Effect of urea as an impurity on the structural, …determined by the Kurtz and Perry powder method 16 using a Nd-YAG Quanta ray laser (with fundamental radiation of wavelength 1064

International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 5 Issue 8, August 2017, ISSN No.: 2348 – 8190

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Effect of urea as an impurity on the structural, optical and electrical properties of potassium pentaborate dihydrate single crystals

S.Benita Jeba Silviya1,C.K.Mahadevan2, T.Balu* A.Moses Ezhil Raj3 and S.Balakumar4

1Department of Physics, V V College of Engineering, Tisayanvilai, Tirunelveli-627657, Tamilnadu,India. 2,4Centre for Scientific & Applied Research, PSN College of Engineering and Technology, Melathediyoor, Tirunelveli-627152,

Tamilnadu, India. *Department of Physics, Aditanar College of Arts and Science, Tiruchendur-628216, Tamilnadu, India.

3Department of Physics, Scott Christian College, Nagercoil-629003, Tamilnadu, India. *[email protected]

ABSTRACT

Potassium pentaborate dihydrate (KB5) crystal is an interesting nonlinear optical (NLO) material. In order to understand the effect of urea as an impurity on the structural, optical and electrical properties of KB5, optically transparent pure and urea added KB5 single crystals were grown by the slow solvent evaporation method and characterized. X-ray diffraction (both single crystal and powder) and FTIR and EDX spectral measurements were carried out to characterize chemically and structurally the grown (pure + 3 urea added) crystals. Optical characterization was done by carrying out UV-Vis spectral and second harmonic generation measurements. Dielectric measurements were carried out at various temperatures in the range 30-90 oC with different frequencies in the range 100Hz-1MHz. Results obtained in the present study indicate that urea addition, without distorting the crystal structure, significantly tunes the optical and electrical properties of the KB5 crystal. Key words: Single crystals, Doped crystals, Crystal growth, Optical properties, Electrical properties.

Graphical abstract

1. Introduction

Potassium pentaborate dihydrate (KB5, KB5O6(OH)4.2H2O) is an interesting and useful ultraviolet (UV) nonlinear optical (NLO) material.1 It is a desirable inorganic material which exhibits a low angular sensitivity.2 It is a positive biaxial

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crystal belonging to the orthorhombic crystal system with a non-centrosymmetric space group Aba2 and lattice parameters: a = 11.065, b = 11.171 and c = 9.054 Ǻ.3 The borate crystal lattice is defined by the rigid polymer boron-oxygen skeleton that gives a series of unique properties and specific features to borate.4

Forming hybrid materials (by impurity addition or forming mixed crystals) is very much necessary for many emerging technologies. Impurity added and single phased mixed crystals without any lattice distortion are expected to have physical properties related to those of the pure crystals. However, the impurity concentration/mixed composition dependence often varies from system to system and from property to property. Once the trend in impurity concentration dependence and/or mixed composition dependence is established, we will have a means to have a tailor-made crystal with a desired value for a given physical property.5- 6 This is as if we have a new crystal for our use.

Several authors have reported the growth at low (near ambient) temperature of single and twinned crystals of KB5.7–11 Poprawski et al 12 have reported the pyroelectric properties, influence of hydrostatic pressure on spontaneous polarization, electro-optic effect and spontaneous birefringence of potassium pentaborate tetrahydrate crystal. Rajasekar et

al 13have reported the growth, thermal and microhardness studies of pure and metal doped KB5 single crystals. Jesudurai et al 14 have reported the growth and characterization of borate mixed crystals of type (NH4)1-xKxB5O8.

Organic compound crystals are often formed by weak vander Waals and hydrogen bonds and hence possess high degree of delocalization. Urea (a small molecular substance) is one among the organic crystals which has been used in practical applications.15

Aiming at discovering new useful materials, in the present study, we have grown KB5 single crystals by the slow solvent evaporation method and investigated the effect of urea as an impurity (added in the solution used for the growth of single crystals) with three different impurity concentrations, viz. 2.5, 5.0 ad 7.5 wt% on the structural, optical and electrical properties of KB5. A total of four (pure + 3 urea added) crystals were grown and characterized chemically, structurally, optically and electrically. Results obtained in our study are reported and discussed herein.

2. Experimental details

2.1 Synthesis and crystal growth

Analytical reagent (AR) grade potassium carbonate, boric acid and urea were used along with de-ionized water for the synthesis and crystal growth.

Required amounts of potassium carbonate and boric acid were dissolved in de-ionized water at room temperature (32 οC) according to the chemical reaction:

K2CO3 + 10 H3BO3 2[KB5O6(OH)4.2H2O]+ 7 H2O + CO2 …..(1)

The above solution was stirred with a magnetic stirrer for 6 h continuously and filtered by using a Whatman filter paper. The filtered solution was taken in a beaker covered with porous paper and kept in a dust free atmosphere. Transparent, colorless and good quality (without any crack or visible unwanted defect) KB5 crystals up to a size of 8 x 7 x 4 mm3 could be harvested in about 32 days.

For the growth of urea added KB5 crystals, the above process was repeated by adding required amount of urea in the solution. Three different concentrations, viz. 2.5, 5.0 and 7.5 wt% of urea were considered. Transparent, colorless and good quality urea added KB5 crystals up to a size of 15 x 8 x 6 mm3 could be harvested in about 26 days.

The four crystals grown in the present study are quite stable in the normal atmospheric condition and are now represented here as Pure KB5, 2.5 wt% urea doped, 5.0 wt% urea doped and 7.5 wt% urea doped respectively for the pure KB5, 2,5 wt% urea added KB5, 5.0 wt% urea added KB5 and 7.5 wt% urea added KB5 crystals. A photograph of the grown crystals is shown in Fig.1.



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Fig.1 Photograph of the grown crystals.

2.2 Characterization Single crystal X-ray diffraction (SXRD) measurement was carried out using a Bruker Kappa Apex II diffractometer with MoKα radiation (λ= 0.71073 Å ) to determine the lattice parameters. X-ray powder diffraction (PXRD) data were collected using an automated X-ray powder diffractometer (PANalytical) in the 2θ range of 10-80 ο with CuKα radiation (λ = 1.5406 Å) to understand the crystallinity of the grown crystals. The observed reflections were indexed using the 2θ (Bragg angle) and d (interplanar spacing) values. Fourier transform infrared (FTIR) spectra were recorded at room temperature by the KBr pellet method using a SHIMADZU spectrometer in the wavenumber range 400 – 4000 cm-1. Energy dispersive X-ray absorption (EDX) spectra were recorded using a scanning electron microscopic system (JEOL JSM-6390). UV-Vis absorption spectra were recorded in the wavelength range 190 – 790 nm using a JASCO UV spectrometer. The forbidden energy gap (Eg) was determined from the cut-off wavelength (λ) using the relation: Eg = hc/λ, where h is the Planck’s constant and c is the speed of light. The second harmonic generation (SHG) efficiency was determined by the Kurtz and Perry powder method 16 using a Nd-YAG Quanta ray laser (with fundamental radiation of wavelength 1064 nm). KDP (KH2PO4) was used as the reference and the results were obtained in terms of KDP unit. The dielectric measurements were carried out to an accuracy of ±2 % by the parallel plate capacitor method using an LCR meter (HIOKI-3532-50) along the direction perpendicular to the major area surface of the crystal at different temperatures in the range 30–90 οC with different frequencies in the range 100 Hz – 1 MHz. Single crystals with high transparency and large surface defect-free size were selected for the electrical measurement. The extended portions of the crystal were removed completely and the opposite faces were polished and coated with good quality silver paste to obtain a good conductive surface layer. A traveling microscope (Least count 0.001 cm) was used to measure the dimensions of the crystals. The dielectric constant ( r) and AC electrical conductivity (σac) were determined from the measured capacitance (C) and dielectric loss factor (tan ) using the relations: r = Cd / ( 0 A) ….. (2) σac = r 0 ω tan ….. ( 3) Here, 0 is the permittivity of free space, A is the area of the crystal touching the electrode, d is the thickness of the crystal and ω (= 2πf, f is the frequency of the applied electric field) is the angular frequency. 3. Results and discussion 3.1 Lattice parameters and chemical nature The lattice parameters obtained through SXRD analysis in the present study are given in Table 1. The lattice parameters obtained for Pure KB5 are found to be in close agreement with those reported in the literature.3 The grown crystals belong to the orthorhombic system with space group Aba2 which is non-centrosymmetric and thus satisfying one of the basic and essential material requirements for the SHG activity of the crystal. The a, b and c parameters (lattice edges) slightly change but the lattice volume does not change significantly due to urea addition. This indicates that the urea molecules have entered into the KB5 crystal matrix but the urea addition does not distort the crystal structure of KB5 significantly.



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Table 1: Lattice parameters obtained for the pure and urea added KB5 crystals.

Crystal a (Å) b (Å) c (Å) V (Å3)

Pure KB5 11.043(3) 11.177(4) 9.042(4) 1116.0(6)

2.5 wt % urea doped 11.075(3) 11.163(2) 9.053(3) 1119.2(4) 5.0 wt % urea doped 11.047(2) 11.160(2) 9.042(1) 1115.0(2) 7.5 wt % urea doped 11.062(6) 11.173(5) 9.045(2) 1118.0(7)

The PXRD patterns observed for all the four crystals grown are shown in Fig.2. Appearance of sharp and strong peaks in the PXRD patterns indicates that the grown crystals exhibit good crystallinity. Small changes in d values and relative intensities observed due to urea addition can be attributed to the presence of urea molecules in the host KB5 crystal matrix.

Fig.2 The PXRD patterns observed for the pure and urea added KB5 crystals.

The FTIR spectra recorded for the pure and urea added KB5 crystals are shown in Fig.3. FTIR spectral analysis was carried out to identify the functional groups present in the crystalline materials considered. FTIR band assignments for all the grown crystals are given in Table 2. The peaks observed at around 3442 and 3058 cm-1 correspond to O-H stretching vibrations.14 The peak at 1434 cm-1 corresponds to B-O terminal asymmetric stretching vibration. The peaks at 1355, 1249 and 1102 cm-1 can be assigned to B-O asymmetric stretching vibrations. Peaks due to B-O ring stretching vibrations are observed at 1019,925 and 782 cm-1. O-B-O terminal bending vibration is observed at 696 cm-1. The sharp peaks observed at 506 and 458 cm-1 can be assigned to O-B-O ring bending vibrations.10 The peaks observed only for the urea added KB5 crystals at 3454, 1685 and 1454 cm-1 can be assigned respectively to N-H, C=O and N-C-N stretching vibrations. Thus, the FTIR spectral analysis indicates the presence of all the functional groups in the pure and urea added KB5 crystals. Also, it indicates the incorporation of urea molecules into the KB5 crystal matrix.



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Fig.3 The FTIR spectra observed for the pure and urea added KB5 crystals.

Table 2: FTIR band assignments for the pure and urea added KB5 crystals

Pure KB5 2.5 wt% urea

doped

5.0 wt% urea

doped

7.5 wt% urea doped

Assignment

- 3454 3454 3454 N-H stretching vibration 3442 3442 3442 3442 O-H Stretching 3374 - - - O-H Stretching 3058 3060 3060 3060 O-H Stretching

- 1685 1685 1685 C=O stretching vibration

1434,1442 1438 1438 1438 B-O terminal asymmetric stretching

- 1454 1454 1454 N-C-N Asymmetric stretching 1355 1353 1353 1353 B-O asymmetric stretching 1249 1250 1250 1250 B-O asymmetric stretching 1102 1102 1102 1102 B-O asymmetric stretching 1019 1030 1030 1030 B-O ring stretching 925 925 925 925 B-O ring stretching 782 789 789 789 B-O ring stretching 696 698 698 698 O-B-O ring asymmetric bending 543 544 544 544 O-B-O terminal bending 509 509 509 509 O-B-O ring bending 453 453 453 453 O-B-O ring bending



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The EDX spectra recorded in the present study are shown in Fig.4. The EDX spectral analysis was used to verify the presence of different elements (K, B and O) in the grown crystals as well as to confirm the incorporation of urea molecules into the KB5 crystal matrix in the case of urea added crystals. Entry of urea molecules into the KB5 crystal matrix in the case of urea added KB5 crystals is confirmed by the observation of less intense peaks due to C and N atoms. These peaks are absent in the case of Pure KB5 crystal. Atomic % and weight % for all the grown crystals are given in Table 3.

Fig.4 EDX spectra observed for the pure and urea added KB5 crystals.

Table 3: Atomic % and weight % for the pure and urea added KB5 crystals

Crystals Elements Energy (Kev) Atomic %

Weight %

Pure KB5 Carbon - - - Nitrogen - - -

2.5 wt % urea doped Carbon 0.110 20.94 19.38 Nitrogen 0.122 10.90 10.78



3.2 Optical Properties The optical behavior of the material basically includes the interaction of light radiation over the range of the electromagnetic spectrum. The ultraviolet light absorbed by the sample gives information about the transmission window which is very essential in many optoelectronic applications.19 The UV-Vis absorption spectra recorded in the present



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study for the pure and urea added KB5 crystals are shown in Fig.5. The observed cut-off wavelengths and forbidden energy gaps are provided in Table 4. Interestingly, urea addition has reduced the optical absorption and cut-off wavelength. This means that the optical transmittance and forbidden energy gap are increased due to urea addition. Also, this increase is in line with the increase in concentration of urea addition. Further, the low value of absorbance in the entire visible range (large window wavelength range) shows the high optical transmission ability of the grown crystals. This property enables the crystals considered in the present study for optoelectronic and SHG applications. The SHG efficiencies observed for the pure and urea added KB5 crystals grown in the present study are given in Table 4. It is interesting to observe that the SHG efficiency also increases, though it is small, in line with the amount of urea added in the solution used for the crystallization process. Thus, the present study indicates that the urea addition leads to significant improvement in the optical properties (optical transmittance, window wavelength range and SHG efficiency) of KB5 single crystals and making them more useful in photonic devices than the pure KB5 crystal.

Fig.5 Optical absorption spectra for the pure and urea added KB5 crystals.

Table 4: Cut-off wavelengths, forbidden energy gaps and SHG efficiencies for the pure and urea added KB5 crystals

Crystal Cut-off wavelength

(nm) Forbidden energy gap

(eV) SHG efficiency

(KDP unit) Pure KB5 219 5.67 0.767

2.5 wt% urea doped 210 5.90 0.785 5.0 wt% urea doped 209 5.94 0.785 7.5 wt% urea doped 205 6.05 0.803

3.2 Electrical properties The dielectric constants ( r) and dielectric loss factors (tan ) observed for the pure and urea added KB5 crystals grown in the present study are shown in Fig.6. The r and tan values are found to decrease with the increase in frequency. The r and tan values increase with the increase in temperature for the Pure KB5 and 7.5 wt% urea doped crystals but these values vary at random with temperature for the 2.5 wt% urea doped and 5.0 wt% urea doped crystals. Moreover, urea addition decreases the above dielectric parameters but not in a systematic way with the concentration of urea addition.

The random variation of r and tan values with temperature and concentration of urea addition can be explained as due to the random distribution of urea molecules occupying the interstitial sites in the crystal matrix. When the concentration increases, the randomness decreases as most of the interstitial sites are occupied. The randomness in urea molecules occupying the interstitial sites leads to randomness in the distribution of hydrogen bonds and thereby leads to



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randomness in polarization which leads to random variation of r and tan values with temperature and concentration of urea addition.

Continuous and gradual decrease of r as well as tan with the increase of frequency suggests that the pure and urea added KB5 crystals considered, like any normal dielectric, may have domains of different sizes and varying relaxation times. The very high value of ϵr at low frequencies may be due to the presence of all the four polarizations, viz. space charge, orientation, electronic and ionic polarizations and its low value at higher frequencies may be due to the loss of significance of these polarizations gradually.17 At low frequencies, dipoles easily respond and align with the applied alternating electric field but as frequency increases dipoles cannot cope up with the pace of fast changing electric field and therefore dielectric constant decreases and become saturated at further higher frequencies.18 The results obtained in the present study suggest that the dielectric constant and dielectric loss factor values are high at low frequencies and they decrease with increase in frequency and attain almost constant value beyond 104 Hz. The observed behavior of tan with frequency supports the enhanced optical quality with less defects and this property is of vital importance for NLO applications.3

Fig.6. Dielectric constant and dielectric loss factor observed for the pure and urea added KB5 crystals.



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The AC electrical conductivities (σac) obtained in the present study are shown in Fig.7. The σac values are found to increase with the increase in frequency as well as temperature. It is also found that, for higher frequencies (1 x 104 Hz or more), σac increases with the increase in concentration of urea addition. When the frequency is 1 x 102 Hz, σac decreases with the increase in concentration of urea addition. When the frequency is 1 x 103 Hz, the σac decreases for the 2.5 wt% urea doped and 5.0 wt% urea doped crystals but increases for the 7.5 wt% urea doped crystal. This means that the variation of σac with concentration of urea addition is not systematic for the lower frequencies.

Fig.7 AC electrical conductivities (σac) and plots between lnσac and 1000/T for the pure and urea added KB5

crystals. The r, tan and σac values observed in the present study compare well with those reported3 for Pure KB5 crystals.

The plots between lnσac and 1000/T (shown in Fig.7) are found to be nearly linear (slight deviation can be seen in few cases) which indicates that the AC electrical conductivity data obtained in the present study can be fitted into the Arrhenius relation for electrical conductivity19-21 :

σac = σoexp(-Eac/(kT)) (3) Here, σo is the pre-exponential factor (a constant depending on the material having the electrical conductivity unit), Eac is the activation energy for AC electrical conduction, k is the Boltzann constant and T is the absolute temperature. From the



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slope (-Eac/k) of the best-fitted line curve the activation energy (Eac) for AC electrical conduction can be evaluated. The AC activation energies estimated in the present study are shown in Fig.8. The Eac value is not found to vary systematically with frequency as well as with concentration of urea addition.

Fig.8 The estimated AC activation energies (Eac) for the pure and urea added KB5 crystals.

The r, tan , σac and Eac values observed in the present study compare well with those reported 3 for Pure KB5 crystals. The KB5 crystals contain water molecules and are hydrogen bonded. In the case of urea added KB5 crystals, the urea molecules mainly occupy the interstitial sites which is expected to disturb the hydrogen bonding system in the host KB5 crystal matrix may be at random. The electrical conduction in KB5 crystals may be determined by the proton transport within the framework of hydrogen bonds as explained in the case of KDP (KH2PO4) and ADP (NH4H2PO4) crystals.19 The low activation energies observed indicate the presence of oxygen vacancies which is a dominating factor for the electrical conduction in pure and urea added KB5 crystals grown in the preset study. Thus, the results obtained indicate that the electrical properties of KB5 crystal could be tuned significantly by urea addition.

4. Conclusion

Good quality, optically transparent and colorless pure and urea added KB5 single crystals with considerable size and good crystallinity could be grown successfully at room temperature by the slow solvent evaporation method. All the four crystals (pure + 3 urea added) grown belong to the orthorhombic crystal system. SXRD, PXRD, FTIR spectral and EDX spectral analyses indicate that, in the case of urea added KB5 crystals, the urea molecules have entered into the host KB5 crystal matrix. Results of UV-Vis absorption spectral and SHG efficiency measurements indicate that the urea addition leads to significant improvement in the optical properties (optical transmittance, window wavelength range and SHG efficiency) of KB5 single crystals and making them more useful in photonic devices than the pure KB5 crystal. Results of AC electrical measurements indicate that the electrical conductivity is due to the proton transport. Also, it is understood that the electrical properties of KB5 crystal could be tuned significantly by urea addition. In effect, the present study indicates that urea addition significantly tunes the optical and electrical properties of KB5 crystal without distorting the crystal structure.

References [1] C.F. Dewey Jr, W.R. Cook Jr, R.T. Hodgson and J.J. Wynne, Frequency doubling in KB5O8.4H2O and NH4B5O8.4H2O to 217.3 nm, Appl. Phys. Lett. 26 (1975) 714-716.



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[2] V.N. Voitsekhovskii, V.P. Nikolaeva and I.A. Velichko, Twinning in potassium pentaborate crystals, Sov. Phys. Crystallogr.27 (1982) 322-323. [3] S. Stella Mary, S. Shahil Kirupavathy, P. Mythili, P. Srinivasan,T. Kanagasekaran and R. Gopalakrishnan , Studies on the growth, optical, electrical and spectral properties of potassium pentaborate (KB5) single crystals, Spectrochim. Acta Part A 71 (2008) 10–16. [4] N.I. Leonyuk, Structural aspects in crystal growth of anhydrous borates, J. Cryst. Growth, 174 (1997) 301-307. [5] J.M. Kavitha and C.K. Mahadevan, Growth and characterization of NixZn1-xSO4.7H2O single crystals, Spectrochim. Acta Part A 128 (2014) 342-350. [6] V. Rajalekshmi and C.K. Mahadevan, Effect of glycine as an impurity on the structural and optical properties of KDP-ADP mixed crystals, Int. J. Innov. Res. Sci. Eng. Technol., 4 (11), (2015) 10957-10970. [7] C. Ramachandra Raja, R.Gobinathan and F.D. Gnanam,Growth and characterization of potassium pentaborate single crystals, Cryst. Res. Technol. 28 (1993) 735-741. [8] K. Thamizharasan, S. Xavier Jesuraja, P. Francis Xavier and P. Sagayaraj, Growth, thermal and microhardness studies of single crystals of potassium pentaborate(KB5), J. Crystal Growth 218 (2000) 323-326. [9] S.A. Rajasekar, K. Thamizharasan, A. Joseph Arul Pragasam, J. Packiam Julies and Sagayaraj, Growth and characterization of pure and doped potassium pentaborate (KB5) single crystals, J.Crystal Growth 247 (2003) 199-206. [10] V. Joseph, S. Gunasekaran and V. Santhanam, Photoconductivity and dielectric studies of potassium pentaborate crystal (KB5), Bull.Mater.Sci. 26 (2003) 383-386. [11] S. Stella Marya, S. Shahil Kirupavathy, P. Mythili, P. Srinivasan, T. Kanagasekaran and R. Gopalakrishnan, Studies on the growth, optical, electrical and spectral properties of potassium pentaborate (KB5) single crystals, Spectrochim. Acta A 71 (2008) 10–16. [12] R. Poprawski, E. Pawlik, S. Matyjasik and Kosturek,Pyroelectric, pressure-electric and optical properties of potassium pentaborate tetrahydrate crystals, Ferroelectrics 159 (1994) 103- 118. [13] S.Abraham Rajasekar, K. Thamizharasan, Joe G. M. Jesudurai, D. Prem Anand and P. Sagayaraj, The role of metallic dopants on the optical and photoconductivity properties of pure and doped potassium pentaborate (KB5) single crystals, Mater.Chem.Phys.84 (2004) 157- 161 . [14] Joe G.M. Jesudurai, K. Prabha, P. Dennis Christy, J. Madhavan and P. Sagayaraj, Synthesis, growth and characterization of new borate-mixed crystals of type (NH4)1−xKxB5O8, Spectrochim. Acta Part A 71 (2008) 1371–1378. [15] G. Bhagavannarayana and S.K. Kushwaha, Enhancement of SHG efficiency by urea doping in ZTS single crystals and its correlation with crystalline perfection as revealed by Kurtz powder and high-resolution X-ray diffraction methods, J. Appl. Cryst. 43 (2010) 154-162. [16] S.K. Kurtz and T.T. Perry, A powder technique for the evaluation of nonlinear optical materials, J. Appl. Phys. 39 (1968) 3798–3814. [17] C. Balarew and R. Duhlew, Application of the hard and soft acids and bases concept to explain ligand coordination in double salt structures, J. Solid State Chem.55 (1984) 1-6. [18] V. Gupta, K.K. Bamzai,P.N. Kotru and B.M. Wanklyn,Dielectric properties, ac Conductivity and thermal behaviour of flux grown cadmium titanate crystals, Mater. Sci. Eng. B 130 (2006) 163-172. [19] M. Meena and C.K. Mahadevan, Growth and electrical characterization of L-arginine added KDP and ADP single crystals, Cryst. Res. Technol. 43 (2008) 166-172. [20] P.V. Dhanaraj, C.K. Mahadevan, G. Bhagavannarayana, P. Ramasamy and N.P. Rajesh, Growth and characterization of KDP crystals with potassium carbonate as additive, J. Cryst. Growth 310 (2008) 5341-5346. [21] J.M. Kavitha and C.K. Mahadevan, Growth and characterization of pure and glycine added morenosite single crystals, Int. J. Eng. Res. Appl. 3(5), (2013) 1931- 1940 .



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Highlights:

High quality urea added potassium pentaborate crystals grown and characterized Doping (3 concentrations) effect on the physicochemical properties understood Presence of urea molecules in the host crystal matrix confirmed by EDX spectra Optical transmittance, window and SHG efficiency improved significantly Crystal structure undistorted but dielectric parameters changed by urea addition


Documents

Effect of urea as an impurity on the structural, …determined by the Kurtz and Perry powder method 16 using a Nd-YAG Quanta ray laser (with fundamental radiation of wavelength 1064