- Email: [email protected]
Growth and characterization of nonlinear optical l -arginine maleate dihydrate single crystals
Available online at www.sciencedirect.com
Materials Letters 62 (2008) 755 – 758 www.elsevier.com/locate/matlet
Growth and characterization of nonlinear optical L-arginine maleate dihydrate single crystals D. Kalaiselvi, R. Mohan Kumar, R. Jayavel ⁎ Crystal Growth Centre, Anna University, Chennai, 600 025, India Received 17 February 2007; accepted 19 June 2007 Available online 27 June 2007
Abstract Bulk single crystals of L-arginine maleate dihydrate (LAMD), an organic nonlinear optical material, have been grown from aqueous solution by slow cooling technique. Single crystals of size 48 × 33 × 7 mm3 were grown over a growth period of 3 weeks. Since LAMD has high solubility in water, the growth parameters have been optimized for the reproducible growth of good quality crystals. The cell parameters were estimated by powder X-ray diffraction analysis. FTIR analysis was used to confirm the presence of various functional groups in the grown crystal. Thermal behavior of the grown crystal was investigated by DTA and TGA. The mechanical property of the grown crystal has been studied using Vicker's microhardness tester. The etching studies have been carried out on the grown crystals. The powder technique of Kurtz and Perry confirms the NLO property of the grown crystal and the SHG efficiency of LAMD was found to be 1.4 times that of KDP crystal. © 2007 Published by Elsevier B.V. Keywords: Growth from solution; Organic compound; Nonlinear optical material
1. Introduction Nonlinear optics is a new frontier of science and technology playing a major role in the emerging era of photonics. Photonics involves the application of photons for information and image processing and is branded to be the technology of the 21st century, wherein nonlinear optical (NLO) processes have applications in the vital functions such as frequency conversion and optical switching [1]. These require materials exhibiting second order NLO effects and hence there is a great need for device quality single crystals. Organic crystals with large nonlinear optical (NLO) effects make them attractive for application in frequency conversion and optical processing. Hence, opto-electronics has stimulated the search for highly nonlinear organic crystals for efficient signal processing [2–4]. Up to now several hundreds of donor and acceptor substituted delocalized Π electron systems have been reported which show NLO properties. In this respect, amino acids are interesting materials for NLO applications. The importance of amino acid for NLO application lies on the fact that almost all amino acids ⁎ Corresponding author. Tel.: +91 44 2220 3571; fax: +91 44 2235 2870. E-mail address: [email protected] (R. Jayavel). 0167-577X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.matlet.2007.06.054
contain an asymmetric carbon atom and crystallize in noncentrosymmetric space group. In solid state, amino acid contains a deprotonated carboxylic acid group (COO−) and protonated amino group (NH3+). This dipolar nature exhibits peculiar physical and chemical properties in amino acids, thus making them ideal candidates for NLO applications. In this present communication, the synthesis, single crystal growth of L-arginine maleate dihydrate (LAMD) from its aqueous solution by slow cooling method has been reported. The grown crystals were then characterized by FTIR, powder X-ray diffraction analysis, DTA–TGA analyses, hardness, etching studies and second harmonic generation efficiency measurements. A preliminary study on L-arginine maleate has already been reported by Tapati Mallik et al. [5]. But in this work, we have grown single crystals in larger size and subjected for detailed characterization studies. 2. Experimental 2.1. Material synthesis and purification Equimolar amount of strongly basic amino acid, L-arginine (Merck, 99%) and weak organic acid, maleic acid (Loba Chemie,
756
D. Kalaiselvi et al. / Materials Letters 62 (2008) 755–758
99.5%) were dissolved in double distilled water to synthesize LAMD as per the following reaction. NH2 CNHNHðCH2 Þ3 CHðNH2 ÞCOOH þ HOOCCHCHCOOH þ 2H2 O→ þ − − ðH2 NÞþ 2 CNHðCH2 Þ3 CHðNH3 Þ COO dHOOCCHCHCOO d2H2 O
During this crystallization, L-arginine maleate transformed to its hydrated form with the addition of two molecules of water of crystallization to its crystal lattice. The synthesized material was then purified by repeated recrystallization process and melting point measured was carried out to monitor the purity of the material. 2.2. Solubility studies and crystal growth L-arginine
maleate was dissolved in double distilled water and kept in a constant temperature bath with a cryostat facility and stirring was achieved continuously for 6 h. Solubility studies for different temperatures (30, 35, 40 and 45 °C) have been carried out by gravimetric analysis and it has been observed that the solubility of LAMD is high in water (31.2 g/ 100 ml of water at 30 °C) compared to organic solvents. The starting material L-arginine maleate was dissolved in double distilled water in accordance with the solubility diagram. 300 ml of saturated solution of LAMD at 40 °C was prepared and the solution was filtered using a filter paper of porosity 0.1 μm. Seeds obtained from slow evaporation technique were used for growth. The growth was carried out in constant temperature bath of controlling accuracy of ± 0.01 °C. A cooling rate of 0.2 °C/day was employed in the initial and final stages of the experiment. Optical quality crystal with dimension of 48 × 33 × 7 mm3 has been grown over a typical growth period of 3 weeks.
Fig. 2. Powder X-ray diffraction pattern of LAMD crystal.
diffractometer with CuKα radiation (λ = 1.5418 Å) was used at a scan speed of 0.02°/s. The FTIR spectrum was recorded using Bruker IFS 66V by KBr pellet technique to confirm the presence of functional groups. Thermal studies were carried out using NETZSCH-Geratebu Gmbh thermal analyser to study the thermal behavior of the grown crystal. Etching studies were carried out on the (–101) plane of L-arginine maleate crystal using water as an etchant, in order to investigate the growth mechanism and surface features. Microhardness measurements of LAMD crystal were carried out using a Leitz Wetzlar Vicker's microhardness tester fitted with a diamond pyramidal indenter attached to an optical microscope. The powder technique of Kurtz and Perry confirms the NLO property of the grown crystal. 3. Results and discussion 3.1. Crystal growth
2.3. Characterization studies The grown crystals were subjected to powder X-ray diffraction analysis to confirm the crystallinity and also to estimate the lattice parameters. A Rich-Seifert powder X-ray
As grown crystals of LAMD is shown in Fig. 1. During single crystal growth of LAMD, it was observed that crystals grow from solution of pH values ranging from 3 to 5, but the best crystals were obtained from the solution with pH value of 3.5 within a very short period. By increasing the pH from 3.5 to 5, no significant change in the quality and size of the crystals was observed. The common problem in organic crystal growth is the formation of several undesirable funguslike organisms, which contaminate the solution and thereby affect the quality of the grown crystal. In the case of LAMD no such microbes were observed even after 3 months. 3.2. Powder X-ray diffraction analysis Powder X-ray diffraction analysis was performed on LAMD crystal with a scan speed of 0.02°/s in the range of 10–60°. Fig. 2 shows the powder X-ray diffractogram of LAMD crystal. All the observed reflections were indexed for triclinic structure and the unit cell parameters were calculated. Table 1 shows the calculated unit cell parameters and agree well with the reported values [6]. 3.3. FTIR absorption studies
Fig. 1. As grown single crystal with dimension of 48 × 33 × 7 mm3 of LAMD grown from solution pH value of 3.5.
Fig. 3 shows the FTIR spectrum of LAMD crystal recorded in the range of 400–4000 cm− 1. The high frequency region of the spectrum
D. Kalaiselvi et al. / Materials Letters 62 (2008) 755–758
757
Table 1 Unit cell parameters of LAMD crystal Unit cell parameters from powder data (present study)
Unit cell parameters from single crystal study (Ref. [6])
a = 5.297 (1) Å b = 8.048 (1) Å c = 9.806 (3) Å α = 106.86 (2)° β = 97.61 (2)° γ = 101.22 (3)°
a = 5.264 (3) Å b = 8.039 (3) Å c = 9.784 (3) Å α = 106.19 (3)° β = 97.24 (3)° γ = 101.66 (2)°
consists of bands due to NH2, NH+3 , NH+2 and COO− stretching vibrations. The lower frequency region contains bands due to deformation vibrations of the various groups. The FTIR spectrum is analysed based on the crystal structure of LAMD [6], which shows that in crystalline state the arginine molecule exists as Zwitterion, both the α-amino group and the guanidyl group accepting proton from own carboxyl group and acid group. An extensive system of hydrogen bonding extends throughout the molecule. Hydrogen bonding results in the modification of stretching frequencies of NH+3 and carboxyl groups. On investigation of the absorption bands of LAMD below 1000 cm− 1, three characteristic bands were identified, one at 866 cm− 1 (Rocking NH2), second one at 698 cm− 1 (COO− in plane deformation) and third at 544 cm− 1 (NH+3 torsion mode). The characteristic deformation bands of NH+3 group present in all amino acids appeared at 1610 and 1501 cm− 1. The peaks at 1096, 1188.9 and 1377 cm− 1 are assigned respectively to C–N stretching, NH+3 rocking and COO− symmetric stretching. The absorption band in the region of 2100–2020 cm− 1 is due to combination of NH+3 deformation and NH+3 torsion and is a very good indicator band for the identification of the charged NH+3 group. Combination band of NH+3 bending vibration appeared at 2622.7 cm− 1. The asymmetric and symmetric stretching of water which give rise to absorption band [7] at 3756 and 3655.2 cm− 1 are shifted to 3504 and 3322.4 cm− 1 due to extensive system of hydrogen bonding. So the bands at 3504 and 3322.4 cm− 1 are assigned to be due to the presence of water molecules. On the basis of available data on the vibrational frequencies of amino acids [8], we have identified the characteristic IR bands for different molecular groups present in L-arginine maleate dihydrate crystals. 3.4. Thermal analysis Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of LAMD crystals were carried out in the temperature range of
Fig. 4. DTA and TGA curves of LAMD.
0–500 °C in inert nitrogen atmosphere at a heating rate of 20 °C/min The TGA and DTA curves of LAMD are shown in Fig. 4. The DTA curve of LAMD shows an endothermic peak at 95.8 °C, which can be attributed to the melting point of the sample. The compound starts to lose water at around 83.8 °C and continues up to 123.8 °C, in which 1/3 water molecule is eliminated at around the melting point. A second dissociation occurs at 182.5–217.5 °C, results in the formation of volatile substances probably carbon dioxide, ammonia and rest part of water molecule. Further heating does not produce any significant endothermic or exothermic peaks in the DTA curve, because DTA becomes inactive due to improper contact with the molten substance, whereas TGA shows complete weight loss. The studies revealed that the LAMD crystal is thermally stable up to 217.5 °C. 3.5. Microhardness measurement Vicker's microhardness test was carried out for the LAMD crystal. The indentations were made at room temperature with a constant indentation time of 5 s. The indentation marks were made on the surface by varying the load from 1 to7 g. The diagonal length of the indentation impression was measured using a Leitz Metallax II microscope. In order to get accurate results for each applied load, several indentations were made on the sample and the average diagonal length (d ) of the indenter impressions were measured. The Vicker's microhardness number Hv of the crystal was calculated using the equation Hv = 1.8544 P / d2 (kg/mm2), where P is the applied load and d is the mean diagonal length of the indenter impression. For LAMD ¯ plane was selected for hardness studies. For applied load crystal, (101) above 7 g, microcracks were observed around the impression. It is observed from the measurement that the hardness value initially increases (Hv = 11.6 kg/mm2) up to the applied load (P = 6 g), and then it decreases (Hv = 10.3 kg/mm2) with the increase of applied load. 3.6. Etching studies
Fig. 3. FTIR spectrum of LAMD.
The nonlinear efficiency of the NLO material mainly depends on the quality of the grown crystals because the segregated impurities and dislocations occur during growth results in the distortion of the optical beam to be processed. So, it is very essential to study the microstructural imperfections or crystal defects in the grown crystals [9]. Micromorphology studies of LAMD crystal were carried out by etching with water as an etchant. Water is a superior etching solution for revealing dislocation etch pits and it is insensitive to surface
758
D. Kalaiselvi et al. / Materials Letters 62 (2008) 755–758
confirm the crystallinity and shows that LAMD crystal has triclinic structure. The presence of various functional groups was confirmed by FTIR spectrum. The thermal analyses reveal the thermal stability of the crystal. The microhardness studies reveal that the hardness value initially increases with the applied load, and then it decreases beyond the applied load of 7 g. Analysis of surface micrographs reveals that LAMD crystal grows by two dimensional layer growth mechanism. The SHG efficiency of LAMD was found to be 1.4 times that of KDP crystal which makes it a promising material for NLO applications. Acknowledgement The authors thank Prof. P. K. DAS, Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore for providing facilities for SHG studies. Fig. 5. Step pattern on (−101) plane of LAMD crystal.
orientation as it produced pits almost on all surfaces [10]. In the present experimental work, transparent crystal free from inclusions and cracks was selected. Etching of the crystal surface was carried out by dipping the crystal in water for few seconds at room temperature and then wiping it with dry filter paper. Etch patterns were observed and photographed under an optical microscope in the reflected light. The ¯ surface microscopic study of the grown crystals showed that the (101) plane consists of macrosteps as shown in Fig. 5. The linear steps are the manifestation of two dimensional layer growth. This is exactly similar to what happens on the {100} faces of LAP [11], LAHF [12] and KTP crystals [13]. The surface micrograph shows that the growth on the ¯ (101) plane takes place predominately by two dimensional layer growth. 3.7. Second harmonic generation efficiency test The SHG efficiency of LAMD was measured by the powder technique of Kurtz and Perry [14]. The second harmonic output was generated by irradiating powder samples by a pulsed laser beam of Nd: YAG laser with a pulse width of 8 ns. KDP sample was used as the reference material and the powder SHG efficiency of LAMD was found to be 1.4 times that of KDP.
4. Conclusions Optical quality bulk single crystals of LAMD were grown by slow cooling technique. Powder X-ray diffraction studies
References [1] P.N. Prasad, D.J. Williams, “Introduction to Nonlinear Optical Effects in Molecules and Polymers", John-Wiley and Sons Inc., New York, 1991, p. 1. [2] R.A. Hann, D. Bloor (Eds.), “Organic Materials for Nonlinear Optics", Special publications, No. 69, The Royal Society of Chemistry, 1989. [3] J. Badan, R. Hierle, A. Perigaud, J. Zyss, in: D.J. Williams (Ed.), Nonlinear Optical Properties of Organic Molecules and Polymeric Materials, American Chemical Symposium Series, 233, American Chemical Society, Washington, DC, 1993. [4] D.S. Chemla, J. Zyss (Eds.), “Nonlinear Optical Properties of Organic Molecules and Crystals", Academic press, New York, 1987. [5] T. Mallik, T. Kar, Cryst. Res. Technol. 40 (2005) 778. [6] T. Mallik, T. Kar, G. Bocelli, A. Musatti, Sci. Technol. Adv. Mater. 6 (2005) 508. [7] G. Herzberg, “Infrared and Raman Spectra of Polyatomic Molecules", Van Nostrand, New York, 1945. [8] R.S. Krishnan, V.N. Sankaranarayanan, K. Krishnan, J. Indian. Inst. Sci. 55 (1973) 66. [9] S. Mukerji, T. Kar, Jpn. J. Appl. Phys. 38 (1999) 832. [10] S. Mukerji, T. Kar, J. Cryst. Growth 200 (1999) 543. [11] G. Dhanaraj, T. Shripathi, H.L. Bhat, J. Crystal Growth 113 (1991) 456. [12] T. Pal, T. Kar, J. Cryst. Growth 276 (2005) 247. [13] G. Dhanaraj, H.L. Bhat, Mater. Lett. 10 (1990) 283. [14] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798.
Materials Letters 62 (2008) 755 – 758 www.elsevier.com/locate/matlet
Growth and characterization of nonlinear optical L-arginine maleate dihydrate single crystals D. Kalaiselvi, R. Mohan Kumar, R. Jayavel ⁎ Crystal Growth Centre, Anna University, Chennai, 600 025, India Received 17 February 2007; accepted 19 June 2007 Available online 27 June 2007
Abstract Bulk single crystals of L-arginine maleate dihydrate (LAMD), an organic nonlinear optical material, have been grown from aqueous solution by slow cooling technique. Single crystals of size 48 × 33 × 7 mm3 were grown over a growth period of 3 weeks. Since LAMD has high solubility in water, the growth parameters have been optimized for the reproducible growth of good quality crystals. The cell parameters were estimated by powder X-ray diffraction analysis. FTIR analysis was used to confirm the presence of various functional groups in the grown crystal. Thermal behavior of the grown crystal was investigated by DTA and TGA. The mechanical property of the grown crystal has been studied using Vicker's microhardness tester. The etching studies have been carried out on the grown crystals. The powder technique of Kurtz and Perry confirms the NLO property of the grown crystal and the SHG efficiency of LAMD was found to be 1.4 times that of KDP crystal. © 2007 Published by Elsevier B.V. Keywords: Growth from solution; Organic compound; Nonlinear optical material
1. Introduction Nonlinear optics is a new frontier of science and technology playing a major role in the emerging era of photonics. Photonics involves the application of photons for information and image processing and is branded to be the technology of the 21st century, wherein nonlinear optical (NLO) processes have applications in the vital functions such as frequency conversion and optical switching [1]. These require materials exhibiting second order NLO effects and hence there is a great need for device quality single crystals. Organic crystals with large nonlinear optical (NLO) effects make them attractive for application in frequency conversion and optical processing. Hence, opto-electronics has stimulated the search for highly nonlinear organic crystals for efficient signal processing [2–4]. Up to now several hundreds of donor and acceptor substituted delocalized Π electron systems have been reported which show NLO properties. In this respect, amino acids are interesting materials for NLO applications. The importance of amino acid for NLO application lies on the fact that almost all amino acids ⁎ Corresponding author. Tel.: +91 44 2220 3571; fax: +91 44 2235 2870. E-mail address: [email protected] (R. Jayavel). 0167-577X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.matlet.2007.06.054
contain an asymmetric carbon atom and crystallize in noncentrosymmetric space group. In solid state, amino acid contains a deprotonated carboxylic acid group (COO−) and protonated amino group (NH3+). This dipolar nature exhibits peculiar physical and chemical properties in amino acids, thus making them ideal candidates for NLO applications. In this present communication, the synthesis, single crystal growth of L-arginine maleate dihydrate (LAMD) from its aqueous solution by slow cooling method has been reported. The grown crystals were then characterized by FTIR, powder X-ray diffraction analysis, DTA–TGA analyses, hardness, etching studies and second harmonic generation efficiency measurements. A preliminary study on L-arginine maleate has already been reported by Tapati Mallik et al. [5]. But in this work, we have grown single crystals in larger size and subjected for detailed characterization studies. 2. Experimental 2.1. Material synthesis and purification Equimolar amount of strongly basic amino acid, L-arginine (Merck, 99%) and weak organic acid, maleic acid (Loba Chemie,
756
D. Kalaiselvi et al. / Materials Letters 62 (2008) 755–758
99.5%) were dissolved in double distilled water to synthesize LAMD as per the following reaction. NH2 CNHNHðCH2 Þ3 CHðNH2 ÞCOOH þ HOOCCHCHCOOH þ 2H2 O→ þ − − ðH2 NÞþ 2 CNHðCH2 Þ3 CHðNH3 Þ COO dHOOCCHCHCOO d2H2 O
During this crystallization, L-arginine maleate transformed to its hydrated form with the addition of two molecules of water of crystallization to its crystal lattice. The synthesized material was then purified by repeated recrystallization process and melting point measured was carried out to monitor the purity of the material. 2.2. Solubility studies and crystal growth L-arginine
maleate was dissolved in double distilled water and kept in a constant temperature bath with a cryostat facility and stirring was achieved continuously for 6 h. Solubility studies for different temperatures (30, 35, 40 and 45 °C) have been carried out by gravimetric analysis and it has been observed that the solubility of LAMD is high in water (31.2 g/ 100 ml of water at 30 °C) compared to organic solvents. The starting material L-arginine maleate was dissolved in double distilled water in accordance with the solubility diagram. 300 ml of saturated solution of LAMD at 40 °C was prepared and the solution was filtered using a filter paper of porosity 0.1 μm. Seeds obtained from slow evaporation technique were used for growth. The growth was carried out in constant temperature bath of controlling accuracy of ± 0.01 °C. A cooling rate of 0.2 °C/day was employed in the initial and final stages of the experiment. Optical quality crystal with dimension of 48 × 33 × 7 mm3 has been grown over a typical growth period of 3 weeks.
Fig. 2. Powder X-ray diffraction pattern of LAMD crystal.
diffractometer with CuKα radiation (λ = 1.5418 Å) was used at a scan speed of 0.02°/s. The FTIR spectrum was recorded using Bruker IFS 66V by KBr pellet technique to confirm the presence of functional groups. Thermal studies were carried out using NETZSCH-Geratebu Gmbh thermal analyser to study the thermal behavior of the grown crystal. Etching studies were carried out on the (–101) plane of L-arginine maleate crystal using water as an etchant, in order to investigate the growth mechanism and surface features. Microhardness measurements of LAMD crystal were carried out using a Leitz Wetzlar Vicker's microhardness tester fitted with a diamond pyramidal indenter attached to an optical microscope. The powder technique of Kurtz and Perry confirms the NLO property of the grown crystal. 3. Results and discussion 3.1. Crystal growth
2.3. Characterization studies The grown crystals were subjected to powder X-ray diffraction analysis to confirm the crystallinity and also to estimate the lattice parameters. A Rich-Seifert powder X-ray
As grown crystals of LAMD is shown in Fig. 1. During single crystal growth of LAMD, it was observed that crystals grow from solution of pH values ranging from 3 to 5, but the best crystals were obtained from the solution with pH value of 3.5 within a very short period. By increasing the pH from 3.5 to 5, no significant change in the quality and size of the crystals was observed. The common problem in organic crystal growth is the formation of several undesirable funguslike organisms, which contaminate the solution and thereby affect the quality of the grown crystal. In the case of LAMD no such microbes were observed even after 3 months. 3.2. Powder X-ray diffraction analysis Powder X-ray diffraction analysis was performed on LAMD crystal with a scan speed of 0.02°/s in the range of 10–60°. Fig. 2 shows the powder X-ray diffractogram of LAMD crystal. All the observed reflections were indexed for triclinic structure and the unit cell parameters were calculated. Table 1 shows the calculated unit cell parameters and agree well with the reported values [6]. 3.3. FTIR absorption studies
Fig. 1. As grown single crystal with dimension of 48 × 33 × 7 mm3 of LAMD grown from solution pH value of 3.5.
Fig. 3 shows the FTIR spectrum of LAMD crystal recorded in the range of 400–4000 cm− 1. The high frequency region of the spectrum
D. Kalaiselvi et al. / Materials Letters 62 (2008) 755–758
757
Table 1 Unit cell parameters of LAMD crystal Unit cell parameters from powder data (present study)
Unit cell parameters from single crystal study (Ref. [6])
a = 5.297 (1) Å b = 8.048 (1) Å c = 9.806 (3) Å α = 106.86 (2)° β = 97.61 (2)° γ = 101.22 (3)°
a = 5.264 (3) Å b = 8.039 (3) Å c = 9.784 (3) Å α = 106.19 (3)° β = 97.24 (3)° γ = 101.66 (2)°
consists of bands due to NH2, NH+3 , NH+2 and COO− stretching vibrations. The lower frequency region contains bands due to deformation vibrations of the various groups. The FTIR spectrum is analysed based on the crystal structure of LAMD [6], which shows that in crystalline state the arginine molecule exists as Zwitterion, both the α-amino group and the guanidyl group accepting proton from own carboxyl group and acid group. An extensive system of hydrogen bonding extends throughout the molecule. Hydrogen bonding results in the modification of stretching frequencies of NH+3 and carboxyl groups. On investigation of the absorption bands of LAMD below 1000 cm− 1, three characteristic bands were identified, one at 866 cm− 1 (Rocking NH2), second one at 698 cm− 1 (COO− in plane deformation) and third at 544 cm− 1 (NH+3 torsion mode). The characteristic deformation bands of NH+3 group present in all amino acids appeared at 1610 and 1501 cm− 1. The peaks at 1096, 1188.9 and 1377 cm− 1 are assigned respectively to C–N stretching, NH+3 rocking and COO− symmetric stretching. The absorption band in the region of 2100–2020 cm− 1 is due to combination of NH+3 deformation and NH+3 torsion and is a very good indicator band for the identification of the charged NH+3 group. Combination band of NH+3 bending vibration appeared at 2622.7 cm− 1. The asymmetric and symmetric stretching of water which give rise to absorption band [7] at 3756 and 3655.2 cm− 1 are shifted to 3504 and 3322.4 cm− 1 due to extensive system of hydrogen bonding. So the bands at 3504 and 3322.4 cm− 1 are assigned to be due to the presence of water molecules. On the basis of available data on the vibrational frequencies of amino acids [8], we have identified the characteristic IR bands for different molecular groups present in L-arginine maleate dihydrate crystals. 3.4. Thermal analysis Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of LAMD crystals were carried out in the temperature range of
Fig. 4. DTA and TGA curves of LAMD.
0–500 °C in inert nitrogen atmosphere at a heating rate of 20 °C/min The TGA and DTA curves of LAMD are shown in Fig. 4. The DTA curve of LAMD shows an endothermic peak at 95.8 °C, which can be attributed to the melting point of the sample. The compound starts to lose water at around 83.8 °C and continues up to 123.8 °C, in which 1/3 water molecule is eliminated at around the melting point. A second dissociation occurs at 182.5–217.5 °C, results in the formation of volatile substances probably carbon dioxide, ammonia and rest part of water molecule. Further heating does not produce any significant endothermic or exothermic peaks in the DTA curve, because DTA becomes inactive due to improper contact with the molten substance, whereas TGA shows complete weight loss. The studies revealed that the LAMD crystal is thermally stable up to 217.5 °C. 3.5. Microhardness measurement Vicker's microhardness test was carried out for the LAMD crystal. The indentations were made at room temperature with a constant indentation time of 5 s. The indentation marks were made on the surface by varying the load from 1 to7 g. The diagonal length of the indentation impression was measured using a Leitz Metallax II microscope. In order to get accurate results for each applied load, several indentations were made on the sample and the average diagonal length (d ) of the indenter impressions were measured. The Vicker's microhardness number Hv of the crystal was calculated using the equation Hv = 1.8544 P / d2 (kg/mm2), where P is the applied load and d is the mean diagonal length of the indenter impression. For LAMD ¯ plane was selected for hardness studies. For applied load crystal, (101) above 7 g, microcracks were observed around the impression. It is observed from the measurement that the hardness value initially increases (Hv = 11.6 kg/mm2) up to the applied load (P = 6 g), and then it decreases (Hv = 10.3 kg/mm2) with the increase of applied load. 3.6. Etching studies
Fig. 3. FTIR spectrum of LAMD.
The nonlinear efficiency of the NLO material mainly depends on the quality of the grown crystals because the segregated impurities and dislocations occur during growth results in the distortion of the optical beam to be processed. So, it is very essential to study the microstructural imperfections or crystal defects in the grown crystals [9]. Micromorphology studies of LAMD crystal were carried out by etching with water as an etchant. Water is a superior etching solution for revealing dislocation etch pits and it is insensitive to surface
758
D. Kalaiselvi et al. / Materials Letters 62 (2008) 755–758
confirm the crystallinity and shows that LAMD crystal has triclinic structure. The presence of various functional groups was confirmed by FTIR spectrum. The thermal analyses reveal the thermal stability of the crystal. The microhardness studies reveal that the hardness value initially increases with the applied load, and then it decreases beyond the applied load of 7 g. Analysis of surface micrographs reveals that LAMD crystal grows by two dimensional layer growth mechanism. The SHG efficiency of LAMD was found to be 1.4 times that of KDP crystal which makes it a promising material for NLO applications. Acknowledgement The authors thank Prof. P. K. DAS, Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore for providing facilities for SHG studies. Fig. 5. Step pattern on (−101) plane of LAMD crystal.
orientation as it produced pits almost on all surfaces [10]. In the present experimental work, transparent crystal free from inclusions and cracks was selected. Etching of the crystal surface was carried out by dipping the crystal in water for few seconds at room temperature and then wiping it with dry filter paper. Etch patterns were observed and photographed under an optical microscope in the reflected light. The ¯ surface microscopic study of the grown crystals showed that the (101) plane consists of macrosteps as shown in Fig. 5. The linear steps are the manifestation of two dimensional layer growth. This is exactly similar to what happens on the {100} faces of LAP [11], LAHF [12] and KTP crystals [13]. The surface micrograph shows that the growth on the ¯ (101) plane takes place predominately by two dimensional layer growth. 3.7. Second harmonic generation efficiency test The SHG efficiency of LAMD was measured by the powder technique of Kurtz and Perry [14]. The second harmonic output was generated by irradiating powder samples by a pulsed laser beam of Nd: YAG laser with a pulse width of 8 ns. KDP sample was used as the reference material and the powder SHG efficiency of LAMD was found to be 1.4 times that of KDP.
4. Conclusions Optical quality bulk single crystals of LAMD were grown by slow cooling technique. Powder X-ray diffraction studies
References [1] P.N. Prasad, D.J. Williams, “Introduction to Nonlinear Optical Effects in Molecules and Polymers", John-Wiley and Sons Inc., New York, 1991, p. 1. [2] R.A. Hann, D. Bloor (Eds.), “Organic Materials for Nonlinear Optics", Special publications, No. 69, The Royal Society of Chemistry, 1989. [3] J. Badan, R. Hierle, A. Perigaud, J. Zyss, in: D.J. Williams (Ed.), Nonlinear Optical Properties of Organic Molecules and Polymeric Materials, American Chemical Symposium Series, 233, American Chemical Society, Washington, DC, 1993. [4] D.S. Chemla, J. Zyss (Eds.), “Nonlinear Optical Properties of Organic Molecules and Crystals", Academic press, New York, 1987. [5] T. Mallik, T. Kar, Cryst. Res. Technol. 40 (2005) 778. [6] T. Mallik, T. Kar, G. Bocelli, A. Musatti, Sci. Technol. Adv. Mater. 6 (2005) 508. [7] G. Herzberg, “Infrared and Raman Spectra of Polyatomic Molecules", Van Nostrand, New York, 1945. [8] R.S. Krishnan, V.N. Sankaranarayanan, K. Krishnan, J. Indian. Inst. Sci. 55 (1973) 66. [9] S. Mukerji, T. Kar, Jpn. J. Appl. Phys. 38 (1999) 832. [10] S. Mukerji, T. Kar, J. Cryst. Growth 200 (1999) 543. [11] G. Dhanaraj, T. Shripathi, H.L. Bhat, J. Crystal Growth 113 (1991) 456. [12] T. Pal, T. Kar, J. Cryst. Growth 276 (2005) 247. [13] G. Dhanaraj, H.L. Bhat, Mater. Lett. 10 (1990) 283. [14] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798.