# Growth and Characterization of a Nonlinear Optical Crystal a Complex Orthonitroaniline with Picric Acid Single Crystal by Vertical Bridgman Technique

S Noormohammad Shareef, K Chidambaram, S Kalainathan

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S Noormohammad Shareef, K Chidambaram, S Kalainathan. Growth and Characterization of a Nonlinear Optical Crystal a Complex Orthonitroaniline with Picric Acid Single Crystal by Vertical Bridgman Technique. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.39.72.787. hal-01502481

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# Growth and Characterization of a Nonlinear Optical Crystal a Complex Orthonitroaniline with Picric Acid Single Crystal by Vertical Bridgman Technique

S. Noormohammad Shareef¹, K Chidambaram², S. Kalainathan¹, a

1. Centre for Crystal Growth, VIT University, Vellore 632 014, Tamil Nadu, India

2. Sensor Laboratory, Department of Physics, SAS, VIT University Vellore - 632 014, Tamil Nadu, India.

a. kalainathan@yahoo.com

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.39.72.787 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.39.72.787)

**Keywords:** Vertical Bridgman Technique, organic compounds, NLO materials characterization.

**ABSTRACT.** Single crystal of organic NLO material Orthonitroaniline with picric acid (2[C<sub>6</sub>H<sub>6</sub>N<sub>2</sub>O<sub>2</sub>]-C<sub>6</sub>H<sub>2</sub>(NO<sub>2</sub>)<sub>3</sub>OH) (ONAP) of dimensions 27mm length and 12mm of diameter was grown from melt using vertical Bridgman technique. The grown crystal was confirmed with monoclinic structure by single crystal X-ray diffraction studies. Fourier transform infrared spectroscopy was used to assign various modes and to identify the functional groups present in the crystal. The presence of title compound was confirmed by NMR studies. The suitability for laser applications for the reported crystal was confirmed with UV-Vis studies.

**Introduction.** Organic crystals have been recognized as the materials of the future, due to their molecular nature, added with the versatility of synthetic chemistry helpful in changing and control their molecular structure, leading to the betterment of NLO properties [1]. The second order nonlinear optical material properties of the molecules are highly studied for potential applications, such as optical signal processing and optoelectronics [2]. Some crystals are highly polar due to their non-centro symmetric crystal nature, photonic crystals having band gap in the visible range are not noticed and is linked with some properties like refractive index, symmetry of the crystal and lattice spacing [3]. The grown crystal ONAP is easily polarisable based on the intermolecular properties. One of the important reasons for the selection of present compound is its inter molecular interaction between the pairs of orthonitroaniline and that of picric acid is well established that picric acid forms crystalline picrates with many organic molecules [3].and also a bulk size crystal is aimed to grow in this technique. The present crystal's melting point was reported to be 80 °C [3] and it was found to be monoclinic crystal system having space group Cc [4]. For few organic materials solution growth methods are not good because of solvent or compound associations forming during the crystal growth and solvent inclusions can reduce the optical quality. Melt growth techniques are rapid and suitable for growing organic crystals [5]. The vertical Bridgman technique is simple and is the best technique for the growth of good organic and inorganic crystals in a limited period. The single wall ampoule with conical tip is very useful to grow good quality transparent crystals. In the present work Orthonitroaniline with picric acid (2[C<sub>6</sub>H<sub>6</sub>N<sub>2</sub>O<sub>2</sub>]-C<sub>6</sub>H<sub>2</sub>(NO<sub>2</sub>)<sub>3</sub>OH) (ONAP) single crystal was grown using the single wall ampoule by the vertical Bridgman technique.

**Experimental procedure.** High purity orthonitroaniline (99%) and picric acid (99%) were weighed according to the stoichiometry of 2:1 [3]. and loaded in a borosil glass ampule. The cone length and angle of the cone are 25mm, 12° respectively and ampoule of 1 mm thick was chosen for ONAP
single crystal growth. Most of the cases ampoules which are conical in shape produce good quality crystals; The conical tip of the ampoule is the place of initiation of the solidification and controls the growth. The crystal quality increases if the cone length is more, and cone angle was found to be the most optimum to obtain inclusion-free good quality transparent crystals [5]. Before filling the chemicals in the ampoule, it was cleaned and dried well in order to prevent the generation of defects due to the presence of impurity. The ampoule then was evacuated to 10<sup>-6</sup>Pa and was placed in a two zone resistance furnace after sealing it, in a favourable melting point inside the furnace which was noted from the profile as shown in the Fig (1). For growing different single crystals distribution of the temperature inside the furnace and its gradient are much important in this technique [6]. A two zone VBT technique was optimized and a temperature gradient of 2°C/cm was maintained. In this VBT set up contains a furnace, with a temperature controller with a variation of 0.1 °C, a translation assembly and a super kandal wire which was used as a heating element. The ampoule was kept at a height of 22cm in the two zone furnace and was slowly translated inside the furnace in order to achieve crystallization. The materials that have lower thermal conductivity take more time to solidify from the melt. For this reason, organic crystals are grown at a slower rate. The growth rates of organic materials should not exceed 1mm/h, while growth rates for metals and inorganic substances may reach 20mm/h [7]. so 0.5mm/hr translation was given, the crystallization of the molten ONAP was initiated by self-nucleation and crystal growth started and continued through the length of the melt. The heating profile was reduced slowly to room temperature at a rate of 4°C/h in order to avoid cracks due to the difference in the thermal expansion coefficient between the glass and the crystal, after a week the run was completed and the crystal was removed using a diamond wheel cutter. Finally a red coloured transparent crystal of 27mm length and 12 mm of diameter was obtained. As grown crystal and cut and polished crystals are shown in the Fig(2).

<figure><img src="image_3.png" /><figcaption>Fig. 1. Temperature profile of two zone vertical bridgman furnace, Fig. 2. photograph of as-grown crystal by vertical Bridgman method inset shows cut and polished Orthonitroaniline crystal.</figcaption></figure>

## Results and Discussions

**Single crystal XRD analysis.** XRD data was retrieved from single crystal X-ray diffractometer (An Envarf Nonius CAD4) for grown ONAP single crystal using MoKα as a source with wave length of 0.717073 Å radiation to identify the structure and lattice parameter values. The grown crystal was found to be monoclinic crystal system with the space group Cc. The observed and reported cell parameter values are tabulated in table 1 the inferred cell parameters values are in good agreement with the reported values [3].
<table><caption>Table 1. Unit cell parameters of ONAP crystal.</caption><thead><tr><td>Parameter</td><td>Present value (VBT)</td><td>Reported value (SSET)[3]</td></tr></thead><tbody><tr><td>a(Å)</td><td>10.4 Å</td><td>10.366 Å</td></tr><tr><td>b(Å)</td><td>15.29 Å</td><td>15.139Å</td></tr><tr><td>c(Å)</td><td>14.97 Å</td><td>14.091 Å</td></tr><tr><td>β(Å)</td><td>114.7 °</td><td>106.76°</td></tr><tr><td>V(Å³)</td><td>2180 Å³</td><td>-</td></tr></tbody></table>

**FT-IR spectrum.** The recorded IR radiation for ONAP crystal is shown below Fig 3. In the frequency region the peaks 3580 cm⁻¹ was attributed due to the O-H stretching vibration of picric acid. The below assigned peaks of various functional groups are shown in Table 2 confirm the formation of the ONAP.

<figure><img src="image_4.png" /><figcaption>Fig. 3. FTIR spectrum of Orthonitroaniline single crystal.</figcaption></figure>

<table><caption>Table 2. Observed spectral data of ONAP.</caption><thead><tr><td>S. No</td><td>FT-IR</td><td>Assignments</td><td>S.No</td><td>FT-IR</td><td>Assignments</td></tr></thead><tbody><tr><td>1</td><td>748.38</td><td>C-H bending</td><td>5</td><td>1622.13</td><td>NH bending</td></tr><tr><td>2</td><td>918.12</td><td>C-H bending</td><td>6</td><td>3093.82</td><td>CH symmetric stretching</td></tr><tr><td>3</td><td>1247.94</td><td>Phenolic CO stretching</td><td>7</td><td>3487.30</td><td>NH₂ vibration</td></tr><tr><td>4</td><td>1344.38</td><td>NO₂ symmetric stretching</td><td></td><td></td><td></td></tr></tbody></table>
**NMR Studies.**¹H nuclear magnetic resonance spectrum of ONAP was recorded by using Bruker 400MHz spectrometer with tetra methyl silane as internal standard. ¹H NMR spectrum was taken by dissolving the material in CDCl₃ solvent and is shown in Fig. 4. A peak observed at 7.26ppm was mainly due to the CDCl₃ solvent and a triplet peak found at 6.67ppm was attributed to the hydrogen atom present in the third position of the title compound. The doublet peak found at 8.09 and 8.07ppm ascribed to hydrogen atom present in second position of the compound. The triplet peak at 7.32, 7.34 and 7.36 ppm was corresponds to the hydrogen atom present in the fourth position of the title material. The doublet peak observed at 6.71 and 6.81ppm shows the presence of hydrogen atom in the fifth position of the ring. The resonance singlet at 9.175ppm was due to the hydrogen atom present in the picric acid. Thus, the above observations indicates the presence of title compound.

<figure><img src="image_5.png" /><figcaption>Fig. 4. NMR spectrum of ONAP.</figcaption></figure>

**UV -Vis-NIR studies.** Transmission range, low cut of wavelength and absorption band of a crystal are important parameters for a crystal to use it in laser frequency conversion applications [8]. The UV-vis spectrum also gives the information about structure of a molecule [9,10]. The uv visible spectral analysis was carried out between 400 and 1100 nm using Perkin-Elmer Lambda-35 spectrophotometer for 1.9 mm thick ONAP crystal which is shown in the Fig .5. The ONAP crystal has lower cut off wavelength at 508 nm, the crystal is found to be transparent in the 508-1100 frequency range. The peak below 400 nm is due to the presence of π-π* transitions. The optical band gap of ONAP crystal was found to be 2eV calculated from Tauc's plot. The lower band gap of the titled compound (ONAP)can be used for visible light responsive devices and ambipolar organic thin films, transistors [10].
<figure><img src="image_6.png" /><figcaption>Fig. 5. (a) UV-Vis spectrum of Orthonitroaniline (b) optical band gap spectrum of Orthonitroaniline.</figcaption></figure>

**Laser damage threshold.** LDT for crystals is important as it constrains on the crystals applications. The exposure of high intense light on the crystals plays an vital role in NLO devices, electro optic modulators and frequency production of third order generations which mainly involves the capacity to withstand high intense laser light [11]. If LDT value is low the crystal will not have much applications even for good SHG crystals[12]. The laser damage threshold (LDT) of NLO components such as frequency doublers parametric oscillators, pockels cells depends on physical and chemical imperfections particularly on growth dislocations and LDT is influenced by the dislocations and the crystal with many dislocations presented low damage threshold[13]. The laser damage threshold study was performed on ONAP crystal using Q-switched Nd:YAG laser of 1064 nm as a fundamental wavelength with a pulse duration of 10 ns. A small dot was visible on the sample when the energy was 129mJ and a crack appeared on the crystal surface. To calculate each laser pulse's energy a combined phototube oscilloscope was used. The power density for ONAP was also calculated. The LDT value of ONAP was found to be 2.57 GW/cm² which is much higher than that of KDP(0.20 GW/cm²), LiNbO3(0.3 GW/cm²), KNbO3 (1GW/cm²) [14].

**Summary.** ONAP single crystal in bulk size was grown for the first time by vertical Bridgman technique. The unit cell parameters are matched with the reported values. The fundamental groups were found from FTIR studies. The band gap and absorption coefficient were determined from UV-Vis spectrum and the material is suitable for laser applications. The NMR study report confirms the presence of titled compound LDT value for the grown crystal was calculated and the crystal can be used in laser device instruments.

**Acknowledgement**

The authors gratefully acknowledge VIT management for providing facility and financial support.

**References**

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Cite the paper
S. Noormohammad Shareef, K Chidambaram, S. Kalainathan, (2017). Growth and Characterization of a Nonlinear Optical Crystal a Complex Orthonitroaniline with Picric Acid Single Crystal by Vertical Bridgman Technique. Mechanics, Materials Science & Engineering, Vol 9. doi 10.2412/mmse.39.72.787