# Growth, Nonlinear, Dielectric Studies on Urea Phosphoric Acid (UP) Single Crystals

N Hema, R Usha, D Shalini, V Revathi Ambika, D Jayalakshmi

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N Hema, R Usha, D Shalini, V Revathi Ambika, D Jayalakshmi. Growth, Nonlinear, Dielectric Studies on Urea Phosphoric Acid (UP) Single Crystals. Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, 10.2412/mmse.77.39.252. [hal-01503644](https://hal.science/hal-01503644)

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# Growth, Nonlinear, Dielectric Studies on Urea Phosphoric Acid (UP) Single Crystals

N. Hema¹, R. Usha¹, D. Shalini¹, V. Revathi Ambika¹, D. Jayalakshmi², a

1. Research Scholar, Dept Of Physics, Queen Mary's College, Chennai, India

2. Asst. Prof, PG & Research, Dept Of Physics, Queen Mary's College, Chennai, India

a. hemavprabu@gmail.com

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

DOI 10.2412/mmse.77.39.252 provided by [Seo4U.link](https://seo4U.link)

**Keywords:** X-ray diffraction, solubility, growth from solutions, organic compounds, nonlinear optical material, second harmonic generation.

**ABSTRACT.** An organic nonlinear optical Urea phosphoric acid (UP) single crystal has been grown by slow evaporation solution growth technique. The cell parameters of UP single crystal were confirmed by single X-ray diffraction analysis. The presence of functional groups was identified by FT-IR spectral analysis. The UV-Vis spectral studies showed that the UP single crystal has wide transmission window in the entire visible region. The dielectric property of UP crystal revealed the normal dielectric behaviour. Mechanical strength of UP crystal was studied. The second order nonlinear coefficient of the grown crystal was also measured for UP single crystal.

**Introduction.** In the recent scenario, an organic nonlinear optical material (NLO) is attracting a great deal of attention due to their potentially high nonlinearity and rapid response in electro-optic modulation, frequency mixing, second harmonic generation and optical parametric oscillation etc over the inorganic NLO material [1-3]. The search for nonlinear optical crystal is, in fact, the search for the “polar crystals” in which the macroscopic properties reflect the internal asymmetric relationship. The structural flexibility of organic chromophores easily modifiable through precise chemical synthesis in view to increase the molecular hyperpolarizability and the possible crafting of chirality centers are remarkable assets compared to the difficulties of the engineering route of inorganic materials, in which the requirements of non-centrosymmetry and high susceptibility have to be accounted [4,5]. Most of the organic nonlinear optical crystals which have low laser damage threshold values, less thermal stability etc., and hence these materials have been limited for device fabrication. In order to overcome these properties, a new class of organic nonlinear material crystal with high laser damage threshold value has been found. In the present investigation, growth, thermal, optical, laser damage threshold dielectric and microhardness have been carried out on the UP single crystal.

**Experimental.**

**Synthesis, solubility.** Commercially available Urea (Merck AR grade 99.5%) and Phosphoric acid (SRL AR grade 99.5%) were very well dissolved in de-ionized water which was thoroughly mixed using a magnetic stirrer and the mixture was heated at 45°C till a white crystalline salt of UP was obtained. The synthesized salt was further purified by repeated crystallization. The reaction scheme and their chemical structures are illustrated below.
$$CO (NH_2)_2 + H_3PO_4 \rightarrow (NH_2)_2 + H_3PO_4$$

The solubility of UP in water (Fig. 1).

<figure><img src="image_3.png" /><figcaption>Fig. 1. Solubility curve of UP.</figcaption></figure>

**Crystal growth.** The saturated solution was prepared in accordance with the solubility curve. Single crystals of UP were grown from their aqueous solution using the slow evaporation method. The prepared solution was kept in a constant temperature water bath of accuracy of ±0.01 °C. After a span of 19 days, a medium size single crystal with the dimensions of 14 x 5 x 4 mm³ was harvested. The photograph of the grown crystal of UP is shown in Fig 2.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Photograph of UP crystal.</figcaption></figure>

**Characterizations.** The grown UP crystals were subjected to X-ray diffraction studies using Bruker kappa APEXII single crystal X-ray diffractometer, using MoKα (λ = 0.71073 Å). The various functional groups of UP crystal were identified by the KBr pellet technique using a Perkin Elmer FTIR spectrometer in the range 4000–450 cm⁻¹. The transmission spectrum of the UP crystal was studied in the range 200–1100 nm by Perkin Elmer spectrometer. The dielectric measurement has been carried out using HIOKI 3532–50 LCR HITESTER instrument. Microhardness measurement of UP crystal was carried out using Leitz-Wetzlar Vickers' microhardness tester fitted with a diamond pyramidal indenter attached to an optical microscope. The Kurtz and Perry powder technique was employed on UP crystal to measure the second harmonic generation efficiency using Nd:YAG laser.

**Single X-ray diffraction analysis.** The cell parameters of the UP crystal were estimated by single crystal X-ray diffraction analysis and cell parameters are a = 17.3813 Å, b = 7.425 Å, c = 8.9323 Å, V= 1158 Å³, with angle β= α= γ = 90°. [6] The grown crystal belongs to orthorhombic crystal system
with space group Pbca. It is observed that lattice parameter values and cell volume of UP are in good agreement with the reported values [7].

**FTIR spectral studies.** The FTIR analysis of UP was carried out between 4000 and 450 cm⁻¹ using Perkin Elmer model spectrometer. The resulting spectrum is shown in figure. The peak at 3778 cm⁻¹ may be assigned to NH asymmetric stretching. The peaks at 3450 and 1662 cm⁻¹ are assigned to OH asymmetric stretching and C=O asymmetric stretching respectively. The peak at 1453 cm⁻¹ is assigned to P=O bending. The C-N asymmetric stretching appears at 1084 cm⁻¹. The peaks at 983,866, 584 and 529 cm⁻¹ were attributed to NCN symmetric stretching, P-O asymmetric stretching, P=O bending and P-O bending vibration respectively. The C-N rocking exhibit at 508 cm⁻¹. Figure 3 shows FTIR spectrum of UP crystal.

<figure><img src="image_5.png" /><figcaption>Fig. 3. FTIR spectrum of UP crystal.</figcaption></figure>

**UV-Vis spectral studies**

UV-Visible transmission spectrum of UP crystal was recorded in the range of 190-900 nm using 0.93 mm thickness crystal sample. UP crystal is active in the UV-Vis region. It has good transparency of about 55% with lower cut-off wavelength 278 nm (Fig. 4).

<figure><img src="image_6.png" /><figcaption>Fig. 4. UV-Vis transmission spectrum of UP crystal.</figcaption></figure>
**Dielectric studies.** The plates were polished and coated with an electronic grade silver paste, which acts as an electrode. Figure 5 shows the variation of dielectric constant ($ε_r$) with log frequency. From the plot, it is observed that the dielectric constant is relatively higher at 100 Hz and further decreases with increase in frequency. This effect can be attributed to the effect of charge distribution by mean carrier hopping on defects. At low frequency, the charge on the defects can be rapidly redistributed so that defects closer to the positive side of the applied field become negatively charged, while defects closer to the negative side of the applied field become positively charged. This leads to a screening of the field and overall reduction in the electric field. The high value of dielectric constant at low frequency can be attributed to the lower electrostatic binding strength, arising due to the space charge polarization near the grain boundary interfaces. In accordance with Miller rule, the lower value of dielectric constant at higher frequencies is a suitable parameter for the enhancement of SHG coefficient [8]. The characteristic of low dielectric loss (Fig. 6) with high frequency for the sample suggests that the crystal possess enhanced optical quality with lesser defects and this parameter play a vital role for the fabrication of nonlinear optical devices [9].

<figure><img src="image_7.png" /><figcaption>Fig. 5. Plot of frequency vs. dielectric constant of UP crystal.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Plot of frequency vs. dielectric loss of UP crystal.</figcaption></figure>
**SHG Measurements.** The Kurtz and Perry technique was used to find the nonlinear property of the crystal. Microcrystalline KDP material was used as a reference material with UP for SHG measurements. A high intensity Nd:YAG laser beam with input pulse of 6.2mJ as a optical source was allowed onto the powder samples. The second harmonic signal (532nm) 56mV and 78.4mV were obtained through KDP and UP samples. Thus the SHG efficiency of the UP crystal is 1.4 times higher than KDP crystal. This result confirms the non centrosymmetric structure and NLO behavior of the as grown UP crystal.

**Summary.** An organic Urea Phosphoric acid (UP) single crystal has been grown by slow evaporation solution growth technique. The cell parameters of UP single crystal were confirmed by single X-ray diffraction analysis. The presence of functional groups was identified by FT-IR spectral analysis. The UV-Vis spectral studies showed that the UP single crystal has wide transmission window in the entire visible region (300-900 nm). The dielectric property of UP crystal revealed the normal dielectric behaviour. The second order nonlinear coefficient of UP crystal was 1.4 times higher than KDP.

**Acknowledgment**

One of the authors (D. Jayalakshmi) is grateful to the University grants commission for financial support under minor research project scheme.

**References**

[1] J. Badan, R. Hierle, A. Perigand, J. Zyss, in: Williams (Ed.), "Nonlinear Optical Properties of Organic Molecules and Polymeric Materials", D.5. Am. Chem. So., Washington, DC, 233(1993).

[2] D.S. Chemla, J. Zyss, "Nonlinear optical properties of organic molecules and crystals", Vol.1, Academic Press, London, 1987.

[3] R.W. Munn, C.N. Ironside, "Principles and applications of nonlinear optical materials", Chapman and Hall, London, (1993)

[4] S.R.Marder, J.W.Perry, W.P.Schaefer, Science 245, 626 (1989)

[5] S.R.Marder, J.W.Perry, W.P.Schaefer, J.Chem.Phys. 2, 985 (1992).

[6] S.A.Martin Britto Dhas, M.Suresh, G.Bhagavannarayana, S.Natarajan, J.Crystal Growth. 309 (2007) 48-52.

[7] G. Smith, Urs D. Wermuth, Acta Cryst. C66 (2010) o5-o10.

[8] Miller C. Appl. Phys. Lett 1964;5: 17-19.

[9] Balarew C, Duhlew R. J. Solid State Chem 1984; 55:1-6.

Cite the paper

N. Hema, R. Usha, D. Shalini, V. Revathi Ambika, D. Jayalakshmi (2017) Growth, Nonlinear, Dielectric Studies on Urea Phosphoric Acid (UP) Single Crystals. Mechanics, Materials Science & Engineering, Vol 9.

Doi 10.2412/mmse.77.39.252