# Thermal and Dielectric Properties Of L-Malic Acid Doped KDP Single Crystals

A Venkatesan, S Arulmani, E Chinnasamy, S Senthil, M E Rajasaravanan

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A Venkatesan, S Arulmani, E Chinnasamy, S Senthil, M E Rajasaravanan. Thermal and Dielectric Properties Of L-Malic Acid Doped KDP Single Crystals. Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, 10.2412/mmse.24.24.878. hal-01503656

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Distributed under a Creative Commons Attribution 4.0 International License
# Thermal and Dielectric Properties Of L-Malic Acid Doped KDP Single Crystals

A. Venkatesan¹,², S. Arulmani³, E. Chinnasamy³, S. Senthil³, M.E. Rajasaravanan², a

1. Department of Physics, Arignar Anna Govt. Arts College, Villuppuram, India

2. Department of Physics, Government Arts College, Salem, Indira

3. Department of Physics, Govt. Arts College for Men (Autonomous), Nandanam, Chennai - 600 035.

a. merajasaravanan@gmail.com

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

DOI 10.2412/mmse.24.24.878 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.24.24.878)

**Keywords:** PXRD, FT-IR, FT-Raman, NLO studies, dielectric studies, UV-visible, TGA/DTA.

**ABSTRACT.** Potassium Dihydrogen Phosphate (KDP) is a popular nonlinear optical materials that is widely used in the field of nonlinear optics for the frequency conversion processes. Optically good quality L-Malic acid doped KDP (LMKDP) crystals have been grown by slow evaporation method at room temperature. The crystallinity of the LMKDP crystals has been studied by powder XRD analysis. The presence of the functional group for LMKDP crystals are qualitatively analyzed from FTIR and FT-RAMAN spectrum. The second harmonic generation (SHG) efficiency was measured by using Kurtz powder technique. The dielectric behavior of grown crystals has been studied in the frequency range from 50 Hz to 50MHz. UV-visible absorption spectrum was recorded to study the optical transparency of grown crystal. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were used to study the thermal properties of the grown crystal.

**Introduction.** In this modern era of information and technology with fast and high data storage capacity, data retrieving, processing and transmission demands the search of new NLO materials with unique physical properties. Hence, there is a great demand for synthesize the new NLO materials and grow their single crystals. KDP is among the most widely used NLO material. Potassium dihydrogen phosphate (KDP) crystals have created considerable interest because of its Piezo-electric, electro-optic, nonlinear optical properties and its extensive application in X-ray monochromators [1]. In addition to their large NLO response, the advantage of organic materials is that they offer high degree of synthetic flexibility to tailor their optical properties through the structural modification and they exhibit high laser damage threshold [2]. In single crystals form, the NLO materials display huge optical nonlinearity, which is of great interest for telecommunication, optical information processing and high optical data storage, etc., [3]. Many methods have been tried to improve the NLO properties of KDP crystal. With the aim of improving the second harmonic generation (SHG) efficiency of KDP, researchers have attempted to modify KDP crystals by doping different types of impurities. The present work, L-Malic acid of 0.2 molar percentages has been doped with KDP material. The grown L-Malic acid doped KDP (LMKDP) crystals were characterized by PXRD, FTIR, FT-RAMAN, NLO, Dielectric studies.

**Experimental Procedure.** In the present work, L-Malic acid of 0.2 molar percentages has been doped with KDP material. The solution was thoroughly stirred for homogenization and then filtered into a borosil beaker using whatmann filter paper. It was porously sealed and placed in a dust free
atmosphere for slow evaporation technique at room temperature. Single colorless and optically transparent crystals were harvested within 30 days.

## Results and Discussion

**Powder X-ray Diffraction Analysis.** Powder X-ray diffraction technique is a powerful tool to analyze the crystalline nature of materials. Powder X-ray diffraction analysis was carried out by using XPERTPRO X-Ray diffractometer with Cukα radiation (λ=1.5406 Å). The cell parameter values for LMKDP are a=b=0.634Å, c=0.524Å with the angles α = β = γ = 90°. The samples were scanned over the range 10 - 70°. The sharp and well defined peaks at specific 2θ values indicate the high crystalline nature of the crystal. Powder XRD graph has been shown in the Fig. 1.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Powder XRD pattern of LMKDP crystals.</figcaption></figure>

**FT-IR and FT-Raman Analysis.** FT-IR and FT-Raman spectra of LMKDP crystal were recorded and the spectrum of LMKDP crystals are shown in Fig.2 and Fig.3. An intense band of strong absorption around 1204.67cm⁻¹ and protonated by the carboxyl group (COOH) gives hydrogen bonding [4], which is attributed to O-H stretching vibration for the confirmative group of water on hydration. The functional groups of the LMKDP crystals have been identified by the spectrum. In the high frequency region of IR spectra, the sharp peaks observed at 540.13 cm⁻¹ are described to PO₄ stretching vibration in the crystal. The O-H deformation and P=O stretching will be observed in the frequency of 1350.67cm⁻¹. In FTIR the absorption band at 1350.67 cm⁻¹ is due to P-O-H bending vibration in the LMKDP. In Raman studies the sharp peak will be observed at 1053.52cm⁻¹ is described to P-O-H stretching molecule in the LMKDP crystals.

<figure><img src="image_4.png" /><figcaption>Fig. 2. FT-IR spectral analysis of LMKDP Crystal.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. FT-RAMAN analysis of LMKDP Crystal.</figcaption></figure>

**SHG Analysis.** Nonlinear optical measurements were carried out by using Kurtz powder technique. In order to confirmed the NLO property, the grown crystals were powdered and subjected to Kurtz and Perry powder technique, which is a powerful tool for initial screening of materials for SHG [5]. A Q-switched Nd:YAG laser beam of 1064 nm wavelength with 1.9 mJ/pulse input power, 8 ns pulse width and repetition rate 10Hz was used to estimate SHG efficiency of the grown LMKDP crystals. KDP crystalline powder was the reference material, the output of SHG range was compared and found that the SHG conversion efficiency of LMKDP is 1.4 times greater than that of reference KDP.

**Dielectric Studies.** Every material has a unique set of electrical characteristics that are dependent on its dielectric properties. The variation of dielectric constant and dielectric loss of the sample with frequency is studied at room temperature by H10K1 3532 LCR HITESTER in the frequency range 50 Hz to 50 MHz and is shown in Fig. 4 and Fig. 5. The high value of dielectric constant at high frequency may be due to the presence of all the four polarizations namely space charge, orientation, electronic and ionic. The dielectric characteristic of a crystal is determined by the dielectric constant and dielectric loss. The dielectric constant (ε') and dielectric loss (ε'') of the sample were calculated using the following equation [6]:

$$ \epsilon' = c*t/\epsilon_0 A \qquad (1) $$

$$ \epsilon'' = \epsilon'/Q \qquad (2) $$

<figure><img src="image_6.png" /><figcaption>Fig. 4. Dielectric constant Vs Log f of LMADP Crystal</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. Dielectric loss Vs Log f of LMADP Crystal.</figcaption></figure>

**Optical Absorption Studies.** The UV-Vis spectrum of LMKDP crystals were recorded by using Perkin Elmer UV-Vis spectrometer (Model: Lambda 35) in the wavelength range of 200-900 nm. Optical transparency range of single crystals plays a very important role in optical technology [7]. UV-Visible techniques are helpful in the investigation of NLO materials making it possible to check, both NLO responses and spectroscopic absorbance in the appropriate wavelength range [8]. Good optical transmittance and lower UV cutoff (219.5 nm) wavelengths are very important properties for the grown crystals [9]. This is one of the most desirable properties of the crystals for the device fabrication. The band gap of the crystal was evaluated by extrapolation of the linear part of the graph and found to be 5.122 eV.

<figure><img src="image_8.png" /><figcaption>Fig. 6. UV-vis absorption of LMADP Crystal.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. UV-vis band gap of LMADP Crystal.</figcaption></figure>
**Thermal Analysis.** The thermal stability of the crystal is a very important factor for potential application. The thermal stability of the title compound was identified by thermogravimetric (TGA) and differential thermal analysis (DTA) studies simultaneously. Thermogravimetric analysis shows that LMKDP is very stable up to 209.49°C and the weight loss starts at this temperature. It is observed from DTA thermogram that an endothermic event begins at 219.14°C and then sharp peak appears at 240.31°C and the second peak appears at 271.27°C. An endothermic event is observed at 240.31°C which is due to melting of the sample. Good crystalline nature of the title compound is clearly noticeable from the sharpness of the endothermic peak. The sharp endothermic peak 240.31°C is due to the decomposition of the sample desired by its DTA analysis and it coincides with the TG curve [10].

<figure><img src="image_10.png" /><figcaption>Fig. 8. TG-DTA spectrum of LMADP Crystal.</figcaption></figure>

**Summary.** The good-quality single crystals of LMKDP were successfully grown by slow evaporation method at room temperature. XRD studies reveal the tetragonal structure of the grown crystals. FT-IR spectrum confirm the presence of functional group in the grown crystal. The UV-Vis spectra showed that the crystals had a wide optical window, no absorbance and good optical transmittance in the entire visible region. The nonlinear optical studies confirm the SHG property of the grown crystal. The SHG efficiency of LMKDP is 1.4 times greater than that of reference KDP. The electrical property of the grown crystal was observed by the dielectric studies. TGA and DTA thermogram revealed the thermal stability of the materials.

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Cite the paper

A. Venkatesan, S. Arulmani, E. Chinnasamy, S. Senthil, M.E. Rajasaravanan (2017). *Thermal and Dielectric Properties of L-Malic Acid Doped KDP Single Crystals*. Mechanics, Materials Science & Engineering, Vol 9. DOI 10.2412/mmse.24.24.878