# Effect of Hydrochloric Acid (HCl) on Synthesis and Anisotropic Phenomena of Triglycine Phosphate (TGP) Single Crystals

M R Meera, S1 Rayar, V R Bena Jothy

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M R Meera, S1 Rayar, V R Bena Jothy. Effect of Hydrochloric Acid (HCl) on Synthesis and Anisotropic Phenomena of Triglycine Phosphate (TGP) Single Crystals. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.60.6.225. [hal-01503808](https://hal.science/hal-01503808)

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# Effect of Hydrochloric Acid (HCl) on Synthesis and Anisotropic Phenomena of Triglycine Phosphate (TGP) Single Crystals

M.R. Meera¹,², S.L. Rayar³, V. Bena Jothy¹,⁴

1. Department of Physics and Research Centre, Women's Christian College, Nagercoil, Tamil Nadu, India

2. Department of Physics, Sree Ayyappa College for Women, Chunkankadai, Nagercoil, Tamil Nadu, India

3. Department of Physics, St. Judes College, Thoothoor, Tamil Nadu, India

a. meeranairrmr17@gmail.com

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

DOI 10.2412/mmse.60.6.225 provided by Seo4U.link

**Keywords:** TGP; NLO; PXRD; FTIR; EDAX; UV; TGA/DTA.

**ABSTRACT.** Effect of Sulphuric acid ($H_2SO_4$) addition on the growth of triglycine phosphate (TGP) crystal has been studied from the aqueous solution by slow evaporation technique. The characteristics absorption bands of pure and $H_2SO_4$ admixed TGP crystals are confirmed by FTIR spectra. UV-visible transmittance spectra were recorded for the samples to analyze the transparency of the grown crystals. The composition of pure and doped TGP crystals have been confirmed by EDAX analysis. The dielectric constants of the crystals have been studied and result suggests that the $H_2SO_4$ is doped into TGP crystal and that the doping increases its dielectric parameters.

**Introduction.** Investigations on semiorganic nonlinear optical (NLO) materials gain importance because of good thermal and mechanical properties with large NLO coefficients [1,2]. Among the amino acids, glycine [amino acetic acid: $NH_2CH_2COOH$] is the simplest amino acid, hence, glycine mixed semi-organic material is a fundamental building block to grow many complex crystals with improved NLO properties. Reported literature shows that glycine sodiumnitrate [3], triglycine sulphate [4] etc. crystals exhibit of some glycine based compounds like triglycine zinc chloride [5], glycine sodium nitrate [6] and triglycine selenate [7] show non-linear effects ferroelectric properties and are used as infrared detectors, transducers and piezoelectric sensors due to the large difference between the inter-molecular and intra molecular chemical bonds [5-7]. One of the hydrogen bonded ferroelectric single crystal Glycine phosphate (GPI) undergoes a continuous ferroelectric-para electric phase transition at 224.7K [8]. Substitutional or interstitial impurities in the host lattice leads to significant changes in the properties of the pure TGS crystals [9-10]. The pyroelectric coefficient of phosphoric acid doped TGS is more than that of pure TGS and observed phase transition temperature shift [11]. In view of this, the present investigation has been focused on the growth of semi-organic Triglycine phosphate (TGP) single crystals doped with sulphuric acid at various concentrations (0.25, 0.50, 0.75 and 1.0 mol %) and this doping efforts on the growth aspects, structural perfection, phase transition temperature, and optical properties were studied by conducting various characterization techniques.

**Experimental Details.** Triglycine phosphate was synthesized by taking the Analar grade glycine (99%) and phosphoric acid in the molar ratio of 3:1. The doped samples were prepared by mixing fixed amount of Sulphuric acid (0.25, 0.50, 0.75, and 1.0 mol %) with glycine and phosphoric acid. The prepared solutions were then filtered and the purity of the salts was achieved by repetitive recrystallization. The filtered solution was kept in a beaker covered with porous papers and kept in a dust-free atmosphere. After a period of 15-20 days, colorless and transparent crystals were obtained.
For the purpose of powder X-ray diffraction analysis of powdered sample of the grown crystals XPert PRO powder X-ray diffractometer is used. Energy Dispersive X-ray analysis were carried out using a Bruker EDS spectrometer in conjunction with the SEM. FT-IR spectra of pure and doped TGP have been recorded using Bruker IFS 66 V Spectrometer in the range 4000-100 cm⁻¹. Thermal analysis were carried out simultaneously employing Perkin Elmer thermogravimetric and differential analyser (Mode: PYRIS DIAMOND) in nitrogen atmosphere heated from 400 to 7300 with a heating rate of 100°C to understand thermal behaviour.

## Results and Discussion

**PXRD Analysis.** Single crystal XRD data of the pure TGP crystal suggests that the crystal belongs to monoclinic structure. In order to confirm the material of the grown crystal, powder XRD data were collected from the pure and hydrochloric added TGP crystals. Powder diffractogram of the pure and H₂SO₄ admixture TGP are shown in Fig 1. Comparing PXRD patterns of doped crystals with those of pure TGP single crystal, small changes in ‘d’ - spacing values were observed, which may be attributed to the presence of dopants in TGP crystal. It is clear that for all the grown crystals a ≠ b ≠ c which ensures that all the grown crystals is of monoclinic structure [12]. Analysis of the PXRD spectra confirmed the excess H₂SO₄ only acted as the additive rather than as dopant.

<figure><img src="image_3.png" /><figcaption>Fig. 1. PXRD (A) Pure TGP (B) TGP + 0.25mol % of H₂SO₄ (C) TGP + 0.50 mol % of H₂SO₄ (D) TGP + 0.75mol % of H₂SO₄ (E) TGP + 1.0 mol % of H₂SO₄.</figcaption></figure>

**FT-IR Analysis.** Fig. 2 shows the FT-IR spectra of pure and H₂SO₄ doped TGP crystals as it were recorded in the region 400-4000 cm⁻¹. P-O stretching vibrations are expected in the region 1040-910 cm⁻¹ [13] which is observed in IR at ~1042 cm⁻¹. OH stretching mode is observed at ~3430 cm⁻¹ and S=O stretching mode is observed at ~1390 cm⁻¹. Its absence in the pure TGP spectrum and its presence in the doped TGP spectra clearly indicate the presence of H₂SO₄ in the lattice of TGP crystal. C=O stretching vibrations in saturated aliphatic aldehydes, ketones and acids have frequencies in range 1740-1700 cm⁻¹. In amides, the frequency is lowered to 1690 cm⁻¹ [14], which is due to existence of resonance structures. C=O stretching band is observed at 1590 cm⁻¹. The deformation vibration of the carboxylate ion is observed at 682.95 cm⁻¹. C=O and NH₂ stretching vibrations are shifted and strongly evidences of intra-molecular interactions.
<figure><img src="image_4.png" /><figcaption>Fig. 2. FT IR Spectra (A) Pure TGP, (B) TGP + 0.25mol % of H2SO4, (C) TGP + 0.50 mol % of H2SO4 (D) TGP + 0.75mol % of H2SO4 (E) TGP + 1.0 mol % of H2SO4.</figcaption></figure>

**Thermal Analysis.** Thermogravimetry (TG) and differential thermal analysis (DTA) of pure and H2SO4 doped TGP were carried out at room temperature to 730°C and the thermograms are depicted in Fig.3. In pure TGP, up to 200°C there is no weight loss which reveals that the compound is devoid of physically adsorbed water and water of crystallization [15]. Slight variations appears in the TGA traces of H2SO4 doped TGP crystals with various concentrations 0.25, 0.50, 0.75 and 1.0 mole %. The decomposition of the compound takes place in single stage. Between 250°C and 500°C the weight loss of 72% is observed or the compound for pure TGP. This weight loss is due to the loss of glycine fragments in the compound. This weight loss is decrease as the concentration increases. The variation of experimental and the formulated weight losses are insignificant. At 730°C, almost 29 % of the compound remains as residue. An exothermic peak observed in DTA at 238°C complement to H2SO4 admixed TGP are due to the decomposition of the compound. Pure TGP crystal showed a major weight loss (~49 wt.%) in the temperature range between 218 and 268°C. This can be attributed to the decomposition of glycine into CO2 and SO4. DTA curve revealed a sharp peak at 213°C corresponding to the decomposition of glycine. Further increase in temperature resulted in the removal of HCl in the form of H2O with a total weight loss of 24 wt.%. Thus DTA curve of TGP crystal also showed to be due to combustion of glycine and removal of carbon.

<figure><img src="image_5.png" /><figcaption>Fig. 3. TG DTA (a) Pure TGP, (b) TGP + 0.25mol % of H2SO4, (c) TGP + 0.50 mol % of H2SO4, (d) TGP + 0.75mol % of H2SO4, (e) TGP + 1.0 mol % of H2SO4.</figcaption></figure>

**Dielectric studies.** Exact phase transition temperature of all ferroelectric crystals are studied and also the effects of dopants on the phase transition temperature were identified using dielectric measurements. The capacitance (Ccrys) and dielectric loss (tan δ) of the grown crystals (pure and H2SO4 added TGP) were measured using an LCRZ meter to an accuracy of ±2°C with frequency range (100Hz to 1MHz) at various temperatures ranging from 30°C to 150°C. The dielectric studies of pure and H2SO4 added TGP crystals were performed by selecting high transparency rectangular shaped crystal plates of appreciable dimensions and the samples were coated with good quality graphite to obtain a good conductive surface layer. The variation of dielectric constant with
temperature for varying frequencies is shown in Fig.4. The dielectric constant is low at low temperature region and then increases with temperature for pure and H₂SO₄ admixed TGP crystals, reaching maximum at the Curie point Tc. This rapid increase may be due to the space charge polarization of thermally generated carriers [16]. The sharp peak at Tc reveals that the nature of the grown crystal was of continuous phase transition. Above Tc, the dielectric constant decreases, following the Curie-Weiss law. It show that all the grown crystals (pure and doped TGP) behave as para electric before 328K-333K and becomes ferroelectric above 328K-333K, at varying frequencies. Thus all grown crystals may exhibit ferroelectric nature at higher temperatures. The variation of dielectric loss with temperature at different frequencies is shown in Fig.5. Purity and perfection of the crystal will depend on the space contribution and it has significant influence in the low frequency region. The dielectric loss is due to the resistive component that makes them loosey, so that they dissipate some of the applied ac energy. The low value of dielectric loss at high frequency suggest that all the samples possess enhanced optical quality with lesser defects and thus have vital importance for NLO applications [17]. The dopants (H₂SO₄) shifted the phase transition temperature slightly, but the shift was not considerable compared with the deuteration effect [18]. Fig. 6 shows the ac electrical conductivity of the sample as a function of temperature. Two different regions can be distinguished. At T, 328-333K (region I), the curve shows a positive temperature coefficient of the conductivity in accordance with conventional semiconducting behavior [19], where the conductivity increases with increasing temperature and thus the electrical activation energy of this sample is positive in region I. On the other hand, the decrease of conductivity at Tc 328-333K (region II) results from the scattering of carriers by phonons due to lattice vibrations at these elevated temperatures. The slope and temperature range of this region varies widely depending on the nature and structure of the sample [20]. From the results, it is observed that the dielectric parameters are found to be increased when TGP crystals are doped with H₂SO₄.

The activation energy of the crystal is also calculated from an Arrhenius plot using the relation:

$$ \sigma_{ac} = \sigma_{0} \exp[-E_{a}/kT] $$

where σ₀ is a constant depending on material, Ea is the activation energy, T the absolute temperature and k is the Boltzmann's constant. The above equation can be rewritten as

$$ \ln \sigma_{ac} = \ln \sigma_{0} - Ea/kT $$

A plot of ln σdc versus 1000/T gives -Ea/k as the slope and ln σdc as the intercept. Activation energies at the ferroelectric phase (above Tc) of the grown crystals were estimated using the slopes of the line plots, [E= -(slope)k x1000] and are found to be 0.5255eV, 0.2039eV, 0.2454eV, 0.2454eV, and 0.5851eV respectively for pure and H₂SO₄ doped TGP (0.25, 0.50, 0.75 and 1.0 mol %) crystals. A similar range of activation energy values have been reported which infer that pure and sulphuric acid added crystals will behave at high temperatures as a super-ionic conductor [5]. The lower activation energies estimated from electrical conductivity studies suggests that the material contains less number of charge carriers for conduction process and the dielectric behaviour is very well understood.
<figure><img src="image_6.png" /><figcaption>Fig. 4. Dielectric constant.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Dielectric loss.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. AC conductivity.</figcaption></figure>

**UV Analysis.** Grown crystals are optically characterized by UV-Vis spectral analysis are shown in Fig 7. The optical spectrum recorded revealed that grown crystals have lower cut off wavelength at 209, 209, 217, 208 and 212 nm respectively for the pure and H2SO4 (0.25, 0.50, 0.75 and 1.0 mole % concentrations) doped TGP which corresponds suitable for optical applications. TGP crystals are useful as a good non-linear material and for optoelectronics applications due to low cut off wavelength and a very good transparency [21].

<table><caption>Table 2. UV Absorbance, band gap.</caption><thead><tr><td>UV</td><td>Absorbance(a.u)</td><td>Band Gap</td></tr></thead><tbody><tr><td>Pure TGP</td><td>209</td><td>5.94</td></tr><tr><td>0.25 mol % H<sub>2</sub>SO<sub>4</sub> doped TGP</td><td>217</td><td>5.72</td></tr><tr><td>0.50 mol % H<sub>2</sub>SO<sub>4</sub> doped TGP</td><td>216</td><td>5.75</td></tr><tr><td>0.75 mol % H<sub>2</sub>SO<sub>4</sub> doped TGP</td><td>210</td><td>5.91</td></tr><tr><td>1.0 mol % H<sub>2</sub>SO<sub>4</sub> doped TGP</td><td>213</td><td>5.83</td></tr></tbody></table>
<figure><img src="image_9.png" /><figcaption>Fig. 7. UV Absorbance Spectra.</figcaption></figure>

**Summary.** Good optical quality crystals of pure and H₂SO₄ (0.25, 0.50, 0.75 and 1.0 mole %) doped TGP crystals were grown successfully from aqueous solution by slow evaporation technique. PXRD analysis reveals high crystalline nature of the grown crystals and are belongs to monoclinic crystal system. Changes in the relative intensity and peak positions of H₂SO₄ only acted as the additive rather than as dopant. The spectral investigations by IR spectroscopy strongly suggested the Zwitter and glycinium ions were present in both pure and H₂SO₄ admixed TGP crystals. Pure and H₂SO₄ doped TGP crystals are useful as a good non-linear material and for optoelectronics applications due to low cut off wavelength and are transparent to the entire visible region, which shows the wide window for nonlinear applications. The observed low dielectric constant and low dielectric loss suggests that the grown crystals are suitable for NLO applications in accordance with Miller's rule.

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Cite the paper

M.R. Meera, S.L. Rayar, V. Bena Jothy (2017). *Effect of Hydrochloric Acid (HCl) on Synthesis and Anisotropic Phenomena of Triglycine Phosphate (TGP) Single Crystals*. Mechanics, Materials Science & Engineering, Vol 9. Doi 10.2412/mmse.60.6.225