Investigations on the vibrational modes and non-linear optical properties of 4-Fluoro Chalcone crystal

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 114–120

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Investigations on the vibrational modes and non-linear optical properties of 4-Fluoro Chalcone crystal S. Prabu a, R. Nagalakshmi b, J. Balaji a, P. Srinivasan a,⇑ a b

University College of Engineering Panruti, Panruti 607 106, India1 Department of Physics, National Institute of Technology, Tiruchirappalli, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Non-linear optical crystals of 4FC has

been grown.  Presence of vibrational modes were

identified and confirmed.  The SHG efficiency and first order

hyperpolarizability is found to be higher than urea.  HOMO–LUMO charge transfer and energy gap has been found.

a r t i c l e

i n f o

Article history: Received 13 December 2013 Received in revised form 3 March 2014 Accepted 18 March 2014 Available online 27 March 2014 Keywords: Organic compound Optical materials Crystal growth FTIR Optical properties

a b s t r a c t Organic Nonlinear Optical (NLO) crystals of 4-fluorochalcone (4FC) were synthesized and grown by slow evaporation solution growth method. The grown crystals have been characterised by powder X-ray diffraction, factor group analysis, FTIR, FT-Raman, UV–Vis Spectroscopy, powder SHG and Vickers microhardness tests. Theoretical quantum chemical analysis were performed to determine the first order hyperpolarizability (b) and HOMO–LUMO analysis of the title compound were computed by GAUSSIAN 03 package. Ó 2014 Elsevier B.V. All rights reserved.

Introduction The second-order nonlinear optical properties of conjugated organic molecules have been the focus of numerous experimental and theoretical investigations due to their potential applications

⇑ Corresponding author. Address: Department of Physics, University College of Engineering Panruti, Panruti 607 106, India. Tel.: +91 9488046400; fax: +91 4142241000. E-mail address: [email protected] (P. Srinivasan). 1 A Constituent College of Anna University Chennai, India. http://dx.doi.org/10.1016/j.saa.2014.03.030 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

in optoelectronics and photonics for applications including highspeed optical communications, integrated optics, and optical data processing and storage [1]. In the organic materials, the NLO effect arises from the hyperpolarizability due to electrons, in contrast to inorganic materials where NLO effects are mainly due to polar optical lattice vibrations [2–3]. However the non-centrosymmetric crystal packing of molecules is the primary requirement for exhibiting second-order nonlinear optical (NLO) activity [4]. The chalcone derivatives have acquired much attention due to their large NLO properties. In most of the NLO chalcone compounds, an intramolecular charge transfer is obtained, where p conjugated

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system acts as a core which gets attached by electron accepting group (A) (carbonyl group) at one end and an electron donating group (D) (the electron rich subsitution on the aromatic rings) at the other end, that forms a D–p–A type molecule and is characterised by an intense electronic charge transfer transition in the UV/ visible spectral region [5–6]. We have investigated in this article one such type of charge transfer molecule namely 4-flurochalcone (4FC) which crystallizes in non-centrosymmetric crystal packing. The crystal structure of the title compound has been already reported elsewhere [7]. In this work we report a systematic study on the synthesis, crystal growth vibrational and optical properties of 4FC crystals.

Experimental Synthesis and crystal growth The 4-Fluoro Chalcone compound was synthesised from a mixture of equimolar quantities of 4-fluorobenzaldehyde and acetophenone. All the reactants were of Analytical grade. The reactants were taken and stirred in ethanol (50 ml) and to this mixture solution, aqueous NaOH (3%) was added drop by drop. After a particular drop, the whole solution turned out to be a precipitate. The precipitate was dried and then recrystallized from ethanol solution. Solubility of the compound was performed with various solvents such as ethanol, acetone, chloroform, methanol and etc. Finally, we found that N,N-Dimethylformamide (DMF) can be best used as an solvent for the growth of 4FC crystals by slow evaporation solution growth method at room temperature.

Fig. 1. The XRD pattern for the studied powder 4FC.

Table 1 Lattice parameter values for 4FC. Lattice parameter

Obtained value

Previously reported value

a b c b V System

25.1746 (1) 5.7482 (1) 7.5982 (1) 94.18 (3) 1096.615 Cc, monoclinic

24.926 (9) 5.6940 (19) 7.749 (3) 94.747 (5) 1096.0 (6) Cc, monoclinic

Characterization Vibrational studies The grown crystals were characterised by powder X-ray diffraction (XRD) analysis (BRUKER, GERMANY (D8 Advance)). The Fourier Transform Infrared (FTIR) spectrum was recorded in the region 400–4000 cm1 using Perkin Elmer, model SPECTRUM RX1 Spectrophotometer. The FT–Raman spectrum was recorded in the region 5000–100 cm1 (BRUKER RFS 27 model interferometer) stokes region using the 1064 nm line of an Nd:YAG laser for excitation operating at 100 mW (srl = 320) power. The absorption spectrum for the title crystal was recorded in the wavelength range from 200 nm to 1000 nm using Labindia UV1300 UV–Vis spectrophotometer. The Second Harmonic Generation (SHG) efficiency of the 4FC was determined by Kurtz and Perry powder technique [8]. Microhardness studies of the 4FC crystals were performed by Matsuzawa (mmT-X) Vickers microhardness tester by varying the applied load from 1 g to 25 g. The indentation time was maintained constant at 5 s. The quantum chemical calculation of the vibrational frequency and conformational dependent NLO activity were performed at both HF (Hartree–fock) and hybrid Density Functional B3LYP (Becke-Lee-Young-Parr) with 6-31G level basis set using Gaussian ‘03W program package.

Factor group analysis The crystal 4FC crystallizes in the monoclinic system with Cc (C4s ) space group and C1(4) site symmetry. The molecule is composed of 28 atoms, each unit cell containing 4 atoms. The total vibrational degrees of freedom is 336, which are distributed into 12 branches and it has been classified into 312 internal vibrational modes and 24 external vibrational modes such as 12 (6A0 + 6A00 ) for translational lattice modes and 12 (6A0 + 6A00 ) for librational lattice modes. All the vibrations within the crystal can be decomposed according to the irreducible representation of the space group as Ctotal = 168A0 + 168A00 which includes the phonon branches of three acoustic modes that can be distributed as Cacou = 2A0 + A00 . Therefore the remaining optical modes of vibrations distributed as Ctotal = 166A0 + 167A00 . The distribution of all the possible vibrations of the title compound are stacked in Table 2. FTIR and Raman spectral analysis The experimental and computed FTIR spectrum of 4FC are shown in Fig. 2 and the experimental and computed Raman spectrum of 4FC are shown in Fig. 3.

Results and discussion X-ray diffraction analysis The powder X-ray diffraction pattern of 4FC is shown in Fig. 1. From the pattern all intensity peaks could be indexed to the monoclinic crystal structure system and it is found that it belongs to Cc space group. The cell parameter values of 4FC were calculated from X-ray powder diffraction data using XRDA software. The obtained values are depicted in Table 1 which are in good agreement with the literature value [7].

Hydrogen bonding. The hydrogen bond interaction in molecules plays a significant role in improving the SHG efficiency of the material. The aromatic CAH stretching vibrational bands occurs in the region above 3000 cm1 which is the characteristic region for identification of CAH stretching vibration. In 4FC the CAH stretching vibration bands were observed at 3136 and 3070 cm1 from the measured FTIR, Fig. 2a and Raman, Fig. 3a bands respectively. The aromatic CAH stretching vibrations were predicted by B3LYP/6-31G method at 3229, 3228, 3228, 3224, 3212, 3206, 3203, 3199, 3195, 3185 and 3174 cm1.

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Table 2 Factor group analysis of 4FC. Factor group species Cs

4-Fluorochalcone C1 site

Total modes

Optical modes

Acousticmodes

Activity

Internalmodes

Externalmode

C

H

F

O

IR

Raman

A0

156

6T, 6R

90

66

6

6

168

166

02

Tx, Ty; Rz

A00

156

6T, 6R

90

66

6

6

168

167

01

Tz; Rx, Ry

x;y x;y x;y ax;y xx ; ayy ; azz ; axy azxz ; azyz

312

12T, 12R

192

156

12

24

336

333

3

The characteristic aliphatic CAH bending vibration bands occurs at 1415, 1344 cm1 and 1413, 1338 cm1 for experimental Raman and FTIR spectrum respectively. This is in good agreement with computed value appearing at 1383, 1371, 1365, 1342, 1334 and 1328 cm1. The CAH in-plane bending vibrations bands appeared at 1157, 1099, 1016, 981 and 1161, 1018, 1000 cm1 for experimental FTIR and Raman spectrum respectively, which is in good agreement with calculated frequency values of 1193, 1192, 1131, 1116, 1062, 1041, 1039, 1018, 1008, 981, 958, 950 and 942 cm1. The CAH out of plane bending occurs in the region of 1000–650 cm1 which appears at 829, 779, 715, 688 and 659 cm1 in experimental FTIR and its Raman counterpart occur at 896 cm1. The calculated values of the same occurs at 910, 887, 861, 850,845, 826, 797, 770, 725, 704, 684, 671 and

651 cm1. These vibrational bands which have high frequencies are ascribed to aliphatic CAH and those of lower frequencies are attributed to aromatic CAH out of plane bending vibrations. The experimental FTIR vibrational bands at 532, 507 cm1 and Raman bands at 638, 534 cm1 are due to out of plane ring bending in heteroaromatics. The computed values of the same arise at 632, 540, 524, 508 and 453 cm1.

CAC stretching vibration. The aromatic CAC stretching (in-ring) vibration appearing at 1604, 1577, 1510, 1448 and 1601, 1588, 1577, 1491, 1448 cm1 are the experimental FTIR and Raman frequencies respectively which are in good agreement with calculated frequencies at 1562, 1540, 1493 and 1461 cm1.

Fig. 2. As experimental (a) and computed (b) FTIR spectra for the investigated 4FC crystals.

S. Prabu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 114–120

C@O stretching vibration. The C@O stretching vibrational bands in the experimental FTIR spectrum appeared as a strong band at 1660 cm1 and Raman counterpart band at 1662 cm1. This is due to the presence of a, b- unsaturated carbonyl group. The calculated frequencies also occur in the same region. Aryl fluoride vibration. The aryl fluorides stretching vibration appears in the region of 1250–1100 cm1. The sharp and strong intensity absorption band appearing at 1215 cm1 in the experimental FTIR spectrum and Raman counterpart band at 1217 cm1 are assigned to monofluorinated benzene ring which is in good agreement with calculated values. The CAF stretching vibration band occurs at 1288 cm1 and 1289 cm1 of the experimental FTIR and Raman spectrum respectively. Similarly the CAH stretching vibration of aryl fluoride occurs in the region of 3250 cm1 and 3100 cm1 which have values higher compared to normal unsubstituted benzene ring. This may be due to the electronegativity effect of fluoride, that enhances frequency range by coupling interactions. The vibrational frequencies of the 4FC compound have also been identified and listed in Table 3.

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C@O group [9–10]. The absorption cut off wavelength of this crystal is found to be 400 nm. For the optical device applications, large polarizabilities need to be accompanied by optical transparency in the near IR region rather than the visible region because, the absorption does not extend into the 1.3–1.5 lm range wavelength was used in optical telecommunication systems [11]. The 4FC crystal has wide range of transparency in visible and near IR spectral region and hence this compound may be used for NLO application over this range. Second Harmonic Generation test The SHG efficiency of 4FC carried out by Kurtz and Perry powder method. The experimental details of the same has been already reported elsewhere [12]. The beam energy of Nd:YAG laser operating at 1064 nm is set to 3.1 mJ/pulse. The SHG efficiency of 4FC is found to be 1.84 times greater than the urea and much greater than KDP. The Donor-p-Acceptor type structure of the title compound is responsible for the charge transfer and very high electronic delocalisation. This could be the reason for optical nonlinearity in the title compound.

UV-spectral analysis First order hyperpolarizability The recorded UV–Vis Spectrum of 4FC is shown in Fig. 4. The absorption peak appearing in the UV region may be ascribed to transition of n ? p to the excitation in the aromatic ring and

The first order hyperpolarizability (b) of molecular system and related properties of 4FC was found by using B3LYB/6-31G and

Fig. 3. As experimental (a) and computed (b) Raman spectra for the investigated 4FC crystals.

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Table 3 The vibrational frequencies of the 4FC compound identified and listed. S. No.

Frequency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

3229.18 3228.6 3228.1 3224.56 3212.05 3206.84 3203.49 3199.38 3195.09 3185.11 3174.62 1744.07 1673.66 1659.75 1653.75 1636 1633.68

Theoretical value

Experimental value

IR intensity

Raman intensity

2.9631 12.5919 10.6237 13.8196 4.6362 36.6096 6.5291 6.3827 12.6982 0.4143 0.9348 122.192 42.4107 197.848 232.775 3.6497 81.7773

213.614 200.421 76.9319 33.0934 148.724 106.59 43.8076 54.1118 148.547 49.7457 26.6881 23.6282 170.253 49.424 3056.43 7.6115 597.916

18 19 20 21

1562.16 1540.47 1493.74 1461.45

140.902 0.6928 14.9505 38.075

58.3661 43.9021 35.3241 46.6135

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

1383.14 1371.2 1365.08 1342.76 1333.97 1328.59 1294.17 1239.16 1238.01 1212.18 1193.58 1192.92 1131.55 1116.9 1062.48 1041.25 1039.2 1018.19 1008.79 981.168 958.323 950.482 942.053 910.782 887.879 861.392 850.806 845.627 826.133 797.355 770.759 725.651 704.551 684.844 671.785 651.217 632.465 540.533 524.225 508.826 453.892 429.316 424.155 414.845 396.412 380.453 303.936 209.599 206.877

97.2519 56.237 4.1688 11.8649 19.3641 43.3373 138.653 114.006 141.543 55.4938 1.446 69.5595 7.269 1.4394 20.9141 132.224 21.2085 7.9491 0.0509 0.4807 0,1088 0.8529 0.3182 1.2925 0.9894 0.3172 23.3787 52.1701 0.5832 32.159 1.2253 18.7479 22.4443 3.1956 22.3581 0.0828 0.7287 21.1293 12.3406 22.1607 8.3672 3.1675 0.1528 0.0692 1.0824 1.3071 8.4954 0.119 0.0629

83.5998 74.836 29.6561 7.5912 109.167 64.9012 16.5058 69.4269 167.498 4.5068 10.2564 112.283 4.1544 3.6982 6.1743 171.331 3.6626 0.6076 1.0164 0.477 0.8734 3.4192 1.7172 22.0627 10.6822 4.797 31.7776 2.8956 5.6978 4.8387 5.8979 3.2459 2.0244 0.7138 1.6002 13.2987 5.3021 15.1587 0.4279 10.1422 0.4462 3.1072 0.1633 0.4595 0.3727 3.1281 1.7832 1.1312 1.2344

IR Frequency

Assingment Raman Frequency

3136.35

3070.12

1660.71

1662.22

1604.77 1577.77

1601.35 1588.68 1577.49

1510.26 1448.54 1413.82

1491.86 1448.90 1415.01

1338.80

1344.70

1288.45

1289.92

1215.15

1217.92 1199.90

1157.29 1099.43

1161.02

1016.49

1018.13 1000.38

981.77

896.54

829.39 779.24 715.59 688.59 659.66 638.26 532.35 507.28

534.90

Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching Aromatic CAH stretching C@O stretching C@O stretching C@O stretching C@O stretching C@C stretching C@C stretching CAC stretching CAC stretching CAC stretching CAC stretching CAC stretching CAC stretching CAH bending CAH bending CAH bending CAH bending CAH bending CAH bending CAH bending CAF stretching Aryl fluorides stretching Aryl fluorides stretching Aryl fluorides stretching CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending CAH out of plane bending Out of plane ring bending Out of plane ring bending Out of plane ring bending Out of plane ring bending Out of plane ring bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending

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S. Prabu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 114–120 Table 3 (continued) S. No.

71 72 73 74 75 76 77

Frequency

200.28 160.245 95.8444 72.415 53.1342 27.5851 20.3796

Theoretical value

Experimental value

IR intensity

Raman intensity

0.9527 1.4559 0.7603 2.0473 0.1941 0.222 0.1406

6.6955 4.9431 1.2155 0.7396 1.2076 4.171 3.5507

IR Frequency

Assingment Raman Frequency 185.64

CAO in-plane bending CAO out of plane bending CAO out of plane bending CAH out of plane bending CAH out of plane bending Lattice vibration Lattice vibration

73.74

Table 5 The dipole moment (l) of 4FC. Dipole (l) component

DFT/B3LYP method

lx ly lz

0.4374866 1.0433713 0.019104

0.3090971 0.6997399 0.5678602

lTotal

1.13 Debye

0.95 Debye

ltot ¼ ðl2x þ l2y þ l2z Þ

Fig. 4. The UV–Vis spectrum of 4FC.

Table 4 The first order hyperpolarizibility (b) of 4FC. b Component

Values

Values HF method

1=2

From B3LYP/6-31G method the calculated first order hyperpolarizability (b) and dipole moment (l) of 4FC are 12.5726  1030 e.s.u and 0.95 Debye respectively. The b and l value of 4FC are 8.40839  1030 e.s.u and 1.13 Debye found by the HF/6-31G method. The first order hyperpolarizability value of title compound is 22.5 times greater than urea and dipole moment is 0.82 time that of urea. The high value of b is associated with intramolecular charge transfer from electron donor to acceptor through p conjugated system. The calculated first order hyperpolarizability (b) and their components were collected in Table 4. The calculated dipole moment (l) and their component values are presented in Table 5. HOMO–LUMO analysis

HF method

DFT/B3LYP method

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz

872.768472 534.0829939 91.3207704 51.151783 21.0685695 13.5944814 6.0220888 7.3296092 8.6580387 0.5129456

769.6734281 1025.906602 764.9973725 439.3069171 336.4941232 36.7461168 48.2828706 133.1121645 48.5490185 26.6048634

bTotal bTotal

973.258477 a.u 8.40839  1030 e.s.u

1455.254673 a.u 12.5726  1030 e.s.u

In molecule, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) allows to the strong interaction between them. This form corresponds to the

HF/6-31G basis set based on finite field approach. The first order hyperpolarizibility is a third rank tensor that can be described by a 3  3  3 matrix. It strongly depends on the method and basis set used. The 27 components of 3D matrix can be reduced to 10 components due to Kleinman symmetry [13]. The components of b are defined as the coefficient of Taylor series expansion of energy in the external electric field. The complete equation for calculating the first order hyperpolarizability from GAUSSIAN 03 output is given as follows [14]. h i1=2 2 2 2 btot ¼ ðbxxx þ bxyy þ bxzz Þ þ ðbyyy þ byzz þ byxx Þ þ ðbzzz þ bzxx þ bzyy Þ and dipole moment (l) was given by

Fig. 5. The variation of Vickers microhardness number (Hv) with applied load.

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frozen orbital approximation, as the ground state properties are used to calculate the excitation values. The density functional methods (which are based on Hohemberg and Kohn theorem) [15] are designed to yield total energies. The molecular orbital picture of 4FC, the molecular HOMOs and molecular LUMOs are generated via Gaussian 03. The acceptor group of the benzoyl moiety which is p natured is located in the LUMO and by contrast, the HOMO is located over the substituted Fluorine (F) placed on the phenylene group. Consequently the HOMO ? LUMO transition implies an electron density transfer from the F atom substituted benzene ring to benzene ring. The HOMO–LUMO energy separation is characterized to the conjugated molecules. which is the electronic absorption corresponding to the intramolecular charge transition. The 4FC molecule HOMO–LUMO energy gap value is 0.153 a.u and 0.275 a.u calculated by using the B3LYP/6-31 and HF/6-31G levels respectively. The energy gap reflects the chemical activity of the molecule and a lower HOMO–LUMO energy gap explains the fact that eventual charge transfer interaction is taking place within the molecule.

Conclusion The organic NLO crystal of 4FC has been grown by slow evaporation solution growth method. The lattice parameter values were determined from powder X-ray diffraction and it peaks in the monoclinic crystal system. The presence of vibrational modes and functional groups were identified and confirmed by the methods of the factor group analysis and both experimental and computed FTIR and Raman spectrum. The crystal has the wide range of optical transparency and cut of wavelength is found to be at 400 nm. The SHG efficiency is 1.84 times greater than urea and first order hyperpolarizability (b) value is 22.5 times greater than urea. The HOMO–LUMO charge transfer has been analysed and energy gap value of 4FC molecule was found. The crystal has satisfied the normal size indentation effect to determined by mechanical hardness studies. From the above substantiations, we could emphasise that the title compound could be better used for nonlinear optical applications. Appendix A. Supplementary material

Microhardness studies The mechanical properties of 4FC crystal was made by Vickers microhardness tester at room temperature. The microhardness studies prevalent in the system have a direct correlation with crystal structure and is very sensitive to the presence of any other phase or phase transition and lattice perfections. Thus microhardness plays the principle role in electron–phonon an-harmonicities due to the crucial role of inter-molecular interactions [16]. The crystal was indented gently by varying load for the indentation time (maintained constant at 5s) and well-defined impressions were considered and the average of all the diagonals (d) was considered. The Vickers microhardness number was calculated by using standard the formula 2

Hv ¼ 1:8544P=d kg mm2 where P is the applied load in kg and d is the mean diagonal length of the indenter impression in millimetre. Above 25 g of the applied load the crystal gets Crack initiation and material starts breaking. The Fig. 5 shows the variation of Hv with applied load. It can be observed from the study that Vickers hardness value decreases with increases load, which satisfies the normal indentation size effect.

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