6
Mole fraction of SCN
0.8
1.0 SCN
Fig. 1. Phase diagram of pentachloropyridine–succinonitrile system; —melting/miscibility temperature.
temperature gradients were established in a vertical two zones furnace, having a ceramic baffle that separates the zones, and the temperature of each zone was maintained using two different temperature controllers. The top zone of the furnace was maintained at a higher temperature (130 1C, i.e. 5 1C higher than that of the melting point of PCP) than the bottom zone (110 1C). The optimized temperature profile for growing PCP and PCP doped with SCN crystals is given in Fig. 2. For growing PCP crystals the ampoule-lowering rate was maintained at 4.0 mm/day. When the crystal growth run was completed, the temperature of the furnace was cooled down to room temperature at a rate of 0.5 1C/h. 2.4. Micro-hardness measurement A Shimadzu Micro Hardness Tester (HMV-2) with a mini load machine and a diamond pyramid indenter was used for the measurement of Vickers hardness and the values were obtained from the following equation, by taking the test load and the
ARTICLE IN PRESS
Temperature (°C)
R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
113
130
2000 nm. The thicknesses of the crystals were around 2.0 mm.
120
2.7. Second harmonic generation (SHG) measurement
110
100
90
Temperature (°C) -5
0 5 10 15 20 25 30 35 40 Length of the furnace (bottom to top, in cm)
Fig. 2. Temperature profile of furnace used for the single crystals growth of PCP and doped PCP.
To evaluate the SHG efficiencies, powders of different crystallite sizes were prepared using different sizes of mesh and Kurtz powder technique was adopted [11]. The thickness of the materials was also maintained the same by making a sandwich between two glass plates. The materials were exposed to the laser beam using DCR-11, Nd:YAG laser with a pulse width of 10 ns and at the repetition rate of 10 Hz. The OPHIR power meter was used to measure the laser output power in the transmission mode. A comparison was made with that of urea powders.
3. Results and discussion mean of the diagonal length of indentation into account: F HV ¼ 0:1892 2 , d where HV, F and d are the Vickers hardness, test load (N) and mean of the indentation diagonal length (mm), respectively. 2.5. XRD characterization The single crystalline nature of the PCP crystal was confirmed using powder XRD. Besides the phase diagram study, the powder XRD was also extended to confirm the nature of the mixture (0.98 mole fraction PCP and 0.02 mole fraction SCN), whether it is a mechanical mixture or a solid solution. 2.6. Optical characterization The optical transmittance spectrum of PCP and PCP doped with SCN were done on polished crystals using a Hitachi U-3000, UV–Vis. spectrometer whereas, to study the range of transparency, UV/Vis/NIR (JASCO model V-570) spectrometer was used in the absorption mode from 190 to
3.1. Phase diagram The solid–liquid and liquid–liquid equilibrium data is given in Fig. 1. This temperature–composition diagram shows the formation of a monotectic and a eutectic at SCN mole fractions of 0.0456 and 0.9658, respectively. The upper critical temperature being 99.0 1C above the monotectic temperature (M h ). The melting temperature of PCP decreases to a small extent with the addition of the second component (SCN). Beyond this composition a slight addition of the second component (SCN) gives an immiscibility region (L1+L2). This phase equilibria study found that there are three isothermal solidification reactions of interest. The first reaction is L ! L1 þL2 . The phase separation kinetics here is complicated. The various possibilities of phase separation along with thermal properties have been discussed earlier [12]. The second reaction is the monotectic reaction L1 ðrich in PCPÞ ! S1 ðrich in PCPÞ þ L2 ðrich in SCNÞ:
ARTICLE IN PRESS R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
114
The third reaction is the eutectic reaction L2 ðrich in SCNÞ
! S1 ðrich in PCPÞ þ S2 ðrich in SCNÞ: The monotectic, eutectic and critical solution temperatures are 121.0, 54.0 and 220.0 1C, respectively. To assess the comparative micro-hardness, optical and NLO properties of pure PCP crystals to that of SCN-doped PCP crystals, the compositional region from PCP up to the monotectic composition was used for growing crystals of doped PCP. 3.2. Studies on PCP, and PCP and SCN solid solution crystals The temperature profile that was employed for crystal growth was recorded after stabilizing it for 24 h. The top zone of the furnace was maintained at a higher temperature and the ampoule containing the melt was allowed to pass from the top to the bottom. The thermal gradient inside the furnace (Fig. 2) was not uniform, and for the portion of interest it could be computed from the graph. However, along the most probable portion of crystal growth, between 124.0 and 115.0 1C, it was found to be 1.41 1C/cm. The rate of ampoule lowering was found suitable when it was in the range of 3.0–5.0 mm/day. A section of the grown PCP crystal is depicted in Fig. 3. It was observed
that the bulk crystal grown was transparent and without cracks inside, whereas crystal cracking was observed on the outer surface, particularly at the crystal—glass wall interface. These cracks might be the consequence of strain (Fig. 3) as it is significant in the neck portion of the ampoule where the radius of the ampoule has a sharp point for facilitating the nucleation. The tetragonal structure [13] of the PCP crystals was confirmed by X-ray powder diffraction, which was also used to study the compositions of PCPand SCN-doped samples up to the monotectic composition. Only peaks associated with PCP were found, indicating that PCP–SCN mixtures up to a monotectic composition are solid solutions. Based on this study, a crystal of PCP doped with SCN (0.02 mole fraction) was grown. The optical spectrum (Fig. 4) was taken on polished plates of the crystal of about 2.0 mm thicknesses. The optical transmittance for PCP and doped PCP crystals was over 70%. The percent optical transmission of SCN-doped PCP crystals was better than that of pure PCP, while the cut-off wavelengths for both the crystals were almost the same (315 nm). The study of transparency range of PCP crystal, using UV/Vis/NIR spectrometer, (inset of Fig. 4) inferred that the crystal is transparent from 2000 to 315 nm, which is quite impressive because it is difficult to find crystals with this wide range of transparency and
1.0 PCP doped with SCN
% Transmittance
0.8 PCP
0.6
0.4
0.2
0.0 100
Fig. 3. Photograph of PCP crystal.
200
300
400 500 600 Wavelength (nm)
700
800
900
Fig. 4. Optical transmittance spectrum of PCP and doped PCP with SCN crystals.
ARTICLE IN PRESS R.N. Rai, K.B.R. Varma / Journal of Crystal Growth 285 (2005) 111–116
transmission [1,4]. Better NLO materials might be synthesized by increasing the conjugated length and by substituting donor and acceptor groups but there is no control on its consequence, i.e. the red shift of the cut-off wavelengths. The powder SHG measurements showed an intense emission of green light for pure PCP as well as solid solutions of SCN-doped crystals. A comparison was made with the SHG emission of known organic material, urea, maintaining the thickness and crystallite size the same. The intensities recorded for PCP and doped PCP crystals were 2.0 and 2.2 times, respectively, that of urea. The powder SHG measurements are not very accurate as the intensity depends on the crystallite size as well as the coherence length [14]. Therefore, the present SHG data obtained in polycrystalline materials mainly confirms the existence of optical nonlinearities and non-centrosymmetric space groups. The micro-hardness of the crystals was measured on the (0 0 1) plane. At a particular load of 98.07 mN for a dwell time of 10 s, the crystal hardness of PCP and PCP doped with SCN were found to be 13.5 and 11.0 HV, respectively. Alloying behavior has shown increase [15] as well as decrease [16] of micro-hardness values as it depends on interactions between the components involved. The slight decrease in hardness value for the doped PCP crystal may be due to the loose packing of the solid solution of PCP and SCN. Generally for miscibility gap systems the interac-
1.2x104
Intensity (a.u.)
1.0x104 8.0x103 6.0x103
115
tion between like molecules becomes more than unlike molecules [12,17]. Furthermore, in present system SCN is a soft material than PCP; therefore, in the doped PCP crystal the presence of SCN molecules in between the lattices of PCP would have reduced the actual interactions between PCP–PCP molecules. The powder XRD was recorded on (0 0 1) polished plane of PCP crystal from 101 to 701 of 2y values and is shown in (Fig. 5). Only one intense peak was observed that infers the crystalline and single orientation of the grown PCP crystal.
4. Conclusions The binary PCP–SCN phase diagram was determined and shows the formation of a monotectic and a eutectic, with a very high miscibility gap between 0.0456 and 0.9658 mole fraction of SCN. We have demonstrated that large (30.0 mm long and 12 mm diameter), high-quality single crystals can be grown from both PCP- and SCNdoped PCP melts. Both crystals, PCP and PCP doped with SCN were found to be transparent from 2000 to 315 nm. Optical transmittance spectrum of PCP and doped PCP showed more than 70% and 80% transmittance, respectively. The measured mechanical harness value of PCP crystals was more than that of PCP doped with SCN crystals. On the other hand, the SHG efficiency of doped PCP was found better than that of pure PCP. The SHG intensity showed by PCP was twice that of the well-known organic NLO material, urea. Thus, the optical and nonlinear optical properties along with micro-hardness of PCP crystals suggest it as a potential crystal to be considered for frequency doubler applications.
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Acknowledgements
2.0x103 0.0 10
20
30
40
50
60
2θ
Fig. 5. Powder XRD pattern of PCP crystal.
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R.N. Rai sincerely thanks the Department of Science and Technology, New Delhi for financial support under the Young Scientist Award. Thanks also to Prof. P.K. Das, Department of Inorganic
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