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Growth and characterization of pure and metal doped KDP crystals in gel medium Publication History Received: 23 January 2015 Accepted: 07 March 2015 Published: 18 March 2015 Citation Usharani Pisipaty, Dhanabalan O, Abirami S. Growth and characterization of pure and metal doped KDP crystals in gel medium. Indian Journal of Science, 2015, 14(42), 71-83

Indian Journal of Science ANALYSIS International Journal for Science ISSN 2319 – 7730 EISSN 2319 – 7749 © 2015 Discovery Publication. All Rights Reserved

GROWTH AND CHARACTERIZATION OF PURE AND METAL DOPED KDP CRYSTALS IN GEL MEDIUM

Usharani Pisipaty1*, O.Dhanabalan1 and S. Abirami1

Department of Chemistry Sri Chandrasekharendra Saraswathi Viswa Mahavidyalaya University

Enathur, Kanchipuram – 631 561, Tamilnadu, India. * Corresponding author. Email: [email protected]

Abstract

Optically good quality pure and metal doped KDP crystals have been grown by microbial free gel growth method at room temperature and their characterization have been studied. Gel method is a very simple method and can be utilized to synthesizecrystals which are having low solubility. The presences of functional groups of crystals are qualitatively analyzed from FTIR spectra. X-ray diffraction study has been carried out in order to see the effect of dopant on the structural parameters of KDP. The powder X-ray diffraction analysis revealed the tetragonal structure of KDP and doped divalent metal ion. Single crystal XRD revealed the lattice parameter values .Thermal properties like decomposition temperature and weight loss have been reported from the TGA and DTA analysis. The quality, colour and transparency of grown crystals have been confirmed using UV-Vis-spectra. The second harmonic generation (SHG) was measured by using Kurtz powder technique. The relative second harmonic generation (SHG) efficiency of metal doped crystals was higher than the pure KDP crystals. The dielectric behavior of pure and metal doped crystals has been studied in the frequency range from 100Hz to 100 KHz.SEM study was performed to indicate the influence of dopants on surface morphology of KDP crystals .

Keywords: Single crystal growth, growth from gel, non-linear, optical properties

Dielectric studies

Introduction: Potassium dihydrogen orthophosphate (KDP) KH2PO4 continues to be an

interesting material both academically and industrially. KDP is a representative of hydrogen bonded materials which possess very good electro – optic and nonlinear optical properties in addition to interesting electrical properties. Due to the interesting properties, structural phase transitions (at curie temperature 123K) and ease of crystallization, KDP and its isomorphs have been the subject of wide variety of investigations for over 50 years. The demand for high quality large single crystals of KDP increase due to the application as frequency conversion crystal in inertial confinement fusion [1-2]. KDP belongs to scalenohedral class of tetragonal crystal systems [3].

The piezo – electric property of KDP crystal makes it useful for the construction of crystal filters and frequency stabilizers in electronic circuit’s .Use of KDP as a tuning element in laser operation, operation of electro – optic devices is based on the pockel’s effect in which the change in dielectric constant ∆ϵr is a linear function of the applied field [4]. Micro electronics industry needs replacement of high dielectric constant materials in multilevel inter connect structures with new low dielectric constant (ϵr) material as an inter layer dielectric (ILD) which surrounds and insulates inter connect wiring .Lowering the ϵ value of the ILD decrease the RC delay , lowers power consumption and reduces “ cross – talk” between inter connects [5-6].

The excellent properties of KDP include transparency in a wide region of optical spectrum, resistance to damage by laser radiation and relatively high nonlinear efficiency, incombination with reproducible growth to large size. Therefore, it is commonly used in several applications such as laser fusion, electro-optical modulation and frequency conversion [7].Many studies on the growth and properties of KDP crystals in the presence of impurities have been reported [8-9].

Potassium dihydrogen phosphate (KDP) crystal draws persistent attention of scientists due to its excellent quality and possibility of growing large- size crystals [10-11]. Microscopically, crystal growth includes crystal morphology, crystal defects, and growth rate, which are all related to the constituent growth units and their chemical bonding process [12-13]. KDP, ADP and DKDP are the only nonlinear crystal currently used for these applications due to their exclusive properties. These properties included transparency in a wide region of the optical spectrum, resistance to damage by laser radiation, and relatively high nonlinear efficiency. In combination with the only nonlinearcrystal, this can be grown to the large size needed for laser radiation conversion in laser fusion systems. The grown crystals were characterized using XRD, TG/DTA,UV-Vis, dielectric constant,dielectric loss and Vickers micro hardness to reveal the structures , thermal properties , optical transmittance, dielectrics, defects and mechanical strength for pure and metal doped KDP crystals.

Experimental Crystal growth

Pure and metal doped KDP single crystals aregrown in sodium meta silicate gel medium using analar grade KDP and MnSO4, MgSO4,ZnSO4,CuSO4with in concentration of500 mg of dopant & sodium meta silicate (1.06g/cm3 ) .During the process pH was maintained at 4.5at room temperature figure(1). Ethyl alcohol of equal volume is added over the set gel without damaging the cell surface .When the alcohol diffuses into the set gel, it reduces the solubility. This induces nucleation and the nuclei are grown into the single crystals. The crystal growth was carried out at room temperature. The growth period was about 21 days for pure and metal doped KDP crystals. Pure and KDP doped crystals are shown in the fig (2).

Fig. 1 Pure KDP and KDP-metal crystals growth near the interface and inside the gel

Fig. 2 Gel grown Pure KDP and KDP-metal crystals

Characterization

Fine powdered specimens of pure and metal single crystals were subjected to Bruker D8 Advance powder X-ray diffractometer with nickel filtered CuKα radiation (35Kv, 30ma) to see the effect of metal doping on the crystal system and lattice parameters of grown crystals. The FT-IR spectra of pure and metal doped KDP crystals were recorded using thermo-Nicolet Avatar 370 spectrophotometer in the range of 400-4000cm-1 with KBr pellet method of resolution 0.9-1cm.Homogeneous powder was prepared and filtered through 120-125 microns test sieve and density filled in a micro-capillary of good quality glass of 1mm inner bore for SHG measurements. The relative efficiency of pure and metal doped KDP single crystals was measured by Kurtz and Perry powder method by using pure KDP as standard reference. The optical transmission spectra were recorded for the pure as well as doped KDP single crystals in the wavelength range of 200-900nm using perkin Elmer lambda 35 UV-Vis. Spectrophotometer at ambient temperature. Vickers micro hardness studies were carried out using Leitz-Wetzlar hardness tester equipped with a diamond square indenter. Different loads ranging from 25, 50 and 100 gm were used for these studies with a constant identification time of 10 sec. for all these crystals.

Results and Discussion

X-ray diffraction The XRD profiles as shown infig indicate that the samples were of single phase without

detectable impurities .There are some vibrations in peak intensified of metal doped crystals and slight shifts in peak position as a result of metal doping . These observations could be attributed to strains in the lattice. The powder XRD patterns are given in fig (3).

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 (Rigaku, D/maz- 2500) was performed with graphite –monochromated CuK α radiation using a tube voltage and current of 40 Kv and 100mA, respectively. Table 1 lists the observed values and intensity of pure and metal doped KDP crystals.

Table: 1 The X- ray powder diffraction data

d(1wt%KDP)

d(1wt%KDP-Mn)

d(1wt%KDP-Mg)

d(1wt%KDP-Zn)

d(1wt%KDP-Cu)

I(1wt%KDP)

I(1wt%KDP-Mn)

I(1wt%KDP-Mg)

I(1wt%KDP-zn)

I(1wt%KDP-Cu)

5.1040 5.0810 5.0914 5.1223 - - - Weak Weak Weak Weak Weak 3.7333 3.7202 3.7268 3.7402 3.7368 Very

strong Very strong

Very strong

Very strong

Very strong

3.0107 3.0039 3.0078 3.0155 - - - Weak Weak Weak Weak • 2.9118 2.9065 2.9081 2.9170 2.9137 Weak Weak Very

strong Weak Weak

2.6380 2.6326 2.6355 2.6421 2.6409 Weak Weak Weak Weak Weak 2.5482 2.5451 2.5508 - - - 2.5521 Weak Weak Weak • Weak 2.3423 2.3408 2.3419 2.3460 -- Weak Weak Weak Weak - 2.2215 -- - - - 2.2234 -- Weak - - Weak -- 1.9836 1.9813 1.9820 1.9857 1.9859 Weak Weak Weak Weak Weak 1.9541 1.9523 1.9526 1.9565 1.9550 Middle Middle Middle Middle Middle

Fig 3. Powder XRD pattern of Pure KDP and KDP-metal crystals

Single crystal XRD studies Single crystal XRD studies give the lattice parameters. The values of the lattice

parameters constants for pure and doped KDP crystals are given in table 2. Using the tetragonal crystallographic question the lattice parameter vales of pure metal doped crystals are calculated and listed in table 1. Results are compared with the JCPDS (inorganic file No.35-807). The diffraction data matches very well with the reported JCPDS data. Lattice parameter from single XRDdiffraction aretabulated in Table 2.The difference in values due to the metal doped can be attributed to the accommodation of the impurity in the crystal.

Table 2.Single crystal XRD lattice parameter

S.No. Sample Lattice parameter α=β=ϒ Cell Volumes

V(A0)3 Structure

a = b(A0) C(A0) 1 Pure KDP 7.43 6.968 90 385 Tetragonal 2 KDP-Mn 7.43 6.96 90 384 Tetragonal 3 KDP-Mg 7.44 6.97 90 386 Tetragonal 4 KDP-Zn 7.46 6.99 90 388 Tetragonal 5 KDP-Cu 7.30 6.78 90 361 Tetragonal

Thermal studies

The thermal analysis gives the stability and thermal decomposition of pure and metal ions doped KDP crystal. The curves of TG/DTA for pure and metal doped KDP systems recorded in nitrogen ambient in the temperature range between 300and 5000c at a heating rate 150c/min. are given in the figure. To analyze the thermal stability and to confirm the melting point of the material ,the thermo gravimetryanalysis (TGA)and differential thermal analysis (DTA) were

carried out using-STA 1500 thermoanalyser at a heating rate of 200 c min-1 in air is shown in figure(4).

For Mn2+ions doped KDP, TGA curve shows complete weight loss and residual weight obtained is 14.3%. The melting point is 2830c.

For Mg2+ions doped KDP. TG curve mass loss begins at about 2850c which corresponds very well with the pure KDP. The total mass loss is 15.0% and saturation occurs at 3500c. This decomposition is due to the KDP in to K2P2H2O7and water as the melting point is about 2850c.

For Cu2+ divalent ion. TGA curve shows complete weight loss and residual weight observed is 13.7%is well much with pure KDP. The melting point is at2840 C. The mass loss starts at 2500C and ends at3500 C.

Similarly for Zn2+ ions doped KDP, TGA curve shows complete weight loss and residual weight obtained is 14.3% the loss melting point reach about 2810c starts at 2200c and ends at 3500c. This is due to the decomposition of volatile substances.

Fig. 4 TG/DTA curve for Pure KDP and KDP-metal crystals

Dielectric studies Dielectric properties are correlated with electro- optic properties of the crystals particularly when they are non- conducting materials. Permittivity characterization may yield

some useful initial information. The graph wasdrawn between the dielectric loss verses log f. The dielectric loss decreased when metal dopants were added to pure KDP. Due to the incorporation of metal ions polarization increases and the electrical conductivity increases. Fig (5)

The magnitude of dielectric constant depends on the degree of polarization, charge displacement in the crystal. The dielectric constant of materials is due to the contribution of electronic , ionic , dipolar and space charge polarizations which depends on the frequencies [14].At low frequencies , all these , polarization are active . The space charge polarization is generally active at lower frequencies and high temperatures [15], in KDP crystals, many reports are available about its dielectric behavior and in our present work the measured dielectric constant values are in good agreement with the reported results [16-17]. The temperature dependence of dielectric constant at frequency 100Hz to 100Kz is shown in fig. (5). Even though KDP has many reports on dielectric loss, the study clearly ensures the crystalline perfection of crystals in our present case; it is observed that the dielectric loss decreases with increasing frequency and low dielectric losses were observed for the gel method crystal compared to the solution growth. The lower value of dielectric constant is a suitable parameter for the enhancement of SHG signals.

The measurement of dielectric constant and loss as a function of frequency at different temperatures give an idea about the electrical processes that are taking place in materials and these parameters were measured on the polished (010) face of the pure and metal doped KDP crystals. Frequency dependences of dielectric constant of these crystals at room temperature are observed from the figures and it is observed that dielectric constant of KDP and metal doped KDP crystals are high at low frequencies and they decreases with increase in frequency. The very high value of ϵr at low frequencies may be due to the presence of all four polarizations namely. Space charge, orientation, electronic and ionic and its low values at higher frequencies may be due to the loss of significance of these polarizations gradually. The nature of decrease of ϵrfrequency suggests that pure and metal doped KDP crystals contain dipoles of continuously varying relaxation times.

Fig.5 Frequency dependence of dielectric constants of pure and metal doped KDPCrystals

Optical properties studies The recorded transmittance of pure and 500mg divalent metal doped KDP crystals in the wavelength 200-1200nm areas shown in figure (6). It can be seen that the crystals have sufficient transmission in the entire visible and infrared region. The metal doped KDP crystal shows improved transmittance compared to pure KDP crystal. The absorbance is very less for the metal doped crystal.

Fig 6. UV spectrum of Pure KDP and KDP-metal crystals

Michrohardness testings The hardness of a material is a measure of its resistance to plastic deformation. The Vickers micro hardness number Hv was calculated by using relation.

1.8544 P Hv = ------------- (Kg/mm2)

d2

P is the indenter load (Kg) and “d’ is the diagonal length of the impression (mm). The plots of Vickers hardness versus load for the gel grown pure and metal doped KDP crystals is shown in fig(7).The metal doped KDP crystals have higher hardness than pure KDP. This is due to the incorporation of divalent cations in to the pure KDP crystals. Table 3: Michrohardness values of pure KDP and metal doped KDP S.No. Sample 25Kg 50Kg 100Kg

1 Pure KDP 52.1 64.8 76.6 2 KDP – Mn 54.3 66.7 87.6 3 KDP – Mg 53.9 64.4 85.84 4 KDP – Zn 39.2 52.0 70.2 5 KDP- Cu 59.3 67.4 94.8

Fig. 7 Micro hardness number (HV) VS load of pure KDP and metal doped crystals FT-IR Analysis The FTIR spectra of pure and metal doped samples were recorded using thermo – Nicolet Avator 370 spectrophotometer in the range of 400 – 4000cm-1 with KBr pellet method of resolution 0.9cm-1. The recorded spectrum reveals the presences of all functional groups occurring in KDP. The spectrum of metal doped crystals indicates an appreciable shift of peak positions to lower and higher values suggesting incorporation of dopants in the crystal lattice. Using the characteristic frequency values and infrared structural correlation chart,the following vibrational assignments aremade.

Table: 4 observed FT-IR frequencies and intensities of pure and metal doped KDP crystals:

Pure KDP (cm-1)

KDP–Mn (cm-1)

KDP–Mg (cm-

1) KDP–Zn

(cm-1) KDP–Cu

(cm-1) Assignment

-- -- 3413 3418 3421 O-H stret.H2 bonded KDP 3190 3185 -- 3020 2928 OH vibration

Fig. 8. FTIR spectra of Pure KDP and KDP-metal crystals

2804 -- 2816 -- 2811 P-O-H sym.stretch -- -- -- -- 2725 P-O-H sym.stretch

-- -- -- -- 2479 P-O-H sym.stretch 1970 -- -- -- 1953 P-O-H sym.stretch

-- -- -- 1734 -- Stretching of KDP 1690 1705 1598 1619 1593 C=o Stretching 1497 1412 1388 1483 1350 P=o stretchofKDP

-- 1288 1347 1271 -- P=o stretch of KDP -- -- 1233 -- -- P=o stretch of KDP

1122 1098 1078 -- -- P=o stretch of KDP -- 908 936 924 -- P-O-H stretch of KDP

830 -- 764 848 847,763 P-O-H stretch of KDP

614 -- 696 610 680,718 PO43-bending and Liberation of

H2O -- 556,469 514 513 -- Liberation of H2O

Non Liner Optics (NLO) The most widely used technique for confirming the SHG efficiency of NLO materials, to identify the materials with non-Centro symmetric crystal structures, is the Kurtz powder technique. In this method ,the powdered sample with average particle sizes in the range 125-150 µm is filled in micro-capillary tube about 1.5mm diameter.

Q-switched Nd: YAG laser emitting a fundamental wavelength of 1064nm with pulse width 8ns was used. The SHG was confirmed by the emission of green radiation (532nm). The input laser energy incident on the sample was 4.5mj/pulse an energy level optimized not to cause any chemical decomposition of the sample .The SHG efficiency of pure KDP and metal doped KDP are shown in table.

Table: 5 Non Liner Optics (NLO)

S.NO Sample SHG efficiency 1 Pure KDP 10.6mV 2 KDP-Mn 9.0mV 3 KDP-Mg 9.4mV 4 KDP-Zn 8.6mV 5 KDP-Cu 9.2Mv

SEM The influence of metal ion dopants on the surface morphology of KDP crystal and divalent metal reveals structure defect centers seen in SEM image fig (9).

Fig. 9. Surface morphology of Pure KDP and KDP-metal crystals

Conclusion Pure KDP crystals and metal doped KDP crystals are grown by gel method. In gel growth, due to the three dimensional structures, the crystals are free from microbes. The powder X-ray diffraction analysis confirmed the crystal system of pure and metal ions doped crystals. It revealed that the tetragonal structure of KDP and divalent metal ions doped KDP crystals. From the peak, it is confirmed that pure and metal doped KDP belong to the tetragonal structure. It also gave idea about the crystal perfection. Single XRD revealed the lattice parameter values, which are matched with the already reported values. The micro hardness ofmetal ions dopedcrystals showed greater hardness than the pure KDP crystals. Incorporation of metal ions in KDP crystal lattice increased the hardness of metal doped crystals. The functional groups present in the pure and metal doped crystals are confirmedby the FTIR spectrum analysis. UV-Visible studies gave the idea about the quality, colour, and transparency of the pure and metal doped KDP crystals. Themetal doped KDP crystals showed improved transmittance. The NLO properties of the metal doped crystalsshowed comparable SHG efficiency with pure KDP crystals. The capacitance and dielectric loss were measured at different frequencies of pure and metal ions doped KDP crystals. The dielectric constants of metal doped KDP crystals were slightly decreased compared to pure KDP crystals. Thelower the value of dielectric constant more is the enhancement of SHG signals. The stability and decomposition of pure and divalent metal ions doped KDP crystals are determined by TG/DTA analysis. SEM study indicates the influence of dopants on the surface morphology of KDP crystals. From this, structural defects in the pure and metal doped crystals were confirmed. References 1. X.Sun, X, Xu, Z.Gao, Y.Fu, S.Wahg, H.Zong, Y.Li, J.Cryst.Growth 217(2000) 404. 2. N.Zaitseva, L.Carman, I.Smolsky, J.Cryst.Growth 241(2002) 363. 3. R.W.G.Wyckoff, Crystal Structures, vo.3.2ndedn, Interscience, New York, 1960. 4. G.T.Boyd, J.Opt.Soc.Am.B.6 (1989) 685. 5. B.T.Hatton, K.Landskron, W.J.Hunks, M.R.Benett, D.Shukaris,D.DPerovic, G.A.Ozinn, Mater.Today 9(3) (2006) 22. 6. M.Priya, C.K.Mahadevan, Archives of Applied science Research, 3(2011):233-240. 7.Dhanaraj PV, Mahadevan CK, BhagavannarayanG,RamasamyP, Rajesh.NP, Growth and characterization of KDP crystals with potassium carbonate as additives,J.Cryst .Growth 2008; 310:5341-5346. 8. Bo Wang. Chang –shuiFang.Sheng –Iai Wang, Xun Sun, Quing-tianGu,Yi-ping Li, Xin-guangXu, Jian-qinZhang,BingLiu,with potassium carbonate as additives .J.Cryst.Growth 2008;310:5341-5346. 9. N.Pattanaboonmee, P.Ramasamy, P.Manyumprocedia Engineering 32(2012) 1019-1025. 10. Dongli Xu, Dongfeng Xue. J.Cryst. Growth 310 (2008) 1385 11.Dogli Xu, Dongfeng Xue, J.Alloys Compd. 449 (2008) 353 12.Dongli Xu, Dongfeng Xue. J.Cryst. Growth 310 (2008) 1385 13.S.Balamurugan, P.Ramasamy, Spectrochimica Acta part A 71 (2009) 1979-1983 14.Meena M, Mahadevan CK.Cryst Res.Technol 2008; 112:1-4 15.P.Rajesh, P.Ramasamy, Spectrochim, Acta part A 74 (2009) 210 16.P.Rajesh, P.Ramasamy, Mater, Lett.63 (2009) 1611. 17.S.Balamurugan, P.Ramasamy, Mater, Chem.phy. 112(2008) I.

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