Bulk Growth and Characterization of Semi-Organic Nonlinear Optical

CRYSTAL GROWTH & DESIGN

Bulk Growth and Characterization of Semi-Organic Nonlinear Optical Bis Thiourea Bismuth Chloride Single Crystals

2007 VOL. 7, NO. 3 501-505

R. Sankar, C. M. Raghavan, and R. Jayavel* Crystal Growth Centre, Anna UniVersity, Chennai-600 025, India ReceiVed July 11, 2006; ReVised Manuscript ReceiVed NoVember 7, 2006

ABSTRACT: Growth and property studies of a new nonlinear optical (NLO) metal-organic crystal, bis thiourea bismuth chloride (BTBC), is reported. BTBC crystals were grown from aqueous solution by a slow-cooling technique. It has been observed that the solution pH influences the growth rate along the [001] and [010] directions. The grown crystals have been characterized for their structural, thermal, mechanical, linear, and NLO properties. BTBC is found to be a uniaxial and optically positive crystal. The birefringence value of BTBC is found to be 0.103. BTBC crystallizes in a hexagonal system, and the complex formation is stabilized by a metal-sulfur bond. BTBC has good optical transmission in the entire visible region and hence is a potential material for nonlinear frequency conversion. 1. Introduction

2. Experimental Procedures

Second-order nonlinear optical (SONLO) materials have recently attracted much attention because of their potential applications in emerging optoelectronic technologies.1,2 Materials with large second-order optical nonlinearities, short transparency cutoff wavelengths, and stable physico-thermal performance are needed to realize many of these applications. The search for new frequency conversion materials over the past decade has concentrated primarily on organics. It has been demonstrated that organic crystals can have very large nonlinear susceptibilities compared with inorganic crystals, but their use is impeded by low optical transparency, poor mechanical properties, low laser damage threshold, and the inability to produce and process large crystals.3,4 Purely inorganic nonlinear optical (NLO) materials typically have excellent mechanical and thermal properties with relatively modest optical nonlinearities because of the lack of extended π-electron delocalization. In semi-organics, polarizable organic molecules are stoichiometrically bound within an organic host.5 In recent years, the NLO properties of semi-organic complex products of thiourea have attracted great interest because these metal-organic complexes combine the high optical nonlinearity and chemical flexibility of organics with the physical ruggedness of inorganics.6,7 The thiourea molecule is an interesting inorganic matrix modifier due to its large dipole moment8 and its ability to form an extensive network of hydrogen bonds. Most thiourea complexes known so far are centrosymmetric; only a few crystallize in a noncentric space group, which is a general requirement for nonlinear optics. Several, metal-organic complexes of thiourea such as zinc thiourea sulfate (ZTS), zinc thiourea chloride (ZTC), and bismuth thiourea cadmium chloride (BTCC) have already been studied.9-11 In this investigation, a new semiorganic NLO crystal of bis thiourea bismuth chloride (BTBC) is reported. Solubility of BTBC was measured in aqueous solution. Bulk crystals of size 1.7 × 1.0 × 0.7 cm3 have been grown by optimized growth conditions.

2.1. Material Synthesis. The BTBC salt was synthesized using high purity bismuth (III) chloride and thiourea in a stoichiometric ratio of 1:2. The precursor compound was prepared according to the reaction,

* To whom correspondence should be addressed. Dr. R. Jayavel, Assistant Professor, Crystal Growth Centre, Anna University, Chennai -600 025. Tel: + 91-44-22203571. Fax: +91-44-22352870. E-mail: [email protected], [email protected].

BiCl3 + 2[CS(NH2)2] + H2O f thiourea bismuth trichloride BiOCl + 2HCl + [CS(NH2)2]2 bismuth oxy-chloride (white precipitate) BiCl3 + 2[CS(NH2)2] + H2O f Bi[CS(NH2)2]2Cl3 bis-thiourea bismuth bismuth thiourea trichloride trichloride The calculated amounts of bismuth(III) chloride and thiourea were dissolved in deionized water according to the first reaction. In this reaction, BTBC was not formed; instead white precipitate of BiOCl was obtained. The resultant BTBC was formed by mixing the salts in the presence of acid medium (HCl) as per the second reaction. This solution was heated and kept for slow evaporation to dryness at room temperature. The purity of the synthesized salt was improved by a successive recrystallization process. 2.2. Solubility Studies. The solubility of BTBC was determined for four different temperatures, 30, 35, 40, and 45 °C, by dissolving the solute in deionized water in an airtight container maintained at a constant temperature with stirring. After saturation was attained, the equilibrium concentration of the solute was analyzed gravimetrically. The same procedure was repeated, and the solubility curves for different temperatures were drawn. Figure 1 shows the solubility curve for BTBC in aqueous solution.

3. Crystal Growth Bulk crystals of BTBC was grown from aqueous solution by a slow-cooling technique, in a constant temperature bath controlled to an accuracy of (0.01 °C. A total of 300 mL of the solution was saturated at 45 °C and then filtered to remove any insoluble impurities. The seed obtained from slow evaporation was employed for the bulk growth. The solution was maintained at 45 °C for 2 days before seeding. The temperature was reduced at a rate of 0.1-0.2 °C per day as the growth progressed. The period of growth ranged from 30 to 35 days. Figure 2 shows the as-grown crystal of BTBC with an optimized solution pH value of 4.75. Red crystals of average size 1.7 × 1.0 × 0.7 cm3 were obtained. These crystals are non-hygroscopic

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Sankar et al.

Figure 1. Solubility curve for BTBC in aqueous solution.

Figure 3. Morphology of as-grown BTBC crystal. The most developed faces of the crystal are {100}, {010}, and {001}.

Figure 4. Molecular structure of BTBC single crystal.

crystal shown in Figure 3 reveals the faster growth rate along the crystallographic c-axis. The molecular structure of BTBC single crystal is given in Figure 4. 4. Characterization Studies Figure 2. (a) Bulk single crystal of BTBC grown by the solvent evaporation technique. (b) Bulk single crystal of BTBC grown by the slow-cooling technique.

and optically transparent. The most developed faces of these crystals are {100}, {010}, and {001}. The as-grown crystal of BTBC is shown in Figure 2a (which shows the slow evaporation method) and Figure 2b (which shows the slow cooling method). The position of the crystal for (100), (010), and (001) planes were observed, and, hence, the crystallographic axes a, b, and c were determined. The crystal surfaces along the crystallographic axes were indexed in a similar manner. The crystals have elongation in the c direction. Morphology of the BTBC

Single-crystal X-ray diffraction analysis was carried out using an Ehraf CAD-4 diffractometer with MoKR (λ ) 0.1770 Å) radiation to identify the structure and to determine the lattice parameter values. Powder X-ray diffraction analysis was also carried out using a Rich Seifert diffractometer with Cu KR (λ ) 1.5418 Å) radiation to verify the correctness of lattice parameter values. To confirm the NLO property, Kurtz powder SHG test was performed on the grown crystals. Refractive indices were measured along the crystallographic axis c (ne ) 1.842) and the direction perpendicular to it (nw ) 1.7110) by the Brewster’s angle method using a light of wavelength 5893 Å. The channel spectrum method was used for the measurement

Bis Thiourea Bismuth Chloride Crystals

Crystal Growth & Design, Vol. 7, No. 3, 2007 503 Table 1. Single-Crystal Data for BTBC Crystal chemical formula molecular weight system cell dimensions V density Z space group

Bi[NH2CSNH2]2Cl3 380.35 hexagonal a ) b ) 13.533 Å c ) 7.109 Å, γ ) 120°; 127.7 Å3 2.402 g/mL 4 P3

Table 2. Assignment of FTIR Frequencies (cm-1)a thiourea

Figure 5. Powder X-ray diffraction pattern of BTBC crystal.

BTBC

3376 3280 3167 1628 1477 1414

3436 3193 3107 1624 1504 1384.8

1082

1095

730

702

488

470.6, 555

assignment N-H stretching vibration (υ N-H) N-H bending vibration (δ N-H) N-C-N stretching vibration (υ N-C-N) NH2 rocking vibration, C-S stretching vibration and N-C-N stretching vibration NH2 rocking vibration υCdS and υS C-N CdS stretching vibration (υCdS) N-C-N bending vibration δs(N-C-N)

δ - deformation, ν - band stretching, F - rocking, s - symmetric, as asymmetric. a

Figure 6. Absorption spectrum of BTBC crystal.

of birefringence. A 500 W halogen light source served as the source. The FTIR spectrum of BTBC crystals was recorded in the range 400-4000 cm-1 employing a Perkin-Elmer spectrometer by KBr pellet method to study the metal complex coordination. Linear optical properties of the crystals were studied using a Shimadzu UV-visible spectrophotometer. Thermogravimetric (TG) and differential thermal analysis (DTA) for the BTBC crystal were carried out using a ZETZSCHGeratebau GmbH Thermal analyzer. The Vickers’s hardness measurement was carried out on the grown crystals to assess the mechanical property. 5. Results and Discussion Powder X-ray diffraction studies confirm the hexagonal structure of the grown crystal. Figure 5 shows the powder X-ray diffraction (XRD) pattern of BTBC. The experimental values were found to agree well with the calculated d values. The unit cell dimensions determined from single-crystal X-ray diffraction analysis are shown in Table 1. Optical transmission spectra were recorded using a 2-mm-thick c-cut crystal plate. The recorded transmission spectrum is shown in Figure 6. From the spectra, it is evident that BTBC crystal has UV cut off below 250 nm, which is sufficient for SHG laser radiation of 1064 nm or other

Figure 7. FTIR spectrum of BTBC crystal.

application in the blue region. Kurtz and Perry powder technique remains an extremely valuable tool for initial screening of materials for second harmonic generation. The fundamental beam 1064 nm from Q-switched Nd:YAG laser (Pro Lab 170 Quanta ray) was used to test the second harmonic generation (SHG) property of the BTBC crystal by the Kurtz powder technique.12 Pulse energy of 4 mJ/pulse and pulse width of 10 ns with a repetition rate of 10 Hz was used. The fundamental beam was filtered by using IR filter. A photo multiplier tube (Philips Photonics) was used as a detector. A KDP sample was used as the reference material, and the output power intensity of BTBC was observed. A second harmonic signal of 562 mV was obtained from BTBC with reference to 275 mV of KDP. Thus, the SHG efficiency of BTBC is roughly two times that of KDP. The Fourier transform infrared (FTIR) spectrum of BTBC is shown in Figure 7. The assignment of various internal modes observed in these FTIR spectra is given in the Table 2. The assignments correspond to various internal vibrations of thiourea. Crystal structure investigations of thiourea have established the coplanarity structure of C, N, and S atoms in the molecule.13 In BTBC complex, there are two possibilities by which the coordination of bismuth with thiourea can occur. It may occur

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either through nitrogen or sulfur of thiourea. The bands observed in the 3000-3500 cm-1 region in the FTIR spectra, at 3028, 3101.3, 3193.9, and 3436.9 cm-1 are characteristics of NH2 asymmetric and symmetric stretching vibrations and are in agreement with other compounds containing thiourea molecule CS(NH2)2.14-17 The strong band observed at 1624 cm-1 in the BTBC complex corresponds to a band at 1628 cm-1 of thiourea, which is attributed to the NH2 deformation vibration similar to those of some metal thiourea complexes.18 The band at 1477 cm-1 assigned to the N-C-N stretching vibration in thiourea is shifted to 1504.4 cm-1 in the BTBC complex. This shift may be due to the greater double bond character of the carbon-tonitrogen bond on complex formation. The bands observed at 1384 and 1400 cm-1 in the complex correspond to the 1414 cm-1 band of thiourea due to the NH2, N-C-N, and CdS stretching vibrations. The sharp and intense band absorption of BTBC at 702 cm-1 corresponds to 730 cm-1 absorption of thiourea. The vibrations of the C-N bond are expected in 460 to 730 cm-1 range. The three bands at 474 cm-1, 511, and 555 cm-1 are assigned to C-N deformation vibration. The shift in the frequency band at low-frequency region also indicates the presence of a sulfur-to-metal bond in BTBC. One of the most important considerations in the choice of a material for NLO applications is its optical damage tolerance. Material damage is of great importance to the design and successful operation of nonlinear devices. Because of the high optical intensities involved in nonlinear processes, the nonlinear materials must be able to withstand high power intensities.19 In the present study, an actively Q-switched diode array side pumped Nd:YAG laser was used for the laser-induced damage threshold studies on the BTBC crystal. The pulse width and the repetition rate of the laser pulses were 65 ns and 10 kHz, respectively, at 1064 nm radiation. For this measurement, 1.64 mm diameter beam was focused onto the sample with a 10-cm focal length lens. The beam spot size on the sample was 0.51 mm. The damage threshold was found to be 25 MW/cm2 is lesser than that of KDP (0.2 GW/cm2), urea (1.5 GW/cm2), BBO (5 GW/cm2), BGHC (9.8 GW/cm2), BTCC (32 GW/cm2), and LAP (10 GW/cm2).20 Refractive indices were measured along the crystallographic axis c (ne ) 1.842) and the direction perpendicular to it (nw ) 1.7110) by the Brewster’s angle method using light of wavelength 5893 Å. Refractive indices bear definite relations to the crystallographic symmetry of a crystal and the magnitude of the indices of refraction are closely related to its structure.21 BTBC is found to be a uniaxial and optically positive crystal. The mineral calcite, also know as Iceland spar is a widely used material in optics because of its birefringence. Its birefringence is so large that a calcite crystal placed over a dot on a page will reveal two distinct images of the dot. The birefringence value of calcite is 0.172.22 The channel spectrum method was used for the measurement of birefringence. A 500 W halogen light source served as the source. The light beam was allowed to pass through a monochromator. Green light of 550 nm was allowed to travel through [010] of the crystal. The birefringence was calculated using the reaction ∆n ) kλ/d where ∆n is the birefringence value, k is the order of the interference maximum, λ is the light wavelength, and d is the sample thickness along the light propagation direction;23 the birefringence value is found to be 0.103 at a wavelength of 550 nm for a sample thickness of 0.11 mm. Melting point of the grown crystals was measured using a melting point apparatus and found to be 215 ( 1 °C. The density F was calculated as 2.398 g/mL from the crystallographic data using the formula F ) MZ/NV, where M is the molecular weight,

Sankar et al.

Figure 8. TG-DTA analysis curve of BTBC crystal.

Z is the number of molecules per unit cell, N is Avogadro’s number, and V is the volume of the unit cell. Experimentally Sink or Swim method24 was employed, and a density value of 2.402 g/mL was determined. The value determined by theoretical and experimental methods agree very well. TGA and DTA are very important for a NLO material to throw light on the thermal stability of the substance. The TG-DTA curves for BTBC were recorded for the range of temperature from 30 to 1300 °C. A ceramic crucible was used for heating, and analyses were performed in an atmosphere of nitrogen at a heating rate of 20 °C min-1. The initial mass of the material subjected to analysis was 49.800 mg. Four weight loss steps were observed from the TGA curve. The rate of weight losses estimated on successive stages, namely, the first, second, third, and fourth are 28.77, 14.9, 32.45, and 0.82%, respectively. Decomposition of carbonyl sulfide and cyanamide (NH2CN) resulted in the first stage of weight loss. The second weight loss is attributed to isothiocyanic acid, which becomes the main product accompanied by CS2, ammonia (NH3). The reason for the third stage of weight loss is the formation of sulfur dioxide as an air oxidation product of bismuth sulfide. During the fourth stage of weight loss, gaseous oxidation products of the organic residues such as CO2 are released in the temperature range of 535-593 °C.25,26 The DTA curve (Figure 8) shows an endothermic reaction between 200 and 220 °C (peak temperatures of 212 and 215 °C are marked) due to the melting point of the material. The decomposition starts right after endothermic melting at 215 °C, while at 593 °C, explicit exothermic heat effects are observed due to the evolution of organic residues, such as CO2 and NH2CN. Mechanical properties of BTBC crystal were studied by making indentations on the (100) plane to evaluate the Vicker’s hardness number. The distance between any two indentations was maintained to be greater than five times that of the diagonal length to avoid any mutual influence of the indentations. The diagonal length of the indentations was measured using a micrometer eye piece. The Vicker’s hardness number was calculated using the expression Hv ) 1.8544(P/d2) kg/mm2 where Hv is the Vicker’s hardness number in kg/mm2, P is the applied load in kg, and d is the average diagonal length of the indentation in mm. The Vicker’s hardness value was measured to be 95 kg/mm2. Figure 9 shows load vs Vicker’s hardness number for the BTBC crystal. At a lower load, there is an increase in the hardness with the load, which can be attributed to the work hardening of the surface layers. Beyond the load of

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Crystal Growth & Design, Vol. 7, No. 3, 2007 505

Delhi, for the award of National Doctoral Fellowship (NDF). The author is thankful to Prof. P. K. Das, Dept. of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore for providing laser facilities. References

Figure 9. Load (P) vs hardness number (Hv) measured on BTBC crystal along the (100) plane.

50 g, significant cracking occurs, which may be due to the release of internal stress generated by indentation. From these studies, it is inferred that the BTAB single crystal is a good engineering material for device fabrication when compared to other NLO crystals such as ATCC (59.44 kg/mm2),27 urea (6.511 kg/mm2), and N-methyl urea (12-19 kg/mm2).28 The better mechanical properties of BTBC may be attributed to the strong bonding between thiourea molecules and antimony ions. 5. Conclusions Single crystals of BTBC, a potential semi-organic NLO material, have been grown from aqueous solution. Bulk crystals were grown by the slow-cooling method. Lattice parameters have been evaluated by single-crystal XRD analysis. Morphology studies reveal that the crystal is elongated and grows faster along the crystallographic c-axis. The damage threshold was found to be 25 MW/cm2. Refractive indices were measured along the crystallographic axis c (ne ) 1.842) and the direction perpendicular to it (nw ) 1.7110) by the Brewster’s angle method using a light of wavelength 5893 Å. The birefringence value is found to be 0.103 at the wavelength of 550 nm for a sample thickness of 0.11 mm. BTBC is found to be a uniaxial and optically positive crystal. The damage threshold was found to be 25 GW/cm2 is higher than that of KDP (0.2 GW/cm2), urea (1.5 GW/cm2), BBO (5 GW/cm2) The hardness value of the BTBC crystals is measured to be higher than LAP. The melting point of BTBC is 215 °C, which is higher than that of LAP crystal (140.8 °C). With promising structural, optical, and mechanical properties, BTBC is a potential material for frequency conversion applications. Acknowledgment. One of the authors (R.S.) acknowledges the All India Council for Technical Education (AICTE), New

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