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Dielectric properties and phase transition of zinc tris(thiourea) sulfate single crystal
ARTICLE IN PRESS Physica B 403 (2008) 3244– 3247
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Dielectric properties and phase transition of zinc tris(thiourea) sulfate single crystal S. Moitra a, S. Bhattacharya b, T. Kar a, A. Ghosh b, a b
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, West Bengal, India Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, West Bengal, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 5 January 2008 Received in revised form 2 April 2008 Accepted 14 April 2008
The dielectric properties and the ferroelectric to paraelectric phase transition of zinc tris(thiourea) sulfate (ZTS) single crystal have been investigated in a wide range of temperatures and frequencies. In the lower frequency region the real part of dielectric permittivity of the ZTS crystal shows a sudden increase at 323 K. Prominent first-order ferroelectric to paraelectric phase transition at 323 K has been observed in the plot of dielectric permittivity versus temperature at different frequencies. It has been observed that the phase transition occurs in ZTS crystal with a low degree of disorder. Surprisingly, it has been observed for ZTS that the value of the dielectric permittivity is only about 10 at high frequencies and is found to increase to 50 at low frequencies. Dielectric loss has higher values in the paraelectric region. & 2008 Elsevier B.V. All rights reserved.
PACS: 77.80.Bh Keywords: Dielectric properties Zinc tris(thiourea) sulfate single crystal Phase transition
1. Introduction Recently, much attention has been paid to zinc tris(thiourea) sulfate (generally known as ZTS), with chemical formula Zn[CS(NH2)2]3SO4, in view of its potential applications in the fields of telecommunications, optical information-storing devices and second harmonic generation [1]. ZTS is a desirable candidate for semi-organic nonlinear optical material, which exhibits a low angular sensitivity and hence proves useful for type-II secondharmonic generation [1–4]. High-damage threshold and wide transparency make it a better alternative for KDP crystals in frequency-doubling and laser fusion [5]. ZTS belongs to the orthorhombic system with the space group Pca21 (point group mm2) [5]. With growing interest in developing new materials for device applications, ZTS has been studied in order to clarify its ferroelectric to paraelectric (FE–PE) phase transition. Recently, materials with a high dielectric permittivity have been attracting much attention due to their applications as dielectric for capacitors, actuators, etc. [6]. Many authors [7] have revealed the FE–PE phase transition in materials like BaTiO3, KNbO3, etc. Lowtemperature order–disorder phase transition at 60 and 122 K of ZTS by Raman scattering has been reported earlier [8]. The authors of that work have found that the mechanism of phase Corresponding author. Tel.: +91 33 24734971; fax: +91 33 24732805.
E-mail address: [email protected] (A. Ghosh). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.04.010
transition of ZTS might be related to the behavior of thiourea molecule. However, there is no other information regarding FE–PE transition in ZTS crystal. In the present paper, the dielectric properties and the FE–PE phase transition of ZTS single crystal have been investigated in a wide range of frequencies and temperatures.
2. Experimental procedure ZTS was synthesized from stoichiometric amount of analar grade thiourea and zinc sulfate heptahydrate. Thiourea and zinc sulfate heptahydrate were taken in the ratio 3:1 and aqueous solutions were prepared separately with the calculated amount of ZnSO4 and thiourea. Both these solutions were then mixed together and heated with constant stirring, followed by slow evaporation of the solution at 35 1C to get a good and homogeneous yield of ZTS. The product was further purified by repeated crystallization. Single crystals of ZTS were grown by solvent evaporation at a constant temperature bath at 35 1C. To grow a large-size crystal we have used the method of slow cooling. The details of the growth procedure were reported elsewhere [9]. The X-ray diffraction (XRD) analysis of powder sample of ZTS was performed at various temperatures. The XRD pattern of ZTS was recorded on a microprocessor-controlled highresolution X-ray diffractometer (Xpert PRO, PANalytical) using nickel filtered CuKa radiation (36 kV, 20 mA). The powder sample
ARTICLE IN PRESS
[008]
[240] [240]
[034]
[141]
[400] [410] [322] [-312]
[123]
[014]
[004]
(a)
(b)
[-232] [-103]
[-312]
[141]
[-401]
[400]
[-131]
[310] [130]
[211]
323K
3. Results and discussion
(c)
353K 22
30
40 2θ (degrees)
50
Fig. 2. XRD patterns of the ZTS crystal at three temperatures: curve a for T ¼ 293 K, curve b for T ¼ 323 K, curve c for T ¼ 353 K.
50 T = 273 K T = 283 K T = 293 K T = 303 K T = 313 K T = 323 K T = 333 K T = 343 K T = 353 K
40
ε11
Fig. 1 shows the FE-SEM profile of a single-crystal ZTS sample at 293 K. The ferroelectric domains [10] on the face of single crystal ZTS sample are clearly observed in Fig. 1. The homogeneity in the volume of the domains has been found in the ZTS sample below room temperature. The domain boundaries are found to vary when the intensity of the electric field is altered. Fig. 2 shows the collected X-ray diffraction (XRD) pattern of the ZTS crystal at three representative temperatures of 293, 323 and 353 K. The powder XRD patterns were indexed and phases at respective temperatures were determined by using the computer program Winplotr [11]. It is observed from the indexed XRD patterns that the ZTS remains in its orthorhombic phase (curve a) at 293 K. However, there is a change in the XRD pattern at 323 K (curve b) and the indexing could not confirm any specific phase; so the specimen is in a mixed phase at 323 K. It was observed that it transforms to a monoclinic phase above 323 K (curve c). Thus, the XRD study confirms a structural phase transition at about 323 K. However, we were unable to determine a unique space group of the monoclinic phase due to limited diffraction peaks observed above 323 K.
[008]
[324] [034]
[410] [322] [032]
[221] [123]
[121] [014]
3245
293K Intensity (ab. unit)
was scanned in steps of 0.021 for a time interval of 2 s. For dielectric studies, optically clear crystals of ZTS were selected and cut normal to the crystallographic ‘‘a’’ direction and gently polished on felt cloth moistened with alcohol water mixture. For electrical measurements both sides of the samples of area 1 cm2 and thickness 0.5 cm were painted with silver paste for electrodes. In order to visualize the ferroelectric domain at room temperature, field emission scanning electron microscopic (FE-SEM) study was performed in a scanning electron microscope (JEOL JSM-6700F). The electrical measurements such as capacitance and conductance were carried out in the frequency range 10 Hz–2 MHz and in the temperature range 100–363 K using an LCR meter (Quad Tech, model 7600) and a closed cycle cryocooler (Janis Inc, model CCS-450), respectively.
[004]
S. Moitra et al. / Physica B 403 (2008) 3244–3247
30
20
10
2
3
4 log10 [f (Hz)]
5
6
Fig. 3. Variation of real part of permittivity (e11) along the growth direction of ZTS crystal with frequency at different temperatures.
Fig. 1. FE-SEM photographs of ZTS sample prepared by distilled water etching. Ferroelectric domain boundaries are observed in the figure.
Fig. 3 shows the variation of the dielectric permittivity e11 of the ZTS crystal as a function of frequency at different temperatures. It is observed that the dielectric permittivity decreases with the increase in frequency, which is a typical characteristic of a dielectric/ferroelectric material [12,13]. The variation of dielectric permittivity, e11, of the ZTS crystal with temperature at three frequencies is shown in Fig. 4. A peak in the dielectric permittivity of the ZTS crystal is observed at Tc ¼ 323 K. After Curie temperature is reached, the dielectric permittivity decreases due to the phase transition from ferroelectric phase to the paraelectric phase [12,13]. Surprisingly, it has been observed for ZTS that the value of the dielectric permittivity is only about 10 at high frequencies and found to increase to 50 at low frequencies. But the dielectric permittivity for other conventional materials [12,13] at ferroelectric transition is usually very
ARTICLE IN PRESS 3246
S. Moitra et al. / Physica B 403 (2008) 3244–3247
100Hz
1kHz
10.8
100kHz
10.4 10.2 10.0
40
1kHz
-1.1
100kHz
10.6
log10 (1/ε11-1/εmax)
log10 [Area (T)]
60
-1.0
100 Hz
11.0
3
4
5
6
ε11
log10 [f (rads-1)]
-1.2
-1.3
-1.4
20
-1.5
-1.6 0.9
1.0
1.1
0 100
150
200
250
300
1.2 1.3 log10 [(T-Tc)]
1.4
1.5
1.6
350 Fig. 5. Variation of (1/e11–1/emax) versus (T–Tc) of the ZTS crystal at different frequencies. The solid lines denote the least square straight-line fits.
T (K) Fig. 4. Variation of real part of permittivity (e11) along the growth direction with temperature (T) at different frequencies. Frequency-dependent area under the above curve is shown in the inset.
1=11 ¼ ðT T c Þ=C
(1)
where C is a constant, which was estimated as 180 K from Fig. 3 for the ZTS crystal. The values of T0, also obtained from Fig. 3, are 315, 311.5 and 299.4 K for 100 Hz, 1 and 100 kHz, respectively. Thus, the values of T0 for all frequencies are less than the value of Tc (323 K). This result indicates an evidence, although not enough, for first-order phase transition in the present ZTS crystal. It is worthy to mention that we have not observed any noticeable temperature hysteresis of e11. The degree of disorder or diffuseness (i.e. diffusivity) g in the phase transition of the ZTS crystal has been calculated in the paraelectric region using the equation [15] 1=11 1=max ¼ ðT T c Þg =C 0
(2)
where C0 and g are constants and e11 is the dielectric permittivity at temperature T and emax is its maximum value at Tc. The values of g, calculated from Fig. 5, are 0.30, 0.50 and 0.57 for 100 Hz, 1 and 100 kHz, respectively. These results confirm that diffuse phase transition occurs in the ZTS sample with a low degree of disorder.
T = 150 K T = 200 K T = 293 K
0.8
T = 303 K T = 313 K tan δ
large (105). The anomaly of this result for the investigated system might be due to the difference in ZTS structure. It is also noted in Fig. 4 that the value of Tc is not dependent on frequency; only the peaks become sharper with the decrease in frequency. A statistical treatment based on the Lorenzian fitting of the experimental data obtained from Fig. 4 reveals that the area under the curves, shown in the inset of Fig. 3, decreases with the increase in frequency. The area under the curves represents the electrical energy stored in the sample in the definite frequency zone due to the thermal motion of charge carriers. As the frequency increases, the capacity of electrical energy storage decreases owing to more dissipation of energy. Taking the variation of 1/e11 with temperature (Curie–Weiss type behavior in the paraelectric region), the meeting point of the tangent to the curve and the temperature axis is designated as T0. The ferroelectric–paraelectric phase transition [14] is considered first order when T0oTc and second order when T0 ¼ Tc, where Tc is the Curie–Weiss temperature (transition temperature) defined by
T = 100 K
1.0
0.6
T = 323 K T = 333 K T = 343 K
0.4
T = 353 K T = 363 K 0.2
0.0 3.0
3.5
4.0 4.5 log10 [f (Hz)]
5.0
5.5
6.0
Fig. 6. Variation of dielectric loss of the ZTS crystal with frequency at different temperatures.
The dielectric loss tan d can be evaluated from the relation [16] tan d ¼ 22 =11
(3)
where e22 and e11 are, respectively, the imaginary and real parts of dielectric permittivity in the growth direction. Fig. 6 represents the variation of the dielectric loss tan d of the ZTS crystal with frequency at different temperatures. It is clear from Fig. 6 that the values of tan d increase with the increase in frequency and show a broad relaxation peak. The height of the relaxation peak increases and the peak becomes broader with increasing temperature. The low value of dielectric loss indicates that the defects in the grown ZTS single crystals are less. It is also evident in Fig. 6 that the values of tan d show a sudden increment at 323 K, at which ferroelectric to paraelectric phase transition occurs. Thus, in the paraelectric region the dielectric loss is high in comparison with their ferroelectric counterpart and it can be concluded that the decrease in the values of the real part of the dielectric permittivity in paraelectric region shows high dielectric losses.
ARTICLE IN PRESS S. Moitra et al. / Physica B 403 (2008) 3244–3247
4. Conclusion In this study the FE–PE phase transition of the ZTS single crystal has been observed. The transition temperature for the FE–PE phase transition is observed to be 323 K. The thermal variation of 1/e11 obeys the Curie–Weiss type behavior in the paraelectric region. It is observed that the phase transition occurs in the ZTS crystal with a low degree of disorder. It is also observed that the phase transition may be first order and that the dielectric loss has high values in the paraelectric region. References [1] H. Kalkan, F. Koksal, Solid State Commun. 105 (1998) 307. [2] V. Venkataramanan, G. Dhanaraj, V.K. Wadhawan, J.N. Sherwood, H.L. Bhat, J. Cryst. Growth 154 (1995) 92. [3] A.V. Alex, J. Philip, J. Appl. Phys. 90 (2001) 720.
3247
[4] V. Venkataraman, C.K. Subramanian, H.L. Bhat, J. Appl. Phys. 77 (1995) 6049. [5] P.M. Ushasree, R. Muralidharan, R. Jayavel, P. Ramasamy, J. Cryst. Growth 210 (2000) 741. [6] K. Wakino, N. Fuzikawa, Electron. Ceram. 2 (1971) 73. [7] D. Gourdain, E. Moya, J.C. Chervin, B. Canny, P. Pruzan, Phys. Rev. B 52 (1995) 3108. [8] M. Oussaid, P. Becker, M. Kemiche, C. Carabatos-Nedelec, Phys. Stat. Sol. 207 (1998) 103. [9] M. Kroon, R. Kroon, R. Sprik, A. Lagendij, J. Appl. Phys. 77 (1995) 806. [10] C. Kittel, Introduction to Sold State Physics, seventh ed., Wiley, New York, 2001, p. 408. [11] T. Ruisnel, J. Rodriguez-Carvajal, Winplotr: a window tool for powder diffraction patterns analysis, in: Materials Science Forum, Proceedings of the Seventh European Diffraction Conference (EPDIC 7), 2007, p. 118. [12] C. Kittel, Introduction to Solid State Physics, seventh ed., Wiley, New York, 2001, p. 408. [13] L.O. Faria, C. Welter, R.L. Moreira, Appl. Phys. Lett. 88 (2006) 192903. [14] J. Ravez, A. Simon, CR Acad. Sci. 325 (1997) 481. [15] K. Uchino, S. Nomura, Ferroelect. Lett. Sec. 44 (1982) 55. [16] H. Lu, X. Zhang, H. Zhang, J. Appl. Phys. 100 (2006) 054104.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Dielectric properties and phase transition of zinc tris(thiourea) sulfate single crystal S. Moitra a, S. Bhattacharya b, T. Kar a, A. Ghosh b, a b
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, West Bengal, India Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, West Bengal, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 5 January 2008 Received in revised form 2 April 2008 Accepted 14 April 2008
The dielectric properties and the ferroelectric to paraelectric phase transition of zinc tris(thiourea) sulfate (ZTS) single crystal have been investigated in a wide range of temperatures and frequencies. In the lower frequency region the real part of dielectric permittivity of the ZTS crystal shows a sudden increase at 323 K. Prominent first-order ferroelectric to paraelectric phase transition at 323 K has been observed in the plot of dielectric permittivity versus temperature at different frequencies. It has been observed that the phase transition occurs in ZTS crystal with a low degree of disorder. Surprisingly, it has been observed for ZTS that the value of the dielectric permittivity is only about 10 at high frequencies and is found to increase to 50 at low frequencies. Dielectric loss has higher values in the paraelectric region. & 2008 Elsevier B.V. All rights reserved.
PACS: 77.80.Bh Keywords: Dielectric properties Zinc tris(thiourea) sulfate single crystal Phase transition
1. Introduction Recently, much attention has been paid to zinc tris(thiourea) sulfate (generally known as ZTS), with chemical formula Zn[CS(NH2)2]3SO4, in view of its potential applications in the fields of telecommunications, optical information-storing devices and second harmonic generation [1]. ZTS is a desirable candidate for semi-organic nonlinear optical material, which exhibits a low angular sensitivity and hence proves useful for type-II secondharmonic generation [1–4]. High-damage threshold and wide transparency make it a better alternative for KDP crystals in frequency-doubling and laser fusion [5]. ZTS belongs to the orthorhombic system with the space group Pca21 (point group mm2) [5]. With growing interest in developing new materials for device applications, ZTS has been studied in order to clarify its ferroelectric to paraelectric (FE–PE) phase transition. Recently, materials with a high dielectric permittivity have been attracting much attention due to their applications as dielectric for capacitors, actuators, etc. [6]. Many authors [7] have revealed the FE–PE phase transition in materials like BaTiO3, KNbO3, etc. Lowtemperature order–disorder phase transition at 60 and 122 K of ZTS by Raman scattering has been reported earlier [8]. The authors of that work have found that the mechanism of phase Corresponding author. Tel.: +91 33 24734971; fax: +91 33 24732805.
E-mail address: [email protected] (A. Ghosh). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.04.010
transition of ZTS might be related to the behavior of thiourea molecule. However, there is no other information regarding FE–PE transition in ZTS crystal. In the present paper, the dielectric properties and the FE–PE phase transition of ZTS single crystal have been investigated in a wide range of frequencies and temperatures.
2. Experimental procedure ZTS was synthesized from stoichiometric amount of analar grade thiourea and zinc sulfate heptahydrate. Thiourea and zinc sulfate heptahydrate were taken in the ratio 3:1 and aqueous solutions were prepared separately with the calculated amount of ZnSO4 and thiourea. Both these solutions were then mixed together and heated with constant stirring, followed by slow evaporation of the solution at 35 1C to get a good and homogeneous yield of ZTS. The product was further purified by repeated crystallization. Single crystals of ZTS were grown by solvent evaporation at a constant temperature bath at 35 1C. To grow a large-size crystal we have used the method of slow cooling. The details of the growth procedure were reported elsewhere [9]. The X-ray diffraction (XRD) analysis of powder sample of ZTS was performed at various temperatures. The XRD pattern of ZTS was recorded on a microprocessor-controlled highresolution X-ray diffractometer (Xpert PRO, PANalytical) using nickel filtered CuKa radiation (36 kV, 20 mA). The powder sample
ARTICLE IN PRESS
[008]
[240] [240]
[034]
[141]
[400] [410] [322] [-312]
[123]
[014]
[004]
(a)
(b)
[-232] [-103]
[-312]
[141]
[-401]
[400]
[-131]
[310] [130]
[211]
323K
3. Results and discussion
(c)
353K 22
30
40 2θ (degrees)
50
Fig. 2. XRD patterns of the ZTS crystal at three temperatures: curve a for T ¼ 293 K, curve b for T ¼ 323 K, curve c for T ¼ 353 K.
50 T = 273 K T = 283 K T = 293 K T = 303 K T = 313 K T = 323 K T = 333 K T = 343 K T = 353 K
40
ε11
Fig. 1 shows the FE-SEM profile of a single-crystal ZTS sample at 293 K. The ferroelectric domains [10] on the face of single crystal ZTS sample are clearly observed in Fig. 1. The homogeneity in the volume of the domains has been found in the ZTS sample below room temperature. The domain boundaries are found to vary when the intensity of the electric field is altered. Fig. 2 shows the collected X-ray diffraction (XRD) pattern of the ZTS crystal at three representative temperatures of 293, 323 and 353 K. The powder XRD patterns were indexed and phases at respective temperatures were determined by using the computer program Winplotr [11]. It is observed from the indexed XRD patterns that the ZTS remains in its orthorhombic phase (curve a) at 293 K. However, there is a change in the XRD pattern at 323 K (curve b) and the indexing could not confirm any specific phase; so the specimen is in a mixed phase at 323 K. It was observed that it transforms to a monoclinic phase above 323 K (curve c). Thus, the XRD study confirms a structural phase transition at about 323 K. However, we were unable to determine a unique space group of the monoclinic phase due to limited diffraction peaks observed above 323 K.
[008]
[324] [034]
[410] [322] [032]
[221] [123]
[121] [014]
3245
293K Intensity (ab. unit)
was scanned in steps of 0.021 for a time interval of 2 s. For dielectric studies, optically clear crystals of ZTS were selected and cut normal to the crystallographic ‘‘a’’ direction and gently polished on felt cloth moistened with alcohol water mixture. For electrical measurements both sides of the samples of area 1 cm2 and thickness 0.5 cm were painted with silver paste for electrodes. In order to visualize the ferroelectric domain at room temperature, field emission scanning electron microscopic (FE-SEM) study was performed in a scanning electron microscope (JEOL JSM-6700F). The electrical measurements such as capacitance and conductance were carried out in the frequency range 10 Hz–2 MHz and in the temperature range 100–363 K using an LCR meter (Quad Tech, model 7600) and a closed cycle cryocooler (Janis Inc, model CCS-450), respectively.
[004]
S. Moitra et al. / Physica B 403 (2008) 3244–3247
30
20
10
2
3
4 log10 [f (Hz)]
5
6
Fig. 3. Variation of real part of permittivity (e11) along the growth direction of ZTS crystal with frequency at different temperatures.
Fig. 1. FE-SEM photographs of ZTS sample prepared by distilled water etching. Ferroelectric domain boundaries are observed in the figure.
Fig. 3 shows the variation of the dielectric permittivity e11 of the ZTS crystal as a function of frequency at different temperatures. It is observed that the dielectric permittivity decreases with the increase in frequency, which is a typical characteristic of a dielectric/ferroelectric material [12,13]. The variation of dielectric permittivity, e11, of the ZTS crystal with temperature at three frequencies is shown in Fig. 4. A peak in the dielectric permittivity of the ZTS crystal is observed at Tc ¼ 323 K. After Curie temperature is reached, the dielectric permittivity decreases due to the phase transition from ferroelectric phase to the paraelectric phase [12,13]. Surprisingly, it has been observed for ZTS that the value of the dielectric permittivity is only about 10 at high frequencies and found to increase to 50 at low frequencies. But the dielectric permittivity for other conventional materials [12,13] at ferroelectric transition is usually very
ARTICLE IN PRESS 3246
S. Moitra et al. / Physica B 403 (2008) 3244–3247
100Hz
1kHz
10.8
100kHz
10.4 10.2 10.0
40
1kHz
-1.1
100kHz
10.6
log10 (1/ε11-1/εmax)
log10 [Area (T)]
60
-1.0
100 Hz
11.0
3
4
5
6
ε11
log10 [f (rads-1)]
-1.2
-1.3
-1.4
20
-1.5
-1.6 0.9
1.0
1.1
0 100
150
200
250
300
1.2 1.3 log10 [(T-Tc)]
1.4
1.5
1.6
350 Fig. 5. Variation of (1/e11–1/emax) versus (T–Tc) of the ZTS crystal at different frequencies. The solid lines denote the least square straight-line fits.
T (K) Fig. 4. Variation of real part of permittivity (e11) along the growth direction with temperature (T) at different frequencies. Frequency-dependent area under the above curve is shown in the inset.
1=11 ¼ ðT T c Þ=C
(1)
where C is a constant, which was estimated as 180 K from Fig. 3 for the ZTS crystal. The values of T0, also obtained from Fig. 3, are 315, 311.5 and 299.4 K for 100 Hz, 1 and 100 kHz, respectively. Thus, the values of T0 for all frequencies are less than the value of Tc (323 K). This result indicates an evidence, although not enough, for first-order phase transition in the present ZTS crystal. It is worthy to mention that we have not observed any noticeable temperature hysteresis of e11. The degree of disorder or diffuseness (i.e. diffusivity) g in the phase transition of the ZTS crystal has been calculated in the paraelectric region using the equation [15] 1=11 1=max ¼ ðT T c Þg =C 0
(2)
where C0 and g are constants and e11 is the dielectric permittivity at temperature T and emax is its maximum value at Tc. The values of g, calculated from Fig. 5, are 0.30, 0.50 and 0.57 for 100 Hz, 1 and 100 kHz, respectively. These results confirm that diffuse phase transition occurs in the ZTS sample with a low degree of disorder.
T = 150 K T = 200 K T = 293 K
0.8
T = 303 K T = 313 K tan δ
large (105). The anomaly of this result for the investigated system might be due to the difference in ZTS structure. It is also noted in Fig. 4 that the value of Tc is not dependent on frequency; only the peaks become sharper with the decrease in frequency. A statistical treatment based on the Lorenzian fitting of the experimental data obtained from Fig. 4 reveals that the area under the curves, shown in the inset of Fig. 3, decreases with the increase in frequency. The area under the curves represents the electrical energy stored in the sample in the definite frequency zone due to the thermal motion of charge carriers. As the frequency increases, the capacity of electrical energy storage decreases owing to more dissipation of energy. Taking the variation of 1/e11 with temperature (Curie–Weiss type behavior in the paraelectric region), the meeting point of the tangent to the curve and the temperature axis is designated as T0. The ferroelectric–paraelectric phase transition [14] is considered first order when T0oTc and second order when T0 ¼ Tc, where Tc is the Curie–Weiss temperature (transition temperature) defined by
T = 100 K
1.0
0.6
T = 323 K T = 333 K T = 343 K
0.4
T = 353 K T = 363 K 0.2
0.0 3.0
3.5
4.0 4.5 log10 [f (Hz)]
5.0
5.5
6.0
Fig. 6. Variation of dielectric loss of the ZTS crystal with frequency at different temperatures.
The dielectric loss tan d can be evaluated from the relation [16] tan d ¼ 22 =11
(3)
where e22 and e11 are, respectively, the imaginary and real parts of dielectric permittivity in the growth direction. Fig. 6 represents the variation of the dielectric loss tan d of the ZTS crystal with frequency at different temperatures. It is clear from Fig. 6 that the values of tan d increase with the increase in frequency and show a broad relaxation peak. The height of the relaxation peak increases and the peak becomes broader with increasing temperature. The low value of dielectric loss indicates that the defects in the grown ZTS single crystals are less. It is also evident in Fig. 6 that the values of tan d show a sudden increment at 323 K, at which ferroelectric to paraelectric phase transition occurs. Thus, in the paraelectric region the dielectric loss is high in comparison with their ferroelectric counterpart and it can be concluded that the decrease in the values of the real part of the dielectric permittivity in paraelectric region shows high dielectric losses.
ARTICLE IN PRESS S. Moitra et al. / Physica B 403 (2008) 3244–3247
4. Conclusion In this study the FE–PE phase transition of the ZTS single crystal has been observed. The transition temperature for the FE–PE phase transition is observed to be 323 K. The thermal variation of 1/e11 obeys the Curie–Weiss type behavior in the paraelectric region. It is observed that the phase transition occurs in the ZTS crystal with a low degree of disorder. It is also observed that the phase transition may be first order and that the dielectric loss has high values in the paraelectric region. References [1] H. Kalkan, F. Koksal, Solid State Commun. 105 (1998) 307. [2] V. Venkataramanan, G. Dhanaraj, V.K. Wadhawan, J.N. Sherwood, H.L. Bhat, J. Cryst. Growth 154 (1995) 92. [3] A.V. Alex, J. Philip, J. Appl. Phys. 90 (2001) 720.
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