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Growth and characterization of pure and doped potassium pentaborate (KB5) single crystals
Journal of Crystal Growth 247 (2003) 199–206
Growth and characterization of pure and doped potassium pentaborate (KB5) single crystals S.A. Rajasekarc, K. Thamizharasana, A. Joseph Arul Pragasamb, J. Packiam Juliusc, P. Sagayarajc,* b
a Department of Physics, Sir Theagaraya College, Chennai 600 021, India Department of Physics, Sathyabama Institute of Science and Technology, Chennai 600 119, India c Department of Physics, Loyola College, Chennai 600 034, India
Received 25 August 2002; accepted 29 September 2002 Communicated by M. Schieber
Abstract Single crystals of pure and Mg+, Ba+, Ca+ and Cu+ doped potassium pentaborate (KB5) have been grown by low temperature solution growth technique. The growth conditions and surface morphology of pure and doped single crystals of KB5 are optimized and the grown crystals are confirmed by XRD. The pure and doped crystals of KB5 are subjected to TGA and DSC studies. Using the TGA and DSC curves, the enthalpies, decomposition temperature (Td ) and weight loss are measured and the results are analysed and discussed. The influence of the presence of added dopants on the microhardness behaviour of Mg+, Ba+, Ca+ and Cu+ doped KB5 crystals are also studied and discussed. The SHG of the pure and doped samples of KB5 is confirmed by Nd:YAG pulsed laser employing the Kurtz powder technique. r 2002 Elsevier Science B.V. All rights reserved. Keywords: A1. Differential scanning calorimetry; A1. Microhardness; A1. Thermogravimetric analysis; B1. Potassium pentaborate pure and doped single crystals
1. Introduction Inorganic borates exist in numerous structural types and some crystals such as KB5 and BBO are excellent non-linear optical (NLO) materials, particularly in the UV region. These borate crystals generally possess chemical stability, high damage threshold and high optical quality, as well *Corresponding author. Tel.: +91-44-2490490; fax: +91-448231684. E-mail address: p [email protected] (P. Sagayaraj).
as wide range of transparency far into the ultraviolet on account of the rather large difference in the electronegativities of B and O atoms [1]. The properties determining an effective NLO material particularly, the borate compound materials have been discussed and reviewed by several researchers [1–4]. By mixing the fundamental and fifth harmonic at room temperature, Umemura and Kato achieved sixth harmonic generation of the Nd:YAG laser frequency at 1.0642 mm in KB5 crystals [5]. The superiority of KB5 crystals over the popular NLO materials like ADP and KDP
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has been proved by the experiments conducted by Kato, it was necessary to refrigerate the sum frequency generator crystals to about 701C for KDP and 551C for ADP to obtain phase matching, but in the case of KB5 it can be achieved at room temperature itself [6,7]. Potassium pentaborate KB5O8 4H2O belongs to the space group (Aba C17 2v) and contains B5O10 groups in its structure [8]. KB5 crystal is uncolored, optically biaxial positive with optic plane 010 [9]. Using the Bond–Valence theory of complex crystals, the origin of non-linearity of the KB5 crystals has been investigated by Xue Dongfeng and Zhang Siyuan and its larger NLO coefficients are also estimated [10]. The growth of single crystals and twinned crystals of KB5 by low temperature solution growth is reported by several workers [8,11]. Kato attributed the primary advantage of KB5 over KDP and ADP to its slightly large birefringence, which permitted generation of shorter wavelengths without refrigeration of crystal [6,7]. The absolute coefficients of two photon absorption (TPA) of KB5 at 270 and 216 nm were measured by Gurzadyan and Ispiryan and the presence of crystal dopes found to increase non-linear losses [12]. Studies of pyroelectric properties, the influence of hydrostatic pressure on spontaneous polarization, electrooptic effect and spontaneous birefringence of potassium pentaborate tetrahydrate crystal are reported by Poprawski et al. [9]. The experiments conducted by Ramachandra Raja et al. confirmed the anisotropic nature of dielectric constant values of KB5 crystals in the frequency range 0.1–100 KHz [13]. Frequencies of the lattice longitudinal optical and transverse optical modes, their intensities, dampings as well as oscillator strengths and induced dipole moments have been determined for KB5 at 90 and 300 K temperatures [14]. Fedorava et al. investigated the Raman polarization spectra of KB5 and deutero analog (DKB5) in the region of stretching vibrations of OH(OD) groups [15]. In this paper, we report the results of the growth of pure and doped (Mg+, Ba+, Ca+ and Cu+) crystals of KB5 along with the effect of doping on the thermal and microhardness behaviour of the grown crystals.
2. Experimental procedure 2.1. Growth of pure and doped KB5 crystals Single crystal of potassium pentaborate with chemical formula KB5O8 4H2O was synthesized by dissolving the appropriate amount of K2CO3 and H3BO3 in double-distilled water. The resultant product of KB5 was found to be homogeneous. In order to obtain single crystals of high quality, purification of starting material has been an important step hence, recrystallization was carried out for more than three times. The solubility of pure and Mg+, Ba+, Ca+ and Cu+ doped KB5 as a function of temperature is shown in Fig. 1. A 250 ml glass beaker covered with a rubber cork (through which a glass stirrer was inserted) was used as nucleation cell. The stirrer was driven by a stepper motor to ensure homogeneous temperature and concentration through out the entire volume of the solution. The nucleation cell was kept on the platform at a constant temperature bath, which controls the temperature with an accuracy of 0.11C. On reaching saturation, the content of the solution was analysed gravimetrically. A sample of the clear supernatant liquid was withdrawn by means of a warmed pipette and a
Fig. 1. Solubility curves for pure and doped (Mg+, Ba+, Ca+ and Cu+) KB5 crystals.
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weighed quantity of the sample was analysed, several trials were carried out to arrive at the optimized conditions. The same process was repeated for the doped KB5 solutions and the solubility curves were drawn. In order to investigate crystal morphology and to grow large optical quality single crystals of KB5, the seed preparation and its selection become very vital. Seed crystals are prepared either by spontaneous nucleation in a temperature controlled saturated solution or cut from previously grown crystals. Reasonable size seeds with natural habits were mounted on acrylic rods mounted at the bottom of the crystallizer, which served as seed holder. The seeds were seasoned at the growth temperature before initiating the growth. The saturated solution of KB5 was prepared using millipore water with low conductivity at 351C in an air tight container and heated to few degrees above the saturation temperatures to enable homogenization of the solution. Constant stirring of the solution in either direction was employed to overcome concentration gradient in the crystallizer. During the growth, from 401C to 351C a cooling rate of 11C per day was used. A slow cooling rate of 0.1–0.31C per day was employed in the range from 351C to room temperature during the entire growth period.
hardness studies. After polishing the samples, indentations were made on the (0 1 0), (1 0 0) and (0 0 1) faces of pure KB5 crystals and on the (0 1 0) orientation for doped crystals. The microhardness experiments were conducted with Leitz microhardness tester fitted with diamond pyramidal indentor. Hardness of the crystals was calculated using the relation
2.2. DSC and TGA studies
3. Results and discussion
Single crystals of pure and doped (Mg+, Ba+, Ca+ and Cu+) KB5 were subjected to DSC and TGA studies. The instruments used are ‘‘PerkinElmer DSC 7’’ Differential Scanning Calorimeter and ‘‘Perkin-Elmer TGA 7’’ Thermogravimetric analyser. The DSC experiments were carried out within the temperature range 50–5501C and the sample was heated at the rate 201C/min under nitrogen atmosphere. For TGA studies, the crystals were taken in an alumina crucible and were heated from 501C to 8001C at a scanning rate 201C/min (in nitrogen atmosphere).
The solubility curves (Fig. 1) for pure and doped KB5 crystals indicate the existence of a positive
Hv ¼ 1:8544 P=d 2 MPa;
ð1Þ
where, Hv is Vickers microhardness number, P is the indentor load and d is the diagonal length of the impression. 2.4. NLO test The SHG of the crystals were tested by Kurtz powder technique. The samples were prepared by sandwiching the graded crystalline powder with average particle size of about 90 mm between two quartz slides using copper spacers of 0.4 mm thickness. The samples were illuminated using a Q-switched Nd:YAG laser emitting 1.06 mm, 40 ns laser pulses with spot radius of 1 mm. The details of the experimental set-up used in the present work is reported elsewhere [17].
2.3. Microhardness studies Single crystals of pure and doped (Mg+, Ba+, Ca+ and Cu+) KB5 were subjected for micro-
Fig. 2. Photograph of as grown crystal of undoped KB5.
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slope. It is seen that the presence of dopants (Mg+, Ba+, Ca+ and Cu+) has decreased the solubility of KB5. Figs. 2 and 3 show the photographs of typical pure and Ba+ doped KB5 crystals grown from aqueous solution by slow cooling method. During the growth experiments,
Fig. 3. Photograph of as grown Ba+ doped KB5 crystal.
particularly with dopants few needled crystals and twinned crystals are also formed along with perfect crystals. The scheme of formation of twinned concretion of KB5 crystals is reported elsewhere [8]. The presence of dopants in general decreases the dimensions of the growing crystals. However, the morphology of doped crystals is quite similar to that of pure ones. In the present work, good size and optically clear undoped crystals of KB5 with dimensions up to 16 15 6 mm3 are grown in a period of 35–60 days. Among the doped crystals, maximum sized crystal (10 12 8 mm3) is harvested with Ba+ doped KB5. The grown crystals are confirmed by XRD and chemical analysis. Fig. 4 shows the indexed powder XRD pattern of KB5 crystal. The incorporation of dopants in KB5 single crystal has been quantified by the inductively coupled plasma (ICP) analysis. The dopant (Mg+, Ba+, Ca+ and Cu+) concentration in the solution was kept a constant (1 mol%).
Fig. 4. Powder XRD pattern of KB5.
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Fig. 5. DSC and TGA curves for pure KB5 crystal.
The results indicated that for 1 mol% of the dopant concentration added, 0.11 mol% of Mg+, 0.15 mol% of Ba+, 0.18 mol% Ca+ and 0.26 mol% of Cu+ ions have, respectively, entered into the crystal lattice of KB5. Hence, it is seen that the amount of dopant ions incorporated into the crystal lattice is far below its original concentration in the solution. The optical transmission spectra of pure and Ba+ doped KB5 single crystals were recorded in the region 200–2000 nm using the VARIAN CARRY 5E MODEL spectrophotometer. It is found that the crystal is transparent over a wavelength range of 300– 1400 nm for both pure and doped samples. The thermal stability of pure and doped (Mg+, Ba+, Ca+and Cu+) KB5 crystals was studied using differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA). It should be noted that the decomposition temperature (Td ) values only provide a helpful upper limit of thermal stability. The DSC and TGA curves of
Table 1 DSC data for pure and doped KB5 crystals Crystal
Sample weight (mg)
Decomposition temperature (Td ) (1C)
DH (J/g)
KB5 pure Mg+ doped KB5 Ba+ doped KB5 Ca+ doped KB5 Cu+ doped KB5
14.86 05.23 15.34 08.25 08.23
207.187 198.670 210.660 206.670 204.330
495.38 405.03 446.80 514.73 536.76
pure KB5 crystals are shown in Fig. 5. The decomposition temperatures (Td ) and enthalpies for KB5 crystals are listed in Table 1. The DSC curve for pure KB5 indicates that there is no phase transition up to 1601C and after that one distinct anomaly of heat is observed at 207.191C and measured enthalpy is equal to 495.38 J/g. The first endothermic corresponds to the loss of 3 mol of
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H2O. Among the doped crystals, Ba+ doped crystal shows maximum stability (180.451C) and in the case of Cu+ doped KB5 crystals the stability is found to be a minimum (154.811C). From Table 1 it is clear that the incorporation of dopants have introduced small shift in the decomposition temperature (Td ). In the case of doped KB5 crystal (except for Ba+ ion) the reduction in the decomposition temperature compared to pure KB5 may be attributed to the decreased bond energy caused by the addition of Mg+, Ca+ and Cu+ ions. In Mg+ doped crystal, the melting point is decreased by 8.511C in comparison to pure KB5 and the weight loss is increased by 2%. It is estimated from the TGA curves that the doped
KB5 crystals also undergo two-stage thermal decomposition similar to the pure one [11,13]. The values of Hv calculated using Eq. (1) for various loads corresponding to each orientation are plotted as given in Fig. 6. Maximum indentor load applied for KB5 crystal was 50 g, above this load microcracks were observed around the impression and hence readings were not taken for higher loads. The graph indicates that the microhardness number decreases with increasing load and for 25–50 g it is almost a constant for KB5 crystals at different orientations. However, the Vickers hardness number (Hv ) value is found to be less for (1 0 0) orientation suggesting the anisotropic nature in the Hv values.
Fig. 6. Variation of Vickers hardness number with load on (0 1 0), (1 0 0) and (0 0 1) orientations for pure KB5.
Table 2 Hardness data for pure and Mg+, Ba+, Ca+ and Cu+ doped KB5 crystals Crystal
KB5 (pure)
(1 mol%) Mg+ doped KB5 (1 mol%) Ba+ doped KB5 (1 mol%) Ca+ doped KB5 (1 mol%) Cu+ doped KB5
Orientation
010 100 001 010 010 010 010
Hardness number Hv (in MPa) for a load (9.8 103 N) of
Work hardening co-efficient ‘n’
5g
10 g
25 g
50 g
808.696 630.63 771.26 1257.634 687.078 1173.354 1268.414
727.944 585.06 742.84 741.958 672.378 947.366 748.524
674.632 487.06 667.38 607.306 596.428 759.99 608.58
499.506 441.98 639.254 500.682 535.276 725.004 579.67
1.67 1.72 1.84 1.44 1.80 1.65 1.43
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By plotting log d versus log p; the values of work-hardening coefficient n are calculated using the least-squares fit method. The hardness data for pure and Mg+, Ba+, Ca+ and Cu+ doped samples of KB5 are presented in Table 2. The Table indicates the influence of added dopant on the Hv and n values. The work-hardening coefficient value is found to be less in the case of Mg+ and Cu+ doped samples of KB5 for the (0 1 0) orientations as compared to that of the pure KB5 crystals for the same orientation (0 1 0). The hardness results of doped crystals reveal that larger the difference between the ionic radii of the ( for K+) and the foreign ion host ion (1.32 A + ( ( for Cu+), the larger is (0.82 A for Mg and 0.72 A the difference in the Hv and the work-hardening coefficient values [16]. A comparison of hardness studies of KB5 with other popular NLO crystals like KDP and ADP, reveals that the hardness of KB5 crystal is comparatively lower than that of KDP and ADP [18]. The work-hardening coefficient of KB5 for all the planes is less than 2, which is identical to an early work on LAHBr [19]. The trend in load dependence of Hv is reverse in the case of KB5 when compared with LAP. In the case of LAP, at lower loads there is increase in Hv with load [20]. It is also observed that the hardness of KB5 is relatively lower than that of KDP, ADP, ZTS and LAP, and hence a special care has to be taken during processing and device fabrication [18–21]. The second harmonic generation developed in the pure KB5 crystals grown in the present work has been confirmed from the emission of green radiation from the powder sample. The amplitude of the green radiation was found increased to 1.1–1.3 times when the powder samples of doped crystals Mg+, Ba+, Ca+ and Cu+ were used, thereby the enhancement of the NLO property of the doped KB5 crystals was confirmed. However, in the case of Ca+ doped KB5 crystal the amplitude of the green radiation was found to be decreased.
4. Conclusion In the present work, pure and doped (Mg+, Ba+, Ca+ and Cu+) single crystals of KB5 are
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grown by slow cooling method in a period of 35– 60 days. The DSC and TGA analysis confirm that the added dopants slightly alter the decomposition temperature and the weight loss of KB5 crystals. The doped crystals of KB5 undergo two-stage thermal decomposition similar to that of pure ones. The microhardness studies of KB5 crystals indicate the anisotropy in the work-hardening coefficients for different orientations. Work hardening coefficient is found to be less than 2 both for pure and doped crystals of KB5. The NLO studies analysed with Kurtz powder technique confirm that the grown crystals are non-linear in nature.
Acknowledgements One of the authors (KT) acknowledges Dr. T.R. Mahalingam, Head, and A.Thiruvenkatasamy Material Chemistry division, IGCAR, Kalpakkam, Dr. Varadharaju, Head, MSRC, IIT, Chennai and Prof. M. Palanisamy, Anna University for support and encouragement.
References [1] Chuangtian Chen, Yicheng Wu, Rukang Li, J. Crystal Growth 99 (1990) 790. [2] Y. Mori, T. Sasaki, Bull. Mater. Sci. 22 (1999) 399. [3] p. Becker, Adv. Mater. 10 (1998) 979. [4] Kechen Wu, Chuangtian Chen, Appl. Phys. A 54 (1992) 209. [5] N. Umemura, K. Kato, Appl. Opt. 35 (1996) 5332. [6] K. Kato, Opt. Commun. 19 (1976) 332. [7] K. Kato, ieee j. Quantum Electron. 8 (1977) 544. [8] V.N. Voitsekhovskii, V.P. Nikolaeva, I.A. Velichko, Sov. Phys. Crystallogr. 27 (1982) 322. [9] R. Poprawski, E. Pawlik, S. Matyjasik, B. Kosturek, Ferroelectrics 159 (1994) 103. [10] Xue Dong Fing, Zhang Siyvan, Phys. Status Solidi B 200 (1997) 351. [11] C. Ramachandra Raja, R. Gobinathan, F.D. Gnanam, Crystal Res. Technol. 28 (1993) 453. [12] C.G. Gurzadyan, R.K. Ispiryan, Int. J. Non-linear Opt. Phys. 3 (1992) 533. [13] C. Ramachandra Raja, R. Gobinathan, F.D. Gnanam, Crystal Res. Technol. 28 (1993) 735. [14] A. Miniewicz, Y. Marqueton, R. Poprawski, Spectrochim. Acta 49-A (1993) 387. [15] E.N. Fedorava, A.P. Elisev, L.I.T. Isasaenko, Appl. Spectrosc. 55 (1991) 867.
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[16] P. Sagayaraj, S. Sivanesan, R. Gobinathan, Crystal Res. Technol. 30 (1995) 427. [17] S. Chenthamarai, D. Jayaraman, C. Subramanian, P. Ramasamy, Mater. Chem. Phys. 8617 (1995) 1–5. [18] S. Anbukumar, S. Vasudevan, P. Ramasamy, J. Mater. Sci. 5 (1986) 223.
[19] S. Mukerji, T. Kar, Crystal Res. Technol. 34 (1999) 1323. [20] V. Venkataramanan, G. Dhanaraj, H.L. Bhat, J. Crystal Growth 140 (1994) 336. [21] P.M. Ushasree, R. Jayavel, P. Ramasamy, Mater. Chem. Phys. 61 (1999) 270.
Growth and characterization of pure and doped potassium pentaborate (KB5) single crystals S.A. Rajasekarc, K. Thamizharasana, A. Joseph Arul Pragasamb, J. Packiam Juliusc, P. Sagayarajc,* b
a Department of Physics, Sir Theagaraya College, Chennai 600 021, India Department of Physics, Sathyabama Institute of Science and Technology, Chennai 600 119, India c Department of Physics, Loyola College, Chennai 600 034, India
Received 25 August 2002; accepted 29 September 2002 Communicated by M. Schieber
Abstract Single crystals of pure and Mg+, Ba+, Ca+ and Cu+ doped potassium pentaborate (KB5) have been grown by low temperature solution growth technique. The growth conditions and surface morphology of pure and doped single crystals of KB5 are optimized and the grown crystals are confirmed by XRD. The pure and doped crystals of KB5 are subjected to TGA and DSC studies. Using the TGA and DSC curves, the enthalpies, decomposition temperature (Td ) and weight loss are measured and the results are analysed and discussed. The influence of the presence of added dopants on the microhardness behaviour of Mg+, Ba+, Ca+ and Cu+ doped KB5 crystals are also studied and discussed. The SHG of the pure and doped samples of KB5 is confirmed by Nd:YAG pulsed laser employing the Kurtz powder technique. r 2002 Elsevier Science B.V. All rights reserved. Keywords: A1. Differential scanning calorimetry; A1. Microhardness; A1. Thermogravimetric analysis; B1. Potassium pentaborate pure and doped single crystals
1. Introduction Inorganic borates exist in numerous structural types and some crystals such as KB5 and BBO are excellent non-linear optical (NLO) materials, particularly in the UV region. These borate crystals generally possess chemical stability, high damage threshold and high optical quality, as well *Corresponding author. Tel.: +91-44-2490490; fax: +91-448231684. E-mail address: p [email protected] (P. Sagayaraj).
as wide range of transparency far into the ultraviolet on account of the rather large difference in the electronegativities of B and O atoms [1]. The properties determining an effective NLO material particularly, the borate compound materials have been discussed and reviewed by several researchers [1–4]. By mixing the fundamental and fifth harmonic at room temperature, Umemura and Kato achieved sixth harmonic generation of the Nd:YAG laser frequency at 1.0642 mm in KB5 crystals [5]. The superiority of KB5 crystals over the popular NLO materials like ADP and KDP
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 9 7 7 - 2
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has been proved by the experiments conducted by Kato, it was necessary to refrigerate the sum frequency generator crystals to about 701C for KDP and 551C for ADP to obtain phase matching, but in the case of KB5 it can be achieved at room temperature itself [6,7]. Potassium pentaborate KB5O8 4H2O belongs to the space group (Aba C17 2v) and contains B5O10 groups in its structure [8]. KB5 crystal is uncolored, optically biaxial positive with optic plane 010 [9]. Using the Bond–Valence theory of complex crystals, the origin of non-linearity of the KB5 crystals has been investigated by Xue Dongfeng and Zhang Siyuan and its larger NLO coefficients are also estimated [10]. The growth of single crystals and twinned crystals of KB5 by low temperature solution growth is reported by several workers [8,11]. Kato attributed the primary advantage of KB5 over KDP and ADP to its slightly large birefringence, which permitted generation of shorter wavelengths without refrigeration of crystal [6,7]. The absolute coefficients of two photon absorption (TPA) of KB5 at 270 and 216 nm were measured by Gurzadyan and Ispiryan and the presence of crystal dopes found to increase non-linear losses [12]. Studies of pyroelectric properties, the influence of hydrostatic pressure on spontaneous polarization, electrooptic effect and spontaneous birefringence of potassium pentaborate tetrahydrate crystal are reported by Poprawski et al. [9]. The experiments conducted by Ramachandra Raja et al. confirmed the anisotropic nature of dielectric constant values of KB5 crystals in the frequency range 0.1–100 KHz [13]. Frequencies of the lattice longitudinal optical and transverse optical modes, their intensities, dampings as well as oscillator strengths and induced dipole moments have been determined for KB5 at 90 and 300 K temperatures [14]. Fedorava et al. investigated the Raman polarization spectra of KB5 and deutero analog (DKB5) in the region of stretching vibrations of OH(OD) groups [15]. In this paper, we report the results of the growth of pure and doped (Mg+, Ba+, Ca+ and Cu+) crystals of KB5 along with the effect of doping on the thermal and microhardness behaviour of the grown crystals.
2. Experimental procedure 2.1. Growth of pure and doped KB5 crystals Single crystal of potassium pentaborate with chemical formula KB5O8 4H2O was synthesized by dissolving the appropriate amount of K2CO3 and H3BO3 in double-distilled water. The resultant product of KB5 was found to be homogeneous. In order to obtain single crystals of high quality, purification of starting material has been an important step hence, recrystallization was carried out for more than three times. The solubility of pure and Mg+, Ba+, Ca+ and Cu+ doped KB5 as a function of temperature is shown in Fig. 1. A 250 ml glass beaker covered with a rubber cork (through which a glass stirrer was inserted) was used as nucleation cell. The stirrer was driven by a stepper motor to ensure homogeneous temperature and concentration through out the entire volume of the solution. The nucleation cell was kept on the platform at a constant temperature bath, which controls the temperature with an accuracy of 0.11C. On reaching saturation, the content of the solution was analysed gravimetrically. A sample of the clear supernatant liquid was withdrawn by means of a warmed pipette and a
Fig. 1. Solubility curves for pure and doped (Mg+, Ba+, Ca+ and Cu+) KB5 crystals.
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weighed quantity of the sample was analysed, several trials were carried out to arrive at the optimized conditions. The same process was repeated for the doped KB5 solutions and the solubility curves were drawn. In order to investigate crystal morphology and to grow large optical quality single crystals of KB5, the seed preparation and its selection become very vital. Seed crystals are prepared either by spontaneous nucleation in a temperature controlled saturated solution or cut from previously grown crystals. Reasonable size seeds with natural habits were mounted on acrylic rods mounted at the bottom of the crystallizer, which served as seed holder. The seeds were seasoned at the growth temperature before initiating the growth. The saturated solution of KB5 was prepared using millipore water with low conductivity at 351C in an air tight container and heated to few degrees above the saturation temperatures to enable homogenization of the solution. Constant stirring of the solution in either direction was employed to overcome concentration gradient in the crystallizer. During the growth, from 401C to 351C a cooling rate of 11C per day was used. A slow cooling rate of 0.1–0.31C per day was employed in the range from 351C to room temperature during the entire growth period.
hardness studies. After polishing the samples, indentations were made on the (0 1 0), (1 0 0) and (0 0 1) faces of pure KB5 crystals and on the (0 1 0) orientation for doped crystals. The microhardness experiments were conducted with Leitz microhardness tester fitted with diamond pyramidal indentor. Hardness of the crystals was calculated using the relation
2.2. DSC and TGA studies
3. Results and discussion
Single crystals of pure and doped (Mg+, Ba+, Ca+ and Cu+) KB5 were subjected to DSC and TGA studies. The instruments used are ‘‘PerkinElmer DSC 7’’ Differential Scanning Calorimeter and ‘‘Perkin-Elmer TGA 7’’ Thermogravimetric analyser. The DSC experiments were carried out within the temperature range 50–5501C and the sample was heated at the rate 201C/min under nitrogen atmosphere. For TGA studies, the crystals were taken in an alumina crucible and were heated from 501C to 8001C at a scanning rate 201C/min (in nitrogen atmosphere).
The solubility curves (Fig. 1) for pure and doped KB5 crystals indicate the existence of a positive
Hv ¼ 1:8544 P=d 2 MPa;
ð1Þ
where, Hv is Vickers microhardness number, P is the indentor load and d is the diagonal length of the impression. 2.4. NLO test The SHG of the crystals were tested by Kurtz powder technique. The samples were prepared by sandwiching the graded crystalline powder with average particle size of about 90 mm between two quartz slides using copper spacers of 0.4 mm thickness. The samples were illuminated using a Q-switched Nd:YAG laser emitting 1.06 mm, 40 ns laser pulses with spot radius of 1 mm. The details of the experimental set-up used in the present work is reported elsewhere [17].
2.3. Microhardness studies Single crystals of pure and doped (Mg+, Ba+, Ca+ and Cu+) KB5 were subjected for micro-
Fig. 2. Photograph of as grown crystal of undoped KB5.
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slope. It is seen that the presence of dopants (Mg+, Ba+, Ca+ and Cu+) has decreased the solubility of KB5. Figs. 2 and 3 show the photographs of typical pure and Ba+ doped KB5 crystals grown from aqueous solution by slow cooling method. During the growth experiments,
Fig. 3. Photograph of as grown Ba+ doped KB5 crystal.
particularly with dopants few needled crystals and twinned crystals are also formed along with perfect crystals. The scheme of formation of twinned concretion of KB5 crystals is reported elsewhere [8]. The presence of dopants in general decreases the dimensions of the growing crystals. However, the morphology of doped crystals is quite similar to that of pure ones. In the present work, good size and optically clear undoped crystals of KB5 with dimensions up to 16 15 6 mm3 are grown in a period of 35–60 days. Among the doped crystals, maximum sized crystal (10 12 8 mm3) is harvested with Ba+ doped KB5. The grown crystals are confirmed by XRD and chemical analysis. Fig. 4 shows the indexed powder XRD pattern of KB5 crystal. The incorporation of dopants in KB5 single crystal has been quantified by the inductively coupled plasma (ICP) analysis. The dopant (Mg+, Ba+, Ca+ and Cu+) concentration in the solution was kept a constant (1 mol%).
Fig. 4. Powder XRD pattern of KB5.
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Fig. 5. DSC and TGA curves for pure KB5 crystal.
The results indicated that for 1 mol% of the dopant concentration added, 0.11 mol% of Mg+, 0.15 mol% of Ba+, 0.18 mol% Ca+ and 0.26 mol% of Cu+ ions have, respectively, entered into the crystal lattice of KB5. Hence, it is seen that the amount of dopant ions incorporated into the crystal lattice is far below its original concentration in the solution. The optical transmission spectra of pure and Ba+ doped KB5 single crystals were recorded in the region 200–2000 nm using the VARIAN CARRY 5E MODEL spectrophotometer. It is found that the crystal is transparent over a wavelength range of 300– 1400 nm for both pure and doped samples. The thermal stability of pure and doped (Mg+, Ba+, Ca+and Cu+) KB5 crystals was studied using differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA). It should be noted that the decomposition temperature (Td ) values only provide a helpful upper limit of thermal stability. The DSC and TGA curves of
Table 1 DSC data for pure and doped KB5 crystals Crystal
Sample weight (mg)
Decomposition temperature (Td ) (1C)
DH (J/g)
KB5 pure Mg+ doped KB5 Ba+ doped KB5 Ca+ doped KB5 Cu+ doped KB5
14.86 05.23 15.34 08.25 08.23
207.187 198.670 210.660 206.670 204.330
495.38 405.03 446.80 514.73 536.76
pure KB5 crystals are shown in Fig. 5. The decomposition temperatures (Td ) and enthalpies for KB5 crystals are listed in Table 1. The DSC curve for pure KB5 indicates that there is no phase transition up to 1601C and after that one distinct anomaly of heat is observed at 207.191C and measured enthalpy is equal to 495.38 J/g. The first endothermic corresponds to the loss of 3 mol of
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H2O. Among the doped crystals, Ba+ doped crystal shows maximum stability (180.451C) and in the case of Cu+ doped KB5 crystals the stability is found to be a minimum (154.811C). From Table 1 it is clear that the incorporation of dopants have introduced small shift in the decomposition temperature (Td ). In the case of doped KB5 crystal (except for Ba+ ion) the reduction in the decomposition temperature compared to pure KB5 may be attributed to the decreased bond energy caused by the addition of Mg+, Ca+ and Cu+ ions. In Mg+ doped crystal, the melting point is decreased by 8.511C in comparison to pure KB5 and the weight loss is increased by 2%. It is estimated from the TGA curves that the doped
KB5 crystals also undergo two-stage thermal decomposition similar to the pure one [11,13]. The values of Hv calculated using Eq. (1) for various loads corresponding to each orientation are plotted as given in Fig. 6. Maximum indentor load applied for KB5 crystal was 50 g, above this load microcracks were observed around the impression and hence readings were not taken for higher loads. The graph indicates that the microhardness number decreases with increasing load and for 25–50 g it is almost a constant for KB5 crystals at different orientations. However, the Vickers hardness number (Hv ) value is found to be less for (1 0 0) orientation suggesting the anisotropic nature in the Hv values.
Fig. 6. Variation of Vickers hardness number with load on (0 1 0), (1 0 0) and (0 0 1) orientations for pure KB5.
Table 2 Hardness data for pure and Mg+, Ba+, Ca+ and Cu+ doped KB5 crystals Crystal
KB5 (pure)
(1 mol%) Mg+ doped KB5 (1 mol%) Ba+ doped KB5 (1 mol%) Ca+ doped KB5 (1 mol%) Cu+ doped KB5
Orientation
010 100 001 010 010 010 010
Hardness number Hv (in MPa) for a load (9.8 103 N) of
Work hardening co-efficient ‘n’
5g
10 g
25 g
50 g
808.696 630.63 771.26 1257.634 687.078 1173.354 1268.414
727.944 585.06 742.84 741.958 672.378 947.366 748.524
674.632 487.06 667.38 607.306 596.428 759.99 608.58
499.506 441.98 639.254 500.682 535.276 725.004 579.67
1.67 1.72 1.84 1.44 1.80 1.65 1.43
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By plotting log d versus log p; the values of work-hardening coefficient n are calculated using the least-squares fit method. The hardness data for pure and Mg+, Ba+, Ca+ and Cu+ doped samples of KB5 are presented in Table 2. The Table indicates the influence of added dopant on the Hv and n values. The work-hardening coefficient value is found to be less in the case of Mg+ and Cu+ doped samples of KB5 for the (0 1 0) orientations as compared to that of the pure KB5 crystals for the same orientation (0 1 0). The hardness results of doped crystals reveal that larger the difference between the ionic radii of the ( for K+) and the foreign ion host ion (1.32 A + ( ( for Cu+), the larger is (0.82 A for Mg and 0.72 A the difference in the Hv and the work-hardening coefficient values [16]. A comparison of hardness studies of KB5 with other popular NLO crystals like KDP and ADP, reveals that the hardness of KB5 crystal is comparatively lower than that of KDP and ADP [18]. The work-hardening coefficient of KB5 for all the planes is less than 2, which is identical to an early work on LAHBr [19]. The trend in load dependence of Hv is reverse in the case of KB5 when compared with LAP. In the case of LAP, at lower loads there is increase in Hv with load [20]. It is also observed that the hardness of KB5 is relatively lower than that of KDP, ADP, ZTS and LAP, and hence a special care has to be taken during processing and device fabrication [18–21]. The second harmonic generation developed in the pure KB5 crystals grown in the present work has been confirmed from the emission of green radiation from the powder sample. The amplitude of the green radiation was found increased to 1.1–1.3 times when the powder samples of doped crystals Mg+, Ba+, Ca+ and Cu+ were used, thereby the enhancement of the NLO property of the doped KB5 crystals was confirmed. However, in the case of Ca+ doped KB5 crystal the amplitude of the green radiation was found to be decreased.
4. Conclusion In the present work, pure and doped (Mg+, Ba+, Ca+ and Cu+) single crystals of KB5 are
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grown by slow cooling method in a period of 35– 60 days. The DSC and TGA analysis confirm that the added dopants slightly alter the decomposition temperature and the weight loss of KB5 crystals. The doped crystals of KB5 undergo two-stage thermal decomposition similar to that of pure ones. The microhardness studies of KB5 crystals indicate the anisotropy in the work-hardening coefficients for different orientations. Work hardening coefficient is found to be less than 2 both for pure and doped crystals of KB5. The NLO studies analysed with Kurtz powder technique confirm that the grown crystals are non-linear in nature.
Acknowledgements One of the authors (KT) acknowledges Dr. T.R. Mahalingam, Head, and A.Thiruvenkatasamy Material Chemistry division, IGCAR, Kalpakkam, Dr. Varadharaju, Head, MSRC, IIT, Chennai and Prof. M. Palanisamy, Anna University for support and encouragement.
References [1] Chuangtian Chen, Yicheng Wu, Rukang Li, J. Crystal Growth 99 (1990) 790. [2] Y. Mori, T. Sasaki, Bull. Mater. Sci. 22 (1999) 399. [3] p. Becker, Adv. Mater. 10 (1998) 979. [4] Kechen Wu, Chuangtian Chen, Appl. Phys. A 54 (1992) 209. [5] N. Umemura, K. Kato, Appl. Opt. 35 (1996) 5332. [6] K. Kato, Opt. Commun. 19 (1976) 332. [7] K. Kato, ieee j. Quantum Electron. 8 (1977) 544. [8] V.N. Voitsekhovskii, V.P. Nikolaeva, I.A. Velichko, Sov. Phys. Crystallogr. 27 (1982) 322. [9] R. Poprawski, E. Pawlik, S. Matyjasik, B. Kosturek, Ferroelectrics 159 (1994) 103. [10] Xue Dong Fing, Zhang Siyvan, Phys. Status Solidi B 200 (1997) 351. [11] C. Ramachandra Raja, R. Gobinathan, F.D. Gnanam, Crystal Res. Technol. 28 (1993) 453. [12] C.G. Gurzadyan, R.K. Ispiryan, Int. J. Non-linear Opt. Phys. 3 (1992) 533. [13] C. Ramachandra Raja, R. Gobinathan, F.D. Gnanam, Crystal Res. Technol. 28 (1993) 735. [14] A. Miniewicz, Y. Marqueton, R. Poprawski, Spectrochim. Acta 49-A (1993) 387. [15] E.N. Fedorava, A.P. Elisev, L.I.T. Isasaenko, Appl. Spectrosc. 55 (1991) 867.
206
S.A. Rajasekar et al. / Journal of Crystal Growth 247 (2003) 199–206
[16] P. Sagayaraj, S. Sivanesan, R. Gobinathan, Crystal Res. Technol. 30 (1995) 427. [17] S. Chenthamarai, D. Jayaraman, C. Subramanian, P. Ramasamy, Mater. Chem. Phys. 8617 (1995) 1–5. [18] S. Anbukumar, S. Vasudevan, P. Ramasamy, J. Mater. Sci. 5 (1986) 223.
[19] S. Mukerji, T. Kar, Crystal Res. Technol. 34 (1999) 1323. [20] V. Venkataramanan, G. Dhanaraj, H.L. Bhat, J. Crystal Growth 140 (1994) 336. [21] P.M. Ushasree, R. Jayavel, P. Ramasamy, Mater. Chem. Phys. 61 (1999) 270.