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Synthesis, growth and characterization of bis (glycine) lithium molybdate—A semi-organic NLO material
Spectrochimica Acta Part A 74 (2009) 955–958
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis, growth and characterization of bis (glycine) lithium molybdate— A semi-organic NLO material T. Balu a , T.R. Rajasekaran b,∗ , P. Murugakoothan c a b c
Department of Physics, Aditanar College of Arts and Science, Tiruchendur 628 216, India Department of Physics, Manonmaniam Sundaranar University, Abishekapatty, Tirunelveli 627012, Tamil Nadu, India Post Graduate and Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 26 July 2009 Accepted 7 August 2009 PACS: 42.70.Mp 81.10.Dn 81.10.Aj
a b s t r a c t Single crystals of bis (glycine) lithium molybdate [BGLM] with dimensions 20 mm × 10 mm × 5 mm were grown by slow evaporation technique. The grown crystals were subjected to powder X-ray diffraction studies. Functional groups of the crystallized molecules were confirmed by FTIR analyses. Transmission range of the crystal was determined by UV–vis–NIR spectra. Vickers microhardness test was performed on the prominent plane (0 1 1) of the grown crystal. The NLO property of the crystal was confirmed by Kurtz SHG test and compared with NLO efficiency of KDP crystal. © 2009 Elsevier B.V. All rights reserved.
Keywords: Solubility X-ray diffraction Growth from solutions Nonlinear optical materials
1. Introduction Since 1961, when the nonlinear optical (NLO) phenomenon was observed for the first time, NLO frequency conversion materials have played more and more important role in many fields, such as laser technology, optical communication, optical data storage etc. [1]. In the context of NLO, organic materials have advantages such as large NLO coefficients and structural diversity or flexibility, compared to the inorganic counterparts [2]. They also have some inherent drawbacks, for example, poor physico-chemical stability and low mechanical strength. As a result, the quest for new frequency conversion materials is presently concentrated on semi-organic crystals due to their large nonlinearity, high resistance to laser induced damage, low angular sensitivity and good mechanical hardness [3–5]. Semi-organics include organic–inorganic salts and metal–organic coordination compounds [6]. Among the organic materials amino acids constitute a family in which glycine is the simplest of all the amino acids. It has been reported that some complexes of amino acids with simple inorganic salt may exhibit ferroelectric properties
[7–9]. Some complexes of glycine with CaCl2 [10], BaCl2 [11], H2 SO4 [12] and CoBr2 [13] form single crystals but none of these are reported to have nonlinear optical property. Single crystals of glycine sodium nitrate [14], glycine lithium sulphate [15] and benzoyl glycine [16] showed non-centrosymmetry and their quadratic nonlinear coefficients were examined. A survey of the literature shows that the X-ray crystal structure of bis (glycine) lithium molybdate [BGLM] was reported by Fleck et al. [17] and no other reports on the crystal growth and characterization of BGLM are available. This paper reports the synthesis, solubility and growth of title compound, whose SHG efficiency has been estimated to be 1.3 times that of potassium dihydrogen phosphate. The powder X-ray diffraction studies show the BGLM belongs to monoclinic system with non-centrosymmetric space group P21 . FTIR study reveals the functional groups of the grown crystal. The optical and mechanical properties of the crystals were studied. 2. Experimental procedure 2.1. Synthesis
∗ Corresponding author. Tel.: +91 462333887; fax: +91 4622322973. E-mail address: [email protected] (T.R. Rajasekaran). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.08.048
Bis (glycine) lithium molybdate (BGLM) growth solution was prepared by dissolving analar grade glycine (Merk) and lithium
956
T. Balu et al. / Spectrochimica Acta Part A 74 (2009) 955–958
Fig. 1. Chemical structure of BGLM.
molybdate (Sigma–Aldrich) in stoichiometric ratio 2:1 in double distilled water and stirred well for about 3 h using a temperature controlled magnetic stirrer to yield a homogeneous mixture of solution. The polycrystalline starting material was synthesized by evaporating the solution to almost dryness at the temperature of 50 ◦ C according to the following reaction, 2[C2 H5 NO2 ] + (glycine)
Li2 MoO4 → (lithium molybdate)
[Li2 Mo (C2 H5 NO2 )2 O4 ] (BGLM)
The purity of synthesized salt was improved by successive recrystallization process. Care was taken during heating of the solution and temperature as low as 50 ◦ C was maintained. The chemical structure of BGLM is represented in Fig. 1.
Fig. 3. As grown crystal of BGLM.
2.3. Crystal growth In the present study, BGLM crystals were grown by slow evaporation technique. Recrystallized salt of BGLM was taken as raw material. Saturated BGLM solutions were prepared at room temperature with water as solvent. The prepared solution was filtered using Whatmann 41 filter paper to remove the suspended impurities. The solution was taken in vessels and closed with perforated covers and kept in dust free atmosphere. A well developed crystal of size 20 mm × 10 mm × 5 mm was harvested in a growth period of 30 days and is shown in Fig. 3. 3. Characterization
2.2. Solubility
3.1. Powder X-ray diffraction analysis
The solubility of BGLM in water was determined as a function of temperature in the temperature range 30–50 ◦ C. To determine the equilibrium concentration, the solution BGLM was prepared using double distilled water as the solvent. The solution was maintained at a constant temperature and continuously stirred using a magnetic stirrer to ensure homogeneous temperature and concentration throughout the volume of the solution. On reaching the saturation, the content of the solution was analyzed gravimetrically [18] and this process was repeated for every temperature. The solubility curve is shown in Fig. 2. The solubility increases linearly with increase of temperature.
The purified samples of the grown crystals have been crushed to a uniform fine powder and subjected to powder X-ray diffraction using a Rich Seifert Powder X-ray diffractometer. The K␣ radiations ( = 1.5406 Å) from a copper target were used. The specimen in the form of a thin film was scanned in the reflection mode in the 2 range 10–70◦ with five decimal accuracy. Fig. 4 represents the powder diffractograph for the grown BGLM crystals. The peaks are indexed using least square fit method. From the powder X-ray diffraction data, the lattice parameters and the cell volume have been calculated and are given in Table 1. These values agreed very well with the reported values [17].
Fig. 2. Solubility of BGLM in aqueous solution.
Fig. 4. Powder XRD pattern of BGLM.
T. Balu et al. / Spectrochimica Acta Part A 74 (2009) 955–958
957
Table 1 Unit cell parameter of BGLM. Parameter
Present study
Reported values [17]
a b c ˇ V
5.177462 Å 7.72368 Å 12.441315 Å 94.24852◦ 496.14926 Å3
5.192410 Å 7.733915 Å 12.49230 Å 94.283◦ 500.2717 Å3
3.2. Density measurements The density was calculated as 2.151 g/cc from crystallographic data using the formula =
MZ , NV
where M is the molecular weight, Z is the number of molecules per unit cell, N is the Avogadro’s number and V is the volume of the unit cell. Experimentally ‘floatation technique’ was employed and the value 2.163 g/cc was obtained for . Carbon tetrachloride (CCl4 ) of density 1.594 g/cc and bromoform (CHBr3 ) of density 2.890 g/cc are respectively the lower density and higher density liquids used. This value agrees very well with the calculated value. 3.3. FTIR analysis Fourier Transform Infrared (FTIR) spectrum was recorded using KBr pellet technique on a JASCO FTIR-410 spectrophotometer. Fig. 5 shows the FTIR spectrum recorded in the range 400–4000 cm−1 to identify the functional groups present in BGLM. The spectrum shows a broad envelope between 2100 and 3700 cm−1 . It includes the asymmetric stretching mode of NH3 + at 3163.51 cm−1 . The absorption peak at 422.39 cm−1 is assigned to the Li–O stretching vibration. The band appeared at 598.31 cm−1 is assigned to the COO− in-plane bending vibration. The peaks at 1613.86 and 1410.75 cm−1 are due to asymmetric and symmetric stretching modes of the carboxylate group (COO− ), respectively. The characterizing bending vibration of CH2 group and CH2 wagging appeared at 1456.46 and 1333.98 cm−1 respectively. The C–N stretching vibration is observed at 1102.08 cm−1 . The band at 689.47 cm−1 is due to the bridged –O–Mo–O stretch. The bands observed at 1033.32, 906.97 and 828.61 cm−1 can be attributed to Mo–O stretch [19,20]. 3.4. Optical assessment For optical application, especially for SHG, the material considered must be transparent in the wavelength region of interest. The
Fig. 6. UV–vis–NIR spectrum of BGLM.
UV–vis–NIR transmittance spectrum (Fig. 6) was recorded using Varian Cary 5E UV spectrophotometer in the wavelength range 200–2500 nm, which covers near ultraviolet (200–400 nm), visible (400–800 nm) and then far-infrared (800–1200 nm) regions. Optically clear single crystal of thickness 3 mm was used for this study. The lower cut-off wavelength of the BGLM crystal occurs at 300 nm. The crystal has sufficient transmission in the entire visible and IR region. The transmission window in the visible region and IR region enables good optical transmission of the second harmonic frequencies of Nd:YAG laser. 3.5. Microhardness studies Microhardness testing is one of the best methods of understanding the mechanical properties of materials such as fracture behavior, yield strength, brittleness index and temperature of cracking [21]. Transparent crystals free from cracks were selected for microhardness measurements. Before indentations, the crystals were carefully lapped and washed to avoid surface effects [22]. Vickers microhardness measurements were carried out on BGLM crystal using a Leitz Weitzler hardness tester fitted with a diamond indentor. The hardness measurements were made on the well-developed (0 1 1) face. The well-polished crystal was mounted on the platform of the microhardness tester and the loads of different magnitudes (5–30 g) were applied over a fixed interval of time. The indentation time was fixed as 10 s. The diagonals of the impressions were measured using a Leitz Metallux II microscope with a calibrated ocular at magnification 500×. Vickers microhardness number was evaluated from the relation Hv = 1.8544
Fig. 5. FTIR spectrum of BGLM.
P d2
kg/mm2 ,
where Hv is the Vickers hardness number, P is the indentor load in kg and d is the diagonal length of the impression in mm. A graph plotted between hardness number (Hv ) and applied load (P) is shown in Fig. 7. The reason for getting lower value of hardness at lower loads may be due to the surface reaction by surface adsorbed moisture, which may reduce the strength of the surface layers. There is an increase in the hardness with load without saturation, which can be attributed to the work hardening of the surface layers. Beyond the load 30 g, significant cracking occurs, which may be due to the release of internal stress generated locally by indentation [23]. The Vickers hardness number (VHN) of BGLM crystal is high compared with the other semi-organic crystals like glycine lithium sulphate (55) and l-arginine hydrobromide (48) [15,24].
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T. Balu et al. / Spectrochimica Acta Part A 74 (2009) 955–958
solubility temperature gradient. Single crystals of BGLM have been grown by slow evaporation technique at room temperature. The lattice parameters obtained from powder X-ray diffraction study agree well with the reported values. The functional groups present in the grown crystal have been confirmed by FTIR spectral analysis. UV–visible spectrum shows that the crystal has a wide transmission range with a lower UV-cut-off of 300 nm. Vickers hardness values measured on (0 1 1) plane reveal its mechanical strength. The SHG efficiency measured by the Kurtz powder test was about 1.3 times that of potassium dihydrogen phosphate. The density of the grown crystal was evaluated as 2.151 g/cc from the crystallographic data and is verified by floatation experiment. Acknowledgements Fig. 7. Vickers hardness versus load for BGLM crystal.
3.6. Second harmonic generation efficiency measurements The first and most widely used technique for confirming the SHG from prospective second order NLO materials is the Kurtz powder technique [25] to identify the materials with non-centrosymmetric crystal structures. The crystalline sample was powdered to particle sizes in the range 125–150 m. To make relevant comparisons with known SHG materials, KDP was also ground and sieved into the same particle size range. The powdered samples were filled air-tight in separate micro-capillary tubes of uniform bore of about 1.5 mm diameter. A high intensity Nd:YAG laser beam ( = 1064 nm) with a pulse width of 8 ns and a repetition rate of 10 Hz was passed through the powdered sample. The SHG radiations of 532 nm (green light) emitted were collected by a photomultiplier tube (PMTHamamatsu-model R 2059). The optical signal incident on the PMT was converted into voltage output at the CRO (Tektronix-TDS 305213). The second harmonic signal of 71 mV was obtained for an input energy of 1.35 mJ/pulse, while the standard KDP crystal gave a SHG signal of 55 mV for the same input energy. The result obtained for BGLM shows a powder SHG efficiency of about 1.3 times that of KDP crystal. 4. Conclusion Potential semi-organic nonlinear optical bis (glycine) lithium molybdate complex (BGLM) was synthesized and its solubility was analyzed in the temperature range 30–50 ◦ C. The solubility curve indicates moderate solubility of BGLM in water with a positive
One of the authors (T. Balu) is grateful to the University Grants Commission (UGC), Government of India for the award of research fellowship under the Faculty Development Program. This work is partially supported by FIST, Department of Science and Technology, India. References [1] K.F. Hulme, Rep. Prog. Phys. 36 (1973) 497. [2] D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, vols. 1 and 2, Academic Press, New York, 1987. [3] G. Xing, M. Jiang, Z. Sao, D. Xu, Chin. J. Lasers 14 (1987) 302. [4] N. Zhang, M. Jiang, D. Yuan, D. Xu, X. Tao, Chin. Phys. Lett. 6 (1989) 280. [5] H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.L. Catella, Appl. Opt. 31 (1992) 5051. [6] H.L. Bhat, Bull. Mater. Sci. 17 (1994) 1233. [7] R. Pepinsky, Y. Okaya, D.P. Eastman, T. Mitsui, Phys. Rev. 107 (1957) 1538. [8] R. Pepinsky, K. Vedam, Y. Okaya, Phys. Rev. 110 (1958) 1309. [9] A. Deepthy, H.L. Bhat, J. Cryst. Growth 226 (2001) 287. [10] S. Natarajan, J.K. Mohan Rao, Z. Kristallogr. 152 (1984) 179. [11] P. Narayanan, S. Venkataraman, Z. Kristallogr. 142 (1975) 52. [12] S. Hoshino, T. Mitsui, F. Jona, R. Pepinsky, Phys. Rev. 107 (1957) 125. [13] K. Ravikumar, S.S. Rajan, Z. Kristallogr. 171 (1985) 201. [14] M. Narayan Bhat, S.M. Dharmaprakash, J. Cryst. Growth 235 (2002) 511. [15] T. Balakrishnan, K. Ramamurthi, Cryst. Res. Technol. 41 (2006) 1184. [16] H.S. Nagaraja, V. Upadhyaya, P. Mohan Rao, S. Aithal, A.P. Bhat, J. Cryst. Growth 193 (1998) 674. [17] M. Fleck, K. Schwendtner, A. Hensler, Acta Cryst. C62 (2006) m122. [18] W.S. Wang, K. Sutter, Ch. Bosshard, Z. Pan, H. Arend, P. Gunter, G. Chapius, F. Nicolo, Jpn. J. Appl. Phys. 27 (1998) 1138. [19] L. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compound, Wiley, New York, 1978. [20] J. Gopalakrishnan, B. Viswanathan, V. Srinivasan, J. Inorg. Nucl. Chem. 32 (1970) 2565. [21] B.R. Lawn, D.R. Puller, J. Mater. Sci. 9 (1975) 2016. [22] B.W. Mott, Micro Indentation Hardness Testing, Butterworths, London, 1956. [23] T. Balakrishnan, K. Ramamurthi, Mater. Lett. 62 (2008) 65. [24] S. Mukerji, T. Kar, Mater. Res. Bull. 35 (2000) 711. [25] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis, growth and characterization of bis (glycine) lithium molybdate— A semi-organic NLO material T. Balu a , T.R. Rajasekaran b,∗ , P. Murugakoothan c a b c
Department of Physics, Aditanar College of Arts and Science, Tiruchendur 628 216, India Department of Physics, Manonmaniam Sundaranar University, Abishekapatty, Tirunelveli 627012, Tamil Nadu, India Post Graduate and Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 26 July 2009 Accepted 7 August 2009 PACS: 42.70.Mp 81.10.Dn 81.10.Aj
a b s t r a c t Single crystals of bis (glycine) lithium molybdate [BGLM] with dimensions 20 mm × 10 mm × 5 mm were grown by slow evaporation technique. The grown crystals were subjected to powder X-ray diffraction studies. Functional groups of the crystallized molecules were confirmed by FTIR analyses. Transmission range of the crystal was determined by UV–vis–NIR spectra. Vickers microhardness test was performed on the prominent plane (0 1 1) of the grown crystal. The NLO property of the crystal was confirmed by Kurtz SHG test and compared with NLO efficiency of KDP crystal. © 2009 Elsevier B.V. All rights reserved.
Keywords: Solubility X-ray diffraction Growth from solutions Nonlinear optical materials
1. Introduction Since 1961, when the nonlinear optical (NLO) phenomenon was observed for the first time, NLO frequency conversion materials have played more and more important role in many fields, such as laser technology, optical communication, optical data storage etc. [1]. In the context of NLO, organic materials have advantages such as large NLO coefficients and structural diversity or flexibility, compared to the inorganic counterparts [2]. They also have some inherent drawbacks, for example, poor physico-chemical stability and low mechanical strength. As a result, the quest for new frequency conversion materials is presently concentrated on semi-organic crystals due to their large nonlinearity, high resistance to laser induced damage, low angular sensitivity and good mechanical hardness [3–5]. Semi-organics include organic–inorganic salts and metal–organic coordination compounds [6]. Among the organic materials amino acids constitute a family in which glycine is the simplest of all the amino acids. It has been reported that some complexes of amino acids with simple inorganic salt may exhibit ferroelectric properties
[7–9]. Some complexes of glycine with CaCl2 [10], BaCl2 [11], H2 SO4 [12] and CoBr2 [13] form single crystals but none of these are reported to have nonlinear optical property. Single crystals of glycine sodium nitrate [14], glycine lithium sulphate [15] and benzoyl glycine [16] showed non-centrosymmetry and their quadratic nonlinear coefficients were examined. A survey of the literature shows that the X-ray crystal structure of bis (glycine) lithium molybdate [BGLM] was reported by Fleck et al. [17] and no other reports on the crystal growth and characterization of BGLM are available. This paper reports the synthesis, solubility and growth of title compound, whose SHG efficiency has been estimated to be 1.3 times that of potassium dihydrogen phosphate. The powder X-ray diffraction studies show the BGLM belongs to monoclinic system with non-centrosymmetric space group P21 . FTIR study reveals the functional groups of the grown crystal. The optical and mechanical properties of the crystals were studied. 2. Experimental procedure 2.1. Synthesis
∗ Corresponding author. Tel.: +91 462333887; fax: +91 4622322973. E-mail address: [email protected] (T.R. Rajasekaran). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.08.048
Bis (glycine) lithium molybdate (BGLM) growth solution was prepared by dissolving analar grade glycine (Merk) and lithium
956
T. Balu et al. / Spectrochimica Acta Part A 74 (2009) 955–958
Fig. 1. Chemical structure of BGLM.
molybdate (Sigma–Aldrich) in stoichiometric ratio 2:1 in double distilled water and stirred well for about 3 h using a temperature controlled magnetic stirrer to yield a homogeneous mixture of solution. The polycrystalline starting material was synthesized by evaporating the solution to almost dryness at the temperature of 50 ◦ C according to the following reaction, 2[C2 H5 NO2 ] + (glycine)
Li2 MoO4 → (lithium molybdate)
[Li2 Mo (C2 H5 NO2 )2 O4 ] (BGLM)
The purity of synthesized salt was improved by successive recrystallization process. Care was taken during heating of the solution and temperature as low as 50 ◦ C was maintained. The chemical structure of BGLM is represented in Fig. 1.
Fig. 3. As grown crystal of BGLM.
2.3. Crystal growth In the present study, BGLM crystals were grown by slow evaporation technique. Recrystallized salt of BGLM was taken as raw material. Saturated BGLM solutions were prepared at room temperature with water as solvent. The prepared solution was filtered using Whatmann 41 filter paper to remove the suspended impurities. The solution was taken in vessels and closed with perforated covers and kept in dust free atmosphere. A well developed crystal of size 20 mm × 10 mm × 5 mm was harvested in a growth period of 30 days and is shown in Fig. 3. 3. Characterization
2.2. Solubility
3.1. Powder X-ray diffraction analysis
The solubility of BGLM in water was determined as a function of temperature in the temperature range 30–50 ◦ C. To determine the equilibrium concentration, the solution BGLM was prepared using double distilled water as the solvent. The solution was maintained at a constant temperature and continuously stirred using a magnetic stirrer to ensure homogeneous temperature and concentration throughout the volume of the solution. On reaching the saturation, the content of the solution was analyzed gravimetrically [18] and this process was repeated for every temperature. The solubility curve is shown in Fig. 2. The solubility increases linearly with increase of temperature.
The purified samples of the grown crystals have been crushed to a uniform fine powder and subjected to powder X-ray diffraction using a Rich Seifert Powder X-ray diffractometer. The K␣ radiations ( = 1.5406 Å) from a copper target were used. The specimen in the form of a thin film was scanned in the reflection mode in the 2 range 10–70◦ with five decimal accuracy. Fig. 4 represents the powder diffractograph for the grown BGLM crystals. The peaks are indexed using least square fit method. From the powder X-ray diffraction data, the lattice parameters and the cell volume have been calculated and are given in Table 1. These values agreed very well with the reported values [17].
Fig. 2. Solubility of BGLM in aqueous solution.
Fig. 4. Powder XRD pattern of BGLM.
T. Balu et al. / Spectrochimica Acta Part A 74 (2009) 955–958
957
Table 1 Unit cell parameter of BGLM. Parameter
Present study
Reported values [17]
a b c ˇ V
5.177462 Å 7.72368 Å 12.441315 Å 94.24852◦ 496.14926 Å3
5.192410 Å 7.733915 Å 12.49230 Å 94.283◦ 500.2717 Å3
3.2. Density measurements The density was calculated as 2.151 g/cc from crystallographic data using the formula =
MZ , NV
where M is the molecular weight, Z is the number of molecules per unit cell, N is the Avogadro’s number and V is the volume of the unit cell. Experimentally ‘floatation technique’ was employed and the value 2.163 g/cc was obtained for . Carbon tetrachloride (CCl4 ) of density 1.594 g/cc and bromoform (CHBr3 ) of density 2.890 g/cc are respectively the lower density and higher density liquids used. This value agrees very well with the calculated value. 3.3. FTIR analysis Fourier Transform Infrared (FTIR) spectrum was recorded using KBr pellet technique on a JASCO FTIR-410 spectrophotometer. Fig. 5 shows the FTIR spectrum recorded in the range 400–4000 cm−1 to identify the functional groups present in BGLM. The spectrum shows a broad envelope between 2100 and 3700 cm−1 . It includes the asymmetric stretching mode of NH3 + at 3163.51 cm−1 . The absorption peak at 422.39 cm−1 is assigned to the Li–O stretching vibration. The band appeared at 598.31 cm−1 is assigned to the COO− in-plane bending vibration. The peaks at 1613.86 and 1410.75 cm−1 are due to asymmetric and symmetric stretching modes of the carboxylate group (COO− ), respectively. The characterizing bending vibration of CH2 group and CH2 wagging appeared at 1456.46 and 1333.98 cm−1 respectively. The C–N stretching vibration is observed at 1102.08 cm−1 . The band at 689.47 cm−1 is due to the bridged –O–Mo–O stretch. The bands observed at 1033.32, 906.97 and 828.61 cm−1 can be attributed to Mo–O stretch [19,20]. 3.4. Optical assessment For optical application, especially for SHG, the material considered must be transparent in the wavelength region of interest. The
Fig. 6. UV–vis–NIR spectrum of BGLM.
UV–vis–NIR transmittance spectrum (Fig. 6) was recorded using Varian Cary 5E UV spectrophotometer in the wavelength range 200–2500 nm, which covers near ultraviolet (200–400 nm), visible (400–800 nm) and then far-infrared (800–1200 nm) regions. Optically clear single crystal of thickness 3 mm was used for this study. The lower cut-off wavelength of the BGLM crystal occurs at 300 nm. The crystal has sufficient transmission in the entire visible and IR region. The transmission window in the visible region and IR region enables good optical transmission of the second harmonic frequencies of Nd:YAG laser. 3.5. Microhardness studies Microhardness testing is one of the best methods of understanding the mechanical properties of materials such as fracture behavior, yield strength, brittleness index and temperature of cracking [21]. Transparent crystals free from cracks were selected for microhardness measurements. Before indentations, the crystals were carefully lapped and washed to avoid surface effects [22]. Vickers microhardness measurements were carried out on BGLM crystal using a Leitz Weitzler hardness tester fitted with a diamond indentor. The hardness measurements were made on the well-developed (0 1 1) face. The well-polished crystal was mounted on the platform of the microhardness tester and the loads of different magnitudes (5–30 g) were applied over a fixed interval of time. The indentation time was fixed as 10 s. The diagonals of the impressions were measured using a Leitz Metallux II microscope with a calibrated ocular at magnification 500×. Vickers microhardness number was evaluated from the relation Hv = 1.8544
Fig. 5. FTIR spectrum of BGLM.
P d2
kg/mm2 ,
where Hv is the Vickers hardness number, P is the indentor load in kg and d is the diagonal length of the impression in mm. A graph plotted between hardness number (Hv ) and applied load (P) is shown in Fig. 7. The reason for getting lower value of hardness at lower loads may be due to the surface reaction by surface adsorbed moisture, which may reduce the strength of the surface layers. There is an increase in the hardness with load without saturation, which can be attributed to the work hardening of the surface layers. Beyond the load 30 g, significant cracking occurs, which may be due to the release of internal stress generated locally by indentation [23]. The Vickers hardness number (VHN) of BGLM crystal is high compared with the other semi-organic crystals like glycine lithium sulphate (55) and l-arginine hydrobromide (48) [15,24].
958
T. Balu et al. / Spectrochimica Acta Part A 74 (2009) 955–958
solubility temperature gradient. Single crystals of BGLM have been grown by slow evaporation technique at room temperature. The lattice parameters obtained from powder X-ray diffraction study agree well with the reported values. The functional groups present in the grown crystal have been confirmed by FTIR spectral analysis. UV–visible spectrum shows that the crystal has a wide transmission range with a lower UV-cut-off of 300 nm. Vickers hardness values measured on (0 1 1) plane reveal its mechanical strength. The SHG efficiency measured by the Kurtz powder test was about 1.3 times that of potassium dihydrogen phosphate. The density of the grown crystal was evaluated as 2.151 g/cc from the crystallographic data and is verified by floatation experiment. Acknowledgements Fig. 7. Vickers hardness versus load for BGLM crystal.
3.6. Second harmonic generation efficiency measurements The first and most widely used technique for confirming the SHG from prospective second order NLO materials is the Kurtz powder technique [25] to identify the materials with non-centrosymmetric crystal structures. The crystalline sample was powdered to particle sizes in the range 125–150 m. To make relevant comparisons with known SHG materials, KDP was also ground and sieved into the same particle size range. The powdered samples were filled air-tight in separate micro-capillary tubes of uniform bore of about 1.5 mm diameter. A high intensity Nd:YAG laser beam ( = 1064 nm) with a pulse width of 8 ns and a repetition rate of 10 Hz was passed through the powdered sample. The SHG radiations of 532 nm (green light) emitted were collected by a photomultiplier tube (PMTHamamatsu-model R 2059). The optical signal incident on the PMT was converted into voltage output at the CRO (Tektronix-TDS 305213). The second harmonic signal of 71 mV was obtained for an input energy of 1.35 mJ/pulse, while the standard KDP crystal gave a SHG signal of 55 mV for the same input energy. The result obtained for BGLM shows a powder SHG efficiency of about 1.3 times that of KDP crystal. 4. Conclusion Potential semi-organic nonlinear optical bis (glycine) lithium molybdate complex (BGLM) was synthesized and its solubility was analyzed in the temperature range 30–50 ◦ C. The solubility curve indicates moderate solubility of BGLM in water with a positive
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