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Influence of pH on the characteristics of zinc tris (thiourea) sulfate (ZTS) single crystals
Materials Chemistry and Physics 61 (1999) 270±274
Materials Science Communication
In¯uence of pH on the characteristics of zinc tris (thiourea) sulfate (ZTS) single crystals P.M. Ushasree, R. Jayavel*, P. Ramasamy Crystal Growth Centre, Anna University, Chennai 600025, India Received 16 April 1999; received in revised form 15 June 1999; accepted 23 June 1999
Abstract Growth of bulk ZTS single crystals from aqueous solution by low temperature solution growth technique has been reported. The in¯uence of pH on the structural and mechanical properties has been discussed. The FTIR spectra taken for the crystals grown at different pH values reveal that while decreasing the pH more number of sulfate ions is introduced in the coordination sphere of ZTS leading to the increase in the growth rate along [100] direction. The dielectric constant for ZTS single crystals has been determined. Vicker's microhardness studies show that the crystals grown at lower pH are harder than that grown at higher pH values. # 1999 Elsevier Science S.A. All rights reserved. Keywords: ZTS single crystals; Characteristics ; pH values
1. Introduction Nonlinear optical materials (NLO) have wide applications in the area of laser technology, optical communication and data storage technology. Recent interest has been centered on semiorganic crystals which have the combined properties of both inorganic and organic crystals. Among the semiorganic nonlinear optical materials, metal complexes of thiourea which have a low UV cut off wavelengths, applicable for high power frequency conversion, are of interest. The chief virtue, as far as today's technology and research are concerned, is that these materials can be used as better alternatives for KDP crystals in frequency doubling and laser fusion experiments due to their higher values of laser damage threshold. Growth of bulk single crystals of these materials has been a subject of perennial concern in order to use these materials for device application. Zinc tris (thiourea) sulfate (ZTS), Zn[CS(NH2)2]3 SO4 is a desirable semiorganic nonlinear optical material which exhibits a low angular sensitivity and hence, proves useful for type-II second harmonic generation (SHG) [1±4]. High damage threshold and wide transparency make it a better substitute for other nonlinear optical
* Corresponding author. Tel.: +91-144-235-2774; fax: +91-144-235-2774 E-mail address: [email protected] (R. Jayavel)
materials [5]. Marcy et al., [4] have reported ZTS as an ef®cient nonlinear optical material for SHG. Venkatramanan et al., [5] have reported that the ZTS crystals have the highest laser induced damage threshold values among the other solution grown NLO crystals. Thermal properties of ZTS crystal have also been reported [6]. The variation of growth rate with a wide range of pH values has already been reported [7]. In the present investigation, the growth of bulk ZTS single crystals and the effect of pH on the structural and mechanical properties have been discussed. Large size crystals are grown with reasonable growth rate along the three crystallographic directions at an optimised pH (3.45). The grown crystals have been subjected to structural and mechanical characterization studies. The dielectric studies have been carried out for the crystal grown at optimised pH. 2. Experimental 2.1. Crystal growth ZTS can be grown from aqueous solution, both by slow cooling and by slow evaporation. The starting material was prepared by evaporating a solution of Analar grade, zinc sulfate heptahydrate and thiourea taken in stoichiometric ratio 1 : 3 in deionised water to almost dryness at room
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 5 0 - 9
P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
temperature. ZTS salt was synthesised according to the reaction: ZnSO 10H2 OZinc sulfate heptahydrate 3CS NH2 2 Thiourea
271
where Hv is the Vicker's microhardness number, P is the applied load and d is the diagonal length of the indentation impression.
4
!ZnCS NH2 2 3 SO4 ZTS
(1)
The purity of the synthesised salt was further increased by successive recrystallization process. Growth was carried out by low temperature solution growth technique by slow cooling, in a constant temperature bath controlled to an accuracy of 0.018C. Crystal growth was performed on the seeds suspended in the solution saturated at 488C and by lowering the temperature from 488C to room temperature at a rate of 0.03±0.28C dayÿ1. The period of growth ranged from 40 to 45 days. Care was taken during heating of the solution and temperature as low as 508C was maintained in order to avoid decomposition. 2.2. Crystal structure Powder X-ray diffraction (XRD) pattern has been recorded by crushing the ZTS single crystal into ®ne powder using a Rich Seifert diffractometer (Model, 2002) with Cu Ê ) radiation. The sample was scanned over Ka ( = 1.5418A the range 10±508 at a rate of 18 minÿ1. Single crystal XRD analysis of ZTS was carried out using Ehraf CAD-4 difÊ ) radiation. fractometer with Mo Ka ( = 0.7170A 2.3. FTIR analysis
3. Results and discussion At stoichiometric pH (5.6), ZTS grows as plates, elongated along b direction with reduced growth rate along aaxis, as shown in Fig. 1a and the growth habit can be changed by adjusting the pH. At a higher pH of 6.54, the ZTS crystal grows as plates with still reduced growth rate along the a-axis. At a lower pH of about 3.15 the growth rate along the a-axis increases and is comparable to that of b and c directions. At this pH, the growth rates along b and c directions are almost equal. The large ZTS crystals grown with an optimised pH of 3.45 are shown in Fig. 1b. In ZTS, the thioureas are planar, with Zn+2 tetrahedrally coordinated by three thiourea sulfur and one sulfate oxygen. The four formula units are contained in the orthorhombic (Pca21) unit cell (point group mm2). Hydrogen bonding exists between thiourea amino hydrogens and sulfate oxygens. The birefringence and the nonlinear coef®cients of the ZTS crystal are largely determined by the nearly coplanar arrangement of the thiourea units within the structure [4]. The observed and calculated d values for different 2q values along with the hkl index of the corresponding re¯ecting planes are tabulated in Table 1. The lattice parameter values recorded from single crystal XRD analysis for the crystals
The Fourier transform infrared (FTIR) spectra of ZTS crystals were recorded in the range 400±4000 cmÿ1 employing a Perkin-Elmer spectroscopy in the form of solids dispersed in KBr pellets. The FTIR spectra of the ZTS crystals grown at different pH values have been recorded in order to qualitatively analyze the incorporation of sulfate into the crystal lattice. 2.4. Dielectric studies The dielectric permittivity data of the ZTS crystals were estimated using LCR bridge. Dielectric measurements were carried out on b cut plates of ZTS in the frequency range of 0.1±100 kHz at room temperature (288C). The dielectric loss and the capacitance were calculated. 2.5. Microhardness studies The polished (100) face of ZTS single crystals grown at pH 3.45, 4.54 and 6.54 were subjected to static indentation tests at room temperature using a Leitz Wetzlar hardness tester ®tted with Vicker's diamond pyramidal indenter. Vicker's microhardness number was then evaluated from the relation.: Hv
1:8544 P kg mmÿ2 d2
(2)
Fig. 1. ZTS crystals grown at (a) pH 5.6 (b) pH 3.45.
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P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
Table 1 Powder XRD for ZTS single crystals 2q (8)
Experimental Ê) values (A
Calculated Ê) d values (A
hkl
13.60 14.95 16.00 19.60 20.30 22.10 26.10 28.50 30.20 31.80 32.90 33.20 35.20 36.50 39.95 43.70 45.00 48.05
6.5107 5.9257 5.5391 4.5291 4.3745 4.0221 3.4140 3.1317 2.9592 2.8139 2.7223 2.6983 2.5495 2.4616 2.2560 2.0713 2.0144 1.8934
6.3794 5.9257 5.5553 5.5225 4.3417 4.0115 3.4012 3.1240 2.9490 2.8066 2.7134 2.7040 2.5546 2.4612 2.2564 2.0673 2.0103 1.8947
110 111 200 210 211 113 213 221 222 313 223 205 215 224 404 405 234 127
grown at different pH are given in Table 2. The single crystal XRD data shows a slight increase in the cell volume with pH increase. Figs. 2a,b,c show the FTIR spectra of the ZTS crystals grown at different pH values. In ZTS the coordination in the complex occurs through sulfur [7]. The ®gures show a broad envelope lying between 2750 cmÿ1 and 3500 cmÿ1 arising out of symmetric and asymmetric modes of NH2 grouping of Zn coordinated thiourea. The absorption band observed at 1628 cmÿ1 can be assigned to the NH2 bending vibration and that observed at 1502 cmÿ1 and 955 cmÿ1 can be assigned to N-C-N stretching vibration. The absorption band observed at 1404 cmÿ1 and 714 cmÿ1 can be assigned to the C=S stretching vibration. The high frequency absorption bands in the spectrum of thiourea were not shifted to lower frequency, but are either narrowed or broadened, which is a clear indication of incorporation of more number of sulfate ions. The presence of peaks around 500 cmÿ1 and 1000 cmÿ1 also indicated the increase in the number of sulfate ion incorporation in the coordination sphere. Comparison of the spectra of ZTS crystals grown at different pH values clearly indicated that as the pH value is decreased the sulfate ion introduction in the coordination sphere increases, resulting in an increase in the growth rate. Hence, the spectra reveal that the increase in growth rate with decrease
Table 2 Lattice parameters of ZTS single crystals grown at different pH values pH
Ê) a (A
Ê) b (A
Ê) c (A
Ê 3) V (A
6.54 4.54 3.45
7.794 7.793 7.796
11.141 11.162 11.163
15.503 15.529 15.526
1346.29 1350.94 1351.39
Table 3 Vicker's microhardness number of ZTS single crystals grown at different pH values pH
Plane
VHN (kg mmÿ2)
6.54 4.54 3.45
100 100 100
58 116 120
in pH is due to the increase in the number of the crystal growth enhancing site, namely, the sulfate ions. Fig. 3 shows the variation of dielectric permittivity ("0 ) with frequency at room temperature. The dielectric permittivity is maximum at low frequencies and decreases with increasing frequency. The increase in the dielectric constant at low frequency is attributed to space charge polarization [8]. Fig. 4 shows the variation of dielectric loss ("00 ) as a function of frequency at room temperature. From a value of 1.38 at 0.1 kHz it decreases to 0.05 at 100 kHz. The low value of dielectric loss indicates that the ZTS single crystals grown have less defects. The Vicker's hardness number (VHN) for ZTS crystals grown at different pH values at a test load of 25 g are given in Table 3. The Vicker's hardness number was found to increase with decrease in pH. The increase in hardness with decrease in pH may be due to the increase in the sulfate ion concentration in the crystal. Also it has been observed that the solubility of ZTS in water decreases with decrease in pH which may be due to the increase in the interatomic binding resulting in a closely packed lattice [9]. This increase in the bond energy with decrease in pH clearly explains the increase in the microhardness number, Hv. The relative increase in hardness value with decrease in pH makes them more suitable for the fabrication of devices applicable for second harmonic generation. 4. Conclusions In¯uence of pH on the growth of bulk ZTS single crystals has been discussed. Sulfate addition has dilated the lattice. The increase in the growth rate along a-axis with decrease in pH is due to the increase in the number of sulfate ion incorporation con®rmed by the FTIR spectral analysis. The dielectric measurements show that the dielectric constant decreases with increase in frequency and the dielectric loss was low indicating that the ZTS crystals have less defects. The crystals grown at an optimised pH of 3.45 are harder than that grown at pH 6.54. Acknowledgements One of the authors (P.M.U) is grateful to the Council of Scienti®c and Industrial Research (CSIR), Government of
P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
Fig. 2. FTIR spectra of ZTS grown at (a) pH 6.54 (b) pH 4.54 and (c) pH 3.45.
273
274
P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
0
Fig. 3. Dielectric permittivity (" ) as a function of frequency at RT for ZTS single crystals.
India for the award of Senior Research Fellowship. This work is ®nancially supported by the All India Council for Technical Education (AICTE), Government of India. References [1] L.F. Warren, in: R.E. Allred, R. J. Martinez, K.B. Wischmann (Eds.), Electronic Materials±Our Future, Proceedings of The Fourth International Sampe Electronics Conference, Society for the Advancement of Material and Process Engineering, Covina, CA, 1990, vol. 4, p. 388. [2] P.R. Newman, L.F. Warren, P. Cunnigham, T.Y. Chang, D.E. Copper, G.L. Burdge, P. Polak dingels, C.K. Lowe-Ma, in: C.Y. Chiang, P.M. Chaikan D.O. Cowan (Ed.), Advanced Organic Solid State Materials,
Fig. 4. Dielectric loss ("00 ) as a function of frequency at RT for ZTS single crystals.
[3] [4] [5] [6] [7] [8] [9]
Materials Research Society Symposium Proceedings, Materials Research Society, vol. 173, Pittsburg, PA, 1990, p. 557. S. Velsko, Laser Program Annual Report, Lawrence UCRL-JC 105000 Lawrence Livermore National Laboratory, Livermore, CA, 1990 H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C. Catella, Appl. Opt. 31 (1992) 5051. V. Venkataraman, C.K. Subramanian, H.L. Bhat, J. Appl. Phys. 77 (1995) 6049. P. Kerkoc, V. Venkataraman, S. Lochran, R.T. Bailey, F.R. Cruickshank, D. Pugh, J.N. Sherwood, J. Appl. Phy. 80 (1996) 6666. P.M. Ushasree, R. Jayavel, C. Subramanian, P. Ramasamy, J. Crystal Growth 197 (1999) 216. .K.V. Roa, A. Smakula, J. Appl. Phys. 36 (1965) 2701. G. Ravi, S. Anbukumar, P. Ramasamy, Mat. Chem. Phys. 37 (1994) 180.
Materials Science Communication
In¯uence of pH on the characteristics of zinc tris (thiourea) sulfate (ZTS) single crystals P.M. Ushasree, R. Jayavel*, P. Ramasamy Crystal Growth Centre, Anna University, Chennai 600025, India Received 16 April 1999; received in revised form 15 June 1999; accepted 23 June 1999
Abstract Growth of bulk ZTS single crystals from aqueous solution by low temperature solution growth technique has been reported. The in¯uence of pH on the structural and mechanical properties has been discussed. The FTIR spectra taken for the crystals grown at different pH values reveal that while decreasing the pH more number of sulfate ions is introduced in the coordination sphere of ZTS leading to the increase in the growth rate along [100] direction. The dielectric constant for ZTS single crystals has been determined. Vicker's microhardness studies show that the crystals grown at lower pH are harder than that grown at higher pH values. # 1999 Elsevier Science S.A. All rights reserved. Keywords: ZTS single crystals; Characteristics ; pH values
1. Introduction Nonlinear optical materials (NLO) have wide applications in the area of laser technology, optical communication and data storage technology. Recent interest has been centered on semiorganic crystals which have the combined properties of both inorganic and organic crystals. Among the semiorganic nonlinear optical materials, metal complexes of thiourea which have a low UV cut off wavelengths, applicable for high power frequency conversion, are of interest. The chief virtue, as far as today's technology and research are concerned, is that these materials can be used as better alternatives for KDP crystals in frequency doubling and laser fusion experiments due to their higher values of laser damage threshold. Growth of bulk single crystals of these materials has been a subject of perennial concern in order to use these materials for device application. Zinc tris (thiourea) sulfate (ZTS), Zn[CS(NH2)2]3 SO4 is a desirable semiorganic nonlinear optical material which exhibits a low angular sensitivity and hence, proves useful for type-II second harmonic generation (SHG) [1±4]. High damage threshold and wide transparency make it a better substitute for other nonlinear optical
* Corresponding author. Tel.: +91-144-235-2774; fax: +91-144-235-2774 E-mail address: [email protected] (R. Jayavel)
materials [5]. Marcy et al., [4] have reported ZTS as an ef®cient nonlinear optical material for SHG. Venkatramanan et al., [5] have reported that the ZTS crystals have the highest laser induced damage threshold values among the other solution grown NLO crystals. Thermal properties of ZTS crystal have also been reported [6]. The variation of growth rate with a wide range of pH values has already been reported [7]. In the present investigation, the growth of bulk ZTS single crystals and the effect of pH on the structural and mechanical properties have been discussed. Large size crystals are grown with reasonable growth rate along the three crystallographic directions at an optimised pH (3.45). The grown crystals have been subjected to structural and mechanical characterization studies. The dielectric studies have been carried out for the crystal grown at optimised pH. 2. Experimental 2.1. Crystal growth ZTS can be grown from aqueous solution, both by slow cooling and by slow evaporation. The starting material was prepared by evaporating a solution of Analar grade, zinc sulfate heptahydrate and thiourea taken in stoichiometric ratio 1 : 3 in deionised water to almost dryness at room
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 5 0 - 9
P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
temperature. ZTS salt was synthesised according to the reaction: ZnSO 10H2 OZinc sulfate heptahydrate 3CS NH2 2 Thiourea
271
where Hv is the Vicker's microhardness number, P is the applied load and d is the diagonal length of the indentation impression.
4
!ZnCS NH2 2 3 SO4 ZTS
(1)
The purity of the synthesised salt was further increased by successive recrystallization process. Growth was carried out by low temperature solution growth technique by slow cooling, in a constant temperature bath controlled to an accuracy of 0.018C. Crystal growth was performed on the seeds suspended in the solution saturated at 488C and by lowering the temperature from 488C to room temperature at a rate of 0.03±0.28C dayÿ1. The period of growth ranged from 40 to 45 days. Care was taken during heating of the solution and temperature as low as 508C was maintained in order to avoid decomposition. 2.2. Crystal structure Powder X-ray diffraction (XRD) pattern has been recorded by crushing the ZTS single crystal into ®ne powder using a Rich Seifert diffractometer (Model, 2002) with Cu Ê ) radiation. The sample was scanned over Ka ( = 1.5418A the range 10±508 at a rate of 18 minÿ1. Single crystal XRD analysis of ZTS was carried out using Ehraf CAD-4 difÊ ) radiation. fractometer with Mo Ka ( = 0.7170A 2.3. FTIR analysis
3. Results and discussion At stoichiometric pH (5.6), ZTS grows as plates, elongated along b direction with reduced growth rate along aaxis, as shown in Fig. 1a and the growth habit can be changed by adjusting the pH. At a higher pH of 6.54, the ZTS crystal grows as plates with still reduced growth rate along the a-axis. At a lower pH of about 3.15 the growth rate along the a-axis increases and is comparable to that of b and c directions. At this pH, the growth rates along b and c directions are almost equal. The large ZTS crystals grown with an optimised pH of 3.45 are shown in Fig. 1b. In ZTS, the thioureas are planar, with Zn+2 tetrahedrally coordinated by three thiourea sulfur and one sulfate oxygen. The four formula units are contained in the orthorhombic (Pca21) unit cell (point group mm2). Hydrogen bonding exists between thiourea amino hydrogens and sulfate oxygens. The birefringence and the nonlinear coef®cients of the ZTS crystal are largely determined by the nearly coplanar arrangement of the thiourea units within the structure [4]. The observed and calculated d values for different 2q values along with the hkl index of the corresponding re¯ecting planes are tabulated in Table 1. The lattice parameter values recorded from single crystal XRD analysis for the crystals
The Fourier transform infrared (FTIR) spectra of ZTS crystals were recorded in the range 400±4000 cmÿ1 employing a Perkin-Elmer spectroscopy in the form of solids dispersed in KBr pellets. The FTIR spectra of the ZTS crystals grown at different pH values have been recorded in order to qualitatively analyze the incorporation of sulfate into the crystal lattice. 2.4. Dielectric studies The dielectric permittivity data of the ZTS crystals were estimated using LCR bridge. Dielectric measurements were carried out on b cut plates of ZTS in the frequency range of 0.1±100 kHz at room temperature (288C). The dielectric loss and the capacitance were calculated. 2.5. Microhardness studies The polished (100) face of ZTS single crystals grown at pH 3.45, 4.54 and 6.54 were subjected to static indentation tests at room temperature using a Leitz Wetzlar hardness tester ®tted with Vicker's diamond pyramidal indenter. Vicker's microhardness number was then evaluated from the relation.: Hv
1:8544 P kg mmÿ2 d2
(2)
Fig. 1. ZTS crystals grown at (a) pH 5.6 (b) pH 3.45.
272
P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
Table 1 Powder XRD for ZTS single crystals 2q (8)
Experimental Ê) values (A
Calculated Ê) d values (A
hkl
13.60 14.95 16.00 19.60 20.30 22.10 26.10 28.50 30.20 31.80 32.90 33.20 35.20 36.50 39.95 43.70 45.00 48.05
6.5107 5.9257 5.5391 4.5291 4.3745 4.0221 3.4140 3.1317 2.9592 2.8139 2.7223 2.6983 2.5495 2.4616 2.2560 2.0713 2.0144 1.8934
6.3794 5.9257 5.5553 5.5225 4.3417 4.0115 3.4012 3.1240 2.9490 2.8066 2.7134 2.7040 2.5546 2.4612 2.2564 2.0673 2.0103 1.8947
110 111 200 210 211 113 213 221 222 313 223 205 215 224 404 405 234 127
grown at different pH are given in Table 2. The single crystal XRD data shows a slight increase in the cell volume with pH increase. Figs. 2a,b,c show the FTIR spectra of the ZTS crystals grown at different pH values. In ZTS the coordination in the complex occurs through sulfur [7]. The ®gures show a broad envelope lying between 2750 cmÿ1 and 3500 cmÿ1 arising out of symmetric and asymmetric modes of NH2 grouping of Zn coordinated thiourea. The absorption band observed at 1628 cmÿ1 can be assigned to the NH2 bending vibration and that observed at 1502 cmÿ1 and 955 cmÿ1 can be assigned to N-C-N stretching vibration. The absorption band observed at 1404 cmÿ1 and 714 cmÿ1 can be assigned to the C=S stretching vibration. The high frequency absorption bands in the spectrum of thiourea were not shifted to lower frequency, but are either narrowed or broadened, which is a clear indication of incorporation of more number of sulfate ions. The presence of peaks around 500 cmÿ1 and 1000 cmÿ1 also indicated the increase in the number of sulfate ion incorporation in the coordination sphere. Comparison of the spectra of ZTS crystals grown at different pH values clearly indicated that as the pH value is decreased the sulfate ion introduction in the coordination sphere increases, resulting in an increase in the growth rate. Hence, the spectra reveal that the increase in growth rate with decrease
Table 2 Lattice parameters of ZTS single crystals grown at different pH values pH
Ê) a (A
Ê) b (A
Ê) c (A
Ê 3) V (A
6.54 4.54 3.45
7.794 7.793 7.796
11.141 11.162 11.163
15.503 15.529 15.526
1346.29 1350.94 1351.39
Table 3 Vicker's microhardness number of ZTS single crystals grown at different pH values pH
Plane
VHN (kg mmÿ2)
6.54 4.54 3.45
100 100 100
58 116 120
in pH is due to the increase in the number of the crystal growth enhancing site, namely, the sulfate ions. Fig. 3 shows the variation of dielectric permittivity ("0 ) with frequency at room temperature. The dielectric permittivity is maximum at low frequencies and decreases with increasing frequency. The increase in the dielectric constant at low frequency is attributed to space charge polarization [8]. Fig. 4 shows the variation of dielectric loss ("00 ) as a function of frequency at room temperature. From a value of 1.38 at 0.1 kHz it decreases to 0.05 at 100 kHz. The low value of dielectric loss indicates that the ZTS single crystals grown have less defects. The Vicker's hardness number (VHN) for ZTS crystals grown at different pH values at a test load of 25 g are given in Table 3. The Vicker's hardness number was found to increase with decrease in pH. The increase in hardness with decrease in pH may be due to the increase in the sulfate ion concentration in the crystal. Also it has been observed that the solubility of ZTS in water decreases with decrease in pH which may be due to the increase in the interatomic binding resulting in a closely packed lattice [9]. This increase in the bond energy with decrease in pH clearly explains the increase in the microhardness number, Hv. The relative increase in hardness value with decrease in pH makes them more suitable for the fabrication of devices applicable for second harmonic generation. 4. Conclusions In¯uence of pH on the growth of bulk ZTS single crystals has been discussed. Sulfate addition has dilated the lattice. The increase in the growth rate along a-axis with decrease in pH is due to the increase in the number of sulfate ion incorporation con®rmed by the FTIR spectral analysis. The dielectric measurements show that the dielectric constant decreases with increase in frequency and the dielectric loss was low indicating that the ZTS crystals have less defects. The crystals grown at an optimised pH of 3.45 are harder than that grown at pH 6.54. Acknowledgements One of the authors (P.M.U) is grateful to the Council of Scienti®c and Industrial Research (CSIR), Government of
P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
Fig. 2. FTIR spectra of ZTS grown at (a) pH 6.54 (b) pH 4.54 and (c) pH 3.45.
273
274
P.M. Ushasree et al. / Materials Chemistry and Physics 61 (1999) 270±274
0
Fig. 3. Dielectric permittivity (" ) as a function of frequency at RT for ZTS single crystals.
India for the award of Senior Research Fellowship. This work is ®nancially supported by the All India Council for Technical Education (AICTE), Government of India. References [1] L.F. Warren, in: R.E. Allred, R. J. Martinez, K.B. Wischmann (Eds.), Electronic Materials±Our Future, Proceedings of The Fourth International Sampe Electronics Conference, Society for the Advancement of Material and Process Engineering, Covina, CA, 1990, vol. 4, p. 388. [2] P.R. Newman, L.F. Warren, P. Cunnigham, T.Y. Chang, D.E. Copper, G.L. Burdge, P. Polak dingels, C.K. Lowe-Ma, in: C.Y. Chiang, P.M. Chaikan D.O. Cowan (Ed.), Advanced Organic Solid State Materials,
Fig. 4. Dielectric loss ("00 ) as a function of frequency at RT for ZTS single crystals.
[3] [4] [5] [6] [7] [8] [9]
Materials Research Society Symposium Proceedings, Materials Research Society, vol. 173, Pittsburg, PA, 1990, p. 557. S. Velsko, Laser Program Annual Report, Lawrence UCRL-JC 105000 Lawrence Livermore National Laboratory, Livermore, CA, 1990 H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C. Catella, Appl. Opt. 31 (1992) 5051. V. Venkataraman, C.K. Subramanian, H.L. Bhat, J. Appl. Phys. 77 (1995) 6049. P. Kerkoc, V. Venkataraman, S. Lochran, R.T. Bailey, F.R. Cruickshank, D. Pugh, J.N. Sherwood, J. Appl. Phy. 80 (1996) 6666. P.M. Ushasree, R. Jayavel, C. Subramanian, P. Ramasamy, J. Crystal Growth 197 (1999) 216. .K.V. Roa, A. Smakula, J. Appl. Phys. 36 (1965) 2701. G. Ravi, S. Anbukumar, P. Ramasamy, Mat. Chem. Phys. 37 (1994) 180.