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Generation of 532 nm laser radiation and phase matching properties of organic nonlinear optical material
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ARTICLE IN PRESS Optik xxx (2013) xxx–xxx
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Generation of 532 nm laser radiation and phase matching properties of organic nonlinear optical material S. Tamilselvan a , M. Vimalan b,∗ , I. Vetha Potheher c,∗∗ , R. Jeyasekaran d , F. Yogam e , J. Madhavan f a
Department of Physics, Arignar Anna Government Arts College, Cheyyar 604 407, India Department of Physics, Thirumalai Engineering College, Kilambi, Kancheepuram 631 551, India c Department of Physics, Anna University BIT Campus, Tiruchirappalli 620 024, India d Department of Physics, V.H.N.S.N. College, Virudhunagar 626 001, India e Department of Physics, Anand Institute of Higher Technology, Chennai 603 103, India f Department of Physics, Loyola College, Chennai 600 034, India b
a r t i c l e
i n f o
Article history: Received 29 January 2013 Accepted 5 June 2013 Available online xxx Keywords: Organic materials Nonlinear optics Laser damage threshold Optical band gap energy Activation energy
a b s t r a c t Organic nonlinear optical single crystal of l-asparagine-l-tartaric acid (LAsT) was grown by slow evaporation technique at room temperature. The grown crystal was confirmed by single crystal X-ray diffraction and FT-IR studies. The direct band gap energy was found to be 5.4 eV. The SHG efficiency of the sample is 3 times higher than that of KDP crystal. The laser damage threshold of the grown crystal was 5.7 GW/cm2 . The grown crystal was thermally stable up to 141 ◦ C. Low dielectric constant at higher frequency was found by dielectric measurements. The activation energy was calculated from Arrhenius relation and it was found to be 0.088 eV. Negative photoconducting nature was obtained by photoconductivity measurements. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction Organic NLO materials have subsequently produced very good materials with highly desirable characteristics. However most of the organic NLO materials are susceptible to damage because of their poor mechanical and thermal properties [1]. In the recent past, amino-acid family single crystals are gaining importance as highly feasible second order NLO materials. Considerable efforts have been made to combine amino-acids with interesting organic and inorganic matrices to produce outstanding materials to challenge the established inorganic materials like niobates and the borates. This is due to the fact that all the amino acids contain chiral carbon and crystallize in noncentrosymmetric space groups. Hence, they are regarded as potential candidates for second harmonic generation (SHG), optical parametric amplification (OPA) and optical parametric oscillation (OPO). The absence of strong conjugated bonds and the presence of chromophores such as amino groups and carboxyl group lead to exhibit wide transparency in the entire UV–vis
∗ Corresponding author. Mobile: +91 89031 93588. ∗∗ Corresponding author. Mobile: +91 99429 94274. E-mail addresses: [email protected] (M. Vimalan), [email protected] (I. Vetha Potheher).
region, thus meeting out one of the essential requirements of non-linear optical activity [2]. The presence of Zwitter ions influences the physical and chemical properties of amino acids. The proton donor carboxyl acid ( COO) group donates its proton to acceptor amino ( NH2 ) group. Thus, an amino acid exists as a dipolar ion in which carboxyl group is present as carboxylate ion and amino group is present as ammonium ion. Due to this dipolar nature, they possess good mechanical and physical properties, viz. crystal hardness and high melting point, which make them ideal candidates for NLO applications. Among the synthetic materials that have been suggested as sources of piezoelectric elements, some tartrates, e.g. Rochelle salt (sodium potassium tartrate tetrahydrate) and ethylene diamine tartrate (EDT) are strongly piezoelectric. It therefore seemed likely that the examination of other organic tartrates might be fruitful [3]. Crystal and molecular structure of Urea-(+) tartaric acid was reported by Wentao et al. [4]. Growth and characterization of organic nonlinear optical crystals of l-tartaric acid-nicotinamide and d-tartaric acid-nicotinamide were done by Jun Shen et al. [5]. The crystal structure of l-asparaginel-tartaric acid (LAsT) was solved by Natarajan et al. [6]. In the compound LAsT, the amino acid molecule exists as a zwitterion, an uncommon ionization state in the crystal structures of aminocarboxylic acid complexes. Usually, a proton transfer is favored from the carboxylic acid to the amino acid in these complexes, the
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Table 1 Single crystal XRD data of LAsT single crystal. Empirical formula
C4 H8 N2 O3 + .C4 H6 O6 −
Crystal system Space group a b c A B Volume Z
Monoclinic P21 5.0864 A˚ 9.6727 A˚ 11.8349 A˚ 90◦ 95.52◦ 90◦ 582.267 (Å3 ) 2
photograph of as grown crystal of LAsT. The grown crystals are stable, do not decompose in air and non-hygroscopic in nature. An important observation during the growth of LAsT is the absence of any kind of microbial contamination during the growth period. 2.3. Characterization
Fig. 1. Photograph of as grown LAsT single crystals.
former exists in the anionic state and the latter in the cationic state. Similar zwitterionic state for the amino acid molecule is observed in l-phenylalanine fumaric acid and l-phenylalanine benzoic acid [7,8]. The aggregation pattern observed in the structure has striking similarities to those observed in other related amino acid-tartaric acid complexes, e.g. sarcosinium tartrate [9] and l-prolinium tartrate [10]. In this view the present investigation focused on the growth of single crystals of l-asparagine-l-tartaric acid (LAsT). It is a potential semi-organic NLO material. The slow evaporation technique was employed for the growth process. The grown crystals were characterized by single crystal X-ray diffraction, FTIR, optical transmission, optical absorption spectroscopy and TG & DTA analysis. Mechanical and electrical properties of the grown crystal was also studied and reported for the first time. 2. Experimental procedure 2.1. Synthesis of LAsT l-Asparagine-l-tartaric acid (LAsT) was synthesized by dissolving one mole of l-asparagine (Merck 99%) in double distilled water containing one mole of l-tartaric acid. The reaction is as follows C4 H8 N2 O3 + C4 H6 O6 → C4 H8 N2 O3 .C4 H6 O6 The synthesized salt was further purified by repeating the crystallization process at least thrice. 2.2. Growth of LAsT single crystals The synthesized salt of LAsT was purified by repeated crystallization and saturated solution was prepared in accordance with the solubility data. In order to achieve single crystals of good optical quality, needle shaped and relatively large size, many growth attempts have been made. The easiest method was to form tiny crystals by spontaneous nucleation; among them seed crystals with perfect shape and free from defects were chosen for growth experiments. The seeds were suspended in the mother solution with nylon thread. Crystals of dimension up to 10 mm × 2 mm × 2 mm were harvested after a period of 25–30 days. Fig. 1 shows the
The grown crystal of LAsT was confirmed by single crystal Xray diffraction analysis. Single crystal XRD data were collected by ENRAF NONIUS CAD4-F single crystal X-ray diffractometer with ˚ radiation. The FT-IR spectrum was recorded Mo K␣ ( = 0.71073 A) using BRUKER IFS-66V FT-IR Spectrometer with KBr pellet technique for the range 4000–450 cm−1 . The optical absorption and study was carried out using a Shimadzu UV-2400 PC Spectrophotometer in the range of 200–1100 nm. The band gap energy was calculated using the relation E = h, where h is Planck’s constant, is frequency. The NLO efficiency of LAsT crystal was evaluated by Kurtz and Perry powder technique [11] using a Q-switched, mode locked Nd:YAG laser emitting 1.06 m, 8 ns laser pulses. The thermal behavior of LAsT crystal was investigated using NETSZCH STA 409C and PERKIN ELMER thermal analyzer. Microhardness studies have been carried out on LAsT crystal using a Vickers microhardness tester fitted with a Vickers diamond pyramidal indenter attached to an incident light microscope. The frequency dependent dielectric constant and dielectric loss of LAsT were measured using Agilent 4284A LCR METER for various temperatures (313–363 K). The capacitance of the sample was noted by varying the frequency from 100 Hz to 1 MHz. The dc conductivity measurements were carried out by conventional two probe technique for temperatures ranging from 313 to 423 K. Photoconducting nature of the sample was studied by Keithley 485 picoammeter. 3. Results and discussion 3.1. Single crystal XRD study The lattice parameters of LAsT are determined using 25 reflections collected through random search routine with graphite ˚ radiation and indexed by monochromated Mo K␣ ( = 0.71073 A) the method of short vectors followed by the least squares refinement. The XRD data reveals that LAsT crystal belongs to monoclinic structure with a noncentrosymmetric space group of P21 . The unit cell parameters of LAsT are given in Table 1. The single crystal Xray diffraction data of the crystal is in good agreement with the reported values and thus confirming the grown crystal [6]. 3.2. FT-IR analysis Fig. 2 shows the FT-IR transmission spectrum of LAsT in the region 4000–400 cm−1 . It is evident from the spectrum that, there is a broad band between 3800 and 2700 cm−1 . It includes OH stretch
Please cite this article in press as: S. Tamilselvan, et al., Generation of 532 nm laser radiation and phase matching properties of organic nonlinear optical material, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.06.024
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Fig. 2. FT-IR spectrum of LAsT.
of water at 3450 cm−1 and N H vibration at 3320 and 3280 cm−1 . The characteristic CH vibrations of tartaric acid produce peaks at 2977 and 2960 cm−1 . In the overtone region, there is a prominent band near 2002 cm−1 due to combination of the asymmetrical NH3 + bending vibration and the torsional oscillation of the NH3 + groups. The C O stretch of tartaric acid gives a peak at 1737 cm−1 . The asymmetric and symmetric COO vibrations produce peaks at 1553 and 1420 cm−1 . The C O stretching and OH deformation produce peak at 1310 cm−1 . The alcoholic C O stretch gives its peak at 1065 cm−1 . From the spectroscopic investigation, the presence of all the fundamental functional groups of the grown sample is confirmed qualitatively. From the IR spectral analysis the presence of water is evident in the lattice of l-asparagine. The main IR spectral data along with the frequency assignments are listed in Table 2. 3.3. Optical studies The UV–vis-NIR spectrum gives information about the structure of molecule because the absorption of UV and visible light involves the promotion of the electrons in the and orbitals from the ground state to higher energy states. Optical absorption spectrum of LAsT is shown in Fig. 3a. The spectrum was recorded in the range of 200–1100 nm. From the Figure, it is observed that there is very low absorption in the visible and NIR region. The UV cut-off wavelength of LAsT is seen around 230 nm which is lower than some of the l-arginine family single crystals [12–14]. There is no absorption in this range, thus enabling the use of this material for second harmonic generation (SHG) applications. The direct band gap energy of LAsT was calculated from the Tauc’s plot [15]. A graph between h and (˛h)2 was plotted and shown in Fig. 3b. Band gap energy
Table 2 FT-IR frequency assignment for LAsT. Wave number (cm−1 ) 3450 3320 3280 3092 2977 2960 2002 1737 1673 1553 1420 1310 1065 690 612 510
Assignment O H stretch of water NH2 symmetric stretching NH2 asymmetric stretching C H symmetric stretching CH vibrations of tartaric acid CH vibrations of tartaric acid NH3 + asymmetric bending C O stretch of tartaric acid C O stretching COO− asymmetric stretching COO− symmetric stretching C O stretching and OH deformation C O stretching of alcoholic COO− bending COO− wagging COO− rocking
Fig. 3. (a) Optical absorption spectrum of LAsT. (b) Tauc’s plot of LAsT.
was calculated by extrapolating the linear portion of the curve to zero absorption and it was found to be 5.4 eV. 3.4. NLO studies For the SHG efficiency measurements, microcrystalline material of KDP is used for comparison. The sample was ground into fine powder and tightly packed in a micro capillary tube. It is mounted in the path of the laser beam of pulse energy 10.8 mJ obtained by splitting the original laser beam. The transmitted light is passed through 532 nm monochromator. The green light at double the incident frequency was collected by photo multiplier tube and converted into electrical signal. The sample is illuminated using Q-switched mode locked Nd:YAG laser with input pulse of 10.8 mJ. This signal was displayed on the oscilloscope. Signal amplitude in milli volts on the oscilloscope indicates the SHG efficiency of the sample. The emission of green radiation from the crystal is confirmed the second harmonic signal generation in the crystal. A second harmonic signal of 161 mV is obtained for LAsT with reference to KDP (53 mV). Thus the SHG efficiency of LAsT is 3 times that of KDP for a particle size between <50 and >150 m. The level of SHG response of a given material is inherently dependent upon its structural attributes. On a molecular scale, the extent of charge transfer across the NLO chromophore determines the level of SHG output, greater the charge transfer, larger is the SHG output. The presence of strong intermolecular interactions, such as hydrogen bonds can extend this level of charge transfer into the supramolecular realm, owing to their electrostatic and directed nature, thereby enhancing the SHG response [16]. High optical quality single crystals cut in phase-matching directions are very essential for NLO applications. The phase matching property was confirmed in LAsT using the same Kurtz powder SHG setup. In such organic crystals, as the thickness of crystal increases, instead of increasing SH intensity, a decrease of SH intensity was observed due to phase mismatch between the fundamental and the SH waves as they propagate inside the crystal. One easy way of confirming the existence of phasematching property in new NLO crystals is by studying the grain size dependence of SH intensity in powder form. The particle size dependency of SHG intensity in
Please cite this article in press as: S. Tamilselvan, et al., Generation of 532 nm laser radiation and phase matching properties of organic nonlinear optical material, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.06.024
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100
240
95 90 2
Hv (Kg/mm )
SHG Output ( I ) (mV)
220
200
180
85 80 75 70
160
65
<50
50-100 100-150 above 150 Particle size (m)
Fig. 4. Phase matching curve of LAsT.
LAsT is shown in Fig. 4. The SHG intensity increases almost linearly with the increase in particle size until <50–>150 m and above this range; it deviates from the linearity and starts to attain saturation. This type of grain size dependence of SH intensity was observed only with phasematchable crystals. This phase-matching property may open up opportunities for LAsT crystal in efficient frequency conversion applications. Most important considerations in the choice of a material for NLO applications are its optical damage tolerance and in NLO material it may severely affect the performance of high power laser systems. High damage threshold is a significant parameter for NLO material. LAsT crystal was prepared for laser damage threshold (LDT measurements. Experimentally, a Q-switched diode array side pumped Nd:YAG laser operating at 532 nm radiation was used with 8 ns pulse width. During laser irradiation, the power meter records the energy density of the input laser beam for which the crystal gets damaged. In the present study, laser damage was found to be 5.7 GW/cm2 . The high damage threshold contributes to attractiveness of the present compound in practical applications. 3.5. Thermal analysis TG-DTA was carried out on the grown LAsT sample for qualitative analysis and the respective curves are shown in Fig. 5. From the DTA curve it has been observed that endothermic peak at about 141 ◦ C corresponds to the melting point of the compound which is coinciding with the stability of the material shown by TGA trace. This is attributed to utilization of the thermal energy to overcome the valence bonding between the l-asparagine cation and tartrate anion, which happens in the initial stage of decomposition. The
10
20
30
40
50
Load P (gm) Fig. 6. Vickers hardness profile of LAsT as a function of applied load.
two broad endothermic peaks at 141 and 275 ◦ C represent the decomposition of molecular fragments in two stages. This weight loss is followed by a major weight loss pattern between 172 and 267 ◦ C occurring in two stages, the total weight loss of these stages correspond to 20% of the substance due to release of CO2 and CO molecules in tartaric acid. The reactions of simplest amino acids induced by heating include the condensation reactions of carboxyl and amino groups leading to the formation of peptide bonds. The total weight loss nearly equals to 90% and the resulting residue (10%) is stable up to 800 ◦ C. It is concluded from the thermal analysis, the material can be useful up to 141 ◦ C for the NLO applications. 3.6. Microhardness study Microhardness plays a key role in device fabrication. Vickers hardness measurements were made on the prominent (1 0 0) plane using Leitz–Wetzler hardness tester fitted with a diamond pyramidal indenter. The static indentations were made for different loads of 10–50 g with a constant indentation time of 10 s for all the trials. Diagonal lengths of the indented impressions obtained at various loads were measured using a calibrated micrometer attached to the eye piece of the microscope. Several indentation were made on LAsT. The average value of the diagonal lengths of the indentation mark for each load was used to calculate the hardness. The variations of Hv for various applied loads are plotted and shown in Fig. 6. The graph indicates that the microhardness number decreases with the increasing load. The decrease in microhardness number with the increasing load satisfies normal indentation size effect (ISE). The work hardening coefficient (n) was calculated by plotting log p vs. log d and it was found to be 1.66. According to Onitsch, if n > 2, the microhardness number Hv increases with increasing load and if n < 2, Hv decreases with increasing load [17]. 3.7. Electrical studies
Fig. 5. TGA and DTA curves of LAsT.
The dielectric study on grown LAsT crystals were carried out using Agilent 4284A LCR meter and was used to measure the capacitance and dielectric loss of the grown crystal as a function of frequency. The observations are made in the frequency range100 Hz–1 MHz in the temperature range 313–363 K. Fig. 7a and b shows the plot of dielectric constant and dielectric loss of LAsT as a function of log frequency for different temperatures (313–363 K). The dielectric constant has a higher value (15.7) at 313 K in the lower-frequency region (100 Hz) and it then decreases (13.47) at 313 K with the applied high frequency (1 MHz). In Miller rule and the Phillips-Van Vechten-Levine-Xue bond theory [18], the lower value of dielectric constant at higher frequencies is a
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suitable parameter for the enhancement of SHG coefficient [19]. The higher value of dielectric constant is due to higher space-charge polarization at lower frequency region. This may be explained on the basis of the mechanism of polarization similar to the conduction process. The electronic exchange of the number of ions in the crystal gives local displacement of the applied field, which gives the polarization. As the frequency increases, at which the space charge cannot sustain and comply the external field. Therefore the polarization decreases and exhibiting the reduction in the value of dielectric constant with increasing frequency. The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations, which depend on frequencies [20]. At low frequencies, all these polarizations are active. The space charge polarization is generally active at lower frequencies and high temperatures [21]. From Fig. 7b, it can be noticed that as the frequency increases the dielectric loss decreases. This behavior is similar to that of the dielectric constant. At low frequencies the dipoles can easily switch alignment with the changing field. As the frequency increases the dipoles are less capable of rotating and maintaining phase with the field; thus they reduce their contribution to the polarization field, and hence the observed reduction in dielectric constant and dielectric loss. The characteristic of low dielectric loss with high frequency for a given sample suggests that the sample possesses enhanced optical quality with lesser defects and this parameter is of vital importance for nonlinear optical materials in their application [22]. Well sized crystal of LAsT was used for dc conductivity study. The conductivity of LAsT is shown in Fig. 8a and b. Fig. 8a represents the temperature dependence of conductivity of the sample is found
Fig. 8. (a) DC electrical conductivity for LAsT crystal. (b) Plot of ln ( dc ) versus 1000/T for LAsT single crystal.
to increase with increase in temperature. The dc activation energy (Fig. 8b) of the LAsT crystal is found to be 0.088 eV. 3.8. Photoconductivity study The applied field was varied from 150 to 3820 V/cm. For measuring the dark current, the sample was kept unexposed from any radiation. After measuring the dark current, the photocurrent was measured by illuminating the sample with a halogen lamp of 100 W power. A spot of light on the sample was focused with the help of a convex lens. The resulting photocurrent was measured by varying 450 400 Id Ip
350 Current (nA)
Fig. 7. (a) Variation of dielectric constant with log frequency at different temperatures for LAsT single crystal. (b) Variation of dielectric loss with log frequency at different temperatures for LAsT single crystal.
5
300 250 200 150 100 50 0 0
500 1000 1500 2000 2500 3000 3500 4000 Applied Field (V/cm)
Fig. 9. Field dependent photoconductivity of LAsT single crystal.
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the applied field for the same range. The variation of dark current and photocurrent with applied field are shown in Fig. 9. It is observed from the plots that the dark current is always higher than the photo current. Thus, LAsT single crystal is found to exhibit negative photoconductivity. 4. Conclusion Single crystals of l-asparagine-l-tartaric acid (LAsT) of dimension 10 mm × 2 mm × 2 mm is conveniently grown by slow evaporation technique at room temperature. A solvent of deionized water is used for the growth process. The single crystal XRD data proves that LAsT crystal belongs to monoclinic in structure with a noncentrosymmetric space group P21 . Optical absorption studies confirm the UV cut-off wavelength of LAsT at 230 nm and the optical band gap energy is found to be 5.4 eV. Thermal analysis revealed that the sample is thermally stable up to 141 ◦ C and various stages of weight losses also discussed. The frequency dependence of the dielectric constant/dielectric loss of LAsT are investigated. The activation energy of the sample is calculated by dc conductivity studies. The negative photoconducting nature of LAsT is studied by photoconductivity investigations. References [1] K. Selvaraju, R. Valluvan, K. Kirubavathi, S. Kumararaman, Investigation on the nucleation kinetics of l-arginine acetate single crystals, Mat. Lett. 61 (2007) 3041–3044. [2] V. Krishnakumar, R. Nagalakshmi, Polarised Raman and infrared spectral analysis of l-alanine oxalate (C5 H9 NO6 )—a non-linear optical single crystal, Spectrochim. Acta A 64 (2006) 736–743. [3] M. Vimalan, T. Rajesh Kumar, S. Tamilselvan, P. Sagayaraj, C.K. Mahadevan, Growth and properties of novel organic nonlinear optical crystal: l-alaninium tartrate (LAT), Physica B 405 (2010) 3907–3913. [4] Y. Wentao, L. Mengkai, M. Fanqing, Crystal and molecular structure of Urea-(+) tartaric acid, Mat. Res. Bull. 31 (1996) 1127–1131. [5] J. Shen, J. Zheng, Y. Che, B. Xi, Growth and properties of organic nonlinear optical crystals: l-tartartic acid-nicotinamide, and d-tartaric acid-nicotinamide, J. Cryst. Growth 257 (2003) 136–140.
[6] S. Natarajan, V. Hema, J. Kalyana Sundar, J. Suresh, P.L. Nilantha Lakshman, l-Asparagine-l-tartaric acid (1/1), Acta Crystallogr. E 66 (2010) 2239. [7] M. Alagar, R.V. Krishnakumar, K. Rajagopal, M. Subha Nandhini, S. Natarajan, l-Phenylalanine fumaric acid, Acta Crystallogr. E 59 (2003) 952–954. [8] J. Suresh, R.V. Krishnakumar, S. Natarajan, l-Phenylalanine-benzoic acid (1/1), Acta Crystallogr. E 61 (2005) 3625–3627. [9] R.V. Krishnakumar, M. Subha Nandhini, S. Natarajan, Sarcosinium tartrate, Acta Crystallogr. C 57 (2001) 165–166. [10] M. Subha Nandhini, R.V. Krishnakumar, S. Natarajan, l-Prolinium tartrate, Acta Crystallogr. C 57 (2001) 423–424. [11] S.K. Kurtz, T.T. Perry, A powder technique for the evaluation of nonlinear optical materials, J. Appl. Phys. 39 (1968) 3798–3813. [12] F. Yogam, I. Vetha Potheher, M. Vimalan, R. Jeyasekaran, T. Rajesh Kumar, P. Sagayaraj, Growth and physicochemical properties of l-phenylalaninium maleate: a novel nonlinear optical crystal, Spectrochim. Acta A 95 (2012) 369–373. [13] K. Vasantha, S. Dhanuskodi, Single crystal growth and characterization of phase-matchable l-arginine maleate: a potential nonlinear optical material, J. Cryst. Growth 269 (2004) 333–341. [14] A. Pricilla Jeyakumari, S. Danushkodi, S. Manivannan, Phase matchable semiorganic NLO material for frequency doubling: l-arginine tetrafluoroborate, Spectrochim. Acta A 63 (2006) 91–95. [15] J. Tauc, in: J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974. [16] H. Zhang, Y. Sun, X. Chen, X. Yan, B. Sun, Crystal growth and characterization of organic single crystal: N-(2-hydroxybenzylidene) acetohydrazide, J. Cryst. Growth 324 (2011) 196–200. [17] E.M. Onitsch, The present status of testing the hardness of materials, Mikroskopie 95 (1959) 12–14. [18] D. Xue, S. Zhang, Chemical bond analysis of the correlation between crystal structure and nonlinear optical properties of complex crystals, Physica B 262 (1999) 78–83. [19] C. Justin Raj, S. Dinakaran, S. Krishnan, B. Milton Boaz, R. Robert, S. Jerome Das, Studies on optical, mechanical and transport properties of NLO active l-alanine formate single crystal grown by modified Sankaranarayanan–Ramasamy (SR) method, Opt. Commun. 281 (2008) 2285–2290. [20] D. Xue, K. Kitamura, Dielectric characterization of the defect concentration in lithium niobate single crystals, Solid State Commun. 122 (2002) 537–541. [21] B.W. Betterman, H. Cole, Dynamical diffraction of X rays by perfect crystals, Rev. Mod. Phys. 36 (1964) 681–717. [22] C. Balarew, R. Duhlew, Application of the hard and soft acids and bases concept to explain Ligand coordination in double salt structures, J. Solid state Chem. 55 (1984) 1–6.
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Generation of 532 nm laser radiation and phase matching properties of organic nonlinear optical material S. Tamilselvan a , M. Vimalan b,∗ , I. Vetha Potheher c,∗∗ , R. Jeyasekaran d , F. Yogam e , J. Madhavan f a
Department of Physics, Arignar Anna Government Arts College, Cheyyar 604 407, India Department of Physics, Thirumalai Engineering College, Kilambi, Kancheepuram 631 551, India c Department of Physics, Anna University BIT Campus, Tiruchirappalli 620 024, India d Department of Physics, V.H.N.S.N. College, Virudhunagar 626 001, India e Department of Physics, Anand Institute of Higher Technology, Chennai 603 103, India f Department of Physics, Loyola College, Chennai 600 034, India b
a r t i c l e
i n f o
Article history: Received 29 January 2013 Accepted 5 June 2013 Available online xxx Keywords: Organic materials Nonlinear optics Laser damage threshold Optical band gap energy Activation energy
a b s t r a c t Organic nonlinear optical single crystal of l-asparagine-l-tartaric acid (LAsT) was grown by slow evaporation technique at room temperature. The grown crystal was confirmed by single crystal X-ray diffraction and FT-IR studies. The direct band gap energy was found to be 5.4 eV. The SHG efficiency of the sample is 3 times higher than that of KDP crystal. The laser damage threshold of the grown crystal was 5.7 GW/cm2 . The grown crystal was thermally stable up to 141 ◦ C. Low dielectric constant at higher frequency was found by dielectric measurements. The activation energy was calculated from Arrhenius relation and it was found to be 0.088 eV. Negative photoconducting nature was obtained by photoconductivity measurements. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction Organic NLO materials have subsequently produced very good materials with highly desirable characteristics. However most of the organic NLO materials are susceptible to damage because of their poor mechanical and thermal properties [1]. In the recent past, amino-acid family single crystals are gaining importance as highly feasible second order NLO materials. Considerable efforts have been made to combine amino-acids with interesting organic and inorganic matrices to produce outstanding materials to challenge the established inorganic materials like niobates and the borates. This is due to the fact that all the amino acids contain chiral carbon and crystallize in noncentrosymmetric space groups. Hence, they are regarded as potential candidates for second harmonic generation (SHG), optical parametric amplification (OPA) and optical parametric oscillation (OPO). The absence of strong conjugated bonds and the presence of chromophores such as amino groups and carboxyl group lead to exhibit wide transparency in the entire UV–vis
∗ Corresponding author. Mobile: +91 89031 93588. ∗∗ Corresponding author. Mobile: +91 99429 94274. E-mail addresses: [email protected] (M. Vimalan), [email protected] (I. Vetha Potheher).
region, thus meeting out one of the essential requirements of non-linear optical activity [2]. The presence of Zwitter ions influences the physical and chemical properties of amino acids. The proton donor carboxyl acid ( COO) group donates its proton to acceptor amino ( NH2 ) group. Thus, an amino acid exists as a dipolar ion in which carboxyl group is present as carboxylate ion and amino group is present as ammonium ion. Due to this dipolar nature, they possess good mechanical and physical properties, viz. crystal hardness and high melting point, which make them ideal candidates for NLO applications. Among the synthetic materials that have been suggested as sources of piezoelectric elements, some tartrates, e.g. Rochelle salt (sodium potassium tartrate tetrahydrate) and ethylene diamine tartrate (EDT) are strongly piezoelectric. It therefore seemed likely that the examination of other organic tartrates might be fruitful [3]. Crystal and molecular structure of Urea-(+) tartaric acid was reported by Wentao et al. [4]. Growth and characterization of organic nonlinear optical crystals of l-tartaric acid-nicotinamide and d-tartaric acid-nicotinamide were done by Jun Shen et al. [5]. The crystal structure of l-asparaginel-tartaric acid (LAsT) was solved by Natarajan et al. [6]. In the compound LAsT, the amino acid molecule exists as a zwitterion, an uncommon ionization state in the crystal structures of aminocarboxylic acid complexes. Usually, a proton transfer is favored from the carboxylic acid to the amino acid in these complexes, the
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Table 1 Single crystal XRD data of LAsT single crystal. Empirical formula
C4 H8 N2 O3 + .C4 H6 O6 −
Crystal system Space group a b c A B Volume Z
Monoclinic P21 5.0864 A˚ 9.6727 A˚ 11.8349 A˚ 90◦ 95.52◦ 90◦ 582.267 (Å3 ) 2
photograph of as grown crystal of LAsT. The grown crystals are stable, do not decompose in air and non-hygroscopic in nature. An important observation during the growth of LAsT is the absence of any kind of microbial contamination during the growth period. 2.3. Characterization
Fig. 1. Photograph of as grown LAsT single crystals.
former exists in the anionic state and the latter in the cationic state. Similar zwitterionic state for the amino acid molecule is observed in l-phenylalanine fumaric acid and l-phenylalanine benzoic acid [7,8]. The aggregation pattern observed in the structure has striking similarities to those observed in other related amino acid-tartaric acid complexes, e.g. sarcosinium tartrate [9] and l-prolinium tartrate [10]. In this view the present investigation focused on the growth of single crystals of l-asparagine-l-tartaric acid (LAsT). It is a potential semi-organic NLO material. The slow evaporation technique was employed for the growth process. The grown crystals were characterized by single crystal X-ray diffraction, FTIR, optical transmission, optical absorption spectroscopy and TG & DTA analysis. Mechanical and electrical properties of the grown crystal was also studied and reported for the first time. 2. Experimental procedure 2.1. Synthesis of LAsT l-Asparagine-l-tartaric acid (LAsT) was synthesized by dissolving one mole of l-asparagine (Merck 99%) in double distilled water containing one mole of l-tartaric acid. The reaction is as follows C4 H8 N2 O3 + C4 H6 O6 → C4 H8 N2 O3 .C4 H6 O6 The synthesized salt was further purified by repeating the crystallization process at least thrice. 2.2. Growth of LAsT single crystals The synthesized salt of LAsT was purified by repeated crystallization and saturated solution was prepared in accordance with the solubility data. In order to achieve single crystals of good optical quality, needle shaped and relatively large size, many growth attempts have been made. The easiest method was to form tiny crystals by spontaneous nucleation; among them seed crystals with perfect shape and free from defects were chosen for growth experiments. The seeds were suspended in the mother solution with nylon thread. Crystals of dimension up to 10 mm × 2 mm × 2 mm were harvested after a period of 25–30 days. Fig. 1 shows the
The grown crystal of LAsT was confirmed by single crystal Xray diffraction analysis. Single crystal XRD data were collected by ENRAF NONIUS CAD4-F single crystal X-ray diffractometer with ˚ radiation. The FT-IR spectrum was recorded Mo K␣ ( = 0.71073 A) using BRUKER IFS-66V FT-IR Spectrometer with KBr pellet technique for the range 4000–450 cm−1 . The optical absorption and study was carried out using a Shimadzu UV-2400 PC Spectrophotometer in the range of 200–1100 nm. The band gap energy was calculated using the relation E = h, where h is Planck’s constant, is frequency. The NLO efficiency of LAsT crystal was evaluated by Kurtz and Perry powder technique [11] using a Q-switched, mode locked Nd:YAG laser emitting 1.06 m, 8 ns laser pulses. The thermal behavior of LAsT crystal was investigated using NETSZCH STA 409C and PERKIN ELMER thermal analyzer. Microhardness studies have been carried out on LAsT crystal using a Vickers microhardness tester fitted with a Vickers diamond pyramidal indenter attached to an incident light microscope. The frequency dependent dielectric constant and dielectric loss of LAsT were measured using Agilent 4284A LCR METER for various temperatures (313–363 K). The capacitance of the sample was noted by varying the frequency from 100 Hz to 1 MHz. The dc conductivity measurements were carried out by conventional two probe technique for temperatures ranging from 313 to 423 K. Photoconducting nature of the sample was studied by Keithley 485 picoammeter. 3. Results and discussion 3.1. Single crystal XRD study The lattice parameters of LAsT are determined using 25 reflections collected through random search routine with graphite ˚ radiation and indexed by monochromated Mo K␣ ( = 0.71073 A) the method of short vectors followed by the least squares refinement. The XRD data reveals that LAsT crystal belongs to monoclinic structure with a noncentrosymmetric space group of P21 . The unit cell parameters of LAsT are given in Table 1. The single crystal Xray diffraction data of the crystal is in good agreement with the reported values and thus confirming the grown crystal [6]. 3.2. FT-IR analysis Fig. 2 shows the FT-IR transmission spectrum of LAsT in the region 4000–400 cm−1 . It is evident from the spectrum that, there is a broad band between 3800 and 2700 cm−1 . It includes OH stretch
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Fig. 2. FT-IR spectrum of LAsT.
of water at 3450 cm−1 and N H vibration at 3320 and 3280 cm−1 . The characteristic CH vibrations of tartaric acid produce peaks at 2977 and 2960 cm−1 . In the overtone region, there is a prominent band near 2002 cm−1 due to combination of the asymmetrical NH3 + bending vibration and the torsional oscillation of the NH3 + groups. The C O stretch of tartaric acid gives a peak at 1737 cm−1 . The asymmetric and symmetric COO vibrations produce peaks at 1553 and 1420 cm−1 . The C O stretching and OH deformation produce peak at 1310 cm−1 . The alcoholic C O stretch gives its peak at 1065 cm−1 . From the spectroscopic investigation, the presence of all the fundamental functional groups of the grown sample is confirmed qualitatively. From the IR spectral analysis the presence of water is evident in the lattice of l-asparagine. The main IR spectral data along with the frequency assignments are listed in Table 2. 3.3. Optical studies The UV–vis-NIR spectrum gives information about the structure of molecule because the absorption of UV and visible light involves the promotion of the electrons in the and orbitals from the ground state to higher energy states. Optical absorption spectrum of LAsT is shown in Fig. 3a. The spectrum was recorded in the range of 200–1100 nm. From the Figure, it is observed that there is very low absorption in the visible and NIR region. The UV cut-off wavelength of LAsT is seen around 230 nm which is lower than some of the l-arginine family single crystals [12–14]. There is no absorption in this range, thus enabling the use of this material for second harmonic generation (SHG) applications. The direct band gap energy of LAsT was calculated from the Tauc’s plot [15]. A graph between h and (˛h)2 was plotted and shown in Fig. 3b. Band gap energy
Table 2 FT-IR frequency assignment for LAsT. Wave number (cm−1 ) 3450 3320 3280 3092 2977 2960 2002 1737 1673 1553 1420 1310 1065 690 612 510
Assignment O H stretch of water NH2 symmetric stretching NH2 asymmetric stretching C H symmetric stretching CH vibrations of tartaric acid CH vibrations of tartaric acid NH3 + asymmetric bending C O stretch of tartaric acid C O stretching COO− asymmetric stretching COO− symmetric stretching C O stretching and OH deformation C O stretching of alcoholic COO− bending COO− wagging COO− rocking
Fig. 3. (a) Optical absorption spectrum of LAsT. (b) Tauc’s plot of LAsT.
was calculated by extrapolating the linear portion of the curve to zero absorption and it was found to be 5.4 eV. 3.4. NLO studies For the SHG efficiency measurements, microcrystalline material of KDP is used for comparison. The sample was ground into fine powder and tightly packed in a micro capillary tube. It is mounted in the path of the laser beam of pulse energy 10.8 mJ obtained by splitting the original laser beam. The transmitted light is passed through 532 nm monochromator. The green light at double the incident frequency was collected by photo multiplier tube and converted into electrical signal. The sample is illuminated using Q-switched mode locked Nd:YAG laser with input pulse of 10.8 mJ. This signal was displayed on the oscilloscope. Signal amplitude in milli volts on the oscilloscope indicates the SHG efficiency of the sample. The emission of green radiation from the crystal is confirmed the second harmonic signal generation in the crystal. A second harmonic signal of 161 mV is obtained for LAsT with reference to KDP (53 mV). Thus the SHG efficiency of LAsT is 3 times that of KDP for a particle size between <50 and >150 m. The level of SHG response of a given material is inherently dependent upon its structural attributes. On a molecular scale, the extent of charge transfer across the NLO chromophore determines the level of SHG output, greater the charge transfer, larger is the SHG output. The presence of strong intermolecular interactions, such as hydrogen bonds can extend this level of charge transfer into the supramolecular realm, owing to their electrostatic and directed nature, thereby enhancing the SHG response [16]. High optical quality single crystals cut in phase-matching directions are very essential for NLO applications. The phase matching property was confirmed in LAsT using the same Kurtz powder SHG setup. In such organic crystals, as the thickness of crystal increases, instead of increasing SH intensity, a decrease of SH intensity was observed due to phase mismatch between the fundamental and the SH waves as they propagate inside the crystal. One easy way of confirming the existence of phasematching property in new NLO crystals is by studying the grain size dependence of SH intensity in powder form. The particle size dependency of SHG intensity in
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100
240
95 90 2
Hv (Kg/mm )
SHG Output ( I ) (mV)
220
200
180
85 80 75 70
160
65
<50
50-100 100-150 above 150 Particle size (m)
Fig. 4. Phase matching curve of LAsT.
LAsT is shown in Fig. 4. The SHG intensity increases almost linearly with the increase in particle size until <50–>150 m and above this range; it deviates from the linearity and starts to attain saturation. This type of grain size dependence of SH intensity was observed only with phasematchable crystals. This phase-matching property may open up opportunities for LAsT crystal in efficient frequency conversion applications. Most important considerations in the choice of a material for NLO applications are its optical damage tolerance and in NLO material it may severely affect the performance of high power laser systems. High damage threshold is a significant parameter for NLO material. LAsT crystal was prepared for laser damage threshold (LDT measurements. Experimentally, a Q-switched diode array side pumped Nd:YAG laser operating at 532 nm radiation was used with 8 ns pulse width. During laser irradiation, the power meter records the energy density of the input laser beam for which the crystal gets damaged. In the present study, laser damage was found to be 5.7 GW/cm2 . The high damage threshold contributes to attractiveness of the present compound in practical applications. 3.5. Thermal analysis TG-DTA was carried out on the grown LAsT sample for qualitative analysis and the respective curves are shown in Fig. 5. From the DTA curve it has been observed that endothermic peak at about 141 ◦ C corresponds to the melting point of the compound which is coinciding with the stability of the material shown by TGA trace. This is attributed to utilization of the thermal energy to overcome the valence bonding between the l-asparagine cation and tartrate anion, which happens in the initial stage of decomposition. The
10
20
30
40
50
Load P (gm) Fig. 6. Vickers hardness profile of LAsT as a function of applied load.
two broad endothermic peaks at 141 and 275 ◦ C represent the decomposition of molecular fragments in two stages. This weight loss is followed by a major weight loss pattern between 172 and 267 ◦ C occurring in two stages, the total weight loss of these stages correspond to 20% of the substance due to release of CO2 and CO molecules in tartaric acid. The reactions of simplest amino acids induced by heating include the condensation reactions of carboxyl and amino groups leading to the formation of peptide bonds. The total weight loss nearly equals to 90% and the resulting residue (10%) is stable up to 800 ◦ C. It is concluded from the thermal analysis, the material can be useful up to 141 ◦ C for the NLO applications. 3.6. Microhardness study Microhardness plays a key role in device fabrication. Vickers hardness measurements were made on the prominent (1 0 0) plane using Leitz–Wetzler hardness tester fitted with a diamond pyramidal indenter. The static indentations were made for different loads of 10–50 g with a constant indentation time of 10 s for all the trials. Diagonal lengths of the indented impressions obtained at various loads were measured using a calibrated micrometer attached to the eye piece of the microscope. Several indentation were made on LAsT. The average value of the diagonal lengths of the indentation mark for each load was used to calculate the hardness. The variations of Hv for various applied loads are plotted and shown in Fig. 6. The graph indicates that the microhardness number decreases with the increasing load. The decrease in microhardness number with the increasing load satisfies normal indentation size effect (ISE). The work hardening coefficient (n) was calculated by plotting log p vs. log d and it was found to be 1.66. According to Onitsch, if n > 2, the microhardness number Hv increases with increasing load and if n < 2, Hv decreases with increasing load [17]. 3.7. Electrical studies
Fig. 5. TGA and DTA curves of LAsT.
The dielectric study on grown LAsT crystals were carried out using Agilent 4284A LCR meter and was used to measure the capacitance and dielectric loss of the grown crystal as a function of frequency. The observations are made in the frequency range100 Hz–1 MHz in the temperature range 313–363 K. Fig. 7a and b shows the plot of dielectric constant and dielectric loss of LAsT as a function of log frequency for different temperatures (313–363 K). The dielectric constant has a higher value (15.7) at 313 K in the lower-frequency region (100 Hz) and it then decreases (13.47) at 313 K with the applied high frequency (1 MHz). In Miller rule and the Phillips-Van Vechten-Levine-Xue bond theory [18], the lower value of dielectric constant at higher frequencies is a
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suitable parameter for the enhancement of SHG coefficient [19]. The higher value of dielectric constant is due to higher space-charge polarization at lower frequency region. This may be explained on the basis of the mechanism of polarization similar to the conduction process. The electronic exchange of the number of ions in the crystal gives local displacement of the applied field, which gives the polarization. As the frequency increases, at which the space charge cannot sustain and comply the external field. Therefore the polarization decreases and exhibiting the reduction in the value of dielectric constant with increasing frequency. The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations, which depend on frequencies [20]. At low frequencies, all these polarizations are active. The space charge polarization is generally active at lower frequencies and high temperatures [21]. From Fig. 7b, it can be noticed that as the frequency increases the dielectric loss decreases. This behavior is similar to that of the dielectric constant. At low frequencies the dipoles can easily switch alignment with the changing field. As the frequency increases the dipoles are less capable of rotating and maintaining phase with the field; thus they reduce their contribution to the polarization field, and hence the observed reduction in dielectric constant and dielectric loss. The characteristic of low dielectric loss with high frequency for a given sample suggests that the sample possesses enhanced optical quality with lesser defects and this parameter is of vital importance for nonlinear optical materials in their application [22]. Well sized crystal of LAsT was used for dc conductivity study. The conductivity of LAsT is shown in Fig. 8a and b. Fig. 8a represents the temperature dependence of conductivity of the sample is found
Fig. 8. (a) DC electrical conductivity for LAsT crystal. (b) Plot of ln ( dc ) versus 1000/T for LAsT single crystal.
to increase with increase in temperature. The dc activation energy (Fig. 8b) of the LAsT crystal is found to be 0.088 eV. 3.8. Photoconductivity study The applied field was varied from 150 to 3820 V/cm. For measuring the dark current, the sample was kept unexposed from any radiation. After measuring the dark current, the photocurrent was measured by illuminating the sample with a halogen lamp of 100 W power. A spot of light on the sample was focused with the help of a convex lens. The resulting photocurrent was measured by varying 450 400 Id Ip
350 Current (nA)
Fig. 7. (a) Variation of dielectric constant with log frequency at different temperatures for LAsT single crystal. (b) Variation of dielectric loss with log frequency at different temperatures for LAsT single crystal.
5
300 250 200 150 100 50 0 0
500 1000 1500 2000 2500 3000 3500 4000 Applied Field (V/cm)
Fig. 9. Field dependent photoconductivity of LAsT single crystal.
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the applied field for the same range. The variation of dark current and photocurrent with applied field are shown in Fig. 9. It is observed from the plots that the dark current is always higher than the photo current. Thus, LAsT single crystal is found to exhibit negative photoconductivity. 4. Conclusion Single crystals of l-asparagine-l-tartaric acid (LAsT) of dimension 10 mm × 2 mm × 2 mm is conveniently grown by slow evaporation technique at room temperature. A solvent of deionized water is used for the growth process. The single crystal XRD data proves that LAsT crystal belongs to monoclinic in structure with a noncentrosymmetric space group P21 . Optical absorption studies confirm the UV cut-off wavelength of LAsT at 230 nm and the optical band gap energy is found to be 5.4 eV. Thermal analysis revealed that the sample is thermally stable up to 141 ◦ C and various stages of weight losses also discussed. The frequency dependence of the dielectric constant/dielectric loss of LAsT are investigated. The activation energy of the sample is calculated by dc conductivity studies. The negative photoconducting nature of LAsT is studied by photoconductivity investigations. References [1] K. Selvaraju, R. Valluvan, K. Kirubavathi, S. Kumararaman, Investigation on the nucleation kinetics of l-arginine acetate single crystals, Mat. Lett. 61 (2007) 3041–3044. [2] V. Krishnakumar, R. Nagalakshmi, Polarised Raman and infrared spectral analysis of l-alanine oxalate (C5 H9 NO6 )—a non-linear optical single crystal, Spectrochim. Acta A 64 (2006) 736–743. [3] M. Vimalan, T. Rajesh Kumar, S. Tamilselvan, P. Sagayaraj, C.K. Mahadevan, Growth and properties of novel organic nonlinear optical crystal: l-alaninium tartrate (LAT), Physica B 405 (2010) 3907–3913. [4] Y. Wentao, L. Mengkai, M. Fanqing, Crystal and molecular structure of Urea-(+) tartaric acid, Mat. Res. Bull. 31 (1996) 1127–1131. [5] J. Shen, J. Zheng, Y. Che, B. Xi, Growth and properties of organic nonlinear optical crystals: l-tartartic acid-nicotinamide, and d-tartaric acid-nicotinamide, J. Cryst. Growth 257 (2003) 136–140.
[6] S. Natarajan, V. Hema, J. Kalyana Sundar, J. Suresh, P.L. Nilantha Lakshman, l-Asparagine-l-tartaric acid (1/1), Acta Crystallogr. E 66 (2010) 2239. [7] M. Alagar, R.V. Krishnakumar, K. Rajagopal, M. Subha Nandhini, S. Natarajan, l-Phenylalanine fumaric acid, Acta Crystallogr. E 59 (2003) 952–954. [8] J. Suresh, R.V. Krishnakumar, S. Natarajan, l-Phenylalanine-benzoic acid (1/1), Acta Crystallogr. E 61 (2005) 3625–3627. [9] R.V. Krishnakumar, M. Subha Nandhini, S. Natarajan, Sarcosinium tartrate, Acta Crystallogr. C 57 (2001) 165–166. [10] M. Subha Nandhini, R.V. Krishnakumar, S. Natarajan, l-Prolinium tartrate, Acta Crystallogr. C 57 (2001) 423–424. [11] S.K. Kurtz, T.T. Perry, A powder technique for the evaluation of nonlinear optical materials, J. Appl. Phys. 39 (1968) 3798–3813. [12] F. Yogam, I. Vetha Potheher, M. Vimalan, R. Jeyasekaran, T. Rajesh Kumar, P. Sagayaraj, Growth and physicochemical properties of l-phenylalaninium maleate: a novel nonlinear optical crystal, Spectrochim. Acta A 95 (2012) 369–373. [13] K. Vasantha, S. Dhanuskodi, Single crystal growth and characterization of phase-matchable l-arginine maleate: a potential nonlinear optical material, J. Cryst. Growth 269 (2004) 333–341. [14] A. Pricilla Jeyakumari, S. Danushkodi, S. Manivannan, Phase matchable semiorganic NLO material for frequency doubling: l-arginine tetrafluoroborate, Spectrochim. Acta A 63 (2006) 91–95. [15] J. Tauc, in: J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974. [16] H. Zhang, Y. Sun, X. Chen, X. Yan, B. Sun, Crystal growth and characterization of organic single crystal: N-(2-hydroxybenzylidene) acetohydrazide, J. Cryst. Growth 324 (2011) 196–200. [17] E.M. Onitsch, The present status of testing the hardness of materials, Mikroskopie 95 (1959) 12–14. [18] D. Xue, S. Zhang, Chemical bond analysis of the correlation between crystal structure and nonlinear optical properties of complex crystals, Physica B 262 (1999) 78–83. [19] C. Justin Raj, S. Dinakaran, S. Krishnan, B. Milton Boaz, R. Robert, S. Jerome Das, Studies on optical, mechanical and transport properties of NLO active l-alanine formate single crystal grown by modified Sankaranarayanan–Ramasamy (SR) method, Opt. Commun. 281 (2008) 2285–2290. [20] D. Xue, K. Kitamura, Dielectric characterization of the defect concentration in lithium niobate single crystals, Solid State Commun. 122 (2002) 537–541. [21] B.W. Betterman, H. Cole, Dynamical diffraction of X rays by perfect crystals, Rev. Mod. Phys. 36 (1964) 681–717. [22] C. Balarew, R. Duhlew, Application of the hard and soft acids and bases concept to explain Ligand coordination in double salt structures, J. Solid state Chem. 55 (1984) 1–6.
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