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Structural, Optical, Microhardness, P-E Hysteresis and SEM studies of Rare- Earth metal Doped Ammonium Penta Borate (APB) Single Crystals

K. Prabha1*, M. Ramesh babu2 and H. Chen2

1Department of Physics, Mother Teresa Women’s University, Kodaikanal, India - 624101. 2State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 10 0190, China.3National Laboratory for Condensed Matter Physics, Institute of Physics, State Key Laboratory, Chinese Academy of Sciences, P. O. Box 603, Beijing 10 0190, China.Received: 28 August 2014: Received in revised form 5 October 2014; Accepted 14 October 2014

AbstractIn the present investigation, influence of rare earth lanthanum (Ld3+) doped ammonium penta borate (APB) have grown by slow solvent evaporation method. The exact weight percentage of the metal dopant was determined by Inductively Coupled Plasma analysis. Single X-ray diffraction studies were carried out for the pure and doped crystals to confirm the structure of the grown crystals. The Nonlinear optical properties were analyzed by SHG studies. The creation of weak lattice stress is confirmed by Vicker’s microhardness test. The ferroelectric properties of pure and La doped single crystals are studied by P-E hysteresis loop. Surface of the pure and doped crystals was analyzed by SEM studies

Keywords: Growth from solution; XRD; Nonlinear optical crystal; Scanning electron microscopy

© 2014 Sri Vidya Mandir Arts & Science College, Uthangarai

*Corresponding author.Tel:+91 9442953257. E-mail address: rk123jay@gmail.com© 2014 Sri Vidya Mandir Arts & Science College, Uthangarai

1. INTRODUCTIONIn the recent past years, efforts have been made

by many researchers to develop ultraviolet lasers for industrial and medical applications. The experiments conducted by Becker (1998) proved that inorganic borate crystals are superior to other commonly used Nonlinear optical (NLO) materials for UV applications [1] In borate crystals, the boron atom usually coordinates with either three or four oxygen atoms forming [BO3]3- or [BO4]5- groups. Ammonium penta borate (APB) is a biaxial crystal of the orthorhombic system and belongs to the mm2 (C2v) class [2]. Rare earth and other element complex borates are of great interest because of their

potential applications in non-linear optics (NLO) and laser engineering [3]. The present study aims to investigate the effect of rare-earth dopant lanthanum on a structural, optical, microhardness, P-E hysteresis and SEM studies of APB.

2. EXPERIMENTAL PROCEDURECrystal Growth

Analytical grade ammonium carbonate ((NH4)2CO3), boric acid (H3BO3) were dissolved in double deionized water according to the stoichiometric ratio. The resulting aqueous solution was filtered and allowed to evaporate under optimized conditions to

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grow crystals by slow evaporation method at room temperature. The reaction formula used for the synthesis of APB is as follows:

(NH4)2CO3 + 10H3BO3

2[NH4(H4B5O10)].2H2O+7H2O + CO2

3 mol % of of lanthanum nitrate is added to the APB solution in order to grow the rare- metal doped crystals. The pure, rare- earth metal doped APB single crystals were grown by slow evaporation technique at room temperature. Figure 1 shows the photographs of the as grown pure and doped APB crystals. The grown pure and doped crystals are found to be transparent and free from defects.

Figure 1 As grown crystals of (a) pure and (b) La3+ doped APB single crystals

3 Results and Discussion3.1 Single Crystal X-ray Diffraction Study

The results of XRD studies confirmed that La3+ doped APB crystallizes in the orthorhombic structure with a non-centrosymmetric space group Aba2. The lattice parameter values of pure and La3+ doped APB single crystals are presented in table 1.

Table 1 Single XRD data of pure and La3+ doped APB

Crystal parameter

Cook & Jaffe (1957)

Pure APB [Present work]

La3+

doped APB [Present work]

a 11.324 Å 11.320 Å 9.219 Åb 11.029 Å 11.027 Å 11.011 Åc 9.325 Å 9.228 Å 11.305 Åα 90.00º 90.00º 90.00ºβ 90.00º 90.00º 90.00ºγ 90.00º 90.00º 90.00º

Table 2 Coercive field (Ec) and Sponataneous polarization (Ps) values of pure and La doped APB single crystals

Crystals Ec (kV/cm) Ps (μC/cm2)Ammonium penta borate (APB)

0.041 6.149x10-4

La doped APB 0.062 7.964x10-4

3.2 Inductive Coupled Plasma Analysis The presence of lanthanum in ammonium penta

borate crystal was confirmed by ICP analysis. The weight percentage of lanthanum in APB crystal system was found to be 0.42 %.

3.3 Optical Absorption StudyThe optical absorption spectra of pure and

lanthanum doped APB single crystals were recorded at room temperature by using VARIAN CARRY 5E MODEL. The cut off wavelengths of pure sample is 230 nm and doped sample is 223 nm.

3.4 Non- Linear Optical PropertiesNonlinear optical (NLO) property of pure and

doped APB were found by subjecting them to Kurtz powder SHG test using the Nd:YAG Q-switched laser

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beam. The pure and doped samples were illuminated using Q-switched, mode locked Nd:YAG laser with input pulse of 6.2 mJ. The second harmonic signal generation in the crystals was confirmed by the emission of green radiation from the crystals. SHG efficiencies of pure and lanthanum doped APB samples are 0.8 and 1.6 times than that of pure KDP respectively. The increase in the SHG efficiency of the doped crystals is due to the presence of metal dopant in the crystal lattice [4].

3.5 Microhardness Studies The microhardness measurements were

carried out using a Leitz Weltzler hardness tester fitted with Vicker’s diamond pyramidal indenter instrument. Indentations were made on the samples and microhardness measurement were applied loads varying from 10 to 100 g for a dwell time of 5 seconds. The Vicker’s microhardness number was calculated using the relation

Hv = 1.8544 (p/d2) Kg/mm2 where Hv is the vicker’s hardness number, p is the indenter load in gm & d is the diagonal length of the impression in mm. The variation of hadrness with indenter load is shown in Fig.2. The hardness of the pure APB is higher than that of La doped APB single crystal. Because of the Lanthanum ions are incorporated in to superficial crystal layer and form defect centres and they generate weak lattice stresses on the surface. [5,6,7].

Figure 2 Microhardness graph of pure and La doped APB single crystals

3.6 P-E Hysteresis Studies The P-E hysteresis loop of pure and La doped

APB crystals were measured at room temperature using RT 6000S, ferroelectric test system is shown in Fig 3a & b. The electrode was prepared the top and bottom side of the samples before measure the P-E loop. Area of the electrode and sample thickness of pure (mm, mm) and La doped samples (mm, mm). A spontaneous polarization (Ps) and coercive field (Ec) values of pure and La doped APB single crystals is shown in table 2. The P-E hysteresis curve for La doped APB shows an increase in coercive field and spontaneous polarization compared to pure ammonium penta borate (APB) single crystals. It may be due to increases of dipole moment with addition of metal in ammonium penta borate single crystals.

(a)

b)Figure 3 P-E hysteresis of (a) pure and (b) La doped

APB single crystals

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3.7 SEM StudyScanning Electron Microscope (SEM) investi-

gations of the as grown samples of La added APB reveal smooth surface whereas they are absent in pure APB crystal. More inter granular pores are present in the pure APB sample. This porous structure can favour the adsorption and condensation of water vapours. The SEM images recorded using JSM 840A are shown in Fig 4.

Figure 4. SEM picture of (a) pure and La doped APB single crystals

4.0. Conclusion Pure and La doped APB single crystals were

harvested by slow solvent evaporation method. Single crystal XRD confirmed the structure of the La3+ doped

and pure APB crystals. The incorporation of La3+ in APB sample was confirmed by inductive coupled plasma analysis (ICP). The measured SHG efficiency of La3+ doped APB crystal was twice that of pure APB single crystal. The optical absorption spectral studies confirmed that the crystals have very low absorption in entire visible region, with the UV cut-off around 230 nm for La3+ doped APB, which is lower than that of pure APB single crystals. The hardness value of La doped APB is lowering than that of pure APB due to the presence of weak lattice stresses on the surface. The ferroelectric properties of the pure and La doped APB were studied by P-E hysteresis loop. The scanning electron microscope pictures show the smooth surface in La doped ammonium penta borate crystals.

ACKNOWLEDGMENTSOne of the authors (K. Prabha) thanks the CAS-

TWAS Fellowship for the financial support to do research in China.

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79−992.

[2] W.R.Cook, and Hans Jaffe, Acta Crystallography, Vol. 10 (1957) pp. 705-707

[3] L.M. Dorozhkin, I.J. Kuratev, N.L. Leonyuk, T.I. Timchenko, T.A.V. Shestakov, Sov. Tech. Phys. Lett. Vol. 7 (1981) 555

[4] P. Praveen Kumar, V. Manivannan, S. Tamilselvan, S. Senthil, Victor Antony Raj, P. Sagayaraj, J. Madhavan, Optics Communications, 281 (2008) 2989-2995.

[5] V. Kannan, R. Bairava Ganesh, R. Sathyalakshmi, N.P. Rajesh, P. Ramasamy, Crystal Research and Technology, Vol. 14 (2006) pp. 678-682.

[6] B. Munirathnamand J. Madhavan, Indian Journal of Science and Technology, Vol.2 (2009) pp 44-45.

[7] P. Pandi, G. Peramaiyan, R. Mohan Kumar and R. Jayavel, Advanced Materials Research, Vol. 584 (2012) pp. 24-28.