Structural and Optical Studies of Potential Ferroelectric Crystal : KDP Doped TGS

Triglycine sulphate (TGS), an important ferroelectric material has been widely used in the fabrication of high sensitivity infrared detectors at room temperature. Single crystals of KDP doped TGS was grown by slow evaporation method at room temperature in this study. The grown crystal was characterized by UV-Vis spectroscopy, FTIR spectroscopy, powder X-ray diffraction studies, and ferroelectric studies. KDP doped TGS crystals were found to be highly transparent and full faced. The experimental results evidence the suitability of the grown crystal for optoelectronic applications.


Introduction
Triglycine sulphate plays a major role in FT-IR instrumentation and infrared detector.It exhibits order disorder phase transition at the Curie point 49 ∘ C. TGS has protonated carboxyl groups and form chain-like system with SO 4 2− group.The neighbouring chains are connected by SO 4 2− groups of adjacent chains and GI group.Such configuration of TGS is regarded as particularly important for the ferroelectric behavior of TGS crystal.The spontaneous polarization reversal in TGS is due to the proton transfer between glycine and glycinium ions [1][2].TGS exhibits order-disorder phase transition at the Curie point.Above Curie temperature, it is in the Paraelectric phase with space group symmetry P2 1 /m and below Curie it is in the ferroelectric phase with space group symmetry P2 1 with two formula units per unit cell [3].All ferroelectric materials are pyro electric, however, not all pyro electric materials are ferroelectric.Below Curie temperature, ferroelectric and pyro electric materials are polar and possess a spontaneous polarization or electric dipole moment [4][5][6][7].However, this polarity can be reoriented or reversed fully or partially through the application of an electric field with ferroelectric materials.TGS has a major disadvantage that it is depolarized by thermal, mechanical, and electrical means.In order to overcome this difficulty, several studies have been attempted with different organic and inorganic dopants to achieve effective internal bias to stabilize the domains to increase the pyro electric and ferroelectric properties of TGS crystals [8][9][10][11][12][13][14].In this article, we report the effect of doping TGS crystal with potassium di hydrogen phosphate (KDP), and the characterization studies adopted for the grown crystal.

Materials and Methods
Analar grade reagent (AR) L-Glycine and concentrated suphuric acid (H 2 SO 4 ) of 99% purity were dissolved in deionized water in the molar ratio of 3:1, and the solution was heated upto 50 0 C to obtain TGS salt.Glycine reacts with sulphuric acid as follows: The synthesized salt was again dissolved in double distilled water and then recrystallized by natural evaporation process.This process was repeated twice to improve the purity of the material.To obtain doped TGS crystal, 1mole% KDP was added to the saturated mother solution.Highly transparent and full faced crystals were obtained within two weeks.The KDP doped TGS crystals are found as colorless and are shown in

UV-Vis spectral analysis
The optical transmission spectra of the grown crystal were carried out in the range of 200nm 800nm covering the entire near-ultraviolet, visible, and near infrared region using SHIMADZU UV-160 Spectrometer to find the transmission range about the suitability of this grown crystal for optical applications.An optically polished single crystal was used for this study.The transmission spectra for pure KDP and KDP doped TGS are shown in Fig. 2. A strong absorption and the UV cut off wavelength lies at 270 nm for KDP-doped TGS crystal, and it reveals the good optical quality of the grown crystal.High transmission in the entire visible region for KDP-doped TGS crystal proves the suitability of grown crystal as UV tunable laser and in second harmonic generation (SHG) device applications.The band gap calculated using the formula, E g = hc/ is found to be 4.6 eV.The absence of absorption and excellent transmission in entire visible region makes this crystal a good candidate for optoelectronic application [16][17][18].

Powder x-ray diffraction
X-ray diffraction technique is a powerful tool to analyze the crystalline nature of the materials.If the material to be investigated is crystalline, well defined peaks will be observed.Powder X-ray diffraction analysis was carried out by using PANalytical X-Ray diffractometer with a CuKα radiation.The sample was scanned over the range 20 0 -70 0 .The XRD pattern of pure KDP and KDP doped TGS crystal is shown in Fig. 3(a   Structure Tetragonal Monoclinic

FTIR spectral study
The infrared spectral analysis is effectively used to understand the chemical bonding and it provides information about molecular structure of the synthesized compound.Fourier transform infrared spectra were recorded in the range 400-4000cm -1 using Perkin Elmer spectrophotometer.The sample used was in pellet form mixed with KBr.The characteristic absorption peaks are observed in the range from 400-4000cm -1 IR and is shown in Fig. 4. The NH, OH and CH absorption occurs at around high frequency of 3163cm -1 .The spectra at 1627cm -1 is assigned for NH 2 bending.The absorption in the range 1712-1867cm -1 is assigned to C=O stretching of carboxylic acid.The broad band around 1100cm -1 is assigned to C-N asymmetric stretch of SO 4 .The peak absorption at 613cm -1 and 501 cm -1 are due to NH 3 oscillation.The observed frequencies and their assignments for KDP doped TGS crystal are compared with those of pure KDP and listed in Table 2.

Photoconductivity studies
Photoconductivity measurements were made using Keithley 485 picoammeter.The dark current was recorded by keeping the sample unexposed to any radiation.Fig. 5 shows the variation of both dark current (I d ) and photocurrent (I p ) with applied field.It is seen from the plots that both I d and I p of the sample increase linearly with applied field.It is observed from the plot that the dark current is always higher than the photo current, thus confirming the negative photoconductivity.The phenomenon of negative photoconductivity is explained by Stockmann model [19].The negative photoconductivity in a solid is due to the decrease in the number of charge carriers or their lifetime, in the presence of radiation [20].For a negative photoconductor, forbidden gap holds two energy levels in which one is placed between the Fermi level and the conduction band while the other is located close to the valence band.The second state has higher capture crosssection for electrons and holes.As it captures electrons from the conduction band and holes from the valence band, the number of charge carriers in the conduction band gets reduced and the current decreases in the presence of radiation.

Ferroelectric P-E hysteresis loop analysis
The properties of ferroelectrics are governed by their crystallographic structure [21][22][23] and strongly influenced by the lattice defects, presence of which in crystals leads to appearing and existence of internal bias field [24].The polarization and electric field (P-E) curve was traced in KDP doped TGS crystals using a standard computer controlled Sawyer-Tower P-E hysteresis analyzer.A good quality crystal was used for the study.The opposite faces were electroded with conducting silver paste.Typical ferroelectric P-E hysteresis loop of grown KDP doped TGS crystal is shown in Fig. 6.It is found that the crystals were able to sustain a field (E c ) up to 5 kV/cm.In addition, a well saturated hysteresis loop was obtained in the grown KDP doped TGS crystal.Higher field was not applied to avoid the damage of the TGS crystal.Fig. 6.Ferroelectric P-E hysteresis loop of grown KDP doped TGS crystal.

Conclusion
KDP doped TGS crystals of good optical quality were grown by slow evaporation method.UV-Vis spectra show that the grown crystal was optically transparent through 200-800 nm and hence suggests the suitability of this material for optical devices.Powder X-ray diffraction analysis confirms the crystalline nature of grown crystal.The FTIR spectral analysis confirms the presence of functional groups in the crystals.The high optical transparency, and low dielectric constant, proves the suitability of the grown TGS crystal doped with KDP for SHG and other optoelectronic device applications.
) and 3(b).The structural data for pure KDP and KDP doped TGS are given in Table1.

Fig. 5 .
Fig. 5. Variation of dark current (I d ) and photocurrent (I p ) with applied field.

Table 1 .
The structural data for pure KDP and KDP doped TGS.

Table 2 .
Frequencies and their assignments for pure TGS and KDP doped TGS crystal.