Effect of Amino Acid Additives on Crystal Growth Parameters and Properties of Ammonium Dihydrogen Phosphate Crystals

The isomorphous ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP) are technologically important crystals grown in large size for various applications. ADP crystal is of more appeal due to its piezo-electric property (Tukubo et al., 1989). Studies on ADP crystals attract interest because of their unique nonlinear optical, dielectric and antiferroelectric properties (Gunning et al., 2001). ADP crystals are widely used as the second, third and fourth harmonic generators for Nd: YAG, Nd: YLF lasers and for electro-optical applications such as Q-switches for Ti: Sapphire, Alexandrite lasers, as well as for acousto-optical applications. ADP crystal has found applications in NLO, electrooptics, transducer devices and as monochromators for X-ray fluorescence analysis. The room temperature structure of ADP determined by X-ray diffraction analysis was reported by Ueda (1948). Tenzer et al (1958) and Hewat (1973) examined the structure by neutron diffraction analysis. The projection of the structure onto the b, c plane is shown in Figure 1. ADP differs from KDP by having extra N–H–O hydrogen bonds which connect PO4 tetrahedra with neighbouring NH4 group. Each oxygen atom is connected with another oxygen atom in the neighboring PO4 ion and with a nitrogen atom in a neighbouring NH4 ion by two Kinds of bonds: (O–H–O) and (N–H–O). According to the positional refinements of each atom in ADP by X-ray diffraction study (Srinivasan, 1997), both above and below the phase transition point, each NH4 ion at the potassium position in KDP structure is shifted to the off-center position by forming two shorter and two longer bonds with four PO4 tetrahedra at low temperature phase. When an oxygen is connected with the shorter N–H–O bond, it tends to keep the other proton off in the O–H–O bond and when with the longer N– H–O bond it tends to take the acid proton nearby. Thus the extra hydrogen bonds produce a distorted NH4 ion lattice at low temperature and co-operate with the acid protons in causing proton configurations different from those found at low temperature in KDP (Matsushita et al., 1987). As a representative hydrogen bonded material, ADP has attracted extensive attention in the investigation of hydrogen bonding behaviors in crystal and the relationship between crystal structures and their properties.


INTRODUCTION
The isomorphous ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP) are technologically important crystals grown in large size for various applications.ADP crystal is of more appeal due to its piezo-electric property (Tukubo et al 1989).Studies on ADP crystals attract interest because of their unique nonlinear optical, dielectric and antiferroelectric properties (Gunning et al 2001).ADP crystals are widely used as the second, third and fourth harmonic generators for Nd: YAG, Nd: YLF lasers and for electro-optical applications such as Q-switches for Ti: Sapphire, Alexandrite lasers, as well as for acousto-optical applications.
ADP crystal has found applications in NLO, electro-optics, transducer devices and as monochromators for X-ray fluorescence analysis.
The room temperature structure of ADP determined by X-ray diffraction analysis was reported by Ueda (1948).Tenzer et al (1958) and Hewat (1973) (N-H-O).According to the positional refinements of each atom in ADP by X-ray diffraction study (Srinivasan 1997), both above and below the phase transition point, each NH 4 ion at the potassium position in KDP structure is shifted to the off-center position by forming two shorter and two longer bonds with four PO 4 tetrahedra at low temperature phase.When an oxygen is connected with the shorter N-H-O bond, it tends to keep the other proton off in the O-H-O bond and when with the longer N-H-O bond it tends to take the acid proton nearby.Thus the extra hydrogen bonds produce a distorted NH 4 ion lattice at low temperature and co-operate with the acid protons in causing proton configurations different from those found at low temperature in KDP (Matsushita et al 1987).As a representative hydrogen bonded material, ADP has attracted extensive attention in the investigation of hydrogen bonding behaviors in crystal and the relationship between crystal structure and their properties.
Several researchers have carried out a lot of studies in pure and doped ADP crystals (Zaitseva et al 2001, Ren et al 2008).In ADP and KDP crystal growth, the metallic cations present in the solutions, especially materials with high valency were considered to strongly affect the growth habit and optical properties of the crystals.The most dangerous impurities which affect the growth habit are trivalent metals Cr 3+ , Fe 3+ and Al 3+ (Alexandru et al 2003).Even after repeated recrystallization, the presence of small amount of those kinds of impurities in the solution suppresses the crystal quality and growth rate.Here comes the importance of beneficial effects of additives in the crystal growth.An additive can suppress, enhance or stop the growth of crystal completely and its effects depend on the additive concentration, supersaturation, temperature and pH of the solution.Some dopants are added to suppress the effect of metal ion impurities on ADP and KDP crystals.For example, EDTA and KCl reduces the effect of metal ion impurities and enhance the metastable zone width and increases the growth rate of the crystals (Rajesh et al 2000, Podder 2002, Meenakshisundaram et al 2009).The addition of such kind of dopants does not remove the impurities present in the solution; it just reacts with the metal ions and is making complexes.By making complex, the ions become bigger in size and it is not possible to enter into the growing crystal (Li et al 2005, Asakuma et al 2007).
Studies have also been made about the effect of additives on growth, habit modification and structure of ADP (Davey et al 1974, Boukhris et al 1998).
The adsorption of impurities at different sites can cause growth inhibitions, even block the growing surface and in consequence stop the growth process.
However, the adsorbed impurities may simultaneously lead to a reduction in the edge free energy, which results in an increase in crystal growth rate (Rak et al 2005).Several dopants help in the growth of ADP crystals with higher growth rate and enhancement in the various properties of the crystals.The growth promoting effect is observed in the presence of organic additives (Kern et al 1992, Bhagavannarayana et al 2006) as well as inorganic additives (Shantha et al 1997, Podder et al 2001).
Amino acid family crystals exhibit excellent nonlinear optical and electro-optical properties.Reports are available in literature on the doping of amino acids in technologically important crystals and the enhancement of the material properties like nonlinear optical and ferroelectric properties.For example, enhancement of SHG efficiency has been reported in L-arginine doped KDP crystals (Parikh et al 2007).Kumaresan et al (2008) reported the doping of amino acids (L-glutamic acid, L-histidine, L-valine) with KDP and studied its properties.The effects on various properties of L-theronine, DL-theronine and L-methionine admixtured TGS crystals were studied and the authors reported that the admixtured TGS crystal has different properties compared to pure TGS crystal (Meera et al 2004).Batra et al (2005) investigated the growth kinetics of KDP and TGS crystals doped with L-arginine phosphate monohydrate.The addition of L-arginine decreases the value of dielectric constant of KDP crystals (Meena et al 2008).
In the light of research work being done on ADP crystals, to improve their growth and other characteristics, it was thought interesting and worthwhile to investigate the effects of amino acid materials L-arginine monohydrochloride (C 6 H 15 N 4 O 2 Cl) and L-alanine (C 3 H 7 NO 2 ) on nucleation studies, growth and properties of ADP crystals for both academic and industrial uses.The reason for choosing the dopants is that L-arginine monohydrochloride and L-alanine are efficient NLO materials under the amino acid category.Monaco et al (1987) discovered NLO material L-arginine monohydrochloride, which belongs to space group P2 1 of monoclinic system with two molecules in the asymmetric unit.L-alanine crystallizes in orthorhombic system with noncentrosymmetric space group P2 1 2 1 2 1 (Razzetti et al 2002).

DETERMINATION OF SOLUBILITY AND METASTABLE ZONE WIDTH
Ammonium dihydrogen phosphate, L-arginine monohydrochloride (LAHCl) and L-alanine of GR grade from Merck and Millipore water of resistivity 18.2 M cm were used for all studies.The solubility was determined gravimetrically for pure ADP and ADP doped with small amount  Considering the principles of homogeneous and heterogeneous nucleation theories, the free energy of formation of a nucleus under heterogeneous nucleation is less than that of a homogeneous condition (Sangwal 1996, Srinivasan et al 1999).Considering the additive added system it can be noticed that the induction period of doped ADP is higher than that of pure and it increases with the increase in the additive concentration.Among the additives, L-alanine has a higher induction period than LAHCl at every concentration.The presence of additives in the system affects the nucleation behavior very considerably.This may be due to the suppression of chemical activity of the metal ions present in the ADP solution (Mullin 1993).

GROWTH RATE MEASUREMENTS
The influence of additives on the growth rate of ADP crystals is determined by the weighing method (Kubota et al 1995).By this method, the growth rate of a crystal is determined by G g = (m -m o )/m o , where m o is the initial mass of the crystal (kg), m is the final mass of the crystal (kg), and is the growth time.In the experiment, growth time is taken as 1 h and size of the seed crystal is 5-10 mm.The seed crystal (of mass m o ) was suspended in the solution in 500 ml glass vessel (working volume: 400 ml) for 1 h ( ) for growth.The solution was continuously stirred throughout the process.
Supercooling was varied from 2 to 10 o C by changing the growth temperature.
The same procedure is done for pure, LAHCl doped ADP and L-alanine doped ADP crystals.Growth rates of pure and doped ADP crystals are shown in the Figure 8.4.which consequently decreases the rates of layer displacement that cause an increase in the growth rate (Sangwal 1996).

CRYSTAL GROWTH
In the present work, ADP crystals doped with 5 mol% LAHCl and L-alanine separately were grown from aqueous solution with an apparatus that rotates the growing crystal.The seed mount platform in this experiment stirs the solution very well and makes the solution more stable, which resulted in better crystal quality.The crystal growth was carried out in a 5000 ml standard crystallizer used for conventional crystal growth by using the method of temperature reduction.The aqueous solutions at the saturation temperature

POWDER XRD STUDIES
The X-ray powder diffraction analysis was used to confirm the physical phase of the product.Grown crystals were ground using an agate mortar and pestle in order to determine the crystal phases by X-ray diffraction.
The XRD analysis (SAIFERT, 2002 DLX model) was performed using a tube voltage and current of 40 kV and 30 mA respectively.Figure 8.6 shows X-ray powder diffraction patterns of ADP doped with LAHCl (5 mol%) and ADP doped with L-alanine (5 mol%) compared with that of pure ADP crystal.

FTIR SPECTRAL ANALYSIS
The influence of additives used in this work on the vibration frequencies of functional groups of pure ADP crystal has been identified by FTIR spectroscopy.The FTIR spectra were recorded in the region 400-4000 cm -1 using a Perkin-Elmer FTIR Spectrum RXI spectrometer by KBr pellet technique.Figure 8.9 shows the FTIR spectra of the pure ADP, ADP doped with LAHCl (1 mol%) and ADP doped with L-alanine (1 mol%).
The broad band in the high energy region is due to O-H vibrations of water, P-O-H group and N-H vibrations of ammonium.The peak at 2370 cm -1 is due to the combination band of vibrations occurring at 1293 and 1290 cm -1 .
The bending vibrations of water give the peak at 1646 cm -1 .The peak at 1402 cm -1 is due to bending vibrations of ammonium.The P-O-H vibrations give the peaks at 1090 and 930 cm -1 .The PO 4 vibrations give their peaks at 544 and 470 cm -1 .

ADP doped with L-alanine
In the spectrum of ADP doped with LAHCl, the intense band appearing at 3442 cm -1 includes O-H vibrations and N-H vibrations of ammonium and amino acid.Although this spectrum carries similar features as that of ADP, there is a distinct evidence for the presence of LAHCl in the lattice of ADP.The peaks appearing at 2928 cm -1 and about 2890 cm -1 are due to CH 2 vibrations of LAHCl.In addition, shift in the peak positions of P-O-H and PO 4 vibrations compared to ADP established the presence of the additive in the lattice of ADP.In the spectrum of ADP with L-alanine also, there is a significant shift in the peak positions.For example, the PO 4 vibration of the parent is shifted from 470 to 419 cm -1 .Similarly the P-O-H vibrations at 1090 and 930 cm -1 of the parent are shifted to 1101 and 913 cm -1 .Such a shift establishes the presence of L-alanine in the lattice of ADP.As the vibrations of L-alanine are not clearly resolved from the spectrum of the parent, it might be in a trace amount below the deductibility limit.There is a slight evidence of CH 2 vibrations of L-alanine just below 3000 cm -1 .All these support the presence of L-alanine in the lattice of ADP.

DIELECTRIC STUDIES
The magnitude of dielectric constant depends on the degree of polarization charge displacement in the crystals.Using Agilent 4284A LCR meter, the capacitances of the pure and doped ADP crystals were measured for temperatures from 313 to 423 K with frequency (f) of 1 kHz.Good quality transparent crystals of size 7 × 7 × 2 mm 3 were used for the measurements.
The dimensions of the samples were determined using a traveling microscope (LC = 0.001 cm).Samples were coated with good quality graphite in order to obtain a good ohmic contact.The measurements were done on a-b directions of the crystals.The samples were annealed up to 423 K to remove water molecules if present.Several trials of experiments were conducted.ADP crystal is 2.16 times that of pure ADP crystal.Greater crystalline perfection may be the reason for the increase in piezoelectric efficiency.

CONCLUSIONS
New additives L-arginine monohydrochloride and L-alanine were added with ADP and found that these additives can affect the nucleation of ADP from aqueous solutions.The addition of these amino acid materials enhances the metastable zone width and induction period of pure ADP solution.Also, during the experiment it was observed that the number of tiny crystals formed by spontaneous nucleation was appreciably reduced in the case of doped solution.It is believed that the addition of these amino acid materials suppresses the activities of the metal ion impurities present in the solution which enables larger metastable zone width and faster growth rate.
HRXRD curves recorded for 5 mol% doped crystals have excellent crystalline perfection.The FTIR spectrum shows that amino acid additives have entered into the ADP crystals.The transmission spectrum reveals that the crystal has sufficient transmission in the entire visible and IR region.The SHG conversion efficiency and piezoelectric coefficient values of doped crystals are higher than that of pure.
examined the structure by neutron diffraction analysis.The projection of the structure onto the b, c plane is shown in Figure 8.1.

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Figure 8.2 in comparison with the pure system.

Figure 8 . 2 Figure 8 . 3
Figure 8.2 Saturation and metastability limit curves of pure, LAHCl and L-alanine added ADP solutions

Figure 8 . 4
Figure 8.4 Variation of mass growth rates for pure and doped ADP crystals 50 o C were filtered by filtration pump and Whatman filter paper under slight pressure in a closed system to remove extraneous solid and colloidal particles.Then the solutions were overheated at 70 o C for 24 h, to make the solution stable against spontaneous nucleation under a high supersaturation (Nakatsuka et al 1997).Then the temperature of the solution was reduced to 3-5 o C higher than saturation point (50 o C) at 1 o C/h.After that temperature was reduced to saturation point at 1 o C/day and the seed crystal was mounted on the platform.The rotation rate of the platform was 40 rpm.From the saturation point, the temperature was decreased at 0.1 o C/day at the beginning of the growth.As the growth progressed, the temperature lowering rate was increased up to 1 o C/day.After the growth period of 30 days, crystals were harvested.The as-grown crystals are shown in Figure 8.5.In the figure, (a) is ADP doped with L-Alanine and (b) is ADP doped with LAHCl.

Figure 8 . 5
Figure 8.5 Photograph of L-alanine doped ADP crystal and LAHCl doped ADP crystal

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Figure 8.6 X-ray powder diffraction patterns of ADP crystals

Figure 8
Figure 8.9 FTIR spectra of pure ADP, ADP doped with LAHCl and

Figure 8 .Figure 8 .
Figure 8.10 shows the temperature dependence of dielectric constants of pure and LAHCl doped (1 and 5 mol%) ADP crystals.Temperature dependence of dielectric constants of pure and L-alanine doped (1 and 5 mol%) ADP crystals are depicted in Figure 8.11.
Figure 8.12 Variation of dielectric loss with temperature for pure and LAHCl doped ADP crystals Figure 8.14 Transmission spectra of pure and doped ADP crystals