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Synthesis, growth, characterization and crystal structure of zinc cadmium thiourea complex Zn0.625Cd1.375(CS(NH2)2)9.4(SO4)
Journal of Crystal Growth 377 (2013) 197–202
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis, growth, characterization and crystal structure of zinc cadmium thiourea complex Zn0.625Cd1.375(CS(NH2)2)9.4(SO4) G. Ramasamy, Subbiah Meenakshisundaram n Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2012 Received in revised form 9 May 2013 Accepted 14 May 2013 Communicated by M. Fleck Available online 24 May 2013
Single crystals of zinc cadmium thiourea complex Zn0.625Cd1.375(CS(NH2)2)9.4(SO4) (ZCTS) are grown by slow evaporation of aqueous solution containing zinc sulfate, cadmium sulfate and thiourea in the ratio 1:1:6. Crystal composition as determined by single crystal X-ray diffraction analysis reveals that it belongs to the monoclinic system with space group P21 and cell parameters are a ¼13.263(5) Å, b ¼ 11.886 (5) Å, c¼ 27.215(5) Å, α ¼γ ¼90.000(5)1, β¼96.586(5)1, V¼4262(3) Å3 and Z¼2. The presence of zinc and cadmium in the final product is further confirmed by inductively coupled plasma (ICP), atomic absorption spectroscopy (AAS) and energy dispersive X-ray spectroscopy (EDS). The vibrational patterns in FT-IR are used for identifying the material and the thermal analysis by TG/DTA indicates the stability of the mixed crystal. The surface morphology changes of the mixed crystal are studied by scanning electron microscopy (SEM). High transmittance in the visible region is observed with a lower optical cut-off at ∼260 nm. The change in intensity patterns in XRD profiles indicates lattice distortion. The relative second harmonic generation (SHG) efficiency measurements reveal that the mixed crystal has a superior activity than that of tris(thiourea)zinc(II) sulfate (ZTS). & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal Growth A1. Crystal structure A2. X-ray diffraction B1. Characterization methods
1. Introduction
2. Experimental
Tris(thiourea)zinc(II) sulfate (ZTS) is a semi-organic nonlinear optical (NLO) material which finds applications in the area of laser technology, optical communications, data storage technology and optical computing because it has high resistance to laser induced damage, high nonlinearity, wide transparency, low angular sensitivity and good mechanical hardness compared to many organic NLO crystals [1–4]. The crystal growth, kinetics and characteriszation of ZTS have been extensively investigated [5–17]. Recently, we have investigated the effect of doping organic additives [18–20], Mn(II) [21], Ce(IV) [22], Cs(I) [23] and benzene on ZTS [24]. Crystal growth, characterization [25,26], and effect of glycine doping [27] on tris(thiourea)cadmium(II) sulfate have been studied. Nucleation kinetics on tris(thiourea)zinc cadmium sulfate [28] has also been reported. In the present investigation, we report the synthesis, growth and characterization of a new mixed crystal Zn0.625Cd1.375(CS(NH2)2)9.4(SO4). The crystal structure is determined to study the coordination around the metal ions.
2.1. Synthesis and crystal growth
n
Corresponding author. Tel.: +91 4144 221670; mobile: +91 9443091274. E-mail addresses: [email protected], [email protected] (S. Meenakshisundaram). 0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.05.021
ZTS [18] and CTS [26] were synthesized as reported earlier. The mixed crystal was synthesized mixing stoichiometric amounts of ZnSO47H2O (Qualigens), CdSO4 4H2O (Qualigens) and thiourea (EM) in the molar ratio of 1:1:6. The reactants were dissolved in triply distilled water, thoroughly mixed using a magnetic stirrer (for 3 h) and to avoid decomposition, low temperature ( o70 1C) was maintained. Zn0.625Cd1.375(CS(NH2)2)9.4(SO4) (ZCTS) was grown by slow evaporation solution growth technique. The solution was tightly covered with a perforated paper and kept in a constant temperature at (35 1C). The crystallization took place in a period of 10–15 days and the crystals were harvested from the aqueous growth medium. Best quality and highly transparent seed crystals are used in the growth of bulk crystals. Photographs of the crystals are shown in Fig. 1. 2.2. Characterization techniques The FT-IR spectrum was recorded by using an AVATAR 330 FT-IR instrument by KBr pellet technique in the range 500– 4000 cm−1. The powder XRD data were analyzed with a PAN analytical, model-X’pert PRO analyzer and room temperature at wavelength of 1.540 Å with a step size of 0.0081, using graphite monochromated Cu Kα radiation in the 2θ range from 101 to 701.
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G. Ramasamy, S. Meenakshisundaram / Journal of Crystal Growth 377 (2013) 197–202
The surface morphologies were observed by using a JEOL JSM 5610 LV SEM with the resolution of 3.0 nm, an accelerating voltage of 20 kV and maximum magnification of 3,00,000 . EDS, a chemical microanalysis technique was performed in conjunction with SEM. ICP studies were recorded by using a Perkin-Elmer Optima 5300 DV spectrometer. AAS was recorded using a VARIAN Model SPECTRAA 220 spectrometer in acetone–air flame. In order to ascertain the structure, purity and identification of the grown crystal, single crystal X-ray diffraction data were collected with a specimen of size ∼0.20 0.15 0.15 mm3 cut out from the grown crystals using an Oxford Diffraction Xcalibur-S CCD system
equipped with graphite monochromated Mo Kα(λ ¼0.71073 Å) radiation at 293(2) K. The structure was solved by direct methods (SHELXS-97) [29] and refined by full-matrix least squares against F2 using SHELXL-97 software [30]. The molecular structure was drawn using ORTEP-3. All non-hydrogen atoms were refined anisotropically. The UV–vis analysis was carried out in the range 200–1100 nm using the Perkin-Elmer Lambda 35 model spectrophotometer. TG and DTA were performed using an STD Q 600 in the temperature range 0–600 1C at a heating rate of 100 1C/min in the nitrogen atmosphere. The second harmonic generation (SHG) test was performed by the Kurtz powder SHG method. An Nd:YAG laser with modulated radiation of 1064 nm was used as the optical source with an input radiation of 2.5 mJ/pulse; the grown crystals were ground to a uniform particle size of 125–150 μm, packed in a micro-capillary of uniform bore and exposed to laser radiation. The output from the sample was monochromated to collect the intensity of the 532 nm component and to eliminate the fundamental. Microcrystalline KDP was used as a reference material.
3. Results and discussion 3.1. FT-IR Comparison of the characteristic vibrational frequencies of mixed crystals reveals slight shifts in some of the characteristic vibrational frequencies (Table 1). Thiourea forms metal complexes by coordination bonds through either sulfur or nitrogen. The symmetric and asymmetric C ¼S stretching vibrations at 740 and 1417 cm−1 of thiourea are shifted to lower frequencies (721 and 1397 cm−1). The NH2 stretching vibrations at 3376 and 3167 cm−1 of thiourea are shifted to higher frequencies (3430 and 3198 cm−1). The C ¼N stretching frequencies at 1089 cm−1 of thiourea are also shifted to higher frequencies (1136 cm−1). These observations suggest that metals coordinate with thiourea through sulfur atom.
3.2. SEM and EDS The SEM pictures of ZTS, CTS and ZCTS are given in Fig. 2(a)–(c). The micrographs of ZTS and CTS show dendritic growth. Large scatter centers and voids are observed in the mixed crystal. The ionic radius of Cd(II) (109 pm) is higher than that of Zn(II) (88 pm) and the coexistence of both Zn and Cd results in distortions in the crystalline matrix as seen in the picture [Fig. 2(c)]. Presence of zinc and cadmium in the mixed crystal is confirmed by EDS [Fig. 2].
Table 1 FT-IR frequencies of some zinc(II) thiourea complexes (cm−1). Sl. Thioureaa ZTSa no.
ZTS (present CTSb CTS work) (present work)
ZCTS Assignment of vibrations
1 2 3 4 5 6 7 8 9 10
3399 3195 – 1624 1122 1502 1398 712 619 471
3430 3198 – 1626 1136 1560 1397 721 622 486
a
Fig. 1. Photographs of (a) ZTS, (b) CTS and (c) ZCTS mixed crystals.
b
3376 3167 3280 1627 1089 1472 1417 740 648 492 Ref. [32]. Ref. [25].
3378 3206 – 1623 1126 1515 1404 717 – -–
3392 3180 – 1623 1124 1490 1408 714 620 498
3396 3174 – 1616 1084 1467 1410 726 626 447
νas(NH2) νs(NH2) νs(NH2) δ(NH2) νs(C–N) ν(N–C–N) νas(C ¼S) νs(C¼S) νas(N–C–S) δas(N–C–N)
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Fig. 2. SEM micrographs and EDS spectra of (a) ZTS (b) CTS and (c) ZCTS mixed crystal.
3.3. AAS and ICP Quantitative analysis has been carried out to determine the amount of zinc and cadmium in the final product using ICP and AAS. The AAS data reveal that the zinc and cadmium ion concentrations in ZCTS crystalline matrix are 10 and 22 ppm, respectively. The ICP results also confirm the above results and the amounts of zinc and cadmium are 13 and 27 ppm, respectively. The chemical composition of the mixed crystal ZCTS is not exactly proportional to the quantity or ratio of salts taken in the growth medium at the time of crystallization process.
3.4. UV–vis spectroscopy The optical absorption spectrum of ZCTS mixed crystal shows good transmittance in the entire visible region and the lower cutoff wavelength is ∼260 nm. The per cent transmittance is slightly
high in the case of ZCTS. As such, this mixed crystal system is quite useful for optical device applications.
3.5. Thermal analysis Fig. 3 shows the TG/DTA curve of ZCTS. It is clear that there is no physically absorbed water in the molecular structure of the crystal. TG curve shows a single stage weight loss at ∼240 1C due to melting and decomposition of ZCTS mixed crystal into fragments and its subsequent volatilization. In DTA, the sharp endothermic peak at ∼240 1C is due to melting and decompostion of the material. Melting point of the material was confirmed by using a Sigma instrument melting point apparatus (238 1C). No decomposition up to the melting point ensures the suitability of the material for application in lasers where the crystals are required to withstand high temperatures. The sharpness of the endothermic peak shows good degree of crystallinity and purity of the crystal.
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Fig. 3. TG/DTA curve of ZCTS mixed crystal.
3.6. XRD analysis The XRD profiles show that all samples are of single phase without any detectable impurity. Bragg’s peaks at specific 2θ angles show high crystallinity of the specimens. The structural analysis of ZCTS was carried out by single crystal XRD analysis. The lattice parameters of ZCTS, ZTS and CTS are given in Table 2 along with corresponding reported values. The structure is determined with a low value of R factor (0.0456) using 33,252 reflections (R(int) ¼ 0.0316) and goodness of fit (F2) ¼1.122, such that this structural elucidation does not come under the category of low symmetry. ZTS belongs to orthorhombic system with space group Pca21 [31,32] whereas CTS belongs to triclinic system with space group P-1[33,34]. Mixed crystal ZCTS crystallizes in the monoclinic system with noncentrosysmmetric space group P21 and the cell parameters are a ¼13.263(5) Å, b ¼11.886(5) Å, c ¼27.215(5) Å, α¼γ ¼90.000(5)1, β¼ 96.586(5)1,V ¼4262(3) Å3 and Z ¼2 with formula weight ¼2208.51 and calculated density ¼ 1.758 Mg m3. Crystal data and structure refinement of ZCTS mixed crystal are given in Table 3. Zn partially replaces the Cd present in tetrahedral geometry with four thiourea units. This is clearly seen in bond angle and bond length table. Cadmium is located in both tetrahedral and square pyramidal/trigonal bipyramidal coordination arrangements (Fig. 4). The partial occupancy is such that the molecular formulae have fractional values (Table 3). The occupation is determined by using a general procedure [35]. Single crystal XRD data are in agreement with the other experimental observations. No significant deviations are observed. The metal is coordinated by four thiourea groups with Zn/Cd– S–C bond angles ranging from 99(3)1 to 104(2)1, S–Zn/Cd–S bond angles ranging from 102.37(7)1 to 106.33(7)1 and Zn/Cd–S bond lengths lying in the range 2.496(2) Å–2.640(2) Å in a distorted tetrahedral geometry. In ZTS, zinc coordinates with three sulfur atoms from thiourea molecules with bond lengths 2.33, 2.32, and 2.31 Å [31]. In five coordinations, the cadmium is coordinated by five thiourea groups. Cd(tu)3SO4 is the first reported fivecoordinated Cd(II) complex [34]. The Cd–S–C bond angles lie in the range from 95.4(2)1 to 112.5(3)1, and S–Cd–S bond angles in the range 86.37(7)1–98.02(7) and 118.50(7)–123.92(6)1. The Cd–S bond lengths in five coordinations of the complex range from 2.6053(19) Å to 2.8250(2) Å in a trigonal bipyramidal geometry. In the case of CTS, the coordination around metal atom is intermediate between square pyramidal and trigonal bipyramidal, although somewhat closer to the former. One sulphur atom from a thiourea molecule coordinates to two adjacent cadmium atoms at distances 2.627 and 2.870 Å. The other three Cd–S bond lengths in the coordination polyhedral are 2.538, 2.627 and 2.627 Å [34]. Strong
Table 2 Comparison of cell parameters of ZTS, CTS, and ZCTS mixed crystals. Crystal
ZTSa
CTSb
ZCTS
System Space group aÅ bÅ cÅ α (deg) β (deg) γ (deg) V Å3 Z
Orthorhombic Pca21 7.773(4) 11.126(5) 15.491(5) 90 90 90 1339.70 4
Tricinic P-1 8.77(2) 9.05(2) 9.83(1) 91.3(2) 111.9(1) 95.5(2) 718.9 2
Monoclinic P21 13.263(5) 11.886(5) 27.215(5) 90.000(5) 96.586(5) 90.000(5) 4262(3) 2
a b
Ref. [31]. Ref. [34].
hydrogen bonds are observed for N–H–O with bond lengths in the range of 2.776(9)–2.998(8) Å. 3.7. SHG In order to confirm the NLO properties, the specimen was subjected to SHG test. The output SHG intensities for ZTS and ZCTS give relative NLO efficiencies. It is interesting to observe that the SHG conversion efficiency of ZCTS (47 mV) is superior to that ZTS (14 mV), a well known NLO material. Generally, high SHG efficiency is related with favorable molecular alignment facilitating nonlinearity. The effect of various dopants on SHG efficiency of ZTS has been listed in Table 4 and enhanced SHG efficiency is observed by doping. It is Interesting to observe that ZTS and ZCTS crystallize in a noncentrosymmetric space group whereas CTS has a centrosymmetric structure. The orthogonal projection of CTS [34] and ORTEP diagram of ZTS [36] are given in Fig. 5. In CTS, there is a mirror plane perpendicular to rotation axis, and an inversion center at the position of the intersection (1̄ ) leading to centrosymmetry and hence it is SHG-inactive. ZTS has an orthorhombic, primitive lattice; its c-glide plane is perpendicular to the a-axis, a-glide plane is perpendicular to b-axis and there is a two-fold screw axis parallel to the c-axis, leading to noncentrosymmetry. ZCTS has point group symmetry 2. Parallel to the b crystallographic axis is a 2-fold screw axis with a rotation of 1801 and a unit of translation of ½ b1 parallel to the b crystallographic axis. It belongs to a noncentrosymmetric space group and is hence SHG-active. Thus, the development of noncentrosymmetry is due to orientation. Higher SHG efficiency of the mixed crystal system compared with ZTS could be justified by
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Table 3 Crystal data and structure refinement of mixed crystal ZCTS. Empirical formula Molecular formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions Volume Z, Calculated density Absorption coefficient F(000) Crystal size Theta range for data collection Limiting indices Reflections collected/unique Completeness to theta ¼25.00 Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I 4Z sigma (I)] R indices (all data) Largest diff. peak and hole Absolute structure parameters
C18H72Cd2.75N36O16S22Zn1.25 2[Zn0.625Cd1.375(CS(NH2)2)9 4SO4] 2208.51 293(2) K 0.71073 Å Monoclinic, P21 a¼ 13.263(5) Å, b¼ 11.886(5) Å, c ¼ 27.215(5) Å, α ¼ γ¼ 90.000(5)1, β¼ 96.586(5)1, 4262(3) Å3 2, 1.758 Mg m−3 1.605 mm−1 2263 0.20 0.15 0.15 mm3 0.75 to 25.00 1 −15o h o15,−13o k o 14, −32o lo 32 33,252/14,300 [R(int) ¼0.0316] 99.6% Semi-empirical from equivalents 0.7948 and 0.7396 1.122 R1¼ 0.0456, wR2¼ 0.1092 R1¼ 0.0677, wR2¼ 0.1207 0.869 and −0.669 A−3 −0.02(2)
Fig. 4. ORTEP diagram of ZCTS.
Table 4 SHG outputs of ZTS with various dopants and metal thiourea sulfate crystals. System
Relative efficiency
Reference
ZCTS mixed crystal ZTS doped with Cs(I) ZTS doped with EDTA ZTS doped with Benzene ZTS doped with Mn(II) ZTS doped with Ce(IV) ZTS doped with KI ZTS doped with KCl ZTS doped with LiBr ZTS doped with Na(I) ZTS doped with urea ZTS doped with L-lysine ZTS doped with glycine
∼3.5 times higher wrt that of ZTS ∼1.2 times higher wrt that of ZTS ∼1.8 times higher wrt that of ZTS ∼1.4 times higher wrt that of ZTS ∼2.1 times higher wrt that of ZTS ∼1.6 times higher wrt that of ZTS ∼1.2 times higher wrt that of KDP ∼1.2 times higher wrt that of KDP Green light emission Slightly greater wrt that of ZTS ∼2.0 times higher wrt that of ZTS Comparable to ZTS ∼4.14 times higher wrt that of ZTS
Present work [23] [19] [19] [21] [22] [11] [12] [14] [13] [15] [16] [17]
facile charge transfer because of incorporation of foreign metal ion. Charge transfer is responsible for SHG or NLO. The foreign metal ion, by its incorporation, tunes the electronic properties.
4. Conclusion Zinc cadmium thiourea sulfate, Zn0.625Cd1.375(CS(NH2)2)9.4SO4 mixed crystals are grown by slow concentration of aqueous solution of the components and characterized using FT-IR, SEM, EDS, AAS, ICP, UV–vis, TG, DTA, SHG and X-ray diffraction studies. The SHG-active mixed crystal belongs to noncentrosymmetric P21 space group with two molecules in the unit cell. The product formation was confirmed by FT-IR and single crystal XRD analysis. Good tansmittance in the visible region is observed by UV–vis
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Fig. 5. (a) Orthogonal projection of CTS (reproduced from Ref. [34]) and (b) ORTEP diagram of ZTS (reproduced from Ref. [36]).
spectrum and the powder X-ray diffraction study shows the good crystallinity of the material. TG and DTA studies reveal the purity of the sample and no decomposition is observed up to the melting point. SEM reveals that the ZCTS crystal facets exhibit more voids and structure defect centers. Relative ease of preparation clubbed with high SHG efficiency makes this mixed crystal a technologically important material. Acknowledgments The authors thank Dr. Babu Varghese, senior scientific officer Grade 1, SAIF, Indian Institute of Technology, Chennai, for the support in structural elucidation by single crystal XRD. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jcrysgro.2013.05.021.
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Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis, growth, characterization and crystal structure of zinc cadmium thiourea complex Zn0.625Cd1.375(CS(NH2)2)9.4(SO4) G. Ramasamy, Subbiah Meenakshisundaram n Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2012 Received in revised form 9 May 2013 Accepted 14 May 2013 Communicated by M. Fleck Available online 24 May 2013
Single crystals of zinc cadmium thiourea complex Zn0.625Cd1.375(CS(NH2)2)9.4(SO4) (ZCTS) are grown by slow evaporation of aqueous solution containing zinc sulfate, cadmium sulfate and thiourea in the ratio 1:1:6. Crystal composition as determined by single crystal X-ray diffraction analysis reveals that it belongs to the monoclinic system with space group P21 and cell parameters are a ¼13.263(5) Å, b ¼ 11.886 (5) Å, c¼ 27.215(5) Å, α ¼γ ¼90.000(5)1, β¼96.586(5)1, V¼4262(3) Å3 and Z¼2. The presence of zinc and cadmium in the final product is further confirmed by inductively coupled plasma (ICP), atomic absorption spectroscopy (AAS) and energy dispersive X-ray spectroscopy (EDS). The vibrational patterns in FT-IR are used for identifying the material and the thermal analysis by TG/DTA indicates the stability of the mixed crystal. The surface morphology changes of the mixed crystal are studied by scanning electron microscopy (SEM). High transmittance in the visible region is observed with a lower optical cut-off at ∼260 nm. The change in intensity patterns in XRD profiles indicates lattice distortion. The relative second harmonic generation (SHG) efficiency measurements reveal that the mixed crystal has a superior activity than that of tris(thiourea)zinc(II) sulfate (ZTS). & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal Growth A1. Crystal structure A2. X-ray diffraction B1. Characterization methods
1. Introduction
2. Experimental
Tris(thiourea)zinc(II) sulfate (ZTS) is a semi-organic nonlinear optical (NLO) material which finds applications in the area of laser technology, optical communications, data storage technology and optical computing because it has high resistance to laser induced damage, high nonlinearity, wide transparency, low angular sensitivity and good mechanical hardness compared to many organic NLO crystals [1–4]. The crystal growth, kinetics and characteriszation of ZTS have been extensively investigated [5–17]. Recently, we have investigated the effect of doping organic additives [18–20], Mn(II) [21], Ce(IV) [22], Cs(I) [23] and benzene on ZTS [24]. Crystal growth, characterization [25,26], and effect of glycine doping [27] on tris(thiourea)cadmium(II) sulfate have been studied. Nucleation kinetics on tris(thiourea)zinc cadmium sulfate [28] has also been reported. In the present investigation, we report the synthesis, growth and characterization of a new mixed crystal Zn0.625Cd1.375(CS(NH2)2)9.4(SO4). The crystal structure is determined to study the coordination around the metal ions.
2.1. Synthesis and crystal growth
n
Corresponding author. Tel.: +91 4144 221670; mobile: +91 9443091274. E-mail addresses: [email protected], [email protected] (S. Meenakshisundaram). 0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.05.021
ZTS [18] and CTS [26] were synthesized as reported earlier. The mixed crystal was synthesized mixing stoichiometric amounts of ZnSO47H2O (Qualigens), CdSO4 4H2O (Qualigens) and thiourea (EM) in the molar ratio of 1:1:6. The reactants were dissolved in triply distilled water, thoroughly mixed using a magnetic stirrer (for 3 h) and to avoid decomposition, low temperature ( o70 1C) was maintained. Zn0.625Cd1.375(CS(NH2)2)9.4(SO4) (ZCTS) was grown by slow evaporation solution growth technique. The solution was tightly covered with a perforated paper and kept in a constant temperature at (35 1C). The crystallization took place in a period of 10–15 days and the crystals were harvested from the aqueous growth medium. Best quality and highly transparent seed crystals are used in the growth of bulk crystals. Photographs of the crystals are shown in Fig. 1. 2.2. Characterization techniques The FT-IR spectrum was recorded by using an AVATAR 330 FT-IR instrument by KBr pellet technique in the range 500– 4000 cm−1. The powder XRD data were analyzed with a PAN analytical, model-X’pert PRO analyzer and room temperature at wavelength of 1.540 Å with a step size of 0.0081, using graphite monochromated Cu Kα radiation in the 2θ range from 101 to 701.
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The surface morphologies were observed by using a JEOL JSM 5610 LV SEM with the resolution of 3.0 nm, an accelerating voltage of 20 kV and maximum magnification of 3,00,000 . EDS, a chemical microanalysis technique was performed in conjunction with SEM. ICP studies were recorded by using a Perkin-Elmer Optima 5300 DV spectrometer. AAS was recorded using a VARIAN Model SPECTRAA 220 spectrometer in acetone–air flame. In order to ascertain the structure, purity and identification of the grown crystal, single crystal X-ray diffraction data were collected with a specimen of size ∼0.20 0.15 0.15 mm3 cut out from the grown crystals using an Oxford Diffraction Xcalibur-S CCD system
equipped with graphite monochromated Mo Kα(λ ¼0.71073 Å) radiation at 293(2) K. The structure was solved by direct methods (SHELXS-97) [29] and refined by full-matrix least squares against F2 using SHELXL-97 software [30]. The molecular structure was drawn using ORTEP-3. All non-hydrogen atoms were refined anisotropically. The UV–vis analysis was carried out in the range 200–1100 nm using the Perkin-Elmer Lambda 35 model spectrophotometer. TG and DTA were performed using an STD Q 600 in the temperature range 0–600 1C at a heating rate of 100 1C/min in the nitrogen atmosphere. The second harmonic generation (SHG) test was performed by the Kurtz powder SHG method. An Nd:YAG laser with modulated radiation of 1064 nm was used as the optical source with an input radiation of 2.5 mJ/pulse; the grown crystals were ground to a uniform particle size of 125–150 μm, packed in a micro-capillary of uniform bore and exposed to laser radiation. The output from the sample was monochromated to collect the intensity of the 532 nm component and to eliminate the fundamental. Microcrystalline KDP was used as a reference material.
3. Results and discussion 3.1. FT-IR Comparison of the characteristic vibrational frequencies of mixed crystals reveals slight shifts in some of the characteristic vibrational frequencies (Table 1). Thiourea forms metal complexes by coordination bonds through either sulfur or nitrogen. The symmetric and asymmetric C ¼S stretching vibrations at 740 and 1417 cm−1 of thiourea are shifted to lower frequencies (721 and 1397 cm−1). The NH2 stretching vibrations at 3376 and 3167 cm−1 of thiourea are shifted to higher frequencies (3430 and 3198 cm−1). The C ¼N stretching frequencies at 1089 cm−1 of thiourea are also shifted to higher frequencies (1136 cm−1). These observations suggest that metals coordinate with thiourea through sulfur atom.
3.2. SEM and EDS The SEM pictures of ZTS, CTS and ZCTS are given in Fig. 2(a)–(c). The micrographs of ZTS and CTS show dendritic growth. Large scatter centers and voids are observed in the mixed crystal. The ionic radius of Cd(II) (109 pm) is higher than that of Zn(II) (88 pm) and the coexistence of both Zn and Cd results in distortions in the crystalline matrix as seen in the picture [Fig. 2(c)]. Presence of zinc and cadmium in the mixed crystal is confirmed by EDS [Fig. 2].
Table 1 FT-IR frequencies of some zinc(II) thiourea complexes (cm−1). Sl. Thioureaa ZTSa no.
ZTS (present CTSb CTS work) (present work)
ZCTS Assignment of vibrations
1 2 3 4 5 6 7 8 9 10
3399 3195 – 1624 1122 1502 1398 712 619 471
3430 3198 – 1626 1136 1560 1397 721 622 486
a
Fig. 1. Photographs of (a) ZTS, (b) CTS and (c) ZCTS mixed crystals.
b
3376 3167 3280 1627 1089 1472 1417 740 648 492 Ref. [32]. Ref. [25].
3378 3206 – 1623 1126 1515 1404 717 – -–
3392 3180 – 1623 1124 1490 1408 714 620 498
3396 3174 – 1616 1084 1467 1410 726 626 447
νas(NH2) νs(NH2) νs(NH2) δ(NH2) νs(C–N) ν(N–C–N) νas(C ¼S) νs(C¼S) νas(N–C–S) δas(N–C–N)
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Fig. 2. SEM micrographs and EDS spectra of (a) ZTS (b) CTS and (c) ZCTS mixed crystal.
3.3. AAS and ICP Quantitative analysis has been carried out to determine the amount of zinc and cadmium in the final product using ICP and AAS. The AAS data reveal that the zinc and cadmium ion concentrations in ZCTS crystalline matrix are 10 and 22 ppm, respectively. The ICP results also confirm the above results and the amounts of zinc and cadmium are 13 and 27 ppm, respectively. The chemical composition of the mixed crystal ZCTS is not exactly proportional to the quantity or ratio of salts taken in the growth medium at the time of crystallization process.
3.4. UV–vis spectroscopy The optical absorption spectrum of ZCTS mixed crystal shows good transmittance in the entire visible region and the lower cutoff wavelength is ∼260 nm. The per cent transmittance is slightly
high in the case of ZCTS. As such, this mixed crystal system is quite useful for optical device applications.
3.5. Thermal analysis Fig. 3 shows the TG/DTA curve of ZCTS. It is clear that there is no physically absorbed water in the molecular structure of the crystal. TG curve shows a single stage weight loss at ∼240 1C due to melting and decomposition of ZCTS mixed crystal into fragments and its subsequent volatilization. In DTA, the sharp endothermic peak at ∼240 1C is due to melting and decompostion of the material. Melting point of the material was confirmed by using a Sigma instrument melting point apparatus (238 1C). No decomposition up to the melting point ensures the suitability of the material for application in lasers where the crystals are required to withstand high temperatures. The sharpness of the endothermic peak shows good degree of crystallinity and purity of the crystal.
200
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Fig. 3. TG/DTA curve of ZCTS mixed crystal.
3.6. XRD analysis The XRD profiles show that all samples are of single phase without any detectable impurity. Bragg’s peaks at specific 2θ angles show high crystallinity of the specimens. The structural analysis of ZCTS was carried out by single crystal XRD analysis. The lattice parameters of ZCTS, ZTS and CTS are given in Table 2 along with corresponding reported values. The structure is determined with a low value of R factor (0.0456) using 33,252 reflections (R(int) ¼ 0.0316) and goodness of fit (F2) ¼1.122, such that this structural elucidation does not come under the category of low symmetry. ZTS belongs to orthorhombic system with space group Pca21 [31,32] whereas CTS belongs to triclinic system with space group P-1[33,34]. Mixed crystal ZCTS crystallizes in the monoclinic system with noncentrosysmmetric space group P21 and the cell parameters are a ¼13.263(5) Å, b ¼11.886(5) Å, c ¼27.215(5) Å, α¼γ ¼90.000(5)1, β¼ 96.586(5)1,V ¼4262(3) Å3 and Z ¼2 with formula weight ¼2208.51 and calculated density ¼ 1.758 Mg m3. Crystal data and structure refinement of ZCTS mixed crystal are given in Table 3. Zn partially replaces the Cd present in tetrahedral geometry with four thiourea units. This is clearly seen in bond angle and bond length table. Cadmium is located in both tetrahedral and square pyramidal/trigonal bipyramidal coordination arrangements (Fig. 4). The partial occupancy is such that the molecular formulae have fractional values (Table 3). The occupation is determined by using a general procedure [35]. Single crystal XRD data are in agreement with the other experimental observations. No significant deviations are observed. The metal is coordinated by four thiourea groups with Zn/Cd– S–C bond angles ranging from 99(3)1 to 104(2)1, S–Zn/Cd–S bond angles ranging from 102.37(7)1 to 106.33(7)1 and Zn/Cd–S bond lengths lying in the range 2.496(2) Å–2.640(2) Å in a distorted tetrahedral geometry. In ZTS, zinc coordinates with three sulfur atoms from thiourea molecules with bond lengths 2.33, 2.32, and 2.31 Å [31]. In five coordinations, the cadmium is coordinated by five thiourea groups. Cd(tu)3SO4 is the first reported fivecoordinated Cd(II) complex [34]. The Cd–S–C bond angles lie in the range from 95.4(2)1 to 112.5(3)1, and S–Cd–S bond angles in the range 86.37(7)1–98.02(7) and 118.50(7)–123.92(6)1. The Cd–S bond lengths in five coordinations of the complex range from 2.6053(19) Å to 2.8250(2) Å in a trigonal bipyramidal geometry. In the case of CTS, the coordination around metal atom is intermediate between square pyramidal and trigonal bipyramidal, although somewhat closer to the former. One sulphur atom from a thiourea molecule coordinates to two adjacent cadmium atoms at distances 2.627 and 2.870 Å. The other three Cd–S bond lengths in the coordination polyhedral are 2.538, 2.627 and 2.627 Å [34]. Strong
Table 2 Comparison of cell parameters of ZTS, CTS, and ZCTS mixed crystals. Crystal
ZTSa
CTSb
ZCTS
System Space group aÅ bÅ cÅ α (deg) β (deg) γ (deg) V Å3 Z
Orthorhombic Pca21 7.773(4) 11.126(5) 15.491(5) 90 90 90 1339.70 4
Tricinic P-1 8.77(2) 9.05(2) 9.83(1) 91.3(2) 111.9(1) 95.5(2) 718.9 2
Monoclinic P21 13.263(5) 11.886(5) 27.215(5) 90.000(5) 96.586(5) 90.000(5) 4262(3) 2
a b
Ref. [31]. Ref. [34].
hydrogen bonds are observed for N–H–O with bond lengths in the range of 2.776(9)–2.998(8) Å. 3.7. SHG In order to confirm the NLO properties, the specimen was subjected to SHG test. The output SHG intensities for ZTS and ZCTS give relative NLO efficiencies. It is interesting to observe that the SHG conversion efficiency of ZCTS (47 mV) is superior to that ZTS (14 mV), a well known NLO material. Generally, high SHG efficiency is related with favorable molecular alignment facilitating nonlinearity. The effect of various dopants on SHG efficiency of ZTS has been listed in Table 4 and enhanced SHG efficiency is observed by doping. It is Interesting to observe that ZTS and ZCTS crystallize in a noncentrosymmetric space group whereas CTS has a centrosymmetric structure. The orthogonal projection of CTS [34] and ORTEP diagram of ZTS [36] are given in Fig. 5. In CTS, there is a mirror plane perpendicular to rotation axis, and an inversion center at the position of the intersection (1̄ ) leading to centrosymmetry and hence it is SHG-inactive. ZTS has an orthorhombic, primitive lattice; its c-glide plane is perpendicular to the a-axis, a-glide plane is perpendicular to b-axis and there is a two-fold screw axis parallel to the c-axis, leading to noncentrosymmetry. ZCTS has point group symmetry 2. Parallel to the b crystallographic axis is a 2-fold screw axis with a rotation of 1801 and a unit of translation of ½ b1 parallel to the b crystallographic axis. It belongs to a noncentrosymmetric space group and is hence SHG-active. Thus, the development of noncentrosymmetry is due to orientation. Higher SHG efficiency of the mixed crystal system compared with ZTS could be justified by
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Table 3 Crystal data and structure refinement of mixed crystal ZCTS. Empirical formula Molecular formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions Volume Z, Calculated density Absorption coefficient F(000) Crystal size Theta range for data collection Limiting indices Reflections collected/unique Completeness to theta ¼25.00 Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I 4Z sigma (I)] R indices (all data) Largest diff. peak and hole Absolute structure parameters
C18H72Cd2.75N36O16S22Zn1.25 2[Zn0.625Cd1.375(CS(NH2)2)9 4SO4] 2208.51 293(2) K 0.71073 Å Monoclinic, P21 a¼ 13.263(5) Å, b¼ 11.886(5) Å, c ¼ 27.215(5) Å, α ¼ γ¼ 90.000(5)1, β¼ 96.586(5)1, 4262(3) Å3 2, 1.758 Mg m−3 1.605 mm−1 2263 0.20 0.15 0.15 mm3 0.75 to 25.00 1 −15o h o15,−13o k o 14, −32o lo 32 33,252/14,300 [R(int) ¼0.0316] 99.6% Semi-empirical from equivalents 0.7948 and 0.7396 1.122 R1¼ 0.0456, wR2¼ 0.1092 R1¼ 0.0677, wR2¼ 0.1207 0.869 and −0.669 A−3 −0.02(2)
Fig. 4. ORTEP diagram of ZCTS.
Table 4 SHG outputs of ZTS with various dopants and metal thiourea sulfate crystals. System
Relative efficiency
Reference
ZCTS mixed crystal ZTS doped with Cs(I) ZTS doped with EDTA ZTS doped with Benzene ZTS doped with Mn(II) ZTS doped with Ce(IV) ZTS doped with KI ZTS doped with KCl ZTS doped with LiBr ZTS doped with Na(I) ZTS doped with urea ZTS doped with L-lysine ZTS doped with glycine
∼3.5 times higher wrt that of ZTS ∼1.2 times higher wrt that of ZTS ∼1.8 times higher wrt that of ZTS ∼1.4 times higher wrt that of ZTS ∼2.1 times higher wrt that of ZTS ∼1.6 times higher wrt that of ZTS ∼1.2 times higher wrt that of KDP ∼1.2 times higher wrt that of KDP Green light emission Slightly greater wrt that of ZTS ∼2.0 times higher wrt that of ZTS Comparable to ZTS ∼4.14 times higher wrt that of ZTS
Present work [23] [19] [19] [21] [22] [11] [12] [14] [13] [15] [16] [17]
facile charge transfer because of incorporation of foreign metal ion. Charge transfer is responsible for SHG or NLO. The foreign metal ion, by its incorporation, tunes the electronic properties.
4. Conclusion Zinc cadmium thiourea sulfate, Zn0.625Cd1.375(CS(NH2)2)9.4SO4 mixed crystals are grown by slow concentration of aqueous solution of the components and characterized using FT-IR, SEM, EDS, AAS, ICP, UV–vis, TG, DTA, SHG and X-ray diffraction studies. The SHG-active mixed crystal belongs to noncentrosymmetric P21 space group with two molecules in the unit cell. The product formation was confirmed by FT-IR and single crystal XRD analysis. Good tansmittance in the visible region is observed by UV–vis
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Fig. 5. (a) Orthogonal projection of CTS (reproduced from Ref. [34]) and (b) ORTEP diagram of ZTS (reproduced from Ref. [36]).
spectrum and the powder X-ray diffraction study shows the good crystallinity of the material. TG and DTA studies reveal the purity of the sample and no decomposition is observed up to the melting point. SEM reveals that the ZCTS crystal facets exhibit more voids and structure defect centers. Relative ease of preparation clubbed with high SHG efficiency makes this mixed crystal a technologically important material. Acknowledgments The authors thank Dr. Babu Varghese, senior scientific officer Grade 1, SAIF, Indian Institute of Technology, Chennai, for the support in structural elucidation by single crystal XRD. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jcrysgro.2013.05.021.
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