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Studies on the Effect of L-Alanine on the Structural, Optical and Thermal Properties of Potassium Acid Phthalate Crystals



Ferdousi Akhtar and Jiban Podder
 
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ABSTRACT

Potassium acid phthalate (KAP) crystals are promising materials as good monochromator for the qualitative and quantitative X-ray analysis of light elements like Fe, Al, Mg, F, Si, etc. in a long and middle range. KAP crystal exhibits a rare blend of ionic and molecular properties and has wide application by virtue of their polar nature. KAP is a non centrosymmetric molecular ionic crystal. L-alanine is an efficient organic non Linear Optical (NLO) compound under the amino acid group. L-alanine doped semi organic material like KAP may be the fundamental building block to develop many complex crystals with improved NLO properties. In this paper an attempt has been made to grow large size optically transparent L-alanine doped KAP crystals by slow evaporation solution growth technique and to see the effects of L-alanine into the pure KAP crystals. The chemical composition of the grown crystals was determined by Energy Dispersive X-ray (EDX) and Fourier Transform Infrared (FTIR) Spectroscopy. The structure of pure and L-alanine doped KAP crystals have been examined by powder X-ray diffraction (XRD) study. Optical properties of the grown crystals were studied using UV-visible specrtroscopy. Using Thermogravimetry (TG) and Differential Thermal Analysis (DTA), the decomposition temperature was obtained. Powder XRD study on grown crystals showed that they belong to an orthorhombic system. The transmission in the visible region and the thermal stability of the crystal was found to increase with the doping concentration of L-alanine into the KAP crystal. Optically transparent, large size and thermally stable L-alanine doped KAP crystals were grown successfully in a laboratory for useful application in opto-electronic devices.

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Ferdousi Akhtar and Jiban Podder, 2011. Studies on the Effect of L-Alanine on the Structural, Optical and Thermal Properties of Potassium Acid Phthalate Crystals. Journal of Applied Sciences, 11: 2974-2983.

DOI: 10.3923/jas.2011.2974.2983

URL: https://scialert.net/abstract/?doi=jas.2011.2974.2983
 
Received: April 18, 2011; Accepted: June 13, 2011; Published: August 06, 2011

INTRODUCTION

Potassium acid phthalate (KAP), K (C6 H4 COOH. COO) is well known material for its application in the production of crystal analyzers for long-wave X-ray spectrometers (Benedict et al., 2003; Krishnan et al., 2008a). KAP crystal possesses piezo-electric, pyro-electric, elastic and non-linear optical properties (Destro et al., 1988; Meera et al., 2004; Misoguti et al., 1996). It exhibits excellent cleavage and crystallized into the orthorhombic form with four molecules per unit cell and the unit cell parameters are; a = 6.320 Å, b = 12.343 Å, c = 5.784 Å, α = β = γ = 90°. Single crystals of KAP exhibit a rare blend of ionic and molecular properties and have wide potential application by virtue of their polar nature. The non-linear susceptibility of KAP originates from the contribution of polarizable aromatic rings of phthalate ions (Uthayarani et al., 2008).

Amino acid family crystals are playing an important role in the field of non-linear optical organic molecular crystal. Among them L-alanine (LA), with chemical formula (CH3 CH NH2 COOH) is the smallest, naturally occurring chiral amino acid with a non-reactive hydrophobic methyl group (-CH3) as a side chain. LA has the zwitterionic form (+NH3-C2H4-COO-) both in crystal and in aqueous solution over a wide range of pH, which favors crystal hardness for device application (Nicoud and Twieg, 1986).

Recently, several new complexes incorporating the amino acid L-alanine have been crystallized and their structural, optical and thermal properties have also been investigated by Kajzar et al. (1995), Kejalakshmy and Srinivasan (2003), Aggarwal et al. (2003) and Srinivasan (2004). The growth of pure L-alanine crystals was reported by Vijayan et al. (2006) and found higher damage threshold than Potassium dihydrogen phosphate (KDP). It was reported that addition of bimetallic impurities influences the growth kinetics of KDP from aqueous solutions (Begum and Podder, 2009; Claude et al., 2006b). The properties of ammonium di hydrogen phosphate crystal were modified by the addition novel Ni, Mg (Claude et al., 2006a). It was also reported that the addition of some of the amino acids as dopant enhances the nonlinear optical (NLO) and ferroelectric properties of semi organic materials (Aggarwal et al., 2003; Vijayan et al., 2006). So L-alanine doped semi organic material (KAP) will be of special interest as a fundamental building block to develop many complex crystals with improved NLO properties. It was reported that the ferroelectric properties of KAP crystal exhibited by bimetallic impurities dopants Cu2+ and Zn2+ (Chithambaram et al., 2010) and electro-optics properties are improved by the addition of trivalent ions Fe3+ and Cr3+ (Kejalakshmy and Srinivasan, 2004). The key factor that affect the transmission characteristics of the 90° bent photonic crystal waveguides was explained by Dekkiche and Naoum (2008) and Nasipuri et al. (2011) and they elucidate the reason for enhancement of crystal size by Microbes.

In this study, pure KAP crystals were grown by slow evaporation process at room temperature (30°C) and the effect of L-alanine as impurity with concentration ranging from 3000-10000 ppm (i.e., 0.3-1.0 mol %) on the structural, optical and thermal properties of KAP have been reported.

MATERIALS AND METHODS

Solubility study: The solubility of pure KAP and L-alanine doped KAP in double distilled water was determined in the temperature range 30-50°C insteps of 5°C using a constant temperature bath of accuracy ±0.01°C. Five hundred milliliter of the saturated solution of pure KAP salt was prepared gravimetrically at 30°C. This solution was stirred well for six hours constantly using magnetic stirrer and then filtered using Whatmann filter paper. This solution was taken in five different beakers of 100 mL and L-alanine was added to each four beaker as 0.3, 0.5, 0.7 and 1 mol%. After making supersaturated solution of KAP, the 5 mL of the solution was pipetted out and poured into a 10 mL beaker of known weight. The solvent was completely evaporated by warming the solution at 50°C. The amount of the salt present in 5 mL of the solution was measured by subtracting the empty beaker’s weight. From this the amount of the salt present in 100 mL of the solution was found out. In the same way, the amount of the salt dissolved in 100 mL at 35, 40, 45 and 50°C was determined. Figure 3 shows the solubility curves of pure and doped KAP salt. It is observed from the solubility graph that the solubility of pure and doped KAP in water increases as the temperature increases and decreases with doping concentration increase.

Table 1: Some physical properties of potassium acid phthalate (C8H5KO4)

Fig. 1: Molecular structure of KAP crystal

From this solubility data it can state that the KAP material has positive temperature coefficient. Table 1 shows some physical properties of KAP.

Crystal growth: The pure KAP (AR grade) and L-alanine (AR grade chemical from SIGMA) doped KAP crystals were grown using a good quality seed crystal at room temperature (30°C) by solvent evaporation method. For the preparation of seed crystals saturated solution of KAP was prepared first and then kept in a petri dish covered with a perforated polyethylene and allowed to grow seed crystals within 4-5 days. The pH of the solution was ranged from 3.93 to 3.97. The purity of the crystals was improved by successive recrystallization process. The growth period takes 25-30 days for bigger size. The grown crystals were found color-less and transparent. The molecular structure of KAP crystal is shown in Fig. 1 and 2 show the photographs of pure and doped KAP crystals.

Characterization: The pure and doped KAP crystals were characterized by Energy Dispersive X-ray Spectroscopy (EDX), Powder X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Optical Transmission Spectrum, Thermo Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA).

Fig. 2: Solution grown pure and doped KAP crystals

Fig. 3: Solubility curves of pure and doped KAP materials

In order to confirm the presence of L-alanine into KAP, crystals were subjected to EDX. EDX patterns were recorded using JEOL-6360 Scanning Electron Microscope. The recorded EDX spectra are shown in Fig. 4-6. Powder XRD was recorded using a Philips X pert PRO X-ray diffractometer with CuKα (λ = 1.5418Å) radiation. The XRD spectrum is shown in Fig. 7. The lattice parameters were calculated from the XRD data and tabulated in Table 2.

Table 2: Unit cell parameters of pure and L-alnine doped KAP crystals

In order to confirm the presence of functional groups in the crystal lattice, FTIR spectrum was recorded by KBr pellet technique using a Shimadzu FT-IR-8900 spectrometer in the wave number range 400-4000 cm-1. The FTIR spectra of pure and doped KAP are shown in Fig. 8. The optical properties of the grown crystals were studied by the transmission spectra using Shimadzu UV-1601 visible spectrometer in the wavelength region from 200 to 1100 nm. The transparent crystals with 2 mm thickness samples were used and mounted in a standard manner so that equal area of samples was exposed to the radiation. The transmission spectra of pure and doped KAP crystals are shown in Fig. 9. Thermal analysis was conducted on pure and doped KAP crystals using simultaneous Thermo Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) using thermal analyzer (model no. TG/DTA- 6300) from 30 to 700°C at the rate of 15°C min-1 in nitrogen atmosphere. The spectra of TGA and DTA of pure and doped KAP crystals are shown in Fig. 10 to 12.

Fig. 4: EDX spectrum of pure KAP crystal

Fig. 5: EDX spectrum of pure KAP+ 0.5 mol% LA crystal

Fig. 6: EDX spectrum of pure KAP+1.0 mol% LA crystal

Fig. 7: XRD spectra of pure and doped KAP crystals

RESULTS

Fourier transform infrared (FTIR) analysis: The FTIR spectra of pure and doped KAP crystals are shown in Fig. 8. The frequencies with their relative intensities obtained in pure and L-alanine doped KAP with different concentrations and their most probable assignments are presented in Table 3. The following vibration assignments showed the hydrogen bonding extends throughout the molecule of KAP. These hydrogen bonding results in the modification of stretching frequencies of O-H and the carboxyl groups. The asymmetric and symmetric stretching modes of C = O (with vertical double bond of oxygen on carbon) were observed at around 1650 cm-1. C = C aromatic ring group appears around the frequency 1481 cm-1. C-H aromatic stretching vibration appears between 1911 and 2025 cm-1. Carboxylic O-H stretching vibration produced resolved multiple bands at 1443 cm-1.

Fig. 8: FTIR spectra of pure and doped KAP crystals

Carbonyl group C = O presents in the range 1300-1385 cm-1It was clearly illustrated that the strong hydrogen bonding interaction of C-H group and the corresponding C-H in plane and out of plane bands were observed as weak bands between 811 and 812 cm-1. All these observations confirmed the presence of the functional groups in all grown crystals.

Powder X-ray diffraction analysis (XRD): The lattice parameters and cell volume of pure and doped KAP crystals are shown in Table 2. It was observed that the lattice parameters and cell volume of L-alanine doped KAP crystals slightly differ from those of pure KAP, which may be attributed to the presence of L-alanine in KAP crystals. The powder XRD analysis confirmed that the crystal structure of KAP is orthorhombic.

UV-VIS studies: The Ultra Violet-Visible optical transmission spectra of pure and doped KAP crystals are shown in Fig. 9. This spectral study might be assisted in understanding electronic structure of the optical band gap of the crystals.

Table 3: Comparison of vibrational frequencies obtained through FTIR studies of pure and doped KAP crystals

Fig. 9: UV-Visible transmissions of pure and doped KAP crystals

It is clear from the Fig. 9 that the percentage of optical transmission increases with the increase of the concentration of L-alanine in KAP crystals. All of them have sufficient transmission in the entire visible and near IR (infrared) region. This is the most desirable property of materials possessing for nonlinear optical (NLO) activity. There is a strong absorption at 280 nm. Absorption in the near ultraviolet region arises from electronic transitions associated within the sample. Using the formula Eg = hc/λ, the band gap energy was found to be 4.42 eV. Hence, it could be concluded that the L-alanine doping play a key role in improving the optical quality of KAP crystals.

Thermo Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA): In this study, the effect of L-alanine doping on the thermal stability of KAP crystals was studied by employing TGA and DTA. Figure 10 indicated the thermo gram and differential thermal analysis for pure KAP crystal. The TGA trace showed the different stages of decomposition. The first stage of decomposition started at 281.0°C and at 302.5°C to be for the major stage of decomposition. It was observed that initially there is loss of water of hydration and then became anhydrous and remained in that form up to the end of the analysis. There is no weight loss below 281.0°C. The DTA curve of pure KAP showed an endothermic peak at 296.8°C.

Fig. 10: TGA/DTA spectrum of pure KAP crystal

Fig. 11: TGA/DTA spectrum of KAP+0.5 mol% LA crystal

Fig. 12: TGA/DTA spectrum of KAP+1.0 mol% LA crystal

This endothermic peak corresponded to the decomposition temperature of the crystal. The TGA/DTA curves for L-alanine (0.5 mol%) doped KAP are presented in Fig. 11. DTA/TGA curve of 1.0 mol% L-alanine doped KAP showed an endothermic peak at 298.2°C (Fig. 12). The major stage of decomposition started at 303.6°C. It was confirmed that there is no phase transition for pure and doped KAP crystals up to temperature range 30 to 201°C.

DISCUSSION

The solubility of KAP determined by the solution method agrees well with that reported in the literature (Vasudevan et al., 2009) and the addition of L-alanine impurities, the solubility of KAP has been slightly changed which is also agreed well with Parikh et al. (2010). Large size, transparent, pure and L-alanine doped KAP crystals were grown by slow evaporation technique within a period of 30 days. Energy dispersive X-ray spectroscopy confirmed the presence of L-alanine into the KAP crystals. The powder X-ray diffraction spectra showed a small shifting of the peak position and also the variation in the intensities, which indicated the incorporation of L-alanine into the KAP crystal. The same result was found in KAP by DL-alanine doping (Uthayarani et al., 2008) and in KDP by doping L-alanine (Parikh et al., 2010). In the FTIR spectra of pure and L-alanine doped KAP crystals, the shifting to higher energy due to L-alanine doping indicated the interaction of O-H grouping of KAP with COO group of L-alanine. But Uthayarani found the opposite to this result. The optical transmission of KAP crystals increased with impurities concentration and the result agreed with Uthayarani et al. (2008), Parikh et al. (2010), Krishnan et al. (2008b), Geetha et al. (2006) and Murugakoothan et al. (1999). Chithambaram et al. (2010) and Kejalakshmy and Srinivasan (2004) reported doped KAP crystals possessed very low absorption in the entire visible region by addition of bivalent and trivalent metal ions. In the present investigation, the effect of L-alanine on the thermal stability of KAP crystals was studied by TGA/DTA. The thermograms showed that the crystal became anhydrous presently than pure KAP which agreed with Vasudevan et al. (2009) but in case of KDP it happened opposite of this study. Since amino acid became unstable at lower temperatures, it weakens KDP crystal and as a result the dehydration process took place earlier and more rapidly with comparison to pure KDP.

CONCLUSIONS

The effect of amino acid (L-alanine) impurity on the growth of KAP from supersaturated solutions has been investigated experimentally by measuring structural, optical and thermal properties. The presence of L-alanine in KAP solution was found to increase the optical transmission and decomposition temperature. This phenomenon may be attributed due to zwitterionic nature of L-alanine molecule (+NH3-C2H4-COO¯). The enhancement of optical transmission of L-alanine doped KAP crystals highlights their prospects of application as NLO materials. Further studies viz., frequency response of the dielectric constant, tanδ, temperature dependant conductivity, resistivity, activation energy and Vicker’s micro hardness are in progress and to be reported soon.

ACKNOWLEDGMENTS

The authors are thankful to Dr. Dilip Kumar Saha, CSO, for XRD measurements and Mr. Harinarayan Das, S.O. and Md. Al-Mamun, SO, Atomic Energy Centre, Dhaka, Bangladesh for EDX measurements.

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