Growth of dl -malic acid-doped ammonium dihydrogen phosphate crystal and its characterization

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 3491–3497

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Growth of DL-malic acid-doped ammonium dihydrogen phosphate crystal and its characterization P. Rajesh, P. Ramasamy  Center for Crystal Growth, SSN College of Engineering, Kalavakkam 603 110, India

a r t i c l e in fo

abstract

Article history: Received 20 October 2008 Received in revised form 16 April 2009 Accepted 20 April 2009 Communicated by M. Roth Available online 3 May 2009

Single crystals of DL-malic acid-doped ammonium dihydrogen phosphate have been grown using slow evaporation method and also by Sankaranarayanan–Ramasamy (SR) method with the vision to improve the properties of the ADP crystals. The characterization of grown crystals was made by single-crystal X-ray diffraction, powder X-ray diffraction, UV–vis. spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DTA), Vicker’s microhardness, dielectric measurements, high-resolution X-ray diffraction (HRXRD) and second-harmonic studies. Structural difference between pure and doped crystal has been studied by XRD analysis. Functional groups were identified by FTIR spectroscopy. The grown crystals were found to be transparent in the entire visible region. Decomposition temperatures of the grown crystals were measured by DTA. Vicker’s hardness study carried out on (0 0 1) face at room temperature shows increased hardness of the doped crystals and SRmethod-grown crystals. Dielectric measurements reveal that SR-method-grown DLM-doped ADP crystals have low dielectric loss. Crystalline perfection of the grown crystals is analyzed using HRXRD. Preliminary measurements indicate that the second harmonic generation efficiency of the doped crystals at a fundamental wavelength of 1064 nm is roughly 1.5 times greater than that of pure ADP. & 2009 Elsevier B.V. All rights reserved.

PACS: 77.22.Gm 78.20. e 81.10.Dn Keywords: A1. X-ray diffraction A2. Growth from solutions A2. Single crystal growth B2. Dielectric materials

1. Introduction Ammonium dihydrogen phosphate (ADP), a hydrogen bonded compound, belongs to isomorphous series of phosphates and arsenates that presents a strong piezoelectric activity. These molecular crystals exhibit low-temperature order–disorder phase transitions. Below 148.5 K, ADP is antiferroelectric and belongs to P212121 space symmetry group while above this temperature it becomes paraelectric having a I 4/2d symmetry [1–3]. Ammonium dihydrogen phosphate and potassium dihydrogen phosphate (KDP) are nonlinear optical materials and have been used as optical modulation Q-switch, quantum electronics and frequency converters. Particularly, optical crystals with lower impurity and higher damage threshold are required for inertial confinement fusion [4,5]. In recent years, various growth methods and apparatus have been continuously developed to improve the crystal quality and the growth rate [6–8]. One of the obvious requirements for a non-linear optical crystal is that it should have excellent optical quality. Sankaranarayanan–Ramasamy (SR) method is an efficient method to grow the large-size good-quality

Corresponding author. Tel./fax: +91 4427475166.

E-mail addresses: [email protected], [email protected] (P. Ramasamy). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.04.020

single crystals. The slow evaporation method yields small-size single crystals with different crystallographic faces. For the phasematching applications where the specimen should have more size along a particular direction, the crystals grown by this method are not economical. In the case of VBT growth, although the size of the crystals is good with the desired orientation, the quality of the crystal is generally not up to mark, as these crystals grow with structural grain boundaries due to unavoidable thermal gradients, which affect the device performance and also it is not possible to grow ADP crystals by this method. The recently invented Sankaranarayanan–Ramasamy method has given the solution, and it is possible to grow bulk-size single crystals along a desired orientation needed for device fabrication [9–12]. The process of crystallization represents an important separation and purification technique for the chemical industry, so its control in the manufacture has been an intriguing field for a long time and has recently received considerable attention from both theoretical and experimental standpoints [13–17]. In addition, the minute amounts of additives, such as KCl and EDTA, can effectively suppress the metal ions and promote the crystal quality [18,19]. Several researchers have carried out a lot of studies in pure and organic and metal ions-doped crystals [20–25]. DL-malic acid (DLM) is an organic material, which crystallizes in combination with other materials. DL-malic acid combined with ammonia solution gives ammonium malate single crystal. L-malic acid with

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urea produces urea L-malic acid single crystal. DL-malic acid with boric acid and rubidium carbonate yields potential semiorganic materials rubidium bis-dl-malato borate for second-order NLO applications [26–28]. However, there is no report available in literature on the effect of DL-malic acid on the crystal growth and various properties of inorganic materials like ADP and KDP. 1 mol% of doping has resulted in higher growth rate and has enhanced the various properties of the crystal [29–32]. Keeping this in our mind, in our laboratory it was proposed to grow ADP crystal added with 1 mol% of DL-malic acid. In this work, we have presented the growth, structural, optical, thermal, mechanical, dielectric and SHG efficiency of DLM-doped ADP single crystals grown by slow evaporation method and also by SR method. The effects of impurity atoms on the quality and performance of the crystals are analyzed. The results of the doped ADP crystals grown by slow evaporation method and SR method are compared with the results of the pure ADP grown by slow evaporation method.

2. Experimental procedure 2.1. Crystal growth The commercially available ADP was used for growth, after repeated recrystallization. Single crystals of pure and DLM-added ADP were grown using deionized water as a solvent by slow evaporation technique. According to the solubility data [29], 500 ml saturated solution of ADP was prepared and filtered at room temperature and the solution was divided equally into two beakers and it was named as A and B. The beaker A was kept closed with porously sealed cover, then 1 mol % of DLM was added into the beaker B and it was closed with the same type of cover. Solutions in both the beakers were allowed to evaporate in an identical condition. After seven days, tiny crystals were seen in the beaker B, where as in A, it was observed one day later only. All crystals reached maximum size in 30 days. The colourless transparent pure ADP crystals harvested were of size up to 15  7  15 mm3 and doped crystals of size up to 30  7  15 mm3. It was observed that the growth rate of DLM added ADP is faster than the pure ADP and comparably big crystals were obtained in DLM-added solution. The DLM-doped ADP is shown in Fig. 1(a). Ammonium dihydrogen phosphate and DL-malic acid used in the present study were from Merck, India and the deionized water was from a Millipore water purification unit. The resistivity of the used deionized water is 18.2 MO cm. 2.2. SR method arrangement SR method consists of ring heaters positioned at the top and bottom of the growth ampoule connected to the temperature controller, and the top heater provides the necessary temperature for solvent evaporation. SR method setup was arranged to grow ADP crystals with the addition of 1 mol% DL-malic acid [7]. Depending on the growth rate of the crystal, the ring heater was moved upwards using a translation mechanism. 2.3. Growth rate Based on the quality of the crystals grown by slow evaporation technique, a suitable seed crystal having a size 4  4  3 mm3 was selected for single crystal growth of /0 0 1S face. To control the spurious nucleation, care has been taken while preparing the growth vessel and the solution. The chosen /0 0 1S plane of the seed crystal was mounted in the bottom of the ampoule without polishing the surface. Using the same material ingredi-

ents used in the slow evaporation method, ADP solution of optimized saturation was prepared and 1 mol% of DLM was added to the solution and it was transferred carefully into the growth vessel. The growth vessel was porously sealed and placed in a dust-free chamber. The growth was initiated with a suitable temperature provided by the ring heater at the top region of the saturated solution under equilibrium condition. The applied temperature on the top of the ampoule was 37 1C and the bottom, 33 1C. The placement of ring heater at the top of the growth solution also controls the spurious nucleation near the surface region of the solution during the entire growth period. Under optimized condition highly transparent crystal growth was seen. The growth rate was 2 mm/d for the given ampoule of diameter 10 mm. After 50 days of the growth a good-quality crystal was harvested with size 10 mm in diameter and 100 mm in length. In the slow evaporation method using the same material ingredients the achieved growth rate is 1 mm/d only. After 30 days of growth the grown crystal size is 30  7  15 mm3. This indicates that, it is possible to grow good quality crystals with higher growth rate in SR method. The quality of the crystal will be poor if a suspension thread is used in crystal growth. This situation is avoided in this method. A larger volume of ADP crystals added with dopants can be grown by slow cooling along with seed rotation method [33]. In this method large quantity of solution in large container is normally used and only a small fraction of the solute is converted into a bulk single crystal. In SR method, this drawback has been eliminated. The maximum amount of solution will become a single crystal. The grown crystals are shown in Figs. 1(b) and (c). Pritula et al. [34] reported that in the presence of urea the growth rate of KDP is increased. The increase in the growth rate is obviously connected with the catalytic action of urea molecules on the surface of the growing face. Such an effect seems to be caused by the adsorption of the organic molecules and the decrease of the surface energy of the growth steps resulting in the rise of the growth rate. Dhanaraj et al. [33] reported that the presence of L-arginine monohydrochloride (LAHCl) and Lanaline enhance the growth rate of ADP single crystals.

3. Characterization 3.1. Single-crystal X-ray diffraction The grown pure and DLM-doped ADP crystals were subjected to single-crystal X-ray diffraction studies using Enraf Nonius CAD4 single-crystal X-ray diffractometer to determine the cell parameters. The cell parameters obtained for the pure ADP are a=b=7.510 A˚, c=7.564 A˚, a=b=g=901 and it belongs to tetragonal system. The unit cell parameters for the doped ADP crystal are a=b=7.495(3) A˚, c=7.540(1) A˚, a=b=g=901. It is seen from the above values that slight variation in the lattice parameters is observed and the grown crystal retains its original structure. This result reveals that, the DLM has entered into the lattice sites of ADP. 3.2. Powder X-ray diffraction analysis Powder X-ray diffraction studies were carried out using Rich Seifert X-ray diffractometer employing CuKa (1.54058 A˚) radiation, scanning angle ranging from 10 to 50 1C at a scan rate 1 1/min to study the crystallinity of the grown crystal. The diffracted peaks are same in pure and doped ADP crystals. The observed prominent peaks of pure and doped ADP are (1 0 1), (2 0 0), (11 2), (2 0 2), (3 0 1) and (3 1 2) but the intensities of the diffracted peaks are found to be varied. The sharp peaks and low full-width at halfmaximum (FWHM) values confirm that crystallinity of the grown

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Fig. 2. Powder XRD spectra of grown crystals.

crystals is good. This has been given in Fig. 2. The observed values are in good agreement with the reported values [35]. 3.3. Fourier transform infrared spectroscopy (FTIR) FTIR spectra of pure and DLM-doped ADP crystal were recorded using a JASCO FT-IR 410 spectrometer in the range 400–4000 cm 1 by KBr pellet technique. The spectra of pure and doped crystals are shown in Fig. 3. The effect of DLM on the functional groups of the pure ADP crystal has been identified by the spectrum. The broad band in the high-energy region is due to the O–H vibrations of water, P–O–H group and N–H vibrations of ammonium. The peaks at 1092 and 932 cm 1 represent P–O–H vibrations. The PO4 vibrations give their peaks at 544 and

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Wavenumber (cm-1) Fig. 3. FTIR spectra of grown crystals.

470 cm 1. The peak at 2370 cm 1 is due to the combination of band of vibrations occurring at 1293 and 1290 cm 1. The broadness is generally considered to be due to hydrogen bonding interaction of H2PO 4, COOH and NH3+ with adjacent molecules. By comparing the spectra of doped and undoped crystals, one can easily find missing absorptions for N–H stretching of NH3+, C–H stretching of CH2 and CH and conclude that DLM doping was successfully achieved. The bending vibrations of water give its peak at 1646 cm 1. The peak at 1402 cm 1 is due to bending vibrations of ammonium. In the structure of ADP strong bonding is there between P and O. The addition of DLM can make a small change in the hydrogen bonding of the crystals. The changes in the hydrogen bonds make some variations in the stretching vibrations and in the peak

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positions. Although this spectrum also carries similar features of that of ADP, there is a distinct evidence for the presence of DLM in the lattice of ADP. The peaks appearing at 2928 and about 2890 cm 1 are due to CH2 vibrations of DLM. In addition, shift in the peak positions of P–O–H and PO4 vibrations compared to ADP established the presence of the additive in the lattice sites of ADP. 3.4. Optical transmittance studies Crystal plates of pure ADP, DLM-doped ADP- and SR-methodgrown DLM-doped ADP with a thickness of 2 mm were cut and polished at face (0 0 1) without any coating for optical measurements. Optical transmission spectra were recorded for all the crystals in the wavelength region from 200 to 1100 nm using Perkin-Elmer Lambda 35 UV–vis spectrometer (Fig. 4). It is observed from the figure that the pure ADP has 50% transmittance and the conventional grown DLM-doped ADP has 52%, and SR-method-grown DLM-doped single crystal has transmittance of 60% in the visible region. In order to confirm the reproducibility, the whole experiment was repeated several times for the crystal plates cut from the different parts of the grown crystals and the same results were observed. The above results indicate that the addition of DLM has increased the transmittance and the SR method also plays important role to enhance the optical transmittance of the crystal. Similar results were reported in KDP and TGS crystals [36,37]. Dhanaraj et al. have reported higher transmission in the visible region for the Lanaline and LAHCl-doped ADP crystals grown by slow cooling along with seed rotation method [33]. 3.5. Thermal analysis Thermal analysis was performed using Perkin-Elmer diamond TG/DTA instrument in nitrogen atmosphere. Fig. 5 Shows the DTA spectra for pure and doped ADP crystals. The DTA curve shows a week endothermic at about 60 1C, which may be due to the presence of DLM inside the crystal and this is followed by a strong endothermic at 207 1C for the DLM-doped ADP corresponding to the decomposition of the crystals. The decomposition temperature of the ADP is decreased by 6 1C when it is doped with DL-malic acid. This appears to be in order as the DLM present inside the crystal could have partially out diffused damaging the 65 SR method grown DLM doped ADP

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lattice partially. The measurement was repeated several times and same results were observed. The melting point of the DLM is 130 1C [Science lab-material safety data sheet]. The presence of DLM appears to decrease the decomposition temperature. 3.6. Measurement of microhardness The good quality crystals are needed for various applications not only with good optical performance but also with good mechanical behaviour. Vicker’s hardness studies have been carried out using the instrument MITUTOYO model MH 120. The indentation hardness was measured as the ratio of applied load to the surface area of the indentation. The conventional- and SRmethod-grown pure and doped crystal of size 5  5  3 mm3 with (0 0 1) face was selected for microhardness studies. Indentations were carried out using Vicker’s indenter for varying loads. For each load (p), several indentations were made and the average value of the diagonal length (d) was used to calculate the microhardness of the crystals. Vicker’s microhardness number was determined using Hv=1.8544p/d2. A plot drawn between the hardness value and corresponding loads is shown in Fig. 6. It is observed from the figure that hardness increases with increase in load for all the crystals and up to 100 g no cracks have been observed for the SR-method-grown DLM-doped crystal. The addition of DLM has enhanced the hardness of the crystal and it is also observed that the mechanical strength of the SR-methodgrown crystal is good compared to the conventional-methodgrown crystals. Hardness is the resistance offered by a solid to the movement of dislocation. Practically, hardness is the resistance offered by a material to localized plastic deformation caused by scratching or by indentation. Due to the application of mechanical stress by the indenter, dislocations are generated locally at the region of the indentation. Higher hardness value for SR-methodgrown crystal indicates that greater stress is required to form dislocation thus confirming greater crystalline perfection. Similar results were reported in KDP crystal [37].

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Dielectric properties are correlated with the electro-optic property of the crystals [38]. The magnitude of dielectric constant depends on the degree of polarization charge displacement in the crystals. The dielectric constant and dielectric loss was measured

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using Agilent 4284-A LCR meter. The dimensions of the used samples were 9  9  2 mm3. Two opposite surfaces across the breadth of the sample were treated with good quality silver paste in order to obtain good ohmic contact. Using the LCR meter, the capacitance of these crystals was measured for the frequencies 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz at various temperatures. The dielectric constant of the crystal was calculated using the relation er=Ccrys/Cair where Ccrys is the capacitance of the crystal and Cair is the capacitance of same dimension of air. Fig. 7(a) shows the dielectric constant of SR-method-grown DLM-doped ADP crystal, conventional-method-grown DLM-doped ADP crystal and conventional-method-grown pure ADP crystal as a function of temperature at the frequency 1 kHz. It is observed from the figure that the dielectric constant increases with increase of temperature. This is normal dielectric behaviour of an antiferroelectric ADP crystal. The same has been observed for all the frequencies (not shown in figure). The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations, which depend on the frequencies. At low frequencies, all these polarizations are active [39,40]. The space charge polarization is generally active at low frequencies and high temperature. It is seen from the Fig. 7(a), that the dielectric constant is higher in SR-method-grown DLM-doped ADP crystal than conventional-method-grown DLM-doped ADP crystal and conventional-method-grown pure ADP crystal at all frequencies. Fig. 7(b) shows the dielectric loss of the grown crystals for various temperatures at the frequency 10 kHz. It is observed from the figure that the dielectric loss increases with increase in temperature for the crystals grown by both of the methods. At higher frequencies it is noted that the dielectric loss is very low (not shown in the figure). It is also observed from the figure that low dielectric loss is obtained at all frequencies in SR-methodgrown ADP crystal compared to the crystal grown by conventional method. The low values of dielectric loss indicate that the grown crystal contains minimum defects [41]. The present study, in effect, indicates that high dielectric constant and low dielectric loss is obtained for the crystal grown by SR method compared to the crystal grown by conventional method. These results clearly indicate that the crystal grown by SR method has better quality than the crystal grown by conventional method. The DLM-doped crystal grown by SR method has higher transparency, higher dielectric constant, higher hardness and lower dielectric loss than the other crystals. This shows that the

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Fig. 7. Temperature dependence of: (a) dielectric constant at frequency 1 kHz; (b) dielectric loss at frequency 10 kHz.

quality of the crystal is comparably higher than other crystals and the crystalline perfection of the SR-method-grown DLM-doped ADP crystal has been increased. The increased crystalline perfection indicates that the dislocation is very low in SR method crystal. The generations of the dislocations are strongly correlated with the formation of the inclusion of the crystals. Depending on the shape of the seed crystal inclusion may arise. The conventional-method-grown crystal has all the facets. The area between these facets develops terraces parallel to the habit faces. In these areas solvent is easily trapped into the crystal. It is also observed that dislocations can originate from the growth sector boundaries [42]. In SR method, as the growth is only on one facet the dislocations of the above causes are avoided.

3.8. HRXRD analysis To reveal the crystalline perfection of the grown crystals, a multicrystal X-ray diffractometer (MCD) developed at NPL [43] has been used to record high-resolution diffraction curves (DCs).

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The high-resolution diffraction curve (DC) was recorded for a typical ADP crystal grown by slow evaporation-method-grown specimen using (2 0 0) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoKa1 radiation. The DC of pure ADP contains a single peak and shows that this specimen is free from structural grain boundaries. It is interesting to see the asymmetry of the DC with respect to the peak position denoted by the dotted line at zero position. For a particular angular deviation (Dy) of glancing angle with respect to the peak position, the scattered intensity is much more in the negative direction in comparison to that of the positive direction. This feature clearly indicates that the crystal contains predominantly vacancy type of defects rather than interstitial defects. This can be well understood by the fact that due to vacancy defects or voids in the crystalline matrix, the lattice around these defects undergo tensile stress and the lattice parameter d (interplanar spacing) increases and leads to give more scattered (also known as diffuse X-ray scattering) intensity at slightly lower Bragg angles (yB) as d and sin yB are inversely proportional to each other in the Bragg equation (2d sin yB=nl; n and l being the order of reflection and wavelength, respectively, which are fixed). However, these point defects with much lesser density as in the present case (if the concentration is high, the FWHM would be much higher and often lead to structural grain boundaries) hardly affect in the performance of the devices made by such crystals. However, the FWHM of this curve is more than that of DLM-doped ADP. Fig. 8 shows the high-resolution DC recorded for 1 mol% DLM-doped ADP specimen using (2 0 0) diffracting planes in symmetrical Bragg geometry by employing the same instrument and same MoKa1 radiation. As seen in the figure the DC is quite sharp without any satellite peaks which may otherwise be observed either due to internal structural grain boundaries [44] or due to epitaxial or a complexating layer which may sometimes form in crystals grown from solution using dopants [45]. The full-width at half-maximum of the diffraction curves is 11 arcsec, which is quite low and close to that expected from the plane wave theory of dynamical X-ray diffraction [46]. The single sharp diffraction curve with very low FWHM indicates that the crystalline perfection is quite good. The specimen is a nearly perfect single crystal without having any internal structural grain boundaries. The DC for SR-grown specimen for the same set of diffracting planes i.e. (2 0 0) were made using the same instrument. This is

Diffracted X-ray intensity [c/s]

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also quite sharp having FWHM of 17 arcsec. But the scattered intensity on both sides of the peak, the magnitude is slightly higher than that of Fig. 8 which indicates that this specimen is grown with slightly more vacancies and interstitials. This may be due to fast growth rate which causes both vacancies and self interstitials.

3.9. SHG conversion efficiency Quantitative measurement of the conversion efficiency of the crystals was determined using the powder technique developed by Kurtz and Perry [47]. The crystal was ground into powder and densely filled into the quartz cell. A fundamental wave with a pulse width of 9 ns, repetition frequency of 10 Hz, a beam diameter of 1 mm and a wave length of 1064 nm radiated from Nd:YAG laser source was focused on the samples by a lens with focal length of 120 mm. An emission of green light was seen in both the samples. The transmitted fundamental wave was absorbed by a CuSO4 solution and the second harmonic signal was detected by a photomultiplier tube and displayed on a storage oscilloscope. ADP crystal powdered to the identical size was used as a reference material in the SHG measurement. The SHG conversion efficiency of DLM-doped crystal grown by both the methods was found to be 1.5 times that of pure ADP. Though thiourea-doped ADP is reported to have SHG efficiency 3 times that of pure ADP [48], DLM-doped ADP has resulted in higher growth rate, large size and higher quality.

4. Conclusion DL-malic acid was successfully doped with ADP and found that the dopant can affect the various properties of pure ADP. Higher growth rate is achieved in SR-method-grown ADP. Single and powder XRD analysis indicates that the structure of the crystal remains same after the addition of 1 mol% of DL-malic acid. The shift in the FTIR spectrum proves the presence of DL-malic acid in the ADP crystal. The transmission spectrum of the crystal reveals that the grown crystal has sufficient transparency in the entire visible region and it is noted that the transparency is higher in SRmethod-grown DLM-doped crystal than the crystal grown by conventional method. Microhardness measurements reveal that the hardness value is increased in DLM-doped crystal and further it is observed that the hardness value is higher in SR-methodgrown DLM-doped crystal than the crystal grown by conventional method. HRXRD analysis confirms the good crystalline perfection of the doped crystals. ADP crystals generate optical second harmonic frequency of an Nd:YAG laser. The SHG conversion efficiency of doped crystal is 1.5 times higher than that of pure ADP. The addition of 1 mol% DL-malic acid in ADP will be useful to grow high-quality, large-size ADP crystals with faster growth rate.

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Fig. 8. Diffraction curve recorded for 1 mol% DLM-doped conventional-methodgrown ADP single crystal for (2 0 0) diffracting planes by employing the multicrystal X-ray diffractometer with MoKa1 radiation.

The authors thank DST, Government of India for funding this research project (No. SR/S2/LOP-0020/2006). The authors are thankful to Dr. G. Bhagavannarayana, Materials Characterization Division, National Physical Laboratory, New Delhi, India for HRXRD analysis and Dr. C. K. Mahadevan, Department of Physics, S.T. Hindu College, India for providing the dielectric measurement facilities.

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