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Uniaxial growth of nonlinear optical active lithium para-nitrophenolate trihydrate single crystal by Sankaranarayanan–Ramasamy (SR) method
ARTICLE IN PRESS
Journal of Crystal Growth 310 (2008) 410–413 www.elsevier.com/locate/jcrysgro
Uniaxial growth of nonlinear optical active lithium para-nitrophenolate trihydrate single crystal by Sankaranarayanan–Ramasamy (SR) method S. Dinakaran, S. Jerome Das Department of Physics, Loyola College, Chennai 600 034, India Received 26 July 2007; received in revised form 6 October 2007; accepted 16 October 2007 Communicated by M. Roth Available online 23 October 2007
Abstract Optically transparent bulk single crystal of lithium para-nitrophenolate trihydrate has been grown along (1 1 0) plane using the uniaxial crystal growth method of Sankaranarayanan–Ramasamy with a slight modification in the growth assembly. The crystal was grown with a growth rate of 7 mm per day up to a dimensions of 80 mm length and 12 mm diameter with in a period of 12 days having cylindrical morphology. The grown crystal was confirmed by single crystal X-ray diffraction analysis. The optical transparency of the crystal was observed by UV–Vis–NIR spectral analysis. The mechanical strength of the grown crystal was tested by Vickers microhardness test along the growth plane (1 1 0). Frequency dependent dielectric studies were carried out along the growth axis. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.h; 78.20.Ci; 81.10.Dn; 42.70.Mp Keywords: A1. Characterization; A1. Crystal growth; A1. Solubility; A2. Growth from solutions; B2. Dielectric materials; B2. Nonlinear optic materials
1. Introduction During the last few years, the semi-organic nonlinear optical (NLO) crystals have attracted much interest due to their superior properties over inorganic counterparts such as high susceptibility, faster response, capability of designing components on the molecular level and NLO co-efficient [1,2]. For practical applications, bulk single crystals are needed with excellent optical transparency. Though the various crystal growth techniques such as solution growth [3], Bridgman–Stockbarger method [4,5], Czochralski method [6] are employed to grow crystals of bulk size, recently discovered unidirectional Sankaranarayanan–Ramasamy (SR) method [7] gains momentum owing to 100% solute-crystal conversion efficiency [8–11] with high degree of transparency [12]. Lithium paranitrophenolate trihydrate (NPLi) is a semi-organic NLO crystal having high figure of merit, deff of about two times Corresponding author. Tel.: +91 44 2817 5662; fax: +91 44 2817 5566.
E-mail addresses: [email protected], [email protected] (S. Jerome Das). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.10.035
than that of lithium niobate [13]. In the present work, the bulk single crystals of NPLi have been grown by SR method with a slight modification in the growth apparatus. The grown crystals were subjected to single crystal X-ray diffraction (XRD) analysis, UV–Vis–NIR spectral analysis, dielectric studies and Vickers hardness test. 2. Experimental procedure 2.1. Material synthesis Lithium para-nitrophenolate trihydrate (NO2–C6H4– OLi 3H2O) has been synthesized using high purity nitrophenol and lithium hydroxide (AR grade) in the stoichiometric ratio 1:1 with de-ionized water. The synthesis was carried out using the reaction NO2 2C6 H4 2OH þ LiOH þ 2H2 O ! NO2 2C6 H4 2OLi 3H2 O
(1)
The purity was improved further by successive recrystallization process.
ARTICLE IN PRESS S. Dinakaran, S. Jerome Das / Journal of Crystal Growth 310 (2008) 410–413
2.2. Solubility studies and seed selection
Thermometer
The solubility of the synthesized salt was carried out using de-ionized water at various temperatures ranging from 303 to 328 K in 5 K intervals by gravimetric analysis [14]. The solubility curve of NPLi is shown in Fig. 1. From the plot, it was observed that the sample shows positive solubility, with 22 g/100 ml at 313 K. Super saturated solution, prepared in accordance with the solubility data was allowed for slow evaporation at 313 K in a constant temperature bath. From the morphology pattern (Fig. 2) of NPLi, the (1 1 0) plane was observed to be quite favorable for the growth experiments. The selected seed of (1 1 0) plane was polished further using de-ionized water and mounted at the bottom of the ampoule. 2.3. Experimental setup and crystal growth The growth setup used here is a modified version of the SR method and the schematic representation of the apparatus is shown in Fig. 3. It consists of heating coil, thermometer, inner container, temperature controller, growth vessel and water bath. A ring heater fixed at the top of cylindrical glass tube of diameter 6 cm and height
40
Solubility (gm/100ml)
35 30 25 20 15 10 5 0 300
305
310 315 320 Temperature (K)
Fig. 1. Solubility curve of NPLi.
411
325
330
Saturated Solution L-Bend Ring Heater Water Inner Container Ampoule Water Bath Growing Crystal Seed
Fig. 3. Crystal growth setup.
40 cm was used as inner container and a short cylindrical constant temperature water bath as outer container. The assembly was designed in such a way to obtain a maximum temperature profile at the top. The ring heater connected to microprocessor-controlled thermocouple provides a constant temperature 318 K at the top of ampoule. A seed was fixed at the bottom of the ampoule and filled with the saturated solution of NPLi which was mounted along the axis of the inner cylinder. The ampoule was designed with an inner L-bend, which controls spontaneous nucleation on the top wall of the ampoule and prevents poly crystallization. The water level inside the water bath was increased with respect to the growth in the ampoule. The temperature gradient creates a concentration gradient along the growth ampoule, having a maximum super saturation at the bottom of the ampoule and a minimum at the top of the ampoule, there by avoiding any possibility of a spurious nucleation along the length of the ampoule. The excess solute generated by evaporation of the solution is driven down the ampoule by the temperature gradient of the setup. Thus, the growth initiated from the seed fixed at the bottom of the ampoule with desired orientation (1 1 0). The growth rate of the crystal was found to be 7 mm per day. The crystals of 80 mm length and 12 mm diameter have been grown successfully with in a period of 12 days. The grown crystal shows a cylindrical morphology and the photograph of the grown crystal is shown in Fig. 4.
3. Results and discussion 3.1. Single crystal X-ray analysis
Fig. 2. Morphology of NPLi single crystal.
Single crystal XRD studies of NPLi was performed with a specimen of dimensions 0.21 0.25 0.32 mm3 using ENRAF NONIUS CAD 4 diffractometer with an incident Mo Ka radiation (l ¼ 0.71703 A˚). Above study reveals that the crystal belongs to monoclinic system with lattice parameters a ¼ 10.867 A˚, b ¼ 7.528 A˚, c ¼ 11.381 A˚.
ARTICLE IN PRESS S. Dinakaran, S. Jerome Das / Journal of Crystal Growth 310 (2008) 410–413
412
Fig. 4. Photograph of grown NPLi single crystal.
350 100 Dielectric constant
300
%T
80
60
40
250 200 150 100 50
20
0 0
0 500
1000 1500 Wavelength (nm)
2000
2
4 log f
6
8
2500 Fig. 6. Variation of dielectric constant with frequency of NPLi crystal.
Fig. 5. Optical transmission spectrum of NPLi crystal.
3.2. UV–Vis–NIR spectral analysis The UV–Vis–NIR spectrum gives valuable information about the absorption of UV and visible light, which involves promotion of electrons in s and p orbitals from the ground state to higher energy state. The UV–Vis–NIR spectrum (Fig. 5) is recorded in the wavelength range between 200 and 2500 nm using Varian Cary 5E UV–Vis– NIR spectrometer. The crystal is highly transparent between the ranges 500 and 1400 nm, which is attributed as an intensive property required for device fabrication. 3.3. Dielectric characterization A study of dielectric response gives information about lattice dynamics in the crystal [15]. Hence, the grown crystal was subjected to dielectric studies using HIOKI 3532-50 LCR HITESTER. An optically clear crystal of rectangular dimensions 1 1 cm2 along (1 1 0) plane was
selected for dielectric studies. Electronic grade silver paste was applied on the polished surface of the sample in order to make firm electrical contact. The experiment was carried out for different frequencies starting from 50 Hz to 5 MHz. The dielectric constant was calculated using the relation ¼
Cd , 0 A
(2)
where C is the capacitance, d is the thickness of the crystal, e0 is the permittivity of free space and A is the area of the crystal. Fig. 6 shows the dielectric constant for different frequencies. From the curve, it was observed that the dielectric constant decreases slowly with increasing frequency and attains saturation at higher frequencies. The high dielectric constant of the crystal at low frequency is attributed due to the presence of all the four polarizations such as electronic, ionic, dipolar and space charge polarization [16].
ARTICLE IN PRESS S. Dinakaran, S. Jerome Das / Journal of Crystal Growth 310 (2008) 410–413
certainly fulfill the demand for the development of costeffective devices. Single crystal X-ray diffraction studies confirm that the crystal belongs to monoclinic system. Optical transmission studies reveal that the crystal is highly transparent between 500 and 1400 nm. Dielectric study shows that the dielectric constant decreases with increase in frequency. The microhardness study reveal the mechanical strength of the crystal.
70 Hardness Hv in Kg/mm2
413
65 60 55 50 45 40
Acknowledgments
35 5
10
15
20
25
30
35
Load P in 10-3 kg Fig. 7. Variation of load P versus HV.
3.4. Microhardness studies The hardness of the material depends on different parameters such as lattice energy, Debye temperature, heat of formation and inter-atomic spacing [17–19]. According to Gong [20], during an indentation process, the external work applied by the indentor is converted to a strain energy component which is proportional to the volume of the resultant impression and the surface energy component proportional to the area of the resultant impression. Microhardness is a general microprobe technique for assessing the bond strength, apart being a measure of bulk strength. The crystal is well polished to avoid surface defects, if any with a thickness variation along the surface less than 10 mm. Microhardness studies were carried out at room temperature along the growth plane (1 1 0) using Shimadzu HMV-2000 fitted with Vickers pyramidal indentor. The load P is varied from 10 to 30 g in 5 g intervals. Twentyfive indentations were made on the sample and the average diagonal length d of the indentation impressions is measured. The Vicker’s microhardness of the material (HV) was determined by the relation [21] P kg=mm2 . (3) 2 d The graph plotted for load P versus hardness is shown in Fig. 7.
H V ¼ 1:8544
4. Conclusions The unidirectional solution growth SR method has been slightly modified for the successful growth of good quality single crystal of NPLi having dimensions of 80 mm length and 12 mm diameter has been successfully grown with in a period of 12 days. The maximum growth rate of 7 mm per day and the achievement of unidirectional growth technology with 100% solute-crystal conversion efficiency will
The authors thank Fr. Albert Muthumalai, Principal, Loyola College, Chennai, and Dr. P. Ramasamy, Dean, Crystal Growth Centre, SSN College of Engineering and Technology and authoritites of SAIF, IIT, Chennai-36, for encouragements. The co-operation rendered by Dr. B. Milton Boaz is acknowledged. References [1] R. Sankar, C.M. Raghavan, R. Mohan Kumar, R. Jayavel, J. Crystal Growth 304 (2007) 156. [2] B. Milton Boaz, J. Mary Linet, B. Varghese, M. Palanichamy, S. Jerome Das, J. Crystal Growth 280 (2005) 448. [3] H.G. Gallagher, R.M. Virceli, J.N. Sherwood, J. Crystal Growth 250 (2003) 486. [4] R. Ramesh Babu, N. Balamurugan, N. Vijayan, R. Gopalakrishnan, G. Bhagavannarayana, P. Ramasamy, J. Crystal Growth 285 (2005) 649. [5] J.C. Brice, Crystal Growth Processes, Wiley, NewYork, 1986. [6] J. Bleay, R.M. Hooper, R.S. Narang, J.N. Sherwood, J. Crystal Growth 43 (1978) 589. [7] K. Sankaranarayanan, P. Ramasamy, J. Crystal Growth 280 (2005) 467. [8] K. Sethuraman, R. Ramesh Babu, R. Gopalakrishnan, P. Ramasamy, J. Crystal Growth 294 (2006) 349. [9] K. Sankaranarayanan, J. Crystal Growth 284 (2005) 203. [10] K. Sankaranarayanan, P. Ramasamy, J. Crystal Growth 292 (2006) 445. [11] C. Justin Raj, S. Jerome Das, J. Crystal Growth 304 (2007) 191. [12] C. Justin Raj, S. Krishnan, S. Dinakaran, R. Uthrakumar, S. Jerome Das, Cryst. Res. Technol., 1–3 (2007) in press, doi:10.1002/ crat.200710968. [13] B. Milton Boaz, A. Leyo Rajesh, S. Xavier Jesu Raja, S. Jerome Das, J. Crystal Growth 262 (2004) 531. [14] P.M. Ushasree, R. Maralidharan, R. Jeyavel, P. Ramasamy, J. Crystal Growth 210 (2000) 741. [15] R. Ramesh Babu, K. Sethuraman, N. Vijayan, G. Bhagavannarayana, R. Gopalakrishnan, P. Ramasamy, Cryst. Res. Technol. 41 (9) (2006) 906. [16] N.V. Prasad, G. Prasad, T. Bhimasankaran, S.V. Suryanarayana, G.S. Kumar, Indian J. Pure Appl. Phys. 34 (1996) 639. [17] D. Arivouli, R. Fornari, J. Kumar, J. Mater. Sci. Lett. 10 (1991) 559. [18] J. Benet Charles, F.D. Gnanam, J. Mater. Sci. Lett. 9 (1990) 165. [19] E. Chacko, J. Mary Linet, S. Mary Navis Priya, C. Vesta, B. Milton Boaz, S. Jerome Das, Indian J. Pure Appl. Phys. 44 (2006) 260. [20] J. Gong, J. Mater. Sci. Lett. 19 (2000) 515. [21] B.W. Mott, Micro Indentation Hardness Testing, Butterworths, London (1956).
Journal of Crystal Growth 310 (2008) 410–413 www.elsevier.com/locate/jcrysgro
Uniaxial growth of nonlinear optical active lithium para-nitrophenolate trihydrate single crystal by Sankaranarayanan–Ramasamy (SR) method S. Dinakaran, S. Jerome Das Department of Physics, Loyola College, Chennai 600 034, India Received 26 July 2007; received in revised form 6 October 2007; accepted 16 October 2007 Communicated by M. Roth Available online 23 October 2007
Abstract Optically transparent bulk single crystal of lithium para-nitrophenolate trihydrate has been grown along (1 1 0) plane using the uniaxial crystal growth method of Sankaranarayanan–Ramasamy with a slight modification in the growth assembly. The crystal was grown with a growth rate of 7 mm per day up to a dimensions of 80 mm length and 12 mm diameter with in a period of 12 days having cylindrical morphology. The grown crystal was confirmed by single crystal X-ray diffraction analysis. The optical transparency of the crystal was observed by UV–Vis–NIR spectral analysis. The mechanical strength of the grown crystal was tested by Vickers microhardness test along the growth plane (1 1 0). Frequency dependent dielectric studies were carried out along the growth axis. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.h; 78.20.Ci; 81.10.Dn; 42.70.Mp Keywords: A1. Characterization; A1. Crystal growth; A1. Solubility; A2. Growth from solutions; B2. Dielectric materials; B2. Nonlinear optic materials
1. Introduction During the last few years, the semi-organic nonlinear optical (NLO) crystals have attracted much interest due to their superior properties over inorganic counterparts such as high susceptibility, faster response, capability of designing components on the molecular level and NLO co-efficient [1,2]. For practical applications, bulk single crystals are needed with excellent optical transparency. Though the various crystal growth techniques such as solution growth [3], Bridgman–Stockbarger method [4,5], Czochralski method [6] are employed to grow crystals of bulk size, recently discovered unidirectional Sankaranarayanan–Ramasamy (SR) method [7] gains momentum owing to 100% solute-crystal conversion efficiency [8–11] with high degree of transparency [12]. Lithium paranitrophenolate trihydrate (NPLi) is a semi-organic NLO crystal having high figure of merit, deff of about two times Corresponding author. Tel.: +91 44 2817 5662; fax: +91 44 2817 5566.
E-mail addresses: [email protected], [email protected] (S. Jerome Das). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.10.035
than that of lithium niobate [13]. In the present work, the bulk single crystals of NPLi have been grown by SR method with a slight modification in the growth apparatus. The grown crystals were subjected to single crystal X-ray diffraction (XRD) analysis, UV–Vis–NIR spectral analysis, dielectric studies and Vickers hardness test. 2. Experimental procedure 2.1. Material synthesis Lithium para-nitrophenolate trihydrate (NO2–C6H4– OLi 3H2O) has been synthesized using high purity nitrophenol and lithium hydroxide (AR grade) in the stoichiometric ratio 1:1 with de-ionized water. The synthesis was carried out using the reaction NO2 2C6 H4 2OH þ LiOH þ 2H2 O ! NO2 2C6 H4 2OLi 3H2 O
(1)
The purity was improved further by successive recrystallization process.
ARTICLE IN PRESS S. Dinakaran, S. Jerome Das / Journal of Crystal Growth 310 (2008) 410–413
2.2. Solubility studies and seed selection
Thermometer
The solubility of the synthesized salt was carried out using de-ionized water at various temperatures ranging from 303 to 328 K in 5 K intervals by gravimetric analysis [14]. The solubility curve of NPLi is shown in Fig. 1. From the plot, it was observed that the sample shows positive solubility, with 22 g/100 ml at 313 K. Super saturated solution, prepared in accordance with the solubility data was allowed for slow evaporation at 313 K in a constant temperature bath. From the morphology pattern (Fig. 2) of NPLi, the (1 1 0) plane was observed to be quite favorable for the growth experiments. The selected seed of (1 1 0) plane was polished further using de-ionized water and mounted at the bottom of the ampoule. 2.3. Experimental setup and crystal growth The growth setup used here is a modified version of the SR method and the schematic representation of the apparatus is shown in Fig. 3. It consists of heating coil, thermometer, inner container, temperature controller, growth vessel and water bath. A ring heater fixed at the top of cylindrical glass tube of diameter 6 cm and height
40
Solubility (gm/100ml)
35 30 25 20 15 10 5 0 300
305
310 315 320 Temperature (K)
Fig. 1. Solubility curve of NPLi.
411
325
330
Saturated Solution L-Bend Ring Heater Water Inner Container Ampoule Water Bath Growing Crystal Seed
Fig. 3. Crystal growth setup.
40 cm was used as inner container and a short cylindrical constant temperature water bath as outer container. The assembly was designed in such a way to obtain a maximum temperature profile at the top. The ring heater connected to microprocessor-controlled thermocouple provides a constant temperature 318 K at the top of ampoule. A seed was fixed at the bottom of the ampoule and filled with the saturated solution of NPLi which was mounted along the axis of the inner cylinder. The ampoule was designed with an inner L-bend, which controls spontaneous nucleation on the top wall of the ampoule and prevents poly crystallization. The water level inside the water bath was increased with respect to the growth in the ampoule. The temperature gradient creates a concentration gradient along the growth ampoule, having a maximum super saturation at the bottom of the ampoule and a minimum at the top of the ampoule, there by avoiding any possibility of a spurious nucleation along the length of the ampoule. The excess solute generated by evaporation of the solution is driven down the ampoule by the temperature gradient of the setup. Thus, the growth initiated from the seed fixed at the bottom of the ampoule with desired orientation (1 1 0). The growth rate of the crystal was found to be 7 mm per day. The crystals of 80 mm length and 12 mm diameter have been grown successfully with in a period of 12 days. The grown crystal shows a cylindrical morphology and the photograph of the grown crystal is shown in Fig. 4.
3. Results and discussion 3.1. Single crystal X-ray analysis
Fig. 2. Morphology of NPLi single crystal.
Single crystal XRD studies of NPLi was performed with a specimen of dimensions 0.21 0.25 0.32 mm3 using ENRAF NONIUS CAD 4 diffractometer with an incident Mo Ka radiation (l ¼ 0.71703 A˚). Above study reveals that the crystal belongs to monoclinic system with lattice parameters a ¼ 10.867 A˚, b ¼ 7.528 A˚, c ¼ 11.381 A˚.
ARTICLE IN PRESS S. Dinakaran, S. Jerome Das / Journal of Crystal Growth 310 (2008) 410–413
412
Fig. 4. Photograph of grown NPLi single crystal.
350 100 Dielectric constant
300
%T
80
60
40
250 200 150 100 50
20
0 0
0 500
1000 1500 Wavelength (nm)
2000
2
4 log f
6
8
2500 Fig. 6. Variation of dielectric constant with frequency of NPLi crystal.
Fig. 5. Optical transmission spectrum of NPLi crystal.
3.2. UV–Vis–NIR spectral analysis The UV–Vis–NIR spectrum gives valuable information about the absorption of UV and visible light, which involves promotion of electrons in s and p orbitals from the ground state to higher energy state. The UV–Vis–NIR spectrum (Fig. 5) is recorded in the wavelength range between 200 and 2500 nm using Varian Cary 5E UV–Vis– NIR spectrometer. The crystal is highly transparent between the ranges 500 and 1400 nm, which is attributed as an intensive property required for device fabrication. 3.3. Dielectric characterization A study of dielectric response gives information about lattice dynamics in the crystal [15]. Hence, the grown crystal was subjected to dielectric studies using HIOKI 3532-50 LCR HITESTER. An optically clear crystal of rectangular dimensions 1 1 cm2 along (1 1 0) plane was
selected for dielectric studies. Electronic grade silver paste was applied on the polished surface of the sample in order to make firm electrical contact. The experiment was carried out for different frequencies starting from 50 Hz to 5 MHz. The dielectric constant was calculated using the relation ¼
Cd , 0 A
(2)
where C is the capacitance, d is the thickness of the crystal, e0 is the permittivity of free space and A is the area of the crystal. Fig. 6 shows the dielectric constant for different frequencies. From the curve, it was observed that the dielectric constant decreases slowly with increasing frequency and attains saturation at higher frequencies. The high dielectric constant of the crystal at low frequency is attributed due to the presence of all the four polarizations such as electronic, ionic, dipolar and space charge polarization [16].
ARTICLE IN PRESS S. Dinakaran, S. Jerome Das / Journal of Crystal Growth 310 (2008) 410–413
certainly fulfill the demand for the development of costeffective devices. Single crystal X-ray diffraction studies confirm that the crystal belongs to monoclinic system. Optical transmission studies reveal that the crystal is highly transparent between 500 and 1400 nm. Dielectric study shows that the dielectric constant decreases with increase in frequency. The microhardness study reveal the mechanical strength of the crystal.
70 Hardness Hv in Kg/mm2
413
65 60 55 50 45 40
Acknowledgments
35 5
10
15
20
25
30
35
Load P in 10-3 kg Fig. 7. Variation of load P versus HV.
3.4. Microhardness studies The hardness of the material depends on different parameters such as lattice energy, Debye temperature, heat of formation and inter-atomic spacing [17–19]. According to Gong [20], during an indentation process, the external work applied by the indentor is converted to a strain energy component which is proportional to the volume of the resultant impression and the surface energy component proportional to the area of the resultant impression. Microhardness is a general microprobe technique for assessing the bond strength, apart being a measure of bulk strength. The crystal is well polished to avoid surface defects, if any with a thickness variation along the surface less than 10 mm. Microhardness studies were carried out at room temperature along the growth plane (1 1 0) using Shimadzu HMV-2000 fitted with Vickers pyramidal indentor. The load P is varied from 10 to 30 g in 5 g intervals. Twentyfive indentations were made on the sample and the average diagonal length d of the indentation impressions is measured. The Vicker’s microhardness of the material (HV) was determined by the relation [21] P kg=mm2 . (3) 2 d The graph plotted for load P versus hardness is shown in Fig. 7.
H V ¼ 1:8544
4. Conclusions The unidirectional solution growth SR method has been slightly modified for the successful growth of good quality single crystal of NPLi having dimensions of 80 mm length and 12 mm diameter has been successfully grown with in a period of 12 days. The maximum growth rate of 7 mm per day and the achievement of unidirectional growth technology with 100% solute-crystal conversion efficiency will
The authors thank Fr. Albert Muthumalai, Principal, Loyola College, Chennai, and Dr. P. Ramasamy, Dean, Crystal Growth Centre, SSN College of Engineering and Technology and authoritites of SAIF, IIT, Chennai-36, for encouragements. The co-operation rendered by Dr. B. Milton Boaz is acknowledged. References [1] R. Sankar, C.M. Raghavan, R. Mohan Kumar, R. Jayavel, J. Crystal Growth 304 (2007) 156. [2] B. Milton Boaz, J. Mary Linet, B. Varghese, M. Palanichamy, S. Jerome Das, J. Crystal Growth 280 (2005) 448. [3] H.G. Gallagher, R.M. Virceli, J.N. Sherwood, J. Crystal Growth 250 (2003) 486. [4] R. Ramesh Babu, N. Balamurugan, N. Vijayan, R. Gopalakrishnan, G. Bhagavannarayana, P. Ramasamy, J. Crystal Growth 285 (2005) 649. [5] J.C. Brice, Crystal Growth Processes, Wiley, NewYork, 1986. [6] J. Bleay, R.M. Hooper, R.S. Narang, J.N. Sherwood, J. Crystal Growth 43 (1978) 589. [7] K. Sankaranarayanan, P. Ramasamy, J. Crystal Growth 280 (2005) 467. [8] K. Sethuraman, R. Ramesh Babu, R. Gopalakrishnan, P. Ramasamy, J. Crystal Growth 294 (2006) 349. [9] K. Sankaranarayanan, J. Crystal Growth 284 (2005) 203. [10] K. Sankaranarayanan, P. Ramasamy, J. Crystal Growth 292 (2006) 445. [11] C. Justin Raj, S. Jerome Das, J. Crystal Growth 304 (2007) 191. [12] C. Justin Raj, S. Krishnan, S. Dinakaran, R. Uthrakumar, S. Jerome Das, Cryst. Res. Technol., 1–3 (2007) in press, doi:10.1002/ crat.200710968. [13] B. Milton Boaz, A. Leyo Rajesh, S. Xavier Jesu Raja, S. Jerome Das, J. Crystal Growth 262 (2004) 531. [14] P.M. Ushasree, R. Maralidharan, R. Jeyavel, P. Ramasamy, J. Crystal Growth 210 (2000) 741. [15] R. Ramesh Babu, K. Sethuraman, N. Vijayan, G. Bhagavannarayana, R. Gopalakrishnan, P. Ramasamy, Cryst. Res. Technol. 41 (9) (2006) 906. [16] N.V. Prasad, G. Prasad, T. Bhimasankaran, S.V. Suryanarayana, G.S. Kumar, Indian J. Pure Appl. Phys. 34 (1996) 639. [17] D. Arivouli, R. Fornari, J. Kumar, J. Mater. Sci. Lett. 10 (1991) 559. [18] J. Benet Charles, F.D. Gnanam, J. Mater. Sci. Lett. 9 (1990) 165. [19] E. Chacko, J. Mary Linet, S. Mary Navis Priya, C. Vesta, B. Milton Boaz, S. Jerome Das, Indian J. Pure Appl. Phys. 44 (2006) 260. [20] J. Gong, J. Mater. Sci. Lett. 19 (2000) 515. [21] B.W. Mott, Micro Indentation Hardness Testing, Butterworths, London (1956).