- Email: [email protected]
Optical, thermal and dielectric properties of Sr(II)-doped bis(thiourea)zinc(II) chloride crystals
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Optical, thermal and dielectric properties of Sr(II)-doped bis(thiourea)zinc(II) chloride crystals K. Muthu a, M. Rajasekar a, K. Meena a, C.K. Mahadevan b, SP. Meenakshisundaram a,⇑ a b
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India Physics Research Centre, S.T. Hindu College, Nagercoil 629 002, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Sr(II)-doped bis(thiourea)zinc(II)
chloride crystals are grown. " Doping decreases the band gap
energy. " Good transparency in the visible
region. " Sr(II)-doping altered the dielectric
properties of the BTZC.
a r t i c l e
i n f o
Article history: Received 16 May 2012 Received in revised form 18 July 2012 Accepted 23 July 2012 Available online 1 August 2012 Keywords: Crystal growth Band gap energy Optical materials Dielectric properties Thermal analysis Strontium doping
a b s t r a c t The influence of strontium doping on the properties of bis(thiourea)zinc(II) chloride (BTZC) crystals has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies confirm the lattice stress as a result of doping. The incorporation of Sr(II) into the crystal lattice was confirmed by energy dispersive X-ray spectroscopy (EDS). Surface morphological changes due to doping of the alkaline earth metal are observed by scanning electron microscopy (SEM). The crystal is transparent in the entire visible region having a lower optical cut-off at 308 nm with a band gap energy of 4.06 eV. The DSC studies reveal the purity of the materials and no decomposition is observed up to the melting point. Dielectric studies show that the isovalent ion Sr(II)-doping altered the dielectric properties of the host crystal. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
Introduction Thiourea is capable of forming a number of coordination compounds with various metals. Bis(thiourea)zinc(II) chloride (BTZC) is a semi-organic nonlinear optical (NLO) material which finds applications in the area of laser technology, optical communication, data storage technology and optical computing because it
⇑ Corresponding author. Tel.: +91 4144 221670. E-mail address: [email protected] (SP. Meenakshisundaram).
has high resistance to laser induced damage, low angular sensitivity and good mechanical hardness compared to many organic NLO crystals [1–4]. Metal ion doped materials are currently receiving a great deal of attention due to the rapid development of laser diodes. Several foreign metallic cations existing in the parent compounds with high valency and small radii will affect the whole growth process and enhances physical properties. Their effects are related with ionic radius, electric charge, and frequency of solvent exchange [5–7]. Doping influences the mechanical, electrical, optical properties and surface morphology depending upon the nature of the host material and the dopant. Recently, we have
1386-1425/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.07.080
826
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
Fig. 2. FT-IR spectra of BTZC crystals: (a) pure (b)10 mol% Sr-doped.
Fig. 1. Photographs of as-grown BTZC crystals: (a) pure (b) 1 mol% Sr-doped (c) 10 mol% Sr-doped.
investigated the effect of metal doping on the properties and crystalline perfection of BTZC crystals [8–10]. The effect of strontium doping level on electrical transport and magnetic properties of La1xSrxMnO3 perovskite nanoparticles [11] and the optical, electrical and mechanical properties of ZTS and calcium tartrate crystals was studied [12,13]. In the present study, we have investigated the effect of alkaline earth metal Sr(II) on the lattice constant, XRD profile, vibrational patterns, morphology, optical, thermal and dielectric properties of the BTZC crystal.
Experimental
Fig. 3. Powder XRD patterns of BTZC crystals: (a) pure (b) 10 mol% Sr-doped.
Synthesis and crystal growth Bis(thiourea)zinc(II) chloride was synthesized [4,14] using AR grade zinc chloride and thiourea in a stoichimetric ratio 1:2.
ZnCl2 þ 2ðSCðNH2 Þ2 Þ ! ZnðSCðNH2 Þ2 Þ2 Cl2 The purity of the synthesized (BTZC) materials was increased by successive recrystallization processes. Crystals were grown by slow evaporation solution growth technique. Doping of strontium (1 and 10 mol%) in the form of strontium chloride (Merck) was done during the crystallization process. The crystallization took place within 15–20 days and the crystals were harvested when they attained an optimal size and shape. Photographs of as-grown pure and Sr(II)-doped crystals are shown in Fig. 1.
Results and discussion FT-IR analysis The FT-IR spectra were recorded for as-grown crystal using an AVATAR 330 FTIR by KBr pellet technique in the range of 400– 4000 cm1 (Fig. 2). A close observation of FT-IR spectra of pure and doped specimens reveal that doping generally results in small shifts in some of the characteristic vibrational frequencies. It could be due to lattice strain developed as a result of doping. The broad envelope positioned in between 2750 and 3500 cm1 corresponds to the symmetric and asymmetric stretching modes of –NH2 group. The CN stretching frequencies of thiourea (1089 and 1472 cm1)
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830 Table 1 Values of lattice constants a (Å), b (Å), c (Å) and cell volume V (Å)3.
a
Property
Pure BTZCa
Sr-doped BTZC (10 mol%)
a b c V
13.012 12.768 5.890 978.64
13.151(2) 12.761(14) 5.924(2) 978.68
827
are shifted to higher frequencies for pure and Sr2+-doped BTZC crystals (1103 and 1495 cm1). The CS stretching frequencies (1417 and 740 cm1) are shifted to lower frequencies (1407 and 715 cm1) for pure and doped samples. These observations suggest that metal coordinate with thiourea through sulfur atom. XRD analysis
Ref. [14].
As-grown pure and Sr(II)-doped BTZC crystals were finely powdered and subjected to powder XRD analysis using a Philips Xpert Pro Triple-axis X-ray diffractometer. The samples were examined with CuKa radiation in a 2h range of 10 to 80°. The powder XRD patterns of Sr(II)-doped samples are compared with that of undoped one (Fig. 3). No new peaks or phases were observed by doping with alkaline earth metal strontium. However, a slight variation in intensity is observed as a result of doping. The most prominent peaks with maximum intensity of the XRD patterns of pure and doped specimens are quite different. The observations could be attributed to strain in the lattice. The grown crystal was subjected to single crystal X-ray diffraction using Bruker AXS (Kappa Apex II) X-ray diffractometer. The grown crystal belongs to orthorhombic system with space group Pnma [14] and cell parameters of doped material show slight variations (Table 1). SEM and EDS The SEM images were taken using a JEOL JSM 5610 LV scanning electron microscope. The effect of the influence of dopants on the surface morphology of BTZC crystal faces reveals structure defect centers as seen in the SEM images (Fig. 4). SEM photographs of the doped specimen results in more scatter centers than that of the undoped specimen. The incorporation of metals into the crystalline matrix was confirmed by EDS (Fig. 5). It appears that the accommodating capability of host crystal is limited and only a small quantity is incorporated into the BTZC crystalline matrix. Optical absorption studies
Fig. 4. SEM micrographs of BTZC crystals: (a) pure (b)10 mol% Sr-doped.
The UV–Visible absorption spectra were recorded using Hitachi UV–Vis spectrophotometer in the spectral range 200–1100 nm. The optical absorption spectra of pure and Sr(II)-doped specimens show considerable transmission in the entire visible region and the dopant Sr(II) slightly alters the lower cut-off wavelength (Fig. 6). The measurement of the absorption coefficient a as a function of frequency m of the incident beam provides a mean to determine
Fig. 5. EDS spectrum of 10 mol% Sr-doped BTZC.
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K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
6 5
Absorbance
pure BTZC Sr-doped BTZC
4 3 2 1 0 200
400
600
800
1000
1200
Wavelength (nm) Fig. 6. UV–Vis spectra of BTZC crystals.
the band gap Eg of a material. The optical band gap in most of the materials can be determined using the Tauc relation, which is expressed as
ðahmÞ ¼ A hm Eg
r
where A is a constant and r is an index which depends on the nature of electronic transition responsible for the optical absorption. Tauc’s plot of the optical absorption spectrum measured at room temperature for Sr-doped and undoped BTZC are given in Fig. 7. The direct optical energy gap can be obtained from the intercept of the resulting straight lines with the energy axis at (ahm)2 = 0 and the band gap energies of the specimens are deduced as 4.29 and 4.06 eV with absorption edges of 289 and 305 nm for pure and Sr(II)-doped BTZC (Fig. 7), respectively. It is interesting to observe that the doping results in decrease in band gap energy of the host material.
Fig. 7. Tauc’s plot of (a) pure (b) 10 mol% Sr-doped BTZC.
Thermal studies DSC analysis was carried out using SDT Q600 (TA instrument) thermal analyzer in the nitrogen atmosphere. Fig. 8 shows typical DSC curves of pure and Sr-doped BTZC (10 mol%) crystals were carried out between 50 and 300 °C. It shows that there is no physically adsorbed water in the molecular structure of crystals. This study reveals the purity and the sharp endothermic peaks indicate the melting point of the materials. No decomposition up to melting point ensures the suitability of the material for application in lasers where the crystals are required to withstand high temperatures. The melting point of BTZC (258 °C) crystal is lowered slightly by doping with Sr2+ (254 °C) and this observation indirectly supports the incorporation of isovalent ion Sr2+ in the host crystal. Dielectric studies Dielectric measurements (capacitance of the crystal, Ccrys and the dielectric loss factor, tan d) were carried out by the parallel palate capacitor method as a function of temperature for various frequencies using a precision LCR meter (AGILENT 4284 A model). Figs. 9a–c are the plots of dielectric constant (er), dielectric loss (tan d) and AC conductivity (rac) versus temperature at different frequencies of as-grown pure and doped BTZC crystals. It is observed that the dielectric constant and dielectric loss decrease with increase in frequency and attain saturation at higher frequencies for both pure and doped specimens. But these values are slightly higher in doped specimen when compared to the pure material. The AC electrical conductivity increases with increase in frequency. This is the normal dielectric behavior of the material. The large va-
Fig. 8. DSC curves of BTZC crystals: (a) pure (b) 10 mol% Sr-doped.
lue of dielectric constant at low frequency and low temperature could be due to the presence of space charge polarizations which depend on the purity and perfection of the as grown crystal [15]. The increase in conductivity could be attributed to reduction in the space charge polarization at higher frequencies [16]. In the present study, dielectric constant varying proportionally with temperature is essentially due to the temperature variation of the polarizability [17]. The low dielectric loss at high frequency
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
829
Fig. 9. Dielectric measurements for BTZC crystals. (a) Plot of dielectric constant vs. temperature (K). (b) Plot of dielectric loss vs. temperature (K). (c) Plot of AC electrical conductivity vs. temperature (K).
indicates that the as-grown crystal has good optical quality with lesser defects. The low er value dielectric materials have potential applications in microelectronic industries. Dielectric constant for KAP exhibits a slight rise with rise in temperature in the range of 338–368 K as a consequence of a decrease in loss tangent [18]. In the present investigation, the dielectric constant for the doped specimen exhibits a slight rise in temperature in the range of 320–420 K. At high frequencies there is no rapid increase in er and tan d for the doped specimen as observed in the KAP system.
ied. A close observation of XRD and FT-IR profiles of doped and undoped specimens reveals some minor structural variations. These studies indicate that the crystal undergoes considerable lattice stress as a result of doping. The energy dispersive X-ray spectroscopic analysis of the doped specimen confirms the presence of strontium in the crystalline matrix of BTZC crystal. SEM images reveal the varied morphology of BTZC crystal in the presence of dopant. Thermal stability of the host material is slightly altered by doping of isovalent ion Sr2+. The band gap energy of the BTZC is considerably decreased by doping and the influence on dielectric properties reveals a normal dielectric behavior.
Conclusions
Acknowledgements
Pure and Sr(II)-doped bis(thiourea)zinc(II) chloride crystals were grown by slow evaporation solution growth technique and the effect of alkaline earth metal on the BTZC crystal has been stud-
The authors thank the Department of Science and Technology (DST), New Delhi, for the financial support through research grant no. SR/S2/LOP-0025/2010 and M.R. is grateful to DST for a project
830
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
fellowship. One of the authors, K.M. is thankful to CSIR, New Delhi, for the award of a Senior Research Fellowship. References [1] S. Velsko, Laser Program Annual Report, Lawrence UCRL-JC 105000, Lawrence Livemore National Laboratory, Livemore CA, 1990. [2] X. Guangcai, M. Minhua, Z. Zongshu, D. Xu, Chin. J. Laser 14 (1987) 357–363. [3] H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C. Caterlla, Appl. Opt. 31 (1992) 5051–5060. [4] P.A. Angeli Mary, S. Dhanuskodi, Cryst. Res. Technol. 36 (2001) 1231–1237. [5] T. Ohachi, M. Hamanada, H. Konda, D. Hayashi, I. Taniguchi, T. Hashimoto, Y. Kotani, J. Cryst. Growth 99 (1990) 72–76. [6] X.Q. Wang, D. Xu, M.K. Lu, D.R. Yuan, J. Huang, S.G. Li, G.W. Lu, H.Q. Sun, S.Y. Guo, G.H. Zhang, X.L. Duan, H.Y. Liu, W.L. Liu, J. Cryst. Growth 247 (2003) 432– 437. [7] R. Uthrakumar, C. Vesta, C. Justin Raj, S. Dinakaran, R. Christhu Dhas, S. Jerome Das, Cryst. Res. Technol. 43 (2008) 428–432.
[8] K. Nithya, B. Karthikeyan, G. Ramasamy, K. Muthu, S.P. Meenakshisundaram, Spectrochim. Acta Part A 79 (2011) 1648–1653. [9] K. Muthu, G. Bhagavannarayana, S.P. Meenakashisundaram, S.C. Mojumdar, J. Therm. Anal. Calorim. 108 (2012) 927–932. [10] K. Muthu, G. Bhagavannarayana, V. Meenatchi, S.P. Meenakshisundaram, S.C. Mojumdar, J. Therm. Anal. Calorim. 108 (2012) 951–958. [11] P.V. Cuong, D.H. Kim, J. Phys. Conference Series 187 (2009) 012090. [12] C. Krishnan, P. Selvarajan, J. Exp. Sci. 1 (2010) 23–26. [13] K. Suryanarayana, S.M. Dharmaprakash, K. Sooryanarayana, Bull. Mater. Sci. 2 (1) (1998) 87–92. [14] R. Rajasekaran, P.M. Ushashree, R. Jayavel, P. Ramasamy, J. Cryst. Growth 29 (2001) 563–567. [15] P.N. Prasad, G. Prasad, T. Bhimasankaran, S.V. Suryanarayan, G.S. Kumar, Indian J. Pure Appl. Phys. 4 (1996) 639. [16] A.K. Jonscher, Nature 267 (1997) 673–679. [17] M. Priya, C.K. Mahadevan, Physica B 403 (2008) 67–74. [18] K.B.R. Varma, A.K. Raychaudhuri, J. Phys. D: Appl. Phys. 22 (1989) 809–811.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Optical, thermal and dielectric properties of Sr(II)-doped bis(thiourea)zinc(II) chloride crystals K. Muthu a, M. Rajasekar a, K. Meena a, C.K. Mahadevan b, SP. Meenakshisundaram a,⇑ a b
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India Physics Research Centre, S.T. Hindu College, Nagercoil 629 002, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Sr(II)-doped bis(thiourea)zinc(II)
chloride crystals are grown. " Doping decreases the band gap
energy. " Good transparency in the visible
region. " Sr(II)-doping altered the dielectric
properties of the BTZC.
a r t i c l e
i n f o
Article history: Received 16 May 2012 Received in revised form 18 July 2012 Accepted 23 July 2012 Available online 1 August 2012 Keywords: Crystal growth Band gap energy Optical materials Dielectric properties Thermal analysis Strontium doping
a b s t r a c t The influence of strontium doping on the properties of bis(thiourea)zinc(II) chloride (BTZC) crystals has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies confirm the lattice stress as a result of doping. The incorporation of Sr(II) into the crystal lattice was confirmed by energy dispersive X-ray spectroscopy (EDS). Surface morphological changes due to doping of the alkaline earth metal are observed by scanning electron microscopy (SEM). The crystal is transparent in the entire visible region having a lower optical cut-off at 308 nm with a band gap energy of 4.06 eV. The DSC studies reveal the purity of the materials and no decomposition is observed up to the melting point. Dielectric studies show that the isovalent ion Sr(II)-doping altered the dielectric properties of the host crystal. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
Introduction Thiourea is capable of forming a number of coordination compounds with various metals. Bis(thiourea)zinc(II) chloride (BTZC) is a semi-organic nonlinear optical (NLO) material which finds applications in the area of laser technology, optical communication, data storage technology and optical computing because it
⇑ Corresponding author. Tel.: +91 4144 221670. E-mail address: [email protected] (SP. Meenakshisundaram).
has high resistance to laser induced damage, low angular sensitivity and good mechanical hardness compared to many organic NLO crystals [1–4]. Metal ion doped materials are currently receiving a great deal of attention due to the rapid development of laser diodes. Several foreign metallic cations existing in the parent compounds with high valency and small radii will affect the whole growth process and enhances physical properties. Their effects are related with ionic radius, electric charge, and frequency of solvent exchange [5–7]. Doping influences the mechanical, electrical, optical properties and surface morphology depending upon the nature of the host material and the dopant. Recently, we have
1386-1425/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.07.080
826
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
Fig. 2. FT-IR spectra of BTZC crystals: (a) pure (b)10 mol% Sr-doped.
Fig. 1. Photographs of as-grown BTZC crystals: (a) pure (b) 1 mol% Sr-doped (c) 10 mol% Sr-doped.
investigated the effect of metal doping on the properties and crystalline perfection of BTZC crystals [8–10]. The effect of strontium doping level on electrical transport and magnetic properties of La1xSrxMnO3 perovskite nanoparticles [11] and the optical, electrical and mechanical properties of ZTS and calcium tartrate crystals was studied [12,13]. In the present study, we have investigated the effect of alkaline earth metal Sr(II) on the lattice constant, XRD profile, vibrational patterns, morphology, optical, thermal and dielectric properties of the BTZC crystal.
Experimental
Fig. 3. Powder XRD patterns of BTZC crystals: (a) pure (b) 10 mol% Sr-doped.
Synthesis and crystal growth Bis(thiourea)zinc(II) chloride was synthesized [4,14] using AR grade zinc chloride and thiourea in a stoichimetric ratio 1:2.
ZnCl2 þ 2ðSCðNH2 Þ2 Þ ! ZnðSCðNH2 Þ2 Þ2 Cl2 The purity of the synthesized (BTZC) materials was increased by successive recrystallization processes. Crystals were grown by slow evaporation solution growth technique. Doping of strontium (1 and 10 mol%) in the form of strontium chloride (Merck) was done during the crystallization process. The crystallization took place within 15–20 days and the crystals were harvested when they attained an optimal size and shape. Photographs of as-grown pure and Sr(II)-doped crystals are shown in Fig. 1.
Results and discussion FT-IR analysis The FT-IR spectra were recorded for as-grown crystal using an AVATAR 330 FTIR by KBr pellet technique in the range of 400– 4000 cm1 (Fig. 2). A close observation of FT-IR spectra of pure and doped specimens reveal that doping generally results in small shifts in some of the characteristic vibrational frequencies. It could be due to lattice strain developed as a result of doping. The broad envelope positioned in between 2750 and 3500 cm1 corresponds to the symmetric and asymmetric stretching modes of –NH2 group. The CN stretching frequencies of thiourea (1089 and 1472 cm1)
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830 Table 1 Values of lattice constants a (Å), b (Å), c (Å) and cell volume V (Å)3.
a
Property
Pure BTZCa
Sr-doped BTZC (10 mol%)
a b c V
13.012 12.768 5.890 978.64
13.151(2) 12.761(14) 5.924(2) 978.68
827
are shifted to higher frequencies for pure and Sr2+-doped BTZC crystals (1103 and 1495 cm1). The CS stretching frequencies (1417 and 740 cm1) are shifted to lower frequencies (1407 and 715 cm1) for pure and doped samples. These observations suggest that metal coordinate with thiourea through sulfur atom. XRD analysis
Ref. [14].
As-grown pure and Sr(II)-doped BTZC crystals were finely powdered and subjected to powder XRD analysis using a Philips Xpert Pro Triple-axis X-ray diffractometer. The samples were examined with CuKa radiation in a 2h range of 10 to 80°. The powder XRD patterns of Sr(II)-doped samples are compared with that of undoped one (Fig. 3). No new peaks or phases were observed by doping with alkaline earth metal strontium. However, a slight variation in intensity is observed as a result of doping. The most prominent peaks with maximum intensity of the XRD patterns of pure and doped specimens are quite different. The observations could be attributed to strain in the lattice. The grown crystal was subjected to single crystal X-ray diffraction using Bruker AXS (Kappa Apex II) X-ray diffractometer. The grown crystal belongs to orthorhombic system with space group Pnma [14] and cell parameters of doped material show slight variations (Table 1). SEM and EDS The SEM images were taken using a JEOL JSM 5610 LV scanning electron microscope. The effect of the influence of dopants on the surface morphology of BTZC crystal faces reveals structure defect centers as seen in the SEM images (Fig. 4). SEM photographs of the doped specimen results in more scatter centers than that of the undoped specimen. The incorporation of metals into the crystalline matrix was confirmed by EDS (Fig. 5). It appears that the accommodating capability of host crystal is limited and only a small quantity is incorporated into the BTZC crystalline matrix. Optical absorption studies
Fig. 4. SEM micrographs of BTZC crystals: (a) pure (b)10 mol% Sr-doped.
The UV–Visible absorption spectra were recorded using Hitachi UV–Vis spectrophotometer in the spectral range 200–1100 nm. The optical absorption spectra of pure and Sr(II)-doped specimens show considerable transmission in the entire visible region and the dopant Sr(II) slightly alters the lower cut-off wavelength (Fig. 6). The measurement of the absorption coefficient a as a function of frequency m of the incident beam provides a mean to determine
Fig. 5. EDS spectrum of 10 mol% Sr-doped BTZC.
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K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
6 5
Absorbance
pure BTZC Sr-doped BTZC
4 3 2 1 0 200
400
600
800
1000
1200
Wavelength (nm) Fig. 6. UV–Vis spectra of BTZC crystals.
the band gap Eg of a material. The optical band gap in most of the materials can be determined using the Tauc relation, which is expressed as
ðahmÞ ¼ A hm Eg
r
where A is a constant and r is an index which depends on the nature of electronic transition responsible for the optical absorption. Tauc’s plot of the optical absorption spectrum measured at room temperature for Sr-doped and undoped BTZC are given in Fig. 7. The direct optical energy gap can be obtained from the intercept of the resulting straight lines with the energy axis at (ahm)2 = 0 and the band gap energies of the specimens are deduced as 4.29 and 4.06 eV with absorption edges of 289 and 305 nm for pure and Sr(II)-doped BTZC (Fig. 7), respectively. It is interesting to observe that the doping results in decrease in band gap energy of the host material.
Fig. 7. Tauc’s plot of (a) pure (b) 10 mol% Sr-doped BTZC.
Thermal studies DSC analysis was carried out using SDT Q600 (TA instrument) thermal analyzer in the nitrogen atmosphere. Fig. 8 shows typical DSC curves of pure and Sr-doped BTZC (10 mol%) crystals were carried out between 50 and 300 °C. It shows that there is no physically adsorbed water in the molecular structure of crystals. This study reveals the purity and the sharp endothermic peaks indicate the melting point of the materials. No decomposition up to melting point ensures the suitability of the material for application in lasers where the crystals are required to withstand high temperatures. The melting point of BTZC (258 °C) crystal is lowered slightly by doping with Sr2+ (254 °C) and this observation indirectly supports the incorporation of isovalent ion Sr2+ in the host crystal. Dielectric studies Dielectric measurements (capacitance of the crystal, Ccrys and the dielectric loss factor, tan d) were carried out by the parallel palate capacitor method as a function of temperature for various frequencies using a precision LCR meter (AGILENT 4284 A model). Figs. 9a–c are the plots of dielectric constant (er), dielectric loss (tan d) and AC conductivity (rac) versus temperature at different frequencies of as-grown pure and doped BTZC crystals. It is observed that the dielectric constant and dielectric loss decrease with increase in frequency and attain saturation at higher frequencies for both pure and doped specimens. But these values are slightly higher in doped specimen when compared to the pure material. The AC electrical conductivity increases with increase in frequency. This is the normal dielectric behavior of the material. The large va-
Fig. 8. DSC curves of BTZC crystals: (a) pure (b) 10 mol% Sr-doped.
lue of dielectric constant at low frequency and low temperature could be due to the presence of space charge polarizations which depend on the purity and perfection of the as grown crystal [15]. The increase in conductivity could be attributed to reduction in the space charge polarization at higher frequencies [16]. In the present study, dielectric constant varying proportionally with temperature is essentially due to the temperature variation of the polarizability [17]. The low dielectric loss at high frequency
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
829
Fig. 9. Dielectric measurements for BTZC crystals. (a) Plot of dielectric constant vs. temperature (K). (b) Plot of dielectric loss vs. temperature (K). (c) Plot of AC electrical conductivity vs. temperature (K).
indicates that the as-grown crystal has good optical quality with lesser defects. The low er value dielectric materials have potential applications in microelectronic industries. Dielectric constant for KAP exhibits a slight rise with rise in temperature in the range of 338–368 K as a consequence of a decrease in loss tangent [18]. In the present investigation, the dielectric constant for the doped specimen exhibits a slight rise in temperature in the range of 320–420 K. At high frequencies there is no rapid increase in er and tan d for the doped specimen as observed in the KAP system.
ied. A close observation of XRD and FT-IR profiles of doped and undoped specimens reveals some minor structural variations. These studies indicate that the crystal undergoes considerable lattice stress as a result of doping. The energy dispersive X-ray spectroscopic analysis of the doped specimen confirms the presence of strontium in the crystalline matrix of BTZC crystal. SEM images reveal the varied morphology of BTZC crystal in the presence of dopant. Thermal stability of the host material is slightly altered by doping of isovalent ion Sr2+. The band gap energy of the BTZC is considerably decreased by doping and the influence on dielectric properties reveals a normal dielectric behavior.
Conclusions
Acknowledgements
Pure and Sr(II)-doped bis(thiourea)zinc(II) chloride crystals were grown by slow evaporation solution growth technique and the effect of alkaline earth metal on the BTZC crystal has been stud-
The authors thank the Department of Science and Technology (DST), New Delhi, for the financial support through research grant no. SR/S2/LOP-0025/2010 and M.R. is grateful to DST for a project
830
K. Muthu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 825–830
fellowship. One of the authors, K.M. is thankful to CSIR, New Delhi, for the award of a Senior Research Fellowship. References [1] S. Velsko, Laser Program Annual Report, Lawrence UCRL-JC 105000, Lawrence Livemore National Laboratory, Livemore CA, 1990. [2] X. Guangcai, M. Minhua, Z. Zongshu, D. Xu, Chin. J. Laser 14 (1987) 357–363. [3] H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C. Caterlla, Appl. Opt. 31 (1992) 5051–5060. [4] P.A. Angeli Mary, S. Dhanuskodi, Cryst. Res. Technol. 36 (2001) 1231–1237. [5] T. Ohachi, M. Hamanada, H. Konda, D. Hayashi, I. Taniguchi, T. Hashimoto, Y. Kotani, J. Cryst. Growth 99 (1990) 72–76. [6] X.Q. Wang, D. Xu, M.K. Lu, D.R. Yuan, J. Huang, S.G. Li, G.W. Lu, H.Q. Sun, S.Y. Guo, G.H. Zhang, X.L. Duan, H.Y. Liu, W.L. Liu, J. Cryst. Growth 247 (2003) 432– 437. [7] R. Uthrakumar, C. Vesta, C. Justin Raj, S. Dinakaran, R. Christhu Dhas, S. Jerome Das, Cryst. Res. Technol. 43 (2008) 428–432.
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