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Study of growth condition and characterization of Monothiourea-Cadmium Sulphate Dihydrate single crystals in silica gel
Materials Chemistry and Physics xxx (2015) 1e8
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Study of growth condition and characterization of Monothiourea-Cadmium Sulphate Dihydrate single crystals in silica gel T. Sivanandan, S. Kalainathan* Centre for Crystal Growth, VIT University, Vellore 632014, Tamil Nadu, 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
The single crystal was grown by gel growth method. The crystal has high transparency from 250 to 1100 nm. High stiffness constant indicates strong binding forces between ions. Low dielectric constant and dielectric loss at high frequencies.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 February 2015 Received in revised form 23 October 2015 Accepted 28 October 2015 Available online xxx
Single crystals of Monothiourea-Cadmium Sulphate Dihydrate belonging to Semiorganic material with nonlinear optical properties have been grown in silica gel medium using gel technique. The grown crystals were subjected to single crystal X-ray diffraction (SXRD) and Fourier transform infrared spectroscopy to elucidate their lattice parameters and functional group confirmation. Vickers microhardness measurements reveal that these materials have reverse indentation size effect and belong to the category of soft materials. Further, mechanical characterizations are also asserted from yield strength and elastic stiffness studies. The dielectric studies at different temperatures and frequency applied are measured and their behavior is analyzed. Thermal behavior of the crystal was investigated by thermogravimetric analysis (TGA) and differential thermal analysis. © 2015 Elsevier B.V. All rights reserved.
Keywords: Optical materials Crystal growth Differential thermal analysis Dielectric properties Optical properties
1. Introduction In the past three decades, numerous research activities have been in progress on nonlinear optical (NLO) crystals, as they play a key role in frequency conversion, optical image processing and optical communication [1e3]. NLO crystals for visible and ultraviolet (UV) region are of great significance for laser processing and
* Corresponding author. E-mail address: [email protected] (S. Kalainathan).
laser spectroscopy [4e9]. The Thiourea molecules are interesting not only due to the structural chemistry but also because of the possibility of formation of organometallic coordination complexes which results in enhanced NLO activity [10,11]. Metalethiourea coordination compounds have the advantage of both organic and inorganic properties. In the metal thiourea, the small p electron enhances the NLO properties [12]. Thiourea molecule is an attracting inorganic matrix modifier, as it has the potential to form a network of hydrogen bond which is due to the high dipole moment [13,14]. In
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the organometallic complex, the metal part belonging to the group (IIB) (Zn, Hg and Cd) is considered because of their closed d10 shell which is responsible for high transparency in the UV region. The physic-chemical behavior and structural analysis of the metal organic materials have also led to the conclusion that the central metal cannot be ignored in calculating the NLO coefficients [15e19]. The capability of Thiourea to form metal complexes through sulphur bonding has been reported [20e25]. Recent investigational reports on metal complexes of thiourea are Thiourea Zinc Chloride (ZTC), Zinc (tris) Thiourea Sulfate (ZTS), Cadmium Thiourea Acetate (CTA), BisThiourea Cadmium Chloride (BTCC), Thiourea Cadmium Iodide, Cadmium Thiourea Sulphate, and TrisThiourea Cadmium Sulfate (TTCS) [26e30]. These complexes are found to exhibit better nonlinear optical properties than potassium dihydrogen phosphate (KDP) [31e33]. Gel growth technique is an alternative technique to solution growth with controlled diffusion, and the growth process is free from convection. The growth of single crystals in gel medium is self-purifying processes, free from thermal strains which is common in crystals grown from melt [34]. Crystals grown by gel technique are superior to those grown by slow evaporation technique and found to have extreme low dislocation densities [35]. The genesis of the compound chosen for research has been crystallized and reported by Cavalca et al. [36]. In the present work, Monothiourea-Cadmium Sulphate Dihydrate crystals were grown by gel technique at room temperature and were characterized through several techniques such as single crystal XRD, spectral, thermal and dielectric measurements.
2. Experimental details 2.1. Synthesis and crystal growth Single crystals of Monothiourea-Cadmium Sulphate Dihydrate were grown inside silica gel medium, in test tubes by single diffusion method. The stock solution was prepared based on the report given by Henisch [35]. The reaction between cadmium sulphate octahydrate and thiourea is given as CdSO4$8H2O þ CS(NH2)2 / Cd[CS(NH2)2]SO4$2H2O þ 6H2O
(1)
To 10 ml of the stock solution (specific gravity 1.03 g/cc) a small quantity of glacial acetic acid was added and the pH was adjusted to 5.5. To this, 25 ml of 2 M thiourea was added and stirred continuously for 4 h using a magnetic stirrer. Then the solution was poured into the test tubes and placed inside constant temperature (40 C) bath for gellation. After the gel formation, an aqueous solution of 25 ml of 2 M cadmium sulphate octahydrate was pipetted out along the side of the tube. Monothiourea-Cadmium Sulphate Dihydrate crystals of 8 mm length were obtained in 15 days. The experiment was also carried out at different pH (Table 1). The gellation time taken was found to be inversely dependent on the pH. Due to this, the mixing of solution (stock solution with acetic acid) with the inner solution was difficult (aqueous solution of thiourea) for pH greater than 5.5. Besides the above said, Monothiourea-Cadmium Sulphate Dihydrate nuclei was found to decrease as the pH
increased. The optimum pH yielding maximum size of transparent title crystal was found to be at 5.5 pH. Photographs, of Monothiourea-Cadmium Sulphate Dihydrate crystal in the gel medium and at different pH are shown in Fig. 1 (aed) respectively. The size of the crystal increases with the increase in pH as shown in Fig. 1 (bed). The increase in size of the crystal could be explained due to variation in the gel structure (i.e. by establishing threedimensional cross-linkages between the molecules). Mover over gel consists of sheet like structure of varying degree of surface roughness and porosity, forming inter-connected cells. The cell walls are thicker for dense cells. The structure of gel depends on the pH during gelation [37] i.e. the pH is below 4, cross-linkage between the polymerization chains is produced and when the pH of the gel is above 4.5 long chain polymerization is produced. Thus it results in the structural change from a box-like network of loosely bound platelets structure with lack of cross linkages which attribute to the increase in crystal size. 2.2. Characterization The grown crystals of Monothiourea-Cadmium Sulphate Dihydrate was subjected to single crystal X-ray diffraction studies using ENRAF NONIUS CAD-4 diffractometer to elucidate their lattice parameters. The powder X-ray diffraction analysis (XRD) on the Monothiourea-Cadmium Sulphate Dihydrate crystal was also carried out using Brucker D8 Advance diffractometer with Cu Ka radiation (l ¼ 1.5406 Å) over the range 10e60 at a scan rate of 0.02 s1. The FTIR spectra of title crystals were recorded in the range of 4000e400 cm1 employing a Perkin Elmer spectrometer by KBr pellet method in order to reveal the metal complex coordination. The optical properties of the title crystal were examined between 190 and 1100 nm using Shimadzu UV-1061 UVevis spectrophotometer. The thermal stability was ascertained from thermogravimetric analysis and differential thermal analysis carried out on the sample using NETZSCH STA 409PC/PG thermal analyzer at a heating rate of 10 C min1 in nitrogen atmosphere. 3. Results and discussion 3.1. Single crystal X-ray diffraction analysis The Single crystal X-ray diffraction results reveal that the crystal belongs to the orthorhombic system with space group Pbca. The lattice parameters was determined to be a ¼ 7.783 Å, b ¼ 13.461 Å, c ¼ 15.966 Å, a ¼ g ¼ b ¼ 90 and its cell volume V ¼ 1692.49 Å3. These results are found to be in good agreement with that of the reported values [36,38]. 3.2. Powder X-ray diffraction analysis Powder X-ray diffraction (PXRD) pattern of the grown crystal is shown in Fig. 2. From the PXRD pattern, we confirm that the grown crystal has high degree of crystallinity which is revealed by the sharp and high-intensity peaks. The peaks were indexed using Powder X software [39] and the indexed peaks were found to match with the JCPDS data [71-2183]. Further, to confirm the
Table 1 Growth parameters of Monothiourea-Cadmium Sulphate Dihydrate. Density
Inner reagent
pH
Outer reagent
Crystal size
Growth period
1.03 1.03 1.03
10 ml of stock solution þ acetic acidþ25 ml of 2 M Thiourea 10 ml of stock solution þ acetic acidþ25 ml of 2 M Thiourea 10 ml of stock solution þ acetic acidþ25 ml of 2 M Thiourea
3.5 4.5 5.5
25 ml of 2 M cadmium Sulphate (Hydrate) 25 ml of 2 M cadmium Sulphate (Hydrate) 25 ml of 2 M cadmium Sulphate (Hydrate)
Crystal of size 2 mm crystals of size 4e6 mm crystal of size 8e10 mm
27 days 21days 15 days
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Fig. 1. Photographs of Monothiourea-Cadmium Sulphate Dihydrate crystals; (a) inside the gel medium (pH 5.5); Crystals grown at pH: (b) 3.5; (c) 4.5 and (d) 5.5.
insert of Fig. 3, confirm the quality of the crystal. 3.3. FT-IR spectral analysis
Fig. 2. Powder X-ray diffraction pattern of Monothiourea-Cadmium Sulphate Dihydrate.
crystallinity of the as grown crystal, the X-ray diffraction data was obtained and is presented in Fig. 3. The intensity of the diffraction peak is high and the symmetric peak proves the high crystalline nature of the grown crystal. The diffraction peak (002) shown in the
Fig. 3. XRD spectrum of Monothiourea-Cadmium Sulphate Dihydrate single crystal.
The powdered title crystal were made into pellets using KBR and subjected to Fourier Transform Infra Red (FTIR) analysis in range 4000e400 cm1 using Perkin Elmer spectrometer. Fig. 4 shows the FTIR spectrum of the title crystal and is found to be in accordance with the earlier literature [40e43] and as well as with the available spectra of thiourea [44]. When compared with the spectra of thiourea [45,46], shift in the peaks was observed, which confirms the metal coordination with thiourea. There are two possible ways by which the coordination of cadmium with thiourea can occur is either through the nitrogen or through the sulfur of thiourea. The broad envelope in between 3107 and 3500 cm1 corresponds to the symmetric and asymmetric stretching modes of NH2 grouping of cadmium-coordinated thiourea [47]. The absorption band observed at 1614 cm1 in the spectrum corresponds to the NH2 bending vibration. The absorption at 1490 cm1 is assigned to the NeCeN stretching vibration. The absorption band at 1408 cm1 assign to the C]S stretching vibration which confirms that the cadmium coordination occur through the sulfur. The CeN symmetric vibration assign to peak at 1111 cm1. In the finger print region, two peaks at 1111 and 713 cm1 for metal thiourea complex are compared to 1089 and 740 cm1 peaks of thiourea. The lowering of C]S stretching frequency from 740 cm1 to 713 cm1 confirms the formation of metalesulphur coordination bond. The peak observed
Fig. 4. FT-IR spectrum of Monothiourea-Cadmium Sulphate Dihydrate.
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Table 2 Comparision of IR bands of Thiourea and Monothiourea-Cadmium Sulphate Dihydrate single crystal. Thiourea [44](cm1)
Monothiourea-Cadmium Sulphate Dihydrate (cm1)
Assignments
3376 3280 3167 1627 1470 1417 1089 740 494
3396 3290 3203 1614 1490 1408 1111 713 497
y(NH2) y(NH2) y(NH2) y(NH2) d (NeCeN) y(C]S) y(CeN) y(C]S) y(NeCeN)
at 987 cm1 is attributed to the triply degenerate symmetric stretching mode of SeO [48,49]. The vibration of thiourea reported in the literature and its comparison with the vibrations of the title crystal is presented in Table 2. Comparison of the vibrational data demonstrates the shift in peak positions which would have occurred due to significant interaction between thiourea and cadmium in the crystal. 3.4. Optical studies The optical absorption spectra of Monothiourea-Cadmium Sulphate Dihydrate single crystals are shown in Fig. 5. When the absorption is monitored from longer wavelength to shorter wavelength, the enhanced absorption is observed between 200 and 280 nm as shown in Fig. 5. From the spectrum, it is noted that at 282 nm a sharp fall of absorbance to nearly zero is observed indicating the cutoff wavelength [50]. It is seen that low percentage of absorption in the entire visible region which is an essential parameter for NLO applications [51]. 3.5. Microhardness studies Smooth and cleaned surfaces of gel-grown Monothiourea-Cadmium Sulphate Dihydrate single crystals were subjected to static indentation tests at room temperature by using Vicker's microhardness tester. Loads ranging from 0.098 to 0.98 N were used for indentation, keeping the indenter at right angle to the surface for 10s in all cases. Vicker's indented impression was approximately
Fig. 5. UVevis spectrum of Monothiourea-Cadmium Sulphate Dihydrate crystal.
Fig. 6. Variation of Hv with Load P of Monothiourea-Cadmium Sulphate Dihydrate crystal.
square in shape. For every indentation the two diagonal lengths (d) of the indented impressions were measured with the calibrated micrometer embedded in the tester. By substituting the average diagonal length in the formula [52], the Vicker's hardness (HV) value is calculated
1:8544 P 103 GNm2 Hv 103 GNm2 ¼ d2
(2)
where P is taken in N and d is in mm. It is reported in the literature that the microhardness of different materials, (i) increases with load, (ii) decreases with load, (iii) independent of load, and (iv) show complex variation with changes in load [53]. The variation in the value of Hv with load P of the crystal is shown in Fig. 6. The Hv value is found to vary from 0.0006 to 0.0014 GN m2 for the increase of load from 0.098 to 0.98 N. The dependence of microhardness on the applied load is known as reverse indentation size effect (RISE). RISE is attributed to the existence of distorted zone near crystal medium interface, specimen chipping, the effect of vibration, etc. The relationship between the load and diagonal length of the indentation is given by Mayer's law
Fig. 7. Variation of log P with log d of Monothiourea-Cadmium Sulphate Dihydrate crystal.
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The elastic stiffness constant for different loads was calculated using wooster's empirical relation [59]
C11 ¼ ðHv Þ7=4
(7)
The calculated yield strength and stiffness constant for different loads are tabulated in Table 3. The result of microhardness measurement reveals the mechanical stability of the metal complex. As discussed earlier that the central metal ion plays an important role in the metal complex. In the crystal, the cadmium is the central metal ion which has not only broken the centrosymmetric nature of thiourea, but also made the crystal mechanically stable. The reason is that the cadmium ion coordinates with the two sulphur atoms of the thiourea ligands to form a tetrahedral system and thus makes the crystal more stable.
3.6. Dielectric studies
Fig. 8. Variation of Load P with d2 of Monothiourea-Cadmium Sulphate Dihydrate crystal.
[54] by which the Mayer index number was calculated
P ¼ kdn
εr ¼ (3)
where P is the load, d is the diagonal length of the impression, K is a constant and n is the Mayer's index also know as work hardening coefficient. The value of n can be determined from the graph between log P and log d plot, as shown in Fig. 7. In the plot, a straight line was obtained after least square fitting. The value of n is calculated to be 2.8. According to onitsch and Hanneman [55,56], if the value of n is more than 1.6, it belongs to soft materials and between 1 and 1.6 it belongs to hard material. The title crystal belongs to the soft material category. According to Hays-Kendall approach [57], load dependence of hardness is given by
P ¼ W þ A1 d2
(4)
where W is the minimum load to initiate plastic deformation in grams and A1 is a load independent constant. These two values are determined from the plot of load P Vs. d2 (Fig. 8). The value of W is the intercept along the load axis and A1 is the slope. The corrected hardness H0 for the crystal is obtained using the relation
Ho ¼ 1854 X A1
(5)
The load-independent values of Ho come out to be 1.04 GNm2 for the grown crystal. From the hardness value, the yield strength was calculated using the relation [58]
sy ¼
Hv 12:5ðn 2Þ n2 ½1 ðn 2Þ 1 ðn 2Þ 2:9
The dielectric constant and dielectric loss of title crystal was studied at different temperature using HIOKI 3532 LCR HITESTER instrument in the frequency range of 50 Hze5 MHz. The dielectric constant was calculated using the equation
(6)
Cd εo A
(8)
where d represents the thickness and A represents the area of the sample. The frequency dependence of dielectric constant is shown in Fig. 9. The maximum dielectric constant (550, 460 and 360) appeared in the lower-frequency region (100 Hz) at the temperature (308, 328 and 348 K). As the frequency increases, there is a moderate decrease in the dielectric constant till 2 MHz and tends to remain constant with further increase in the frequency at the temperatures (308 Ke348 K) of study. The dielectric constant value being high at low frequency is attributed to the space charge polarizations [60]. Crystal exhibits normal dielectric behavior, i.e., beyond a certain frequency of the electric field, the dipole does not follow the alternating field. The low value of dielectric constant at the higher frequency region may be caused by the inability of the dipoles to comply with the external field. As a result, it will have minimum losses as compared to the material having high dielectric constant [61,62]. Hence, the title crystal may be used for high speed electro optic modulations. The dielectric loss as a function of frequency at different temperatures is shown in Fig. 10. It could be infered that the dielectric loss strongly depends on the frequency of the applied field which is similar to the dielectric constant in the ionic system [63]. Thus, it is concluded that the crystal is optically good for nonlinear optical device [64]. To calculate the electronic properties of Penn gap, Fermi energy and electronic polarizability, only one particular value of the dielectric constant at high frequency was used. The theoretical calculation shows that the high frequency dielectric constant depends on the plasma energy, penn gap and the Fermi energy. The Penn gap is determined using the dielectric constant along with the Plasma energy value [65]. The plasma energy ħup is calculated using the equation [66]
Table 3 Mechanical parameters of Monothiourea-Cadmium Sulphate Dihydrate crystal. Load P in g
Yield strength sy (GN/m2)
Elastic stiffness constant C11 (1015 Pa)
10 25 50 100
94.2860 110.7044 135.1943 190.4870
1.9209 2.5421 3.6086 6.5363
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3=4 EF ¼ 0:2984 ħup
(11)
Electronic Polarizability a [69] can be obtained using the equation
" a¼
ħu2p S0
#
ħu2p S0 þ Ep2
X
M X 0:396 X 1024 cm1 P
(12)
where So is a constant for a particular material and which is determined by the equation
Ep 1 Ep 2 þ 3 4EF 4EF
S0 ¼ 1
(13)
The a value is calculated according to the equation
a¼ Fig. 9. Variation of dielectric constant with log frequency of Monothiourea-Cadmium Sulphate Dihydrate crystal.
3M ε∞ 1 4pNa r ε∞ þ 2
(14)
All the above parameters were calculated and are shown in Table 4. The dielectric constant of materials is a crucial parameter for calculating the physical or electronic properties of materials. These values of gel grown crystal are compared with the values of standard KDP material and the electronic parameters are found to be higher than those of KDP (Table 4). 3.7. Thermal analysis
Fig. 10. Variation of dielectric loss with log frequency of Monothiourea-Cadmium Sulphate Dihydrate crystal.
ħup ¼ 28:8
1=2 Zr M
(9)
where Z is the total number of valence electrons, r is the density and M is the molecular weight of the single crystal, respectively. According to the Penn model [67], the Plasma energy in terms of Penn gap and Fermi energy [68] in eV is given as
Ep ¼ and
ħup ðε∞ 1Þ1=2
(10)
The thermal stability of the title crystal was ascertained from thermogravimetric (TGA) and differential thermal analysis simultaneously. Fig. 11 shows the TG-DTA curve for the grown crystal. Initially, dehydration takes place in the temperature range of 90e170 C, which corresponds to a mass loss of 7.87% [70]. In the temperature range between 170 and 240 C, mass loss of 25.02% occur which could be associated to evolution of carbon disulphide (CS2) and ammonia (NH3) gas, indicating the decomposition of thiourea molecule. 10.55% loss in mass is observed between 280 and 480 C, which may be due to decomposition of metal oxide present in the sample [71]. A weight loss of 5.04% is observed between 500 C to 650 C corresponding to the ignition of CO2. In the final stage, metal sulphate decomposes in the temperature range of 700 Ce990 C with an associated weight loss of 16.65% [72]. In the DTA curve, the endothermic event is observed at 700 C which is due to the melting point of the crystal. Thermal stability of the title crystal is relatively good when compared to other thiourea based complex [73,74]. 4. Conclusion Monothiourea-Cadmium Sulphate Dihydrate can be obtained in an acidic medium by using gel technique. Single crystal XRD analysis revealed that the Monothiourea-Cadmium Sulphate Dihydrate crystallizes in orthorhombic crystal system. PXRD study revealed the crystalline nature. IR spectrum confirms the presence of all the expected functional groups. The UVeVisible studies
Table 4 Comparision of Electronic properties of Monothiourea-Cadmium Sulphate Dihydrate and KDP crystal. Parameters
Values of Monothiourea-Cadmium Sulphate Dihydrate
Values of KDP
Plasma energy (eV) Penn gap (eV) Fermi gap (eV) Polarizability (cm3) Penn analysis Polarizability Clausius-Mossotti Equation
21.892 3.4146 17.636 4.902 1023 4.94 1023
17.2 2.37 12.102 2.12 1023 2.14 1023
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Fig. 11. TG/DTA thermogram of Monothiourea-cadmium Sulphate Dihydrate.
confirm the grown crystal may be useful in the optical application as it has a low cut off wavelength (285 nm). Dielectric studies indicate that the dielectric constant and dielectric loss decreases with the increasing frequency and hence it is significant to use in the microelectronic industry. The plasma energy, penn gap and electronic polarizability have been determined. The TG/DTA analysis reveals that the gel grown crystal has high thermal stability. Acknowledgments The author expresses thanks to SAIF, IIT Madras for providing single crystal XRD data. The author also expresses thanks to VIT University for its constant encouragement and support. References [1] M.H. Jiang, D. Xu, G.C. Xing, Z.S. Shao, Synth. Cryst. 1 (1985) 3e4. [2] W.B. Hou, M.H. Jiang, D.R. Yuan, D. Xu, N. Zhang, M.G. Liu, X.T. Tao, Mater. Res. Bull. 28 (1993) 645e649. [3] S.G. Bhat, S.M. Dharmaprakash, J. Cryst, Growth 81 (1997) 390e395. [4] S. Suresh, D. Arivuoli, J. Min, Mater. Character. Eng. 10 (2011) 517e526. [5] Preeti Singh, Mohd Hasmuddin, Mohd. Shakir, N. Vijayan, M.M. Abdullah, V. Ganesh, M.A. Wahab, Mater. Chem. Phys. 142 (2013) 154e164. [6] N. Vijayan, R. Ramesh Babu, R. Gopalakrishnan, P. Ramasamy, W.T.A. Harrison, J. Cryst. Growth 262 (2004) 490e498. [7] M.D. Aggarwal, J. Choi, W.S. Wang, K. Bhat, R.B. Lal, Angela D. Shields, Benjamin G. Penn, Donald O. Frazier, J. Cryst. Growth 204 (1999) 179e182. [8] Guohong Zou, Ling Huang, Ning Ye, Chensheng Lin, Wendan Cheng, Hui Huang, J. Am. Chem. Soc. 135 (2013) 18560e18566. [9] Guohong Zou, Ning Ye, Ling Huang, Xinsong Lin, J. Am. Chem. Soc. 133 (2011) 20001e20007. [10] T.C. Sabari Girisun, S. Dhanuskodi, Mater. Res. Bull. 45 (2010) 88e91. [11] R. Sankar, C.M. Raghavan, R. Mohan Kumar, R. Jayavel, J. Cryst. Growth 305 (2007) 156e161. [12] M.H. Jiang, Q. Fang, Adv. Mater. 11 (1999) 1147e1154. [13] K.H. Hellwege, A.M. Hellwege, Landolt-bornstein, Group II, 14, 1982, p. 584. [14] S. Selvakumar, S.M. Ravi Kumar, K. Rajarajan, A. Joseph Arul Pragasam, S.A. Rajasekar, K. Thamizharasan, P. Sagayaraj, Cryst. Growth Des. 6 (2012) 2607e2610. [15] C. Ramanchandra Raja, R.S. Sundararajan, V. Krishnakumar, Spectrochim. Acta A 71 (2009) 1634e1637. [16] K. Ambujam, P.C. Thomas, S. Aruna, D. Prem Anand, P. Sagayaraj, Cryst. Res. Technol. 41 (2006) 1082e1088. [17] S.G. Bhat, S.M. Dharmaprakah, J. Cryst. Growth 181 (1997) 390e394. [18] V. Kannan, N.P. Rajesh, R.B. Ganesh, P. Ramasamy, J. Cryst. Growth 269 (2004) 565e569. [19] G. Senthil Murugan, P. Ramasamy, J. Cryst. Growth 311 (2009) 585e588. [20] K. Nithya, B. Karthikeyan, G. Ramasamy, K. Muthu, S.P. Meenakshisundaram, Spectrochim. Acta Part A 79 (2011) 1648e1653. [21] W.B. Hou, D. Xu, D.R. Yuan, M.G. Liu, Cryst. Res. Technol. 29 (1994) 939e945. [22] J.M. Alia, H.G.M. Edwards, M.D. Stoev, Spectrochim. Acta A 55 (1999)
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Study of growth condition and characterization of Monothiourea-Cadmium Sulphate Dihydrate single crystals in silica gel T. Sivanandan, S. Kalainathan* Centre for Crystal Growth, VIT University, Vellore 632014, Tamil Nadu, 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
The single crystal was grown by gel growth method. The crystal has high transparency from 250 to 1100 nm. High stiffness constant indicates strong binding forces between ions. Low dielectric constant and dielectric loss at high frequencies.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 February 2015 Received in revised form 23 October 2015 Accepted 28 October 2015 Available online xxx
Single crystals of Monothiourea-Cadmium Sulphate Dihydrate belonging to Semiorganic material with nonlinear optical properties have been grown in silica gel medium using gel technique. The grown crystals were subjected to single crystal X-ray diffraction (SXRD) and Fourier transform infrared spectroscopy to elucidate their lattice parameters and functional group confirmation. Vickers microhardness measurements reveal that these materials have reverse indentation size effect and belong to the category of soft materials. Further, mechanical characterizations are also asserted from yield strength and elastic stiffness studies. The dielectric studies at different temperatures and frequency applied are measured and their behavior is analyzed. Thermal behavior of the crystal was investigated by thermogravimetric analysis (TGA) and differential thermal analysis. © 2015 Elsevier B.V. All rights reserved.
Keywords: Optical materials Crystal growth Differential thermal analysis Dielectric properties Optical properties
1. Introduction In the past three decades, numerous research activities have been in progress on nonlinear optical (NLO) crystals, as they play a key role in frequency conversion, optical image processing and optical communication [1e3]. NLO crystals for visible and ultraviolet (UV) region are of great significance for laser processing and
* Corresponding author. E-mail address: [email protected] (S. Kalainathan).
laser spectroscopy [4e9]. The Thiourea molecules are interesting not only due to the structural chemistry but also because of the possibility of formation of organometallic coordination complexes which results in enhanced NLO activity [10,11]. Metalethiourea coordination compounds have the advantage of both organic and inorganic properties. In the metal thiourea, the small p electron enhances the NLO properties [12]. Thiourea molecule is an attracting inorganic matrix modifier, as it has the potential to form a network of hydrogen bond which is due to the high dipole moment [13,14]. In
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the organometallic complex, the metal part belonging to the group (IIB) (Zn, Hg and Cd) is considered because of their closed d10 shell which is responsible for high transparency in the UV region. The physic-chemical behavior and structural analysis of the metal organic materials have also led to the conclusion that the central metal cannot be ignored in calculating the NLO coefficients [15e19]. The capability of Thiourea to form metal complexes through sulphur bonding has been reported [20e25]. Recent investigational reports on metal complexes of thiourea are Thiourea Zinc Chloride (ZTC), Zinc (tris) Thiourea Sulfate (ZTS), Cadmium Thiourea Acetate (CTA), BisThiourea Cadmium Chloride (BTCC), Thiourea Cadmium Iodide, Cadmium Thiourea Sulphate, and TrisThiourea Cadmium Sulfate (TTCS) [26e30]. These complexes are found to exhibit better nonlinear optical properties than potassium dihydrogen phosphate (KDP) [31e33]. Gel growth technique is an alternative technique to solution growth with controlled diffusion, and the growth process is free from convection. The growth of single crystals in gel medium is self-purifying processes, free from thermal strains which is common in crystals grown from melt [34]. Crystals grown by gel technique are superior to those grown by slow evaporation technique and found to have extreme low dislocation densities [35]. The genesis of the compound chosen for research has been crystallized and reported by Cavalca et al. [36]. In the present work, Monothiourea-Cadmium Sulphate Dihydrate crystals were grown by gel technique at room temperature and were characterized through several techniques such as single crystal XRD, spectral, thermal and dielectric measurements.
2. Experimental details 2.1. Synthesis and crystal growth Single crystals of Monothiourea-Cadmium Sulphate Dihydrate were grown inside silica gel medium, in test tubes by single diffusion method. The stock solution was prepared based on the report given by Henisch [35]. The reaction between cadmium sulphate octahydrate and thiourea is given as CdSO4$8H2O þ CS(NH2)2 / Cd[CS(NH2)2]SO4$2H2O þ 6H2O
(1)
To 10 ml of the stock solution (specific gravity 1.03 g/cc) a small quantity of glacial acetic acid was added and the pH was adjusted to 5.5. To this, 25 ml of 2 M thiourea was added and stirred continuously for 4 h using a magnetic stirrer. Then the solution was poured into the test tubes and placed inside constant temperature (40 C) bath for gellation. After the gel formation, an aqueous solution of 25 ml of 2 M cadmium sulphate octahydrate was pipetted out along the side of the tube. Monothiourea-Cadmium Sulphate Dihydrate crystals of 8 mm length were obtained in 15 days. The experiment was also carried out at different pH (Table 1). The gellation time taken was found to be inversely dependent on the pH. Due to this, the mixing of solution (stock solution with acetic acid) with the inner solution was difficult (aqueous solution of thiourea) for pH greater than 5.5. Besides the above said, Monothiourea-Cadmium Sulphate Dihydrate nuclei was found to decrease as the pH
increased. The optimum pH yielding maximum size of transparent title crystal was found to be at 5.5 pH. Photographs, of Monothiourea-Cadmium Sulphate Dihydrate crystal in the gel medium and at different pH are shown in Fig. 1 (aed) respectively. The size of the crystal increases with the increase in pH as shown in Fig. 1 (bed). The increase in size of the crystal could be explained due to variation in the gel structure (i.e. by establishing threedimensional cross-linkages between the molecules). Mover over gel consists of sheet like structure of varying degree of surface roughness and porosity, forming inter-connected cells. The cell walls are thicker for dense cells. The structure of gel depends on the pH during gelation [37] i.e. the pH is below 4, cross-linkage between the polymerization chains is produced and when the pH of the gel is above 4.5 long chain polymerization is produced. Thus it results in the structural change from a box-like network of loosely bound platelets structure with lack of cross linkages which attribute to the increase in crystal size. 2.2. Characterization The grown crystals of Monothiourea-Cadmium Sulphate Dihydrate was subjected to single crystal X-ray diffraction studies using ENRAF NONIUS CAD-4 diffractometer to elucidate their lattice parameters. The powder X-ray diffraction analysis (XRD) on the Monothiourea-Cadmium Sulphate Dihydrate crystal was also carried out using Brucker D8 Advance diffractometer with Cu Ka radiation (l ¼ 1.5406 Å) over the range 10e60 at a scan rate of 0.02 s1. The FTIR spectra of title crystals were recorded in the range of 4000e400 cm1 employing a Perkin Elmer spectrometer by KBr pellet method in order to reveal the metal complex coordination. The optical properties of the title crystal were examined between 190 and 1100 nm using Shimadzu UV-1061 UVevis spectrophotometer. The thermal stability was ascertained from thermogravimetric analysis and differential thermal analysis carried out on the sample using NETZSCH STA 409PC/PG thermal analyzer at a heating rate of 10 C min1 in nitrogen atmosphere. 3. Results and discussion 3.1. Single crystal X-ray diffraction analysis The Single crystal X-ray diffraction results reveal that the crystal belongs to the orthorhombic system with space group Pbca. The lattice parameters was determined to be a ¼ 7.783 Å, b ¼ 13.461 Å, c ¼ 15.966 Å, a ¼ g ¼ b ¼ 90 and its cell volume V ¼ 1692.49 Å3. These results are found to be in good agreement with that of the reported values [36,38]. 3.2. Powder X-ray diffraction analysis Powder X-ray diffraction (PXRD) pattern of the grown crystal is shown in Fig. 2. From the PXRD pattern, we confirm that the grown crystal has high degree of crystallinity which is revealed by the sharp and high-intensity peaks. The peaks were indexed using Powder X software [39] and the indexed peaks were found to match with the JCPDS data [71-2183]. Further, to confirm the
Table 1 Growth parameters of Monothiourea-Cadmium Sulphate Dihydrate. Density
Inner reagent
pH
Outer reagent
Crystal size
Growth period
1.03 1.03 1.03
10 ml of stock solution þ acetic acidþ25 ml of 2 M Thiourea 10 ml of stock solution þ acetic acidþ25 ml of 2 M Thiourea 10 ml of stock solution þ acetic acidþ25 ml of 2 M Thiourea
3.5 4.5 5.5
25 ml of 2 M cadmium Sulphate (Hydrate) 25 ml of 2 M cadmium Sulphate (Hydrate) 25 ml of 2 M cadmium Sulphate (Hydrate)
Crystal of size 2 mm crystals of size 4e6 mm crystal of size 8e10 mm
27 days 21days 15 days
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Fig. 1. Photographs of Monothiourea-Cadmium Sulphate Dihydrate crystals; (a) inside the gel medium (pH 5.5); Crystals grown at pH: (b) 3.5; (c) 4.5 and (d) 5.5.
insert of Fig. 3, confirm the quality of the crystal. 3.3. FT-IR spectral analysis
Fig. 2. Powder X-ray diffraction pattern of Monothiourea-Cadmium Sulphate Dihydrate.
crystallinity of the as grown crystal, the X-ray diffraction data was obtained and is presented in Fig. 3. The intensity of the diffraction peak is high and the symmetric peak proves the high crystalline nature of the grown crystal. The diffraction peak (002) shown in the
Fig. 3. XRD spectrum of Monothiourea-Cadmium Sulphate Dihydrate single crystal.
The powdered title crystal were made into pellets using KBR and subjected to Fourier Transform Infra Red (FTIR) analysis in range 4000e400 cm1 using Perkin Elmer spectrometer. Fig. 4 shows the FTIR spectrum of the title crystal and is found to be in accordance with the earlier literature [40e43] and as well as with the available spectra of thiourea [44]. When compared with the spectra of thiourea [45,46], shift in the peaks was observed, which confirms the metal coordination with thiourea. There are two possible ways by which the coordination of cadmium with thiourea can occur is either through the nitrogen or through the sulfur of thiourea. The broad envelope in between 3107 and 3500 cm1 corresponds to the symmetric and asymmetric stretching modes of NH2 grouping of cadmium-coordinated thiourea [47]. The absorption band observed at 1614 cm1 in the spectrum corresponds to the NH2 bending vibration. The absorption at 1490 cm1 is assigned to the NeCeN stretching vibration. The absorption band at 1408 cm1 assign to the C]S stretching vibration which confirms that the cadmium coordination occur through the sulfur. The CeN symmetric vibration assign to peak at 1111 cm1. In the finger print region, two peaks at 1111 and 713 cm1 for metal thiourea complex are compared to 1089 and 740 cm1 peaks of thiourea. The lowering of C]S stretching frequency from 740 cm1 to 713 cm1 confirms the formation of metalesulphur coordination bond. The peak observed
Fig. 4. FT-IR spectrum of Monothiourea-Cadmium Sulphate Dihydrate.
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Table 2 Comparision of IR bands of Thiourea and Monothiourea-Cadmium Sulphate Dihydrate single crystal. Thiourea [44](cm1)
Monothiourea-Cadmium Sulphate Dihydrate (cm1)
Assignments
3376 3280 3167 1627 1470 1417 1089 740 494
3396 3290 3203 1614 1490 1408 1111 713 497
y(NH2) y(NH2) y(NH2) y(NH2) d (NeCeN) y(C]S) y(CeN) y(C]S) y(NeCeN)
at 987 cm1 is attributed to the triply degenerate symmetric stretching mode of SeO [48,49]. The vibration of thiourea reported in the literature and its comparison with the vibrations of the title crystal is presented in Table 2. Comparison of the vibrational data demonstrates the shift in peak positions which would have occurred due to significant interaction between thiourea and cadmium in the crystal. 3.4. Optical studies The optical absorption spectra of Monothiourea-Cadmium Sulphate Dihydrate single crystals are shown in Fig. 5. When the absorption is monitored from longer wavelength to shorter wavelength, the enhanced absorption is observed between 200 and 280 nm as shown in Fig. 5. From the spectrum, it is noted that at 282 nm a sharp fall of absorbance to nearly zero is observed indicating the cutoff wavelength [50]. It is seen that low percentage of absorption in the entire visible region which is an essential parameter for NLO applications [51]. 3.5. Microhardness studies Smooth and cleaned surfaces of gel-grown Monothiourea-Cadmium Sulphate Dihydrate single crystals were subjected to static indentation tests at room temperature by using Vicker's microhardness tester. Loads ranging from 0.098 to 0.98 N were used for indentation, keeping the indenter at right angle to the surface for 10s in all cases. Vicker's indented impression was approximately
Fig. 5. UVevis spectrum of Monothiourea-Cadmium Sulphate Dihydrate crystal.
Fig. 6. Variation of Hv with Load P of Monothiourea-Cadmium Sulphate Dihydrate crystal.
square in shape. For every indentation the two diagonal lengths (d) of the indented impressions were measured with the calibrated micrometer embedded in the tester. By substituting the average diagonal length in the formula [52], the Vicker's hardness (HV) value is calculated
1:8544 P 103 GNm2 Hv 103 GNm2 ¼ d2
(2)
where P is taken in N and d is in mm. It is reported in the literature that the microhardness of different materials, (i) increases with load, (ii) decreases with load, (iii) independent of load, and (iv) show complex variation with changes in load [53]. The variation in the value of Hv with load P of the crystal is shown in Fig. 6. The Hv value is found to vary from 0.0006 to 0.0014 GN m2 for the increase of load from 0.098 to 0.98 N. The dependence of microhardness on the applied load is known as reverse indentation size effect (RISE). RISE is attributed to the existence of distorted zone near crystal medium interface, specimen chipping, the effect of vibration, etc. The relationship between the load and diagonal length of the indentation is given by Mayer's law
Fig. 7. Variation of log P with log d of Monothiourea-Cadmium Sulphate Dihydrate crystal.
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5
The elastic stiffness constant for different loads was calculated using wooster's empirical relation [59]
C11 ¼ ðHv Þ7=4
(7)
The calculated yield strength and stiffness constant for different loads are tabulated in Table 3. The result of microhardness measurement reveals the mechanical stability of the metal complex. As discussed earlier that the central metal ion plays an important role in the metal complex. In the crystal, the cadmium is the central metal ion which has not only broken the centrosymmetric nature of thiourea, but also made the crystal mechanically stable. The reason is that the cadmium ion coordinates with the two sulphur atoms of the thiourea ligands to form a tetrahedral system and thus makes the crystal more stable.
3.6. Dielectric studies
Fig. 8. Variation of Load P with d2 of Monothiourea-Cadmium Sulphate Dihydrate crystal.
[54] by which the Mayer index number was calculated
P ¼ kdn
εr ¼ (3)
where P is the load, d is the diagonal length of the impression, K is a constant and n is the Mayer's index also know as work hardening coefficient. The value of n can be determined from the graph between log P and log d plot, as shown in Fig. 7. In the plot, a straight line was obtained after least square fitting. The value of n is calculated to be 2.8. According to onitsch and Hanneman [55,56], if the value of n is more than 1.6, it belongs to soft materials and between 1 and 1.6 it belongs to hard material. The title crystal belongs to the soft material category. According to Hays-Kendall approach [57], load dependence of hardness is given by
P ¼ W þ A1 d2
(4)
where W is the minimum load to initiate plastic deformation in grams and A1 is a load independent constant. These two values are determined from the plot of load P Vs. d2 (Fig. 8). The value of W is the intercept along the load axis and A1 is the slope. The corrected hardness H0 for the crystal is obtained using the relation
Ho ¼ 1854 X A1
(5)
The load-independent values of Ho come out to be 1.04 GNm2 for the grown crystal. From the hardness value, the yield strength was calculated using the relation [58]
sy ¼
Hv 12:5ðn 2Þ n2 ½1 ðn 2Þ 1 ðn 2Þ 2:9
The dielectric constant and dielectric loss of title crystal was studied at different temperature using HIOKI 3532 LCR HITESTER instrument in the frequency range of 50 Hze5 MHz. The dielectric constant was calculated using the equation
(6)
Cd εo A
(8)
where d represents the thickness and A represents the area of the sample. The frequency dependence of dielectric constant is shown in Fig. 9. The maximum dielectric constant (550, 460 and 360) appeared in the lower-frequency region (100 Hz) at the temperature (308, 328 and 348 K). As the frequency increases, there is a moderate decrease in the dielectric constant till 2 MHz and tends to remain constant with further increase in the frequency at the temperatures (308 Ke348 K) of study. The dielectric constant value being high at low frequency is attributed to the space charge polarizations [60]. Crystal exhibits normal dielectric behavior, i.e., beyond a certain frequency of the electric field, the dipole does not follow the alternating field. The low value of dielectric constant at the higher frequency region may be caused by the inability of the dipoles to comply with the external field. As a result, it will have minimum losses as compared to the material having high dielectric constant [61,62]. Hence, the title crystal may be used for high speed electro optic modulations. The dielectric loss as a function of frequency at different temperatures is shown in Fig. 10. It could be infered that the dielectric loss strongly depends on the frequency of the applied field which is similar to the dielectric constant in the ionic system [63]. Thus, it is concluded that the crystal is optically good for nonlinear optical device [64]. To calculate the electronic properties of Penn gap, Fermi energy and electronic polarizability, only one particular value of the dielectric constant at high frequency was used. The theoretical calculation shows that the high frequency dielectric constant depends on the plasma energy, penn gap and the Fermi energy. The Penn gap is determined using the dielectric constant along with the Plasma energy value [65]. The plasma energy ħup is calculated using the equation [66]
Table 3 Mechanical parameters of Monothiourea-Cadmium Sulphate Dihydrate crystal. Load P in g
Yield strength sy (GN/m2)
Elastic stiffness constant C11 (1015 Pa)
10 25 50 100
94.2860 110.7044 135.1943 190.4870
1.9209 2.5421 3.6086 6.5363
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T. Sivanandan, S. Kalainathan / Materials Chemistry and Physics xxx (2015) 1e8
3=4 EF ¼ 0:2984 ħup
(11)
Electronic Polarizability a [69] can be obtained using the equation
" a¼
ħu2p S0
#
ħu2p S0 þ Ep2
X
M X 0:396 X 1024 cm1 P
(12)
where So is a constant for a particular material and which is determined by the equation
Ep 1 Ep 2 þ 3 4EF 4EF
S0 ¼ 1
(13)
The a value is calculated according to the equation
a¼ Fig. 9. Variation of dielectric constant with log frequency of Monothiourea-Cadmium Sulphate Dihydrate crystal.
3M ε∞ 1 4pNa r ε∞ þ 2
(14)
All the above parameters were calculated and are shown in Table 4. The dielectric constant of materials is a crucial parameter for calculating the physical or electronic properties of materials. These values of gel grown crystal are compared with the values of standard KDP material and the electronic parameters are found to be higher than those of KDP (Table 4). 3.7. Thermal analysis
Fig. 10. Variation of dielectric loss with log frequency of Monothiourea-Cadmium Sulphate Dihydrate crystal.
ħup ¼ 28:8
1=2 Zr M
(9)
where Z is the total number of valence electrons, r is the density and M is the molecular weight of the single crystal, respectively. According to the Penn model [67], the Plasma energy in terms of Penn gap and Fermi energy [68] in eV is given as
Ep ¼ and
ħup ðε∞ 1Þ1=2
(10)
The thermal stability of the title crystal was ascertained from thermogravimetric (TGA) and differential thermal analysis simultaneously. Fig. 11 shows the TG-DTA curve for the grown crystal. Initially, dehydration takes place in the temperature range of 90e170 C, which corresponds to a mass loss of 7.87% [70]. In the temperature range between 170 and 240 C, mass loss of 25.02% occur which could be associated to evolution of carbon disulphide (CS2) and ammonia (NH3) gas, indicating the decomposition of thiourea molecule. 10.55% loss in mass is observed between 280 and 480 C, which may be due to decomposition of metal oxide present in the sample [71]. A weight loss of 5.04% is observed between 500 C to 650 C corresponding to the ignition of CO2. In the final stage, metal sulphate decomposes in the temperature range of 700 Ce990 C with an associated weight loss of 16.65% [72]. In the DTA curve, the endothermic event is observed at 700 C which is due to the melting point of the crystal. Thermal stability of the title crystal is relatively good when compared to other thiourea based complex [73,74]. 4. Conclusion Monothiourea-Cadmium Sulphate Dihydrate can be obtained in an acidic medium by using gel technique. Single crystal XRD analysis revealed that the Monothiourea-Cadmium Sulphate Dihydrate crystallizes in orthorhombic crystal system. PXRD study revealed the crystalline nature. IR spectrum confirms the presence of all the expected functional groups. The UVeVisible studies
Table 4 Comparision of Electronic properties of Monothiourea-Cadmium Sulphate Dihydrate and KDP crystal. Parameters
Values of Monothiourea-Cadmium Sulphate Dihydrate
Values of KDP
Plasma energy (eV) Penn gap (eV) Fermi gap (eV) Polarizability (cm3) Penn analysis Polarizability Clausius-Mossotti Equation
21.892 3.4146 17.636 4.902 1023 4.94 1023
17.2 2.37 12.102 2.12 1023 2.14 1023
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T. Sivanandan, S. Kalainathan / Materials Chemistry and Physics xxx (2015) 1e8
7
Fig. 11. TG/DTA thermogram of Monothiourea-cadmium Sulphate Dihydrate.
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Please cite this article in press as: T. Sivanandan, S. Kalainathan, Study of growth condition and characterization of Monothiourea-Cadmium Sulphate Dihydrate single crystals in silica gel, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.058