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Enhanced properties of cadmium mercury thiocyanate bis(N-methyl formamide): A promising non-linear optical crystal
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Enhanced properties of cadmium mercury thiocyanate bis(N-methyl formamide): A promising non-linear optical crystal A. Subashini , K. Rajarajan , Suresh Sagadevan , Preeti Singh , Jiban Podder , Faruq Mohammad PII: DOI: Reference:
S0577-9073(19)31018-4 https://doi.org/10.1016/j.cjph.2019.12.017 CJPH 1040
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
12 October 2019 2 December 2019 17 December 2019
Please cite this article as: A. Subashini , K. Rajarajan , Suresh Sagadevan , Preeti Singh , Jiban Podder , Faruq Mohammad , Enhanced properties of cadmium mercury thiocyanate bis(Nmethyl formamide): A promising non-linear optical crystal, Chinese Journal of Physics (2019), doi: https://doi.org/10.1016/j.cjph.2019.12.017
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Highlights
CMTN crystals have been prepared by a slow solvent evaporation method.
The melting point of the CMTN is 152.5 o C
The SHG efficiency of CMTN was found to be 5 times that of KDP.
The UV cut off wavelength was found to be 335 nm.
The surface analysis was done by etching studies.
Enhanced properties of cadmium mercury thiocyanate bis(N-methyl formamide): A promising non-linear optical crystal A. Subashini1, K. Rajarajan2, Suresh Sagadevan3*, Preeti Singh4 , Jiban Podder5 and Faruq Mohammad6 1
Department of Physics, Velammal Institute of Technology, Chennai-601 204, India 2
Department of Physics, Rajeswari Vedachalam Government Arts College, Chengalpet-603 001, India
3*
Nanotechnology & Catalysis Research Centre, University of Malaya, Malaysia
4
Bio/Polymers Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, India
5
Department of Physics, Bangladesh University of Engineering and Technology, Dhaka 1000,
Bangladesh 6
Surfactants Research Chair, Department of Chemistry, College of Science, King Saud
University, Riyadh, Kingdom of Saudi Arabia 11451
Corresponding author email: [email protected]
Abstract The present study deals with the synthesis and characterization of cadmium mercury thiocyanate bis(N-methyl formamide) or CMTN crystals where they were grown in two critical steps. In the first step, cadmium mercury thiocyanate (CMTC) single crystals were grown by intriguing cadmium chloride, mercuric chloride, and ammonium thiocyanate in 1:1:4 ratio and mixed with solvent by a slow solvent evaporation technique. The second step involves the reaction between CMTC and N-methyl formamide (NMF) in 1:2 ratio leading to the formation of CMTN crystals. The growth parameters of CMTN grown crystals were optimized at different pHs (1- 5) and the solubility curve has also been reported. On characterization, the orthorhombic crystallinity having Pna21 space group of as-grown CMTN crystals has been revealed by single X-ray diffraction analysis (XRD) and the lattice cell parameters are found to be a = 7.722 Å, b = 15.195 Å, c = 16.162Å, and α = β = γ = 90˚.
Single the phase crystallinity of CMTN is observed by powder XRD pattern and the increase in the intensity of index peaks shows that there exists good coordination between the CMTC and NMF compounds. The FTIR analysis supported for the presence of surface ligands groups of thiocyanate, while the Raman spectroscopy confirmed for the coordination of thiocyanate ions in the CMTN compound and thus both established for the metal-ligand bonding. The UV-vis spectroscopy showed the optical transparency of CMTN to have the cutoff wavelength at 335 nm and the Kurtz powder method for studying the second harmonic generation (SHG) output power is 5 times higher than the reference. Further increase of dielectric constant and dielectric loss with respect to the changes in frequency makes it a suitable material for the construction of photonic and non-linear optical (NLO) devices. Keywords: cadmium mercury thiocyanate bis(N-methyl formamide), Solution growth, Nonlinear optics, Dielectric studies, 1. Introduction In recent years, the non-linear optical (NLO) crystals are being identified scientifically as the most essential crystals due to their advancing applications in many different fields including the remote sensing, modulation of laser frequency, driven fusion, optical computing and information, color displays and medical diagnosis tools etc [1]. In that way, the studies on NLOs and the corresponding description on the light-matter interactions in 2D crystalline materials have promoted a diverse range of photonic applications. As an example, the Mxene and antimony were recently developed as the novel 2D materials and have attracted considerable attention because of their highly tunable and most compatible electronic/optical properties [2-3]. Along with, the Perovskite metasurface is also a promising approach for enhancing the photonic activity in many optical devices where in one of the study, the D titanium disulfide (tis2) was applied for the maintenance of strong light absorption properties in the visible to IR (infrared) region [4]. For the non-linear photonic
applications, this is a highly attractive feature, but the mechanism behind the interaction of broadband with that of light matter is still uncertain and due this fact, the non-linear photonic devices are not developed yet [5]. Thus the quest for NLO crystals that has the capacity to exhibit excellent second-order non-linearity has become one of the research interests. Based on the idea of synthesizing the novel non-centrosymmetric (NCS) compounds, some reports deal with the mixing of inorganic metal oxides with asymmetric p-conjugated organic molecules [6-7]. The three different inorganic metal oxides employed for such applications include, (a) planar (BO3)3- compounds, materials having the p-conjugated system, (b) second order Jahn– Teller (SOJT) compounds, the materials with distorted d0 early transition metal cations (Ti4+, V5+, Mo6+, and W6+), and (c) the materials which contain the stereo chemically active lone pairs (Pb2+, Sb3+, Te4+, Se4+, and I5+) [8]. Thus formed materials are expected to maintain the cumulative properties of both organics (high optical non-linearity and chemical flexibility) and inorganics (short-wavelength transmittance). Since the organometallic compounds also offer some mixed properties related to both organics and inorganics, i.e. architectural flexibility, ease of fabrication, and other custom made properties as similar to that of organics, while the thermal and temporal stabilities linked to the inorganic compounds [9]. When considering the supercapacitor applications which store the energy, the transition metal oxides (TMOs) are more potential because of the promising electrical conductivity, high electrochemical response (by providing Faradaic reactions), low costs associated with the manufacturing, easy processability etc. Further in that view, the metalorganic frameworks (MOFs) are formed by the simple attachment of inorganic metal ions with that of organic linkers via strong chemical bonds and the formed MOF structures maintain specific size and shape, textural and 3D characteristics to meet the demands of specific application type. Because of the maintenance of distinct properties of the MOFs, they
can also find applications in the gas adsorption-desorption, controlled drug delivery, catalysis, optoelectronics etc [10]. The NCS organometallic compounds can be formed by combining distorted inorganic polyhedra with some asymmetric conjugated organic molecules [11-12]. The bimetallic thiocyanate materials, for example AB(SCN)4 type (where A = Zn, Cd, Mn, Fe, Co etc, and B = Cd, Hg etc] are potentially useful for the generation of compact blue-violet light. For these applications, the p-conjugated –S=C=N- ligand forms a bridge between -N and –S bonded tetrahedral groups where they all converges to generate a 3D framework. These crystals possess two central metal ions with the thiocyanate as a bridging ligand which leads to the formation of a 3D network-like structure. The high non-linearity in these crystals is mainly due to the metal-to-ligand charge transfer and the distorted tetrahedral structure and is very difficult to grow the individual high optical quality crystals. Further to enhance the optical properties of some of the organic additives like dimethyl sulfoxide (DMSO), Nmethylformamide (NMF), glycol monomethyl ether (GME) etc, they are being conjugated with ligand crystals. However, one of the bimetallic thiocyanate crystals, cadmium mercury thiocyanate (CMTC) and its corresponding Lewis base adducts are known to show very good second-order NLO properties. This CMTC compound is known to show 11.3 times more efficiency as compared to that of urea [7]. Some of its Lewis base adducts that are reported to exhibit good second harmonic generation (SHG) efficiency are cadmium mercury thiocyanate bis-dimethylsulfoxide (CMTD) [13], cadmium mercury tetrathiocyanate glycol monomethyl ether (CMTG) [14], manganese mercury thiocyanate bis-dimethyl sulfoxide (MMTD) [15], manganese mercury tetrathiocyanate glycol monomethyl ether (MMTG) [16], diaqua (thiocyanate)manganese mercury-N,N-dimethyl acetamide (MMTWD) [17], and tetrathiourea mercury(II) tetrathiocyanato manganate (TMTM) [18].
In this category, N-methyl formamide (NMF), a Lewis base with two donor atoms, nitrogen and oxygen has been combined with CMTC. NMF can coordinates with the metal atom either through N or O or both. When CMTC reacts with NMF, it coordinates with the newly added ligand through the oxygen donor, resulting in the formation of an NCS crystal cadmium mercury thiocyanate bis(N-methyl formamide) (CMTN). The synthesis and the structure of complex CMTN were reported by Liu et al [19]. The complex shows SHG efficiency 0.8 times that of urea and possesses good physicochemical stability up to 128.5ºC. Its UV transparency cutoff is 358 nm [18]. Further, to improve the properties of CMT crystals, we have continued our study with the CMTN crystals where a change of synthesis procedure and associated structural characterization were applied. Following the growth of CMTN crystals by slow solvent evaporation technique, they were characterized thoroughly with the use of many different instrumental techniques like powdered X-ray diffraction (XRD), single crystal XRD, Fourier transform infrared (FTIR), Raman spectroscopy, thermal properties etc. In addition, the NLO properties of the single crystal have been confirmed by SHG test and the dielectric studies are also being analyzed. 2. Experimental details 2.1 Synthesis of CMTN crystals The CMTN crystals were obtained by following the method as described by Liu et al [19] and the chemical reactions involved in the synthesis are as follows. CdCl2 +HgCl2+ 4(NH4SCN)
CdHg(SCN)4
+4NH4Cl
(1) CdHg(SCN)4+ 2C2H5NO (2)
CdHg(SCN)4(C2H5NO)2
(CMTN)
The first step involves the synthesis of CMTC by taking cadmium chloride, mercuric chloride, and ammonium thiocyanate in the 1:1:4 ratios. For the excellent yields and high quality, the synthesized CMTC powder was purified by means of recrystallization and further allowed for the growth of CMTN. The second step involves the reaction between CMTC and NMF in 1:2 ratios that leads to the formation of CMTN. 2.2 Optimization of the growth parameters In solution growth, it is important to optimize the growth parameters like solvent, pH, and temperature before proceeding to the bulk crystal growth. For growing the CMTN crystals, we have selected the solution mixture consisting of water and NMF solvents in different volume ratios. The result indicates that the mixed solvent of water and NMF (v/v = 2:1) is found to be the best to grow large size single crystals of CMTN. Similarly, for optimizing the solution pH, various pHs up to 5 (1, 1.5, 2, 2.5, 3, 3.5, 4 and 5) were selected and the dilutions were made using dilute HCl. Considering the stability of solution and the crystal morphologies obtained at various pHs, the suitable pH for the growth of bulk CMTN was optimized as 3.5. Further, the solubility of CMTN in H2O: NMF (2:1) solvent mixture at pH 3.5 and at different temperatures was studied where the solubility curve is shown in Fig. 1. Considering the temperature graph, the stability of the solution seems to be less at high temperature conditions and so 30˚C temperature was set as the optimal growth temperature.
Fig.1. Solubility curve for CMTN with H2O: NMF (2:1) as a solvent, pH 3.5. 2.3 Crystal growth Under the optimized conditions of water and NMF as the mixed solvents, pH 3.5, and 30oC temperature, the growth was carried out by a solvent evaporation method. For that, a 300 mL saturated solution of CMTN was prepared having the earlier said conditions (water and NMF solvents, pH 3.5) and the maintenance of 30oC as a constant temperature was achieved with an accuracy of ±0.01oC using the water bath. Under these conditions, the seed generated by the spontaneous nucleation was tied and suspended into the reaction solution with the help of a nylon thread. Fig. 2a shows the bulk crystal having the dimensions 20 x 20 x 8 mm3 was harvested after the 25 days of growth period and the Fig. 2b shows the morphology of as grown crystal.
Fig. 2. As grown single crystal (a) and the corresponding morphology (b) of CMTN. 2.4 Instrumental characterization For identifying the crystal structure of the CMTC, single crystal X-ray diffraction analysis (XRD) was applied and for that, ENRAF-NONIUS CAD4-F X-ray diffractometer instrument was used. The powdered XRD analysis was carried on a SEIFERT JSO–DEBYE FLEX 2002 Diffractometer that uses Cu Kα radiation (1.5418 Å), scan rate of 0.001°/s, 2θ of 10–70° angular range, and at ambient temperature. For the sample preparation, the product was crushed using the mortar and were filtered through the sieve until the homogeneous particle sizes were obtained [19]. The Fourier transform infrared (FTIR) spectrum of CMTN was recorded using BRUKER IFS 66 V Spectrophotometer in the 4000 - 500 cm-1 wavenumber range. The Raman spectrum of CMTN single crystal that was recorded using Bruker instruments in the range of 50 to 4000 cm-1. For the optical measurements, the Varian Carry 5E model spectrophotometer in the wavelength range of 200–1200 nm was applied. The powder Kurtz method was used for studying the SHG efficiency and for that, finely ground powder of CMTN was packed in between two glass plates and then the plate placed between the source and detector. For measuring the SHG signal, the fundamental beam of Q-switched Nd: YAG laser of wavelength 1064 nm was used. The incident beam selected contains the pulse energy of 2.1 mJ/pulse, pulse width of 8 ns and a repetition rate of 10 Hz. The output SHG signal was separated from the fundamental beam by the filter and the photomultiplier tube was selected as the detector. The output radiation coming out of the crystal was measured using EPM 2000 power meter model J-50-MB-YAG. The output generated from the CMTN was compared with the output generated from the reference urea. The thermal stability by means of TGA-DSC (thermogravimetric analysis-differential scanning calorimetry) analysis were carried out up to 1000oC in the nitrogen atmosphere at a heating rate of 20oC/min. Etching is a simple and very sensitive tool to understand the growth
mechanism and to assess the quality of the crystal grown. Etching is nothing but the dissolution of the crystal surface and can be done by choosing appropriate solvents. In this work, water and acetone were chosen as the etchants and for observing the crystal surfaces, the optical microscopy was used. For the dielectric studies, the HIOKI 3532-50 LCR HITESTER instrument was employed and for the analysis, the cut and polished sample having the dimensions of 5 x 4 x 4 mm3 was used. The electronic grade silver paste that was applied to either of the samples acts as the electrodes. The capacitance and the dielectric losses for the testing samples were measured in the varying frequency range of 20 Hz to 1 MHz. 3. Results and discussion 3.1 Single crystal XRD The bulk CMTN crystals grown by the solvent evaporation method maintains the welldefined morphology and holds a uniform growth rate all along the three crystallographic directions. The single crystal XRD analysis used to confirm the structure of CMTN revealed the orthorhombic crystallinity having Pna21 space group where the lattice parameters were determined to be, a =15.195 Å, b = 7.722 Å, c = 16.162 Å, and α=β=γ=90˚. The comparison of CMTN cell parameters with that of the literature study [19] tabulated in Table 1 confirms that the results are in a good agreement and further provides evidence for the NCS nature of the grown crystal which supports the SHG generation. Table 1. Crystal data of CMTN single crystal. Present study
Reported [19]
Molecular formula C8H10N6O2S4 CdHg C8H10N6O2S4 CdHg Molecular weight
Cell parameter
663.465
663.465
a = 15.195 A˚
a = 16.266 A˚
b = 7.722 A˚
b = 7.776 A˚
c = 16.162 A˚
c = 15.319 A˚
α = β = γ =90˚
α = β = γ =90˚
Crystal structure
Orthorhombic
Orthorhombic
Volume
1896 Å3
1937.5 Å3
Space group
Pna21
Pna21
The schematic representation of the comparison of molecular structures shown in Figure 3(ab) consists of distorted octahedral (Fig. 3a) and distorted tetrahedral (Fig. 3b) for CMTC and CMTN respectively. In the crystal structure of CMTC (Fig. 3a), according to the hard acidbase concept, Cd2+ being a hard acid gets coordinated with the hard base N (of the thiocyanate ligand) and similarly for CMTN (Fig. 3b), Hg2+ being a soft base coordinates with the soft base S (of the SCN). However, when a new ligand NMF is added to the complex, there is a possibility of it to coordinate with Cd2+ or Hg2+.
Fig. 3. Schematic representation of the molecular structures of (a) CMTC, and (b) CMTN. Considering the NMF groups, it has two donor atoms N and O, both these donor atoms are hard in nature and have a pronounced affinity to coordinate with the Cd2+ which is a hard acid. NMF coordinates with Cd2+ through oxygen and acts as a monodentate ligand. Thus Cd2+ is coordinated to four thiocyanate (NCS) ligands and two NMF ligands to form a slightly distorted octahedral structure. The two NMF are in cis configuration thereby leading to only a slight distortion in the octahedral. Hg2+ is coordinated to four SCN ligands and forms a distorted tetrahedral structure and is linked to Cd2+ through thiocyanate bridging (Cd-NCS-Hg-).
3.2 Powdered XRD studies Figure 4 shows the powdered XRD patterns of CMTN single crystals and from the analysis of results, it is inferred that the diffraction pattern contains all the original peaks related to pure cadmium mercury thiocyanate and in addition, the data is in a good agreement with the earlier report [21]. The peaks indexed with the help of X’pert High score software provided the information that the intensity of some of the peaks like (1 1 2), (2 0 3), and (3 1 1) has increased significantly due to the doping of NMF.
Fig. 4. Powdered XRD pattern of CMTN single crystals. 3.3 FTIR analysis Fig. 5 shows the FTIR spectrum of CMTN and from the figure, the presence of both the ligands of –SCN and NMF was confirmed from the spectrum. The thiocyanate ligand gave the following peaks; C-N stretching mode peak at 2114 cm-1, bending mode of SCN at 452 cm-1, and C-S stretching vibration peak at 741 cm-1. When compared to its parent complex CMTC [6], there is a considerable shift in the peaks corresponding to the C-S stretching, C-N
stretching mode, and SCN’s bending mode due to the formation of a new complex with NMF. The C=O stretching of the NMF ligand is found to be shifted to a lower wavenumber (1668 to 1660 cm-1 in CMTN] in the complex confirming its bonding with the metal ion. Table 2 shows the comparison of FTIR data of CMTN with CMTC and NMF. CMTW 100 90 80 70
%T
60 50 40 30 20 10 0 4000
3500
3000
2500
2000
1500
1000
500
Wavelenght cm-1
Fig.5. FTIR spectrum of CMTN single crystal Table 2. Comparison of the FTIR spectral data of CMTN with that of CMTC and NMF. Wavenumber (cm-1) Assignment
CMTN (cm-1)
NH stretching
3370
NMF (SDBS database) (cm-1) 3298
2 CNH bending
3068
3067
CH stretching
2938, 2915, 2886
2945, 2879
CN stretching
2114
C=O stretching
1660
1668
CNH bending
1534
1542
CH3 bending
1410
1413, 1386
C-N stretching
1244, 1150, 1015
1245, 1161, 1016
CNC stretching
952
959
CMTC [6]
2143, 2120
-
2 SCN bending
918,886
928, 883
-
NH wagging
774, 714, 626
CS stretching
741
773
-
SCN bending
452,437
463, 440
-
770, 701, 662
3.4 Raman spectroscopy Raman spectrum for the CMTC compound shown in Figure 6 confirms the coordination of ligands with metal ions as the peaks in the lower wavenumber region of the spectrum corresponds to the metal-ligand bonding. The sharp and intense peaks at 162 and 236 cm-1 is due to the bending vibrations of S-Hg-S and N-Mn-N respectively. A weak peak at 374 cm-1 can be due to O-Cd-O bending vibration and the C-N and C-S stretching vibration of thiocyanate ligand were observed at 2132 cm-1 and 748 cm-1 respectively; however, the bending mode of SCN does not appear in the spectrum. All the important modes of vibrations corresponding to NMF appear in the spectrum confirming the presence of NMF in the complex. As compared to the parent CMTC complex, the stretching vibrations of C-N and CS bonds are comparable. The extra O-Cd-O bending mode in the complex confirms the formation of the new complex. Further the comparison of peaks in the Raman spectrums of CMTN and CMTC single crystals are tabulated in Table 3 where the results confirms that the obtained results are in a good agreement with the literature report [22].
Fig. 6. Raman spectrum of CMTN single crystals. Table 3. Comparison of the Raman spectral data of CMTN and CMTC. Wavenumber(cm-1) Assignment
CMTN
CMTC
γ NH
-
-
2δ C-NH
-
-
γ CH
2939, 2916, 2871
-
γ C=O
1649
-
δ C-NH
1542
-
δ CH3
1375
-
γ C-N
2144, 2132
2101, 2125, 2137, 2173
δ N-H
748
-
γ C-N-C
954
-
γ C-N
2144, 2132
-
2δ SCN
-
887, 933
γ C-S
748
777
δ SCN
-
443, 464
δ (O-Cd-O)
374
-
δ (N-Cd-N)
236
230, 232, 235
δ (S-Hg-S)
162
149, 158, 174, 176
3.5 Optical properties In optical technology, the major governing factor is the optical transparency range of single crystals and when it comes to the NLO crystals, the intrinsic defects that use the intermolecular voids and phonon subsystem plays the major role. Hence for researchers, it is important to grow defect-free crystals that can be utilized for the fabrication of optical devices [23]. In that way, the CMTN single crystals having ~2 mm thickness are mainly used for various optical applications. Figure 7 shows the optical absorption spectrum of CMTN where the result indicates that the crystal maintains good transparency in the range of 335 1100 nm and has the cutoff UV transparency at 335 nm. Comparing the UV spectra of CMTN with its parent CMTC (371 nm) [7], there is a shift in the cutoff wavelength and this shift can be attributed to the charge transfer occurring from metal to ligand or ligand to metal induced by the new ligand NMF introduced into the structure of CMTN.
Fig.7. UV-visible spectrum of CMTN single crystal. The following equation (1) used to calculate the band gap of crystal,
Eg
1.243 103
max
(1)
With the use of above formula, a 3.71 eV band gap value was obtained, which is typical for a majority of dielectric materials. The generation of such a high band gap provides the information that the defect concentration in CMTN crystal is very low and this crystal as similar to many dielectric materials induces polarization following the exposure to UV radiation. In addition to the high band gap, the missing of strong absorption peaks in the visible region confirms further for the applicability of these crystals towards the photonic and optical applications. 3.6 Studies of NLO properties The powder Kurtz method used for studying the SHG efficiency of CMTN material provided the information that the CMTN’s conversion efficiency is 5 times to that of KDP (potassium dihydrogen phosphate). Also, as compared to CMTC, the SHG efficiency of CMTN is less. This can be explained on the basis of the structure of the two complexes. In CMTN, the Cd2+ forms CdN4O2 a slightly distorted octahedral geometry whereas in CMTC, the Cd2+ ions are bonded to the N (SCN) forming a distorted tetrahedral geometry. Moreover, the two NMF ligands are in the cis configuration which might have resulted in the decrease in high noncentrosymmetric nature observed in its parent complex [6]. This fact can also be proved by comparing the structure of CMTN with that of CMTG, where the SHG of CMTG is 3 times higher than urea [24]. Similarly, in CMTD material, the metal ions of Cd2+ are coordinated with two oxygen and four nitrogen atoms which would have contributed to the higher distortion in the octahedral (CdN4O2) formed, finally increasing its efficiency. Although the efficiency of CMTN is less compared to CMTC and CMTG, its efficiency is greater than that of KDP and urea. As compared with the Lewis base adduct of the bimetallic thiocyanate series, the CMTN crystal is found to show better efficiency than
that of diaqua tetrakis (thiocyanato) cobalt(II) mercury(II) N-methyl-2-pyrolidone (CMTWMP) [25] and comparable with that of tetrathiourea cadmium(II) tetrathiocyanato zinc(II)
(TCTZ)
[16]
and
diaqua
(thiocyanato)
manganese(II)
mercury(II)]-N,N-
dimethylacetamide (MMTWD) [26]. 3.7 Thermal studies Thermal stability of CMTN was studied by TG-DSC and DSC analysis and is shown in Figure 8(a-b) respectively. From the Fig. 8a which shows the TG-DSC curve of CMTN, it is clear that the complex is thermally stable up to 128.5oC and further increase of temperature is supporting for a significant weight loss. The first step involves the decomposition of the organic ligand and then the bimetallic thiocyanate complex starts decomposing. The complex starts decomposing at 150oC where the two molecules of NMF get fragmented. This decomposition temperature is comparable with that of the value reported by Liu et al [19]. During the temperature range of 150 - 210oC, there is a weight loss of 8.20%, this can be accounted for the decomposition of two NMF molecules, which matches well with the theoretical value of 8.22%. The next step involves the breakdown of the 3D structure of CdHg(SCN)4 and the formation of metal sulfides, carbon disulfide, nitrogen gas, and dicyanogen. The most probable and expected reactions involved in this decomposition process are given below. CdHg (SCN)4(C2H5NO)2 CdHg (SCN)4
CdHg(SCN)4 +2C2H5NO CdS + HgS + CS2 +(1/4)N2 + (3/2) (CN)2
(3) (4)
The gaseous products formed during the decomposition are eliminated and the HgS sublimes in the next step leaving the residue. The final residue is CdS (22.56%) which agrees well with the theoretical value of CdS (23.58%). The thermal stability of CMTN was found to be good and quite comparable with another Lewis base adducts like CWTWMP (150oC) [24], MMTD (145oC), TCTZ (156oC) [26], and MMTG (145oC) [16]. It is also inferred from
the study that CMTN (152.5C) is less stable thermally than that of CMTC (202.9oC) [7] and MMTWD (70oC) [26]. Similarly, the DSC curve of CMTN shown in Fig. 8b indicates that the melting point of CMTN is 152.5oC. But the TGA curve provided the information that the weight loss is getting started around 150oC temperature, meaning that the complex is actually decomposing before its melting.
Fig. 8. (a) TGA-DSC, and (b) DSC curves for the CMTN single crystals.
3.8 Etching studies For the etching studies, the as-grown crystals free from any visible defects were selected and etched with water and acetone for 2 minutes. The crystal was immediately dried with the tissue paper and was observed under optical microscopy. Figure 9(a-b) shows the surface nature of as-grown CMTN crystals at two different spots and Figure 10(a-b) shows the etch patterns observed in CMTN when water and acetone (respectively) were used as etchants. From the comparison of images in Figures 9 and 10, the step-growth pattern can be seen in Fig. 10, while the same was missing in Fig. 9. In water, the etching patterns are more of hexagonal shape, whereas, in acetone it gave a tetrahedral etch pattern. Even though different etch patterns were observed in both the solvents, the spiral growth pattern is clearly seen in both (Fig. 10). The difference in the shape of the etch pattern may be due to the difference in
the solubility of CMTN in both the solvents. The inset of Fig. 10a shows the clear picture of spiral growth and the crystal surface with dislocations. These dislocations will create steps in the surface which grows on by the incorporation of the growth units. Once the first step starts advancing, it gives rise to the second step and the process continues and thus generating the spiral pattern around the dislocation core [28]. Here, in this case, we can clearly see the hexagonal shape of growth spirals on the etched surface of CMTN where these hexagonal spirals reflect the growth morphology of the crystal as in bulk. The hexagonal spiral steps convey that the CMTN involves the spiral growth mechanism. Apart from the spiral center, step patterns created due to the spirals are observed at the edges of the crystals. From the etching studies, it is inferred that the spiral growth mechanism is operating towards the crystal growth.
Fig. 9. Surfaces of as-grown CMTN crystals at two different spots of (a) and (b).
Fig. 10. Surface of as-grown CMTN crystals etched for 2 minutes with (a) water, and (b) acetone as the etchants. 3.10 Dielectric studies Variations in the dielectric constant and dielectric loss of CMTN crystal with respect to varying frequencies at room temperature are shown in the Fig. 11(a-b) respectively. In general, for any material, the existence of dielectric constant is mostly due to the influence of frequency on the ionic, electric, orientational and space charge polarizations. The frequency effect of dielectric constant for the CMTN crystal shown in Fig. 11a provides the information that the value is maximum at the lowest frequency. With an increase of applied frequency, we observed a gradual decrease in the dielectric constant and becoming constant when reaching to higher frequencies. The increased value of dielectric constant at the lowest frequency can be due to the fact where all kinds of polarizations are highly active[29]. As the frequency increases, the polarization decreases, hence there is a decrease in the dielectric constant at higher frequencies [30]. The dielectric loss shown in Fig. 11b also follows the same trend with that of the dielectric constant, it decreases with an increase of frequency and after a certain frequency, it become saturated. The low dielectric constant and dielectric loss of the CMTN crystal conveys that the sample possesses enhanced optical properties and thereby supporting for potency towards the NLO applications.
Fig.11. Comparison of the variations in the dielectric constant (a), and (b) dielectric loss of CMTN with respect to frequencies.
4. Conclusion In conclusion, we indicate an optically transparent NLO crystal, CMTN crystals which have been prepared by a slow solvent evaporation method and the bulk crystals were successfully grown by optimizing the growth parameters. The CMTN crystals having the dimensions of 20 x 20 x 8 mm3 were obtained using water as a solvent, at pH 3.5 and at 30oC temperature. The single crystal XRD confirmed the CMTN’s cell parameters to be in the orthorhombic symmetry with the NCS space group Pna21, while the single-phase crystallinity was observed by the powder XRD pattern. Further, the bonding between the metal ion and the ligands were confirmed from the FTIR and Raman spectral analyses. The thermal studies indicated that the melting point of CMTN is 152.5oC and in addition, the CMTN complex was found to decompose before its melting. The optical absorption studies revealed that the CMTN crystals has good optical transparency and has a 335 nm cutoff wavelength. The studies of NLO property indicated that the CMTN’s SHG efficiency is 5 times higher than that of KDP reference. The surface analysis done by the etching studies revealed for the spiral growth patterns and spiral mechanisms involved in the growth process. The low dielectric constant and dielectric loss values suggest that the CMTN posse’ optical properties. Finally, a large crystal of CMTN with good optical quality can be obtained under optimum conditions. All these above mention properties confirm CMTN to be useful material for optical applications makes it a potential candidate material for NLO applications.
Competing interest: The authors declare no conflict of interest. Acknowledgements:
The King Saud university author is grateful to the Deanship of Scientific Research, King Saud University for funding through Vice Deanship of Scientific Research Chairs.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Dr.Suresh Sagadevan
Enhanced properties of cadmium mercury thiocyanate bis(N-methyl formamide): A promising non-linear optical crystal A. Subashini , K. Rajarajan , Suresh Sagadevan , Preeti Singh , Jiban Podder , Faruq Mohammad PII: DOI: Reference:
S0577-9073(19)31018-4 https://doi.org/10.1016/j.cjph.2019.12.017 CJPH 1040
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
12 October 2019 2 December 2019 17 December 2019
Please cite this article as: A. Subashini , K. Rajarajan , Suresh Sagadevan , Preeti Singh , Jiban Podder , Faruq Mohammad , Enhanced properties of cadmium mercury thiocyanate bis(Nmethyl formamide): A promising non-linear optical crystal, Chinese Journal of Physics (2019), doi: https://doi.org/10.1016/j.cjph.2019.12.017
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Highlights
CMTN crystals have been prepared by a slow solvent evaporation method.
The melting point of the CMTN is 152.5 o C
The SHG efficiency of CMTN was found to be 5 times that of KDP.
The UV cut off wavelength was found to be 335 nm.
The surface analysis was done by etching studies.
Enhanced properties of cadmium mercury thiocyanate bis(N-methyl formamide): A promising non-linear optical crystal A. Subashini1, K. Rajarajan2, Suresh Sagadevan3*, Preeti Singh4 , Jiban Podder5 and Faruq Mohammad6 1
Department of Physics, Velammal Institute of Technology, Chennai-601 204, India 2
Department of Physics, Rajeswari Vedachalam Government Arts College, Chengalpet-603 001, India
3*
Nanotechnology & Catalysis Research Centre, University of Malaya, Malaysia
4
Bio/Polymers Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, India
5
Department of Physics, Bangladesh University of Engineering and Technology, Dhaka 1000,
Bangladesh 6
Surfactants Research Chair, Department of Chemistry, College of Science, King Saud
University, Riyadh, Kingdom of Saudi Arabia 11451
Corresponding author email: [email protected]
Abstract The present study deals with the synthesis and characterization of cadmium mercury thiocyanate bis(N-methyl formamide) or CMTN crystals where they were grown in two critical steps. In the first step, cadmium mercury thiocyanate (CMTC) single crystals were grown by intriguing cadmium chloride, mercuric chloride, and ammonium thiocyanate in 1:1:4 ratio and mixed with solvent by a slow solvent evaporation technique. The second step involves the reaction between CMTC and N-methyl formamide (NMF) in 1:2 ratio leading to the formation of CMTN crystals. The growth parameters of CMTN grown crystals were optimized at different pHs (1- 5) and the solubility curve has also been reported. On characterization, the orthorhombic crystallinity having Pna21 space group of as-grown CMTN crystals has been revealed by single X-ray diffraction analysis (XRD) and the lattice cell parameters are found to be a = 7.722 Å, b = 15.195 Å, c = 16.162Å, and α = β = γ = 90˚.
Single the phase crystallinity of CMTN is observed by powder XRD pattern and the increase in the intensity of index peaks shows that there exists good coordination between the CMTC and NMF compounds. The FTIR analysis supported for the presence of surface ligands groups of thiocyanate, while the Raman spectroscopy confirmed for the coordination of thiocyanate ions in the CMTN compound and thus both established for the metal-ligand bonding. The UV-vis spectroscopy showed the optical transparency of CMTN to have the cutoff wavelength at 335 nm and the Kurtz powder method for studying the second harmonic generation (SHG) output power is 5 times higher than the reference. Further increase of dielectric constant and dielectric loss with respect to the changes in frequency makes it a suitable material for the construction of photonic and non-linear optical (NLO) devices. Keywords: cadmium mercury thiocyanate bis(N-methyl formamide), Solution growth, Nonlinear optics, Dielectric studies, 1. Introduction In recent years, the non-linear optical (NLO) crystals are being identified scientifically as the most essential crystals due to their advancing applications in many different fields including the remote sensing, modulation of laser frequency, driven fusion, optical computing and information, color displays and medical diagnosis tools etc [1]. In that way, the studies on NLOs and the corresponding description on the light-matter interactions in 2D crystalline materials have promoted a diverse range of photonic applications. As an example, the Mxene and antimony were recently developed as the novel 2D materials and have attracted considerable attention because of their highly tunable and most compatible electronic/optical properties [2-3]. Along with, the Perovskite metasurface is also a promising approach for enhancing the photonic activity in many optical devices where in one of the study, the D titanium disulfide (tis2) was applied for the maintenance of strong light absorption properties in the visible to IR (infrared) region [4]. For the non-linear photonic
applications, this is a highly attractive feature, but the mechanism behind the interaction of broadband with that of light matter is still uncertain and due this fact, the non-linear photonic devices are not developed yet [5]. Thus the quest for NLO crystals that has the capacity to exhibit excellent second-order non-linearity has become one of the research interests. Based on the idea of synthesizing the novel non-centrosymmetric (NCS) compounds, some reports deal with the mixing of inorganic metal oxides with asymmetric p-conjugated organic molecules [6-7]. The three different inorganic metal oxides employed for such applications include, (a) planar (BO3)3- compounds, materials having the p-conjugated system, (b) second order Jahn– Teller (SOJT) compounds, the materials with distorted d0 early transition metal cations (Ti4+, V5+, Mo6+, and W6+), and (c) the materials which contain the stereo chemically active lone pairs (Pb2+, Sb3+, Te4+, Se4+, and I5+) [8]. Thus formed materials are expected to maintain the cumulative properties of both organics (high optical non-linearity and chemical flexibility) and inorganics (short-wavelength transmittance). Since the organometallic compounds also offer some mixed properties related to both organics and inorganics, i.e. architectural flexibility, ease of fabrication, and other custom made properties as similar to that of organics, while the thermal and temporal stabilities linked to the inorganic compounds [9]. When considering the supercapacitor applications which store the energy, the transition metal oxides (TMOs) are more potential because of the promising electrical conductivity, high electrochemical response (by providing Faradaic reactions), low costs associated with the manufacturing, easy processability etc. Further in that view, the metalorganic frameworks (MOFs) are formed by the simple attachment of inorganic metal ions with that of organic linkers via strong chemical bonds and the formed MOF structures maintain specific size and shape, textural and 3D characteristics to meet the demands of specific application type. Because of the maintenance of distinct properties of the MOFs, they
can also find applications in the gas adsorption-desorption, controlled drug delivery, catalysis, optoelectronics etc [10]. The NCS organometallic compounds can be formed by combining distorted inorganic polyhedra with some asymmetric conjugated organic molecules [11-12]. The bimetallic thiocyanate materials, for example AB(SCN)4 type (where A = Zn, Cd, Mn, Fe, Co etc, and B = Cd, Hg etc] are potentially useful for the generation of compact blue-violet light. For these applications, the p-conjugated –S=C=N- ligand forms a bridge between -N and –S bonded tetrahedral groups where they all converges to generate a 3D framework. These crystals possess two central metal ions with the thiocyanate as a bridging ligand which leads to the formation of a 3D network-like structure. The high non-linearity in these crystals is mainly due to the metal-to-ligand charge transfer and the distorted tetrahedral structure and is very difficult to grow the individual high optical quality crystals. Further to enhance the optical properties of some of the organic additives like dimethyl sulfoxide (DMSO), Nmethylformamide (NMF), glycol monomethyl ether (GME) etc, they are being conjugated with ligand crystals. However, one of the bimetallic thiocyanate crystals, cadmium mercury thiocyanate (CMTC) and its corresponding Lewis base adducts are known to show very good second-order NLO properties. This CMTC compound is known to show 11.3 times more efficiency as compared to that of urea [7]. Some of its Lewis base adducts that are reported to exhibit good second harmonic generation (SHG) efficiency are cadmium mercury thiocyanate bis-dimethylsulfoxide (CMTD) [13], cadmium mercury tetrathiocyanate glycol monomethyl ether (CMTG) [14], manganese mercury thiocyanate bis-dimethyl sulfoxide (MMTD) [15], manganese mercury tetrathiocyanate glycol monomethyl ether (MMTG) [16], diaqua (thiocyanate)manganese mercury-N,N-dimethyl acetamide (MMTWD) [17], and tetrathiourea mercury(II) tetrathiocyanato manganate (TMTM) [18].
In this category, N-methyl formamide (NMF), a Lewis base with two donor atoms, nitrogen and oxygen has been combined with CMTC. NMF can coordinates with the metal atom either through N or O or both. When CMTC reacts with NMF, it coordinates with the newly added ligand through the oxygen donor, resulting in the formation of an NCS crystal cadmium mercury thiocyanate bis(N-methyl formamide) (CMTN). The synthesis and the structure of complex CMTN were reported by Liu et al [19]. The complex shows SHG efficiency 0.8 times that of urea and possesses good physicochemical stability up to 128.5ºC. Its UV transparency cutoff is 358 nm [18]. Further, to improve the properties of CMT crystals, we have continued our study with the CMTN crystals where a change of synthesis procedure and associated structural characterization were applied. Following the growth of CMTN crystals by slow solvent evaporation technique, they were characterized thoroughly with the use of many different instrumental techniques like powdered X-ray diffraction (XRD), single crystal XRD, Fourier transform infrared (FTIR), Raman spectroscopy, thermal properties etc. In addition, the NLO properties of the single crystal have been confirmed by SHG test and the dielectric studies are also being analyzed. 2. Experimental details 2.1 Synthesis of CMTN crystals The CMTN crystals were obtained by following the method as described by Liu et al [19] and the chemical reactions involved in the synthesis are as follows. CdCl2 +HgCl2+ 4(NH4SCN)
CdHg(SCN)4
+4NH4Cl
(1) CdHg(SCN)4+ 2C2H5NO (2)
CdHg(SCN)4(C2H5NO)2
(CMTN)
The first step involves the synthesis of CMTC by taking cadmium chloride, mercuric chloride, and ammonium thiocyanate in the 1:1:4 ratios. For the excellent yields and high quality, the synthesized CMTC powder was purified by means of recrystallization and further allowed for the growth of CMTN. The second step involves the reaction between CMTC and NMF in 1:2 ratios that leads to the formation of CMTN. 2.2 Optimization of the growth parameters In solution growth, it is important to optimize the growth parameters like solvent, pH, and temperature before proceeding to the bulk crystal growth. For growing the CMTN crystals, we have selected the solution mixture consisting of water and NMF solvents in different volume ratios. The result indicates that the mixed solvent of water and NMF (v/v = 2:1) is found to be the best to grow large size single crystals of CMTN. Similarly, for optimizing the solution pH, various pHs up to 5 (1, 1.5, 2, 2.5, 3, 3.5, 4 and 5) were selected and the dilutions were made using dilute HCl. Considering the stability of solution and the crystal morphologies obtained at various pHs, the suitable pH for the growth of bulk CMTN was optimized as 3.5. Further, the solubility of CMTN in H2O: NMF (2:1) solvent mixture at pH 3.5 and at different temperatures was studied where the solubility curve is shown in Fig. 1. Considering the temperature graph, the stability of the solution seems to be less at high temperature conditions and so 30˚C temperature was set as the optimal growth temperature.
Fig.1. Solubility curve for CMTN with H2O: NMF (2:1) as a solvent, pH 3.5. 2.3 Crystal growth Under the optimized conditions of water and NMF as the mixed solvents, pH 3.5, and 30oC temperature, the growth was carried out by a solvent evaporation method. For that, a 300 mL saturated solution of CMTN was prepared having the earlier said conditions (water and NMF solvents, pH 3.5) and the maintenance of 30oC as a constant temperature was achieved with an accuracy of ±0.01oC using the water bath. Under these conditions, the seed generated by the spontaneous nucleation was tied and suspended into the reaction solution with the help of a nylon thread. Fig. 2a shows the bulk crystal having the dimensions 20 x 20 x 8 mm3 was harvested after the 25 days of growth period and the Fig. 2b shows the morphology of as grown crystal.
Fig. 2. As grown single crystal (a) and the corresponding morphology (b) of CMTN. 2.4 Instrumental characterization For identifying the crystal structure of the CMTC, single crystal X-ray diffraction analysis (XRD) was applied and for that, ENRAF-NONIUS CAD4-F X-ray diffractometer instrument was used. The powdered XRD analysis was carried on a SEIFERT JSO–DEBYE FLEX 2002 Diffractometer that uses Cu Kα radiation (1.5418 Å), scan rate of 0.001°/s, 2θ of 10–70° angular range, and at ambient temperature. For the sample preparation, the product was crushed using the mortar and were filtered through the sieve until the homogeneous particle sizes were obtained [19]. The Fourier transform infrared (FTIR) spectrum of CMTN was recorded using BRUKER IFS 66 V Spectrophotometer in the 4000 - 500 cm-1 wavenumber range. The Raman spectrum of CMTN single crystal that was recorded using Bruker instruments in the range of 50 to 4000 cm-1. For the optical measurements, the Varian Carry 5E model spectrophotometer in the wavelength range of 200–1200 nm was applied. The powder Kurtz method was used for studying the SHG efficiency and for that, finely ground powder of CMTN was packed in between two glass plates and then the plate placed between the source and detector. For measuring the SHG signal, the fundamental beam of Q-switched Nd: YAG laser of wavelength 1064 nm was used. The incident beam selected contains the pulse energy of 2.1 mJ/pulse, pulse width of 8 ns and a repetition rate of 10 Hz. The output SHG signal was separated from the fundamental beam by the filter and the photomultiplier tube was selected as the detector. The output radiation coming out of the crystal was measured using EPM 2000 power meter model J-50-MB-YAG. The output generated from the CMTN was compared with the output generated from the reference urea. The thermal stability by means of TGA-DSC (thermogravimetric analysis-differential scanning calorimetry) analysis were carried out up to 1000oC in the nitrogen atmosphere at a heating rate of 20oC/min. Etching is a simple and very sensitive tool to understand the growth
mechanism and to assess the quality of the crystal grown. Etching is nothing but the dissolution of the crystal surface and can be done by choosing appropriate solvents. In this work, water and acetone were chosen as the etchants and for observing the crystal surfaces, the optical microscopy was used. For the dielectric studies, the HIOKI 3532-50 LCR HITESTER instrument was employed and for the analysis, the cut and polished sample having the dimensions of 5 x 4 x 4 mm3 was used. The electronic grade silver paste that was applied to either of the samples acts as the electrodes. The capacitance and the dielectric losses for the testing samples were measured in the varying frequency range of 20 Hz to 1 MHz. 3. Results and discussion 3.1 Single crystal XRD The bulk CMTN crystals grown by the solvent evaporation method maintains the welldefined morphology and holds a uniform growth rate all along the three crystallographic directions. The single crystal XRD analysis used to confirm the structure of CMTN revealed the orthorhombic crystallinity having Pna21 space group where the lattice parameters were determined to be, a =15.195 Å, b = 7.722 Å, c = 16.162 Å, and α=β=γ=90˚. The comparison of CMTN cell parameters with that of the literature study [19] tabulated in Table 1 confirms that the results are in a good agreement and further provides evidence for the NCS nature of the grown crystal which supports the SHG generation. Table 1. Crystal data of CMTN single crystal. Present study
Reported [19]
Molecular formula C8H10N6O2S4 CdHg C8H10N6O2S4 CdHg Molecular weight
Cell parameter
663.465
663.465
a = 15.195 A˚
a = 16.266 A˚
b = 7.722 A˚
b = 7.776 A˚
c = 16.162 A˚
c = 15.319 A˚
α = β = γ =90˚
α = β = γ =90˚
Crystal structure
Orthorhombic
Orthorhombic
Volume
1896 Å3
1937.5 Å3
Space group
Pna21
Pna21
The schematic representation of the comparison of molecular structures shown in Figure 3(ab) consists of distorted octahedral (Fig. 3a) and distorted tetrahedral (Fig. 3b) for CMTC and CMTN respectively. In the crystal structure of CMTC (Fig. 3a), according to the hard acidbase concept, Cd2+ being a hard acid gets coordinated with the hard base N (of the thiocyanate ligand) and similarly for CMTN (Fig. 3b), Hg2+ being a soft base coordinates with the soft base S (of the SCN). However, when a new ligand NMF is added to the complex, there is a possibility of it to coordinate with Cd2+ or Hg2+.
Fig. 3. Schematic representation of the molecular structures of (a) CMTC, and (b) CMTN. Considering the NMF groups, it has two donor atoms N and O, both these donor atoms are hard in nature and have a pronounced affinity to coordinate with the Cd2+ which is a hard acid. NMF coordinates with Cd2+ through oxygen and acts as a monodentate ligand. Thus Cd2+ is coordinated to four thiocyanate (NCS) ligands and two NMF ligands to form a slightly distorted octahedral structure. The two NMF are in cis configuration thereby leading to only a slight distortion in the octahedral. Hg2+ is coordinated to four SCN ligands and forms a distorted tetrahedral structure and is linked to Cd2+ through thiocyanate bridging (Cd-NCS-Hg-).
3.2 Powdered XRD studies Figure 4 shows the powdered XRD patterns of CMTN single crystals and from the analysis of results, it is inferred that the diffraction pattern contains all the original peaks related to pure cadmium mercury thiocyanate and in addition, the data is in a good agreement with the earlier report [21]. The peaks indexed with the help of X’pert High score software provided the information that the intensity of some of the peaks like (1 1 2), (2 0 3), and (3 1 1) has increased significantly due to the doping of NMF.
Fig. 4. Powdered XRD pattern of CMTN single crystals. 3.3 FTIR analysis Fig. 5 shows the FTIR spectrum of CMTN and from the figure, the presence of both the ligands of –SCN and NMF was confirmed from the spectrum. The thiocyanate ligand gave the following peaks; C-N stretching mode peak at 2114 cm-1, bending mode of SCN at 452 cm-1, and C-S stretching vibration peak at 741 cm-1. When compared to its parent complex CMTC [6], there is a considerable shift in the peaks corresponding to the C-S stretching, C-N
stretching mode, and SCN’s bending mode due to the formation of a new complex with NMF. The C=O stretching of the NMF ligand is found to be shifted to a lower wavenumber (1668 to 1660 cm-1 in CMTN] in the complex confirming its bonding with the metal ion. Table 2 shows the comparison of FTIR data of CMTN with CMTC and NMF. CMTW 100 90 80 70
%T
60 50 40 30 20 10 0 4000
3500
3000
2500
2000
1500
1000
500
Wavelenght cm-1
Fig.5. FTIR spectrum of CMTN single crystal Table 2. Comparison of the FTIR spectral data of CMTN with that of CMTC and NMF. Wavenumber (cm-1) Assignment
CMTN (cm-1)
NH stretching
3370
NMF (SDBS database) (cm-1) 3298
2 CNH bending
3068
3067
CH stretching
2938, 2915, 2886
2945, 2879
CN stretching
2114
C=O stretching
1660
1668
CNH bending
1534
1542
CH3 bending
1410
1413, 1386
C-N stretching
1244, 1150, 1015
1245, 1161, 1016
CNC stretching
952
959
CMTC [6]
2143, 2120
-
2 SCN bending
918,886
928, 883
-
NH wagging
774, 714, 626
CS stretching
741
773
-
SCN bending
452,437
463, 440
-
770, 701, 662
3.4 Raman spectroscopy Raman spectrum for the CMTC compound shown in Figure 6 confirms the coordination of ligands with metal ions as the peaks in the lower wavenumber region of the spectrum corresponds to the metal-ligand bonding. The sharp and intense peaks at 162 and 236 cm-1 is due to the bending vibrations of S-Hg-S and N-Mn-N respectively. A weak peak at 374 cm-1 can be due to O-Cd-O bending vibration and the C-N and C-S stretching vibration of thiocyanate ligand were observed at 2132 cm-1 and 748 cm-1 respectively; however, the bending mode of SCN does not appear in the spectrum. All the important modes of vibrations corresponding to NMF appear in the spectrum confirming the presence of NMF in the complex. As compared to the parent CMTC complex, the stretching vibrations of C-N and CS bonds are comparable. The extra O-Cd-O bending mode in the complex confirms the formation of the new complex. Further the comparison of peaks in the Raman spectrums of CMTN and CMTC single crystals are tabulated in Table 3 where the results confirms that the obtained results are in a good agreement with the literature report [22].
Fig. 6. Raman spectrum of CMTN single crystals. Table 3. Comparison of the Raman spectral data of CMTN and CMTC. Wavenumber(cm-1) Assignment
CMTN
CMTC
γ NH
-
-
2δ C-NH
-
-
γ CH
2939, 2916, 2871
-
γ C=O
1649
-
δ C-NH
1542
-
δ CH3
1375
-
γ C-N
2144, 2132
2101, 2125, 2137, 2173
δ N-H
748
-
γ C-N-C
954
-
γ C-N
2144, 2132
-
2δ SCN
-
887, 933
γ C-S
748
777
δ SCN
-
443, 464
δ (O-Cd-O)
374
-
δ (N-Cd-N)
236
230, 232, 235
δ (S-Hg-S)
162
149, 158, 174, 176
3.5 Optical properties In optical technology, the major governing factor is the optical transparency range of single crystals and when it comes to the NLO crystals, the intrinsic defects that use the intermolecular voids and phonon subsystem plays the major role. Hence for researchers, it is important to grow defect-free crystals that can be utilized for the fabrication of optical devices [23]. In that way, the CMTN single crystals having ~2 mm thickness are mainly used for various optical applications. Figure 7 shows the optical absorption spectrum of CMTN where the result indicates that the crystal maintains good transparency in the range of 335 1100 nm and has the cutoff UV transparency at 335 nm. Comparing the UV spectra of CMTN with its parent CMTC (371 nm) [7], there is a shift in the cutoff wavelength and this shift can be attributed to the charge transfer occurring from metal to ligand or ligand to metal induced by the new ligand NMF introduced into the structure of CMTN.
Fig.7. UV-visible spectrum of CMTN single crystal. The following equation (1) used to calculate the band gap of crystal,
Eg
1.243 103
max
(1)
With the use of above formula, a 3.71 eV band gap value was obtained, which is typical for a majority of dielectric materials. The generation of such a high band gap provides the information that the defect concentration in CMTN crystal is very low and this crystal as similar to many dielectric materials induces polarization following the exposure to UV radiation. In addition to the high band gap, the missing of strong absorption peaks in the visible region confirms further for the applicability of these crystals towards the photonic and optical applications. 3.6 Studies of NLO properties The powder Kurtz method used for studying the SHG efficiency of CMTN material provided the information that the CMTN’s conversion efficiency is 5 times to that of KDP (potassium dihydrogen phosphate). Also, as compared to CMTC, the SHG efficiency of CMTN is less. This can be explained on the basis of the structure of the two complexes. In CMTN, the Cd2+ forms CdN4O2 a slightly distorted octahedral geometry whereas in CMTC, the Cd2+ ions are bonded to the N (SCN) forming a distorted tetrahedral geometry. Moreover, the two NMF ligands are in the cis configuration which might have resulted in the decrease in high noncentrosymmetric nature observed in its parent complex [6]. This fact can also be proved by comparing the structure of CMTN with that of CMTG, where the SHG of CMTG is 3 times higher than urea [24]. Similarly, in CMTD material, the metal ions of Cd2+ are coordinated with two oxygen and four nitrogen atoms which would have contributed to the higher distortion in the octahedral (CdN4O2) formed, finally increasing its efficiency. Although the efficiency of CMTN is less compared to CMTC and CMTG, its efficiency is greater than that of KDP and urea. As compared with the Lewis base adduct of the bimetallic thiocyanate series, the CMTN crystal is found to show better efficiency than
that of diaqua tetrakis (thiocyanato) cobalt(II) mercury(II) N-methyl-2-pyrolidone (CMTWMP) [25] and comparable with that of tetrathiourea cadmium(II) tetrathiocyanato zinc(II)
(TCTZ)
[16]
and
diaqua
(thiocyanato)
manganese(II)
mercury(II)]-N,N-
dimethylacetamide (MMTWD) [26]. 3.7 Thermal studies Thermal stability of CMTN was studied by TG-DSC and DSC analysis and is shown in Figure 8(a-b) respectively. From the Fig. 8a which shows the TG-DSC curve of CMTN, it is clear that the complex is thermally stable up to 128.5oC and further increase of temperature is supporting for a significant weight loss. The first step involves the decomposition of the organic ligand and then the bimetallic thiocyanate complex starts decomposing. The complex starts decomposing at 150oC where the two molecules of NMF get fragmented. This decomposition temperature is comparable with that of the value reported by Liu et al [19]. During the temperature range of 150 - 210oC, there is a weight loss of 8.20%, this can be accounted for the decomposition of two NMF molecules, which matches well with the theoretical value of 8.22%. The next step involves the breakdown of the 3D structure of CdHg(SCN)4 and the formation of metal sulfides, carbon disulfide, nitrogen gas, and dicyanogen. The most probable and expected reactions involved in this decomposition process are given below. CdHg (SCN)4(C2H5NO)2 CdHg (SCN)4
CdHg(SCN)4 +2C2H5NO CdS + HgS + CS2 +(1/4)N2 + (3/2) (CN)2
(3) (4)
The gaseous products formed during the decomposition are eliminated and the HgS sublimes in the next step leaving the residue. The final residue is CdS (22.56%) which agrees well with the theoretical value of CdS (23.58%). The thermal stability of CMTN was found to be good and quite comparable with another Lewis base adducts like CWTWMP (150oC) [24], MMTD (145oC), TCTZ (156oC) [26], and MMTG (145oC) [16]. It is also inferred from
the study that CMTN (152.5C) is less stable thermally than that of CMTC (202.9oC) [7] and MMTWD (70oC) [26]. Similarly, the DSC curve of CMTN shown in Fig. 8b indicates that the melting point of CMTN is 152.5oC. But the TGA curve provided the information that the weight loss is getting started around 150oC temperature, meaning that the complex is actually decomposing before its melting.
Fig. 8. (a) TGA-DSC, and (b) DSC curves for the CMTN single crystals.
3.8 Etching studies For the etching studies, the as-grown crystals free from any visible defects were selected and etched with water and acetone for 2 minutes. The crystal was immediately dried with the tissue paper and was observed under optical microscopy. Figure 9(a-b) shows the surface nature of as-grown CMTN crystals at two different spots and Figure 10(a-b) shows the etch patterns observed in CMTN when water and acetone (respectively) were used as etchants. From the comparison of images in Figures 9 and 10, the step-growth pattern can be seen in Fig. 10, while the same was missing in Fig. 9. In water, the etching patterns are more of hexagonal shape, whereas, in acetone it gave a tetrahedral etch pattern. Even though different etch patterns were observed in both the solvents, the spiral growth pattern is clearly seen in both (Fig. 10). The difference in the shape of the etch pattern may be due to the difference in
the solubility of CMTN in both the solvents. The inset of Fig. 10a shows the clear picture of spiral growth and the crystal surface with dislocations. These dislocations will create steps in the surface which grows on by the incorporation of the growth units. Once the first step starts advancing, it gives rise to the second step and the process continues and thus generating the spiral pattern around the dislocation core [28]. Here, in this case, we can clearly see the hexagonal shape of growth spirals on the etched surface of CMTN where these hexagonal spirals reflect the growth morphology of the crystal as in bulk. The hexagonal spiral steps convey that the CMTN involves the spiral growth mechanism. Apart from the spiral center, step patterns created due to the spirals are observed at the edges of the crystals. From the etching studies, it is inferred that the spiral growth mechanism is operating towards the crystal growth.
Fig. 9. Surfaces of as-grown CMTN crystals at two different spots of (a) and (b).
Fig. 10. Surface of as-grown CMTN crystals etched for 2 minutes with (a) water, and (b) acetone as the etchants. 3.10 Dielectric studies Variations in the dielectric constant and dielectric loss of CMTN crystal with respect to varying frequencies at room temperature are shown in the Fig. 11(a-b) respectively. In general, for any material, the existence of dielectric constant is mostly due to the influence of frequency on the ionic, electric, orientational and space charge polarizations. The frequency effect of dielectric constant for the CMTN crystal shown in Fig. 11a provides the information that the value is maximum at the lowest frequency. With an increase of applied frequency, we observed a gradual decrease in the dielectric constant and becoming constant when reaching to higher frequencies. The increased value of dielectric constant at the lowest frequency can be due to the fact where all kinds of polarizations are highly active[29]. As the frequency increases, the polarization decreases, hence there is a decrease in the dielectric constant at higher frequencies [30]. The dielectric loss shown in Fig. 11b also follows the same trend with that of the dielectric constant, it decreases with an increase of frequency and after a certain frequency, it become saturated. The low dielectric constant and dielectric loss of the CMTN crystal conveys that the sample possesses enhanced optical properties and thereby supporting for potency towards the NLO applications.
Fig.11. Comparison of the variations in the dielectric constant (a), and (b) dielectric loss of CMTN with respect to frequencies.
4. Conclusion In conclusion, we indicate an optically transparent NLO crystal, CMTN crystals which have been prepared by a slow solvent evaporation method and the bulk crystals were successfully grown by optimizing the growth parameters. The CMTN crystals having the dimensions of 20 x 20 x 8 mm3 were obtained using water as a solvent, at pH 3.5 and at 30oC temperature. The single crystal XRD confirmed the CMTN’s cell parameters to be in the orthorhombic symmetry with the NCS space group Pna21, while the single-phase crystallinity was observed by the powder XRD pattern. Further, the bonding between the metal ion and the ligands were confirmed from the FTIR and Raman spectral analyses. The thermal studies indicated that the melting point of CMTN is 152.5oC and in addition, the CMTN complex was found to decompose before its melting. The optical absorption studies revealed that the CMTN crystals has good optical transparency and has a 335 nm cutoff wavelength. The studies of NLO property indicated that the CMTN’s SHG efficiency is 5 times higher than that of KDP reference. The surface analysis done by the etching studies revealed for the spiral growth patterns and spiral mechanisms involved in the growth process. The low dielectric constant and dielectric loss values suggest that the CMTN posse’ optical properties. Finally, a large crystal of CMTN with good optical quality can be obtained under optimum conditions. All these above mention properties confirm CMTN to be useful material for optical applications makes it a potential candidate material for NLO applications.
Competing interest: The authors declare no conflict of interest. Acknowledgements:
The King Saud university author is grateful to the Deanship of Scientific Research, King Saud University for funding through Vice Deanship of Scientific Research Chairs.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Dr.Suresh Sagadevan