# Crystal Growth, Optical, Dielectric, Mechanical and Second Harmonic Generation Characterization of 2,5-Dimethylanilinium Dihydrogen Phosphate Single Crystal

A Mani, K Rajesh, P Praveen Kumar

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A Mani, K Rajesh, P Praveen Kumar. Crystal Growth, Optical, Dielectric, Mechanical and Second Harmonic Generation Characterization of 2,5-Dimethylanilinium Dihydrogen Phosphate Single Crystal Mechanics, Materials Science & Engineering Journal, 2017, 9. hal-01503637

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# Crystal Growth, Optical, Dielectric, Mechanical and Second Harmonic Generation Characterization of 2,5-Dimethylanilinium Dihydrogen Phosphate Single Crystal

A. Mani¹,², K. Rajesh³, P. Praveen Kumar¹,⁴

1. Department of Physics, Presidency College, Chennai, India

2. Department of Physics, Sri Venkateswaraa College of Technology, Sriperumbudur, India

3. Department of Physics, AMET University, Chennai, India

a. ppkpresidency@gmail.com

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

DOI 10.2412/mmse.95.39.669 provided by Seo4U.link

**Keywords:** crystal growth, X-ray diffraction, optical properties, mechanical properties, NLO crystals.

**ABSTRACT.** Single crystals of 2,5-dimethylanilinium dihydrogen phosphate (2,5-DADP) were grown by slow evaporation solution growth technique in room temperature. The crystalline nature of the grown crystal was confirmed from the single crystal X-ray data. The grown crystal 2,5-DADP was found to crystallize in orthorhombic system with non-centrosymmetric space group P2₁2₁2₁. Optical transparency of the grown crystal was studied by UV-Vis-NIR spectroscopy. The dielectric loss and dielectric constant measurement as a frequency and temperature were measured on 2,5-DADP single crystal. Microhardness measurements revealed that 2,5-DADP belongs to a soft material category. The second harmonic generation of the crystal was confirmed and the efficiency was measured using Kurtz Perry powder method.

**Introduction.** Nonlinear optical (NLO) materials are a new frontier of science and development for optoelectronics due to their potential applications such as optical computing, 3D optical data storage, color displays, optical power limiting, optical communications, laser fusion, etc. [1]. Inorganic nonlinear optical single crystals have usually high melting point, high mechanical strength, and high degree of chemical inertness, but very poor second and third harmonic generation efficiencies. The nonlinearity of inorganic materials is low compared to organic NLO crystals [2]. In contrast, organic crystals exhibit large NLO coefficients and synthetic flexibility but they have very poor transparency, short optical band gap, laser damage threshold, thermaland mechanical properties [3]. The search for new NLO materials with improved stability (thermal, mechanical and chemical) and a wide transparency window has resulted in the development of the new class of materials called semi-organics. Materials with combined described charteristicnamed semi-organic, can be grown easily by solution growth technique [4].

A number of phosphate acid complexes have been studied as promising materials for second harmonic generation [5-7]. 2,5-Dimethylanilinium dihydrogen phosphate (2,5-DADP) belongs to this large semi-organic NLO family. The crystal structure of this compound was elucidated by K.Kaabi et al[8]. S. Guidara et al [9] have reported the DFT calculation and structural parameter calculations of the title compound. Since few properties of 2,5-DADP crystals have been reported earlier, in the present work a systematic study has been carry outvarious properties of the material. In the present investigation, attempts were made to grow good quality single crystals of 2,5-DADP by the slow evaporation solution growth method. The crystals were characterized using single crystal XRD, UV-
Vis-NIR, dielectric, mechanical studies were carried out. The second harmonic generation property of the crystal was tested.

**Crystal growth.** Single crystals of 2,5-dimethylanilinium dihydrogen phosphate (2,5-DADP) were grown by the slow evaporation solution growth technique. The title compound was prepared by slow addition of phosphoric acid to an ethanolic solution of 2,5-dimethylaniline in a 1:1 molar ratio. The chemical reaction is

$$2,5-(CH_3)_2C_6H_3NH_2 + H_3PO_4 \rightarrow [2,5-(CH_3)_2C_6H_3NH_3]H_2PO_4$$

The solution was stirred continuously using a magnetic stirrer. A crystalline precipitate was formed. After adding distilled water, the resulting solution was mixed well using a magnetic stirrer to ensure homogeneous concentration in the entire volume of the solution. The prepared solution was filtered and kept undisturbed at room temperature. The slow evaporation of solvent during 20 days leads to the formation of colorless and transparent prismatic crystals of 2,5-DADP. Repeated recrystallization yielded to good quality crystal as shown in Fig. 1b.

**Single crystal X-ray diffraction.** Single crystal X-ray diffraction (XRD) analysis on 2,5-DADP crystal was carried out using EnrafNonius CAD4-MV31diffractometer with MoKα (λ = 0.71073 Å) radiation. The X-ray diffraction study confirmed that the crystal belongs to orthorhombic system with the non-centrosymmetric space group P212121. The lattice parameters of 2,5-DADP crystal were measured as a = 5.826 Å, b = 21.063 Å, c = 8.467 Å, α = β = γ = 90°, and the cell volume V = 1039.011 Å³. These values agree well with the reported value [8].

**UV-vis-NIR spectral study.** The optical transmittance study is used to identify the optical transmission range and cut-off wavelength of crystals because a nonlinear optical crystal can be of practical use if it has wide transparency window. The transmission spectra were recorded using Perkin Elmer lambda 35 UV-vis-NIR spectrometer in the spectral region 190-110 nm with spectral resolution 2 nm as shown in Fig. 1a. The optical cut-off wavelength of the 2,5-DADP crystal was found to be 325 nm, and the crystals had good optical transmittance between 325 and 1100 nm (Vis-NIR region).

<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) Optical transmittance spectrum (b) Photograph of as-grown 2,5-DADP single crystal (c) a plot of $(\alpha h\nu)^2$ against photon energy.</figcaption></figure>

The optical absorption coefficient (α) was determined using the relation,
$$ \alpha = \frac{2.303 \log\left(\frac{1}{T}\right)}{t} \qquad (1) $$

where T is the transmittance and t is the thickness of the crystal.

As a direct band gap ($E_g$) has been determined from the transmission spectra, the optical absorption coefficient of the crystal (α) near the absorption edge is given by the following relation:

$$ \alpha h\nu = A (h\nu - E_g)^{1/2} \qquad (2) $$

where A is an optical transition dependent constant, $E_g$ is the optical band gap of the crystal, h is the Planck's constant, and v is the frequency of incident photons.

The band gap of the 2,5-DADP crystal was estimated by plotting $(\alpha h\nu)^2$ against the photon energy $(h\nu)$ (Tauc's plot) as shown in Fig. 1c. The band-gap energy ($E_g$) of the grown crystal was estimated by exploring a straight line in the linear region near the onset of the absorption edge to the energy axis $(\alpha h\nu)^2= 0$. From Fig. 1c, the estimated band-gap energy was found to be 3.79 eV. The wide band gap of the 2,5-DADP crystal confirms the large transmittance window in the visible region.

**Dielectric studies.** Dielectric behavior was studied using a HIOKI 3532-50 LCR HITESTER. The precisely cut and polished crystal was mounted in the sample holder. Measurements were made in the frequency range of 100 Hz–5 MHz at three different temperatures.

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) Variation of dielectric constant with log frequency (b). Variation of dielectric loss with log frequency at different temperatures.</figcaption></figure>

Dielectric properties of 2,5-DADP (dielectric constant and dielectric loss) were recorded with respect to frequency and temperature (Figs. 2a and 2b). It is noticed that both dielectric constant and dielectric loss show similar behavior with respect to frequency. Values of dielectric constant and dielectric loss are high at low frequency, and decreases with increase in frequency. Further increase in frequency does not have any impact on the dielectric constant and dielectric loss and the values remain constant. Though, all types of polarization are active at low frequency region, high value of dielectric constant is mainly attributed to the space charge polarization. The large value of dielectric constant at low frequency is due to the presence of space charge polarization. The low values of both dielectric constant and dielectric loss at high frequencies confirm that the optical quality of 2,5-DADP is quite
good and thus suitable for NLO application. The unique behavior of 2,5-DADP mentioned earlier also points towards its suitability for applications like insulation for wires and cables, ferroelectric, photonic and electro-optic, sensor devices and capacitors.

**Vickers Microhardness Test.** Analysis of mechanical property of the grown crystal is also important for the fabrication of electronic and optical devices. Microharness studies have been carried out on a selected well transparent single crystal using microharness tester, fitted with a Vickers diamond pyramidal indenter. The indentations were made on the 2,5-DADP crystals with applied load ranging from 5 g to 50 g. The time of indentation was kept constant for 5 s. The values of Vickers microhardness at different loads were calculated using the relation,

$$H_v = \frac{1.8544 \times P}{d^2} (Kg/mm^2) \quad (3)$$

where *P* is the applied load and *d* is the mean diagonal length of the indenter impression.

Fig. 3a shows the variation of hardness with the applied load. It is observed that the hardness of 2,5-DADP increases by increasing load up to 50 g which indicates the reverse indentation size effect. The cracks start to occur after the load 50 g.

<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) Plot of Hardness number vs. Load P. (b) Plot of log d vs log P of 2,5-DADP crystal.</figcaption></figure>

This may be due to the release of internal stress generated locally by indentation. The work hardening coefficient (n) of the material was calculated using the relation $P = kd^n$, where k is the arbitrary constant of a given material, and n is the work hardening coefficient. Onitsch [10] and Hanneman have pointed out that n lies between 1 and 1.6 for moderately hard materials and more than 1.6 for soft category materials, where n is Meyer's index. According to Meyer's law, the relation between load and the size of indentation can be correlated. The graph plotted between log P vs log d (Fig. 3b) gives the work hardening coefficient (n), and n is found to be 2.87 followed by the least-squares fitting method.

**NLO studies.** The NLO property of the crystal was confirmed by Kurtz powder technique. The determination of SHG intensity of the crystals using powder technique was developed by Kurtz and Perry [11]. The crystals are ground to powder and packed between two transparent glass slides. The first harmonic output of 1064 nm from a Nd:YAG laser was made to fall normally on the prepared sample with a pulse width of 8 ns. The power of incident laser beam was measured as 4.3 mJ/pulse. The output radiation from the crystal was allowed to fall on a photomultiplier tube which converts the light signal into electrical signal. The second harmonic signal generated in the crystal was
confirmed from the emission of green radiation by the sample. It is found that the SHG efficiency of the crystal is 1.4 times higher than that of KDP.

**Summary.** Optical defect free crystal of 2,5-Dimethylanilinium dihydrogen phosphate (2,5-DADP) was grown by slow evaporation solution growth method. The crystal structure was confirmed by single crystal XRD analysis. The optical studies reveal that the grown crystals have a cut-off wavelength of 325 nm and the band gap is found to be 3.79 eV. Frequency and temperature dependent dielectric studies showed that 2,5-DADP crystal exhibit normal dielectric behavior and established the aptness for NLO applications. Vickers microhardness study indicates the mechanical stability of the crystal. SHG efficiency of the grown crystal is 1.4 times that of KDP.

**Reference**

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Cite the paper

A. Mani, K. Rajesh, P. Praveen Kumar (2017). *Crystal Growth, Optical, Dielectric, Mechanical and Second Harmonic Generation Characterization of 2,5-Dimethylanilinium Dihydrogen Phosphate Single Crystal*. Mechanics, Materials Science & Engineering, Vol 9. DOI 10.2412/mmse.95.39.669