Crystal growth, structural, optical, thermal and

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Optical Materials 73 (2017) 154e162

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

Crystal growth, structural, optical, thermal and dielectric properties of lithium hydrogen oxalate monohydrate single crystal Senthilkumar Chandran, Rajesh Paulraj*, P. Ramasamy Centre for Crystal Growth, Department of Physics, SSN College of Engineering, Kalavakkam, Tamilnadu, 603 110, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 July 2017 Received in revised form 29 July 2017 Accepted 31 July 2017

The vibrational groups of the lithium hydrogen oxalate monohydrate have been investigated by FTIR and FT- Raman analyses. It has low absorbance in the UV-Vis-NIR region. The laser damage threshold study confirms that the material withstands upto 30 mJ with time of 7 s, after that circular dot damage is seen on the surface. The dark region of the surface damage spot occurs due to the thermal effects. The material is thermally stable upto 93  C and there is no weight loss below this temperature. The dielectric studies were carried out at the frequency regions of 1 kHze1 MHz and different temperatures from 40  C to 80  C. Semi-organic non-linear optical (NLO) single crystal lithium hydrogen oxalate monohydrate has been grown by slow evaporation solution growth technique. The Hirshfeld surface analysis was performed to understand the different intermolecular interactions in the title compound. The fingerprint plots contain the highest portion of H/O/O/H (48.3%) interactions. © 2017 Elsevier B.V. All rights reserved.

Keywords: Optical properties Dielectric properties Thermal stability Nonlinear optical material Laser damage threshold

1. Introduction Different optoelectronic materials have motivated researchers to grow semi-organic single crystals for advanced high technology devices. In the recent years, second and third order nonlinear optical materials have evolved as one of the most interesting fields of research for various applications such as amplitude modulation, phase modulation, optical communication, optical electronics, optical data storage, laser frequency shifting, optical limiting, optical data processing and so on. The semi-organic crystals are combining the advantages of both organic and inorganic materials which have high thermal stability, large transmittance range, high laser damage threshold, mechanical strength and so on [1e5]. Lithium hydrogen oxalate monohydrate is one of the semi organic single crystals and crystallizes in triclinic crystal system with the space group P1. It owns strong pyroelectric, dielectric, piezoelectric, elastic and thermoelastic properties. Due to these promising properties, lithium hydrogen oxalate monohydrate crystal makes it a good candidate for image processing, ultrasonic transducers, impact detector and so on [6e9]. Lithium hydrogen oxalate monohydrate has asymmetric hydrogen bond chains along [101] plane [5,6]. There is increasing stress on the development of hydrogen bonds

* Corresponding author. E-mail address: [email protected] (R. Paulraj). http://dx.doi.org/10.1016/j.optmat.2017.07.051 0925-3467/© 2017 Elsevier B.V. All rights reserved.

such as OeH/O, NeH/O and CeH/O, which are known to have specific effects on crystal packing modes. The hydrogen bonding materials have promising piezoelectrics and NLO properties [10,11]. The Hirshfeld surface (HS) analysis is a valuable tool for identifying intermolecular interactions and provides quantitative information about the intermolecular interactions maintaining a whole-ofmolecule approach [12]. The photoinduced absorption is one of the important factors for increasing second and third order nonlinear optical susceptibility. This originates from the photoinduced electronephonon anharmonicities [13,14]. Crystal growth is a non-equilibrium process and thus prone to defects incorporation during growth. Defects can be classified as point defects, linear defects, planar defects and volume defects. All these defects influence the quality of the crystal. Hence, evaluating defects provides elaborate information about the crystal quality and functional characteristics of the crystal. Analysis of defects in the crystals is crucial because present and future applications demand defects free single crystals [15e17]. In a previous paper we reported nucleation kinetics, crystal perfection, transmittance, photoconductivity properties of lithium hydrogen oxalate monohydrate crystal [9]. In this article the authors describe single crystal growth of lithium hydrogen oxalate monohydrate crystal by slow evaporation solution growth technique and their intermolecular interactions, FTIR, FT-Raman, optical band gap, thermal, laser damage threshold and dielectric properties.

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2. Experimental section 2.1. Crystal growth Lithium hydrogen oxalate monohydrate crystal was grown by slow evaporation solution growth technique. To synthesize lithium hydrogen oxalate monohydrate, equimolar amount of lithium sulphate monohydrate and oxalic acid dihydrate were dissolved in deionized water with resistivity of 18.2 MU cm. Then, the solution was stirred well using a magnetic stirrer to ensure a homogeneous temperature and concentration throughout the entire volume of the solution. It was filtered using Whatman filter paper (Grade 1 with pore size of 11 mm) to remove impurities and transferred to beaker. According to the solubility data [9], the saturated solution was prepared (40  C) and it was kept in constant temperature bath at 40  C. After several days good quality crystals were harvested, which are shown in Fig. 1.

2.2. Characterization technique The grown crystal was subjected to single crystal X-ray diffraction analysis to confirm the crystal structure by employing Bruker AXS Kappa APEX II CCD Diffractometer, equipped with graphitemonochromated MoKa radiation (l ¼ 0.7107 Å). Using XPERTPRO X-ray diffractometer (l ¼ 1.5406 Å, CuKa radiation), the Xray powder diffraction pattern was recorded for the finely powdered lithium hydrogen oxalate monohydrate. It was scanned for 2q values from 10 to 80 at a rate of 2 /m and observed reflection peaks were indexed using the ‘TWO THETA’ refinement software. Hirshfeld surface and its two-dimensional fingerprint plots were determined using Crystal Explorer 3.1 [18] using crystallographic data. Crystallographic data is downloaded from CCDC and crystal structure was generated using Mercury 3.8 software (Fig. 2). FTIR spectrum of lithium hydrogen oxalate monohydrate crystal was recorded in the range between 4000 cm1 and 500 cm1 with Bruker alpha spectrometer using the attenuated total reflectance (ATR) mode. FT-Raman spectrum was carried out by BRUKER RFS 27 FT-Raman spectrophotometer in the spectral range between 4000 cm1 and 100 cm1 with resolution of 2 cm1 using the Nd:YAG 1064 nm laser source. The TG/DTA was traced for the powdered lithium hydrogen oxalate monohydrate sample of weight 4.55 mg between 40 and 550  C at a heating rate of 10  C/ min in nitrogen atmosphere using Perkin Elmer Diamond instrument. A Q-switched Nd:YAG laser of wavelength 532 nm, 7 ns pulse

Fig. 2. An ORTEP view of the lithium hydrogen oxalate monohydrate structure: Atom numbering scheme (asymmetric unit).

width and 10 Hz repetition was employed for the laser damage threshold studies (LDT). The lithium hydrogen oxalate monohydrate crystal was polished using its mother (lithium hydrogen oxalate monohydrate) solution and alumina powder to avoid impurities and imperfections on the crystal surface. Dielectric measurements were carried out using the Agilent Model 4284A LCR meter from 1 kHz to 1 MHz in the temperature range between 40  C and 80  C. Square shape crystal (2 mm thickness, 1 cm length and breadth) coated with silver paste on both sides for electrical contact was used. 3. Result and discussion 3.1. XRD analysis The title compound crystallizes in the triclinic system with the space group P1 with lattice parameters a ¼ 3.45 Å b ¼ 5.09 Å, c ¼ 6.14 Å and a ¼ 78.47,b ¼ 84.84 , g ¼ 81.47 and V ¼ 103 Å3. The

Fig. 1. As grown single crystals of lithium hydrogen oxalate monohydrate.

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S. Chandran et al. / Optical Materials 73 (2017) 154e162

different functionalities can affect crystal packing behaviour (such as molecular shape, some degree of chemical information and some aspects of the crystal environment) [19]. It visualizes the intermolecular interactions within a crystal structure by employing three dimensional (3D) surface and two dimensional (2D) fingerprint plots. Initially it was used to find out the molecular dipole, quadruple moments in molecular crystals and study polymorphs of small molecules [20,21]. Hirshfeld surfaces are defined by the partitioning of space within a crystal where the ratio of promolecule to procrystal electron densities is WðrÞ ¼ 0:5, which covers the 0.002 a.u. electron density isosurface. The weight function for a particular molecule can be defined as [22]:

WðrÞ ¼

X

,

X

ra ðrÞ

aεmolecule

ra ðrÞ

aεcrystal

. ¼ rpromolecule ðrÞ rprocrystal ðrÞ

(1)

Fig. 3. PXRD pattern of lithium hydrogen oxalate monohydrate.

sharp nature of the peaks at specific 2q values in the PXRD pattern shows (Fig. 3) the good crystalline nature of the grown crystal. The prominent peaks of lithium hydrogen oxalate monohydrate are (1 0 1), (0 1 0), (0 0 1) , (1 0 1), (1 2 0) and (2 1 0), respectively These values are in good agreements with reported values [4,7, JCPDS no: 49-1209, IUCr A09153]. 3.2. Hirshfeld surface analysis Hirshfeld surface analysis is an effective way to analyzing how

where ra ðrÞ is spherical atomic electron distribution located at the ath nucleus. The Hirshfeld dnorm surfaces, shape index and curvedness of crystal are shown in Fig. 4. The dnorm surfaces are applied for the identification of very close intermolecular interactions. The dnorm can be shown as [23]:

dnorm ¼

di  rivdw rivdw

þ

de  revdw revdw

(2)

where rivdw and revdw are the van der Walls radii of the atoms internal and external to the molecular surfaces. The colour code

Fig. 4. Different views of the Hirshfeld surfaces (a) de (b) dnorm (c) Shape index and (d) Curvedness.

S. Chandran et al. / Optical Materials 73 (2017) 154e162

data is preferred to show the intermolecular contacts on the Hirshfeld surface as red for contacts shorter than the sum of the van der Waals radii of the two atoms resulting in a negative value (0.58) and blue for contacts longer than the sum of the van der Waals radii of the two atoms resulting in a positive value (0.85). White represents the distance of contacts close to the van der Waals radii. The surfaces are exhibited as transparent to allow visualization of the lithium hydrogen oxalate monohydrate molecule, around which they were calculated. To realize the complete picture of intermolecular interactions in the simplest visual manner, the de and di points are exhibited for each surface point of the molecular surface through fingerprint plots. Fig. 5 shows the two dimensional fingerprint plots of lithium hydrogen oxalate monohydrate molecule. Fig. 4(b) exhibits the red regions of the dnorm surface, which are assigned to the H/O/ O/H and Li/O/O/Li intermolecular contacts, respectively. The fingerprint plots show that H/O/O/H interactions contain highest portion (48.3%) of total surfaces. The proportion of Li/O/ O/Li interactions comprise 22.9% of the Hirshfeld surface. The H/H interactions which are reflected in the middle of scattered points in the two dimensional fingerprint plot include only 5.9% of the total Hirshfeld surface. Apart from the above interactions, the presence of p/p (C/C and C/H), lone pair…p (C/O) and lone pair … lone pair (O/O) interactions are also noted. These interactions cover 22.8% of total Hirshfeld surface. Fig. 6 shows the percentage contribution of a variety of contacts in the lithium hydrogen oxalate monohydrate crystal. To analyze the local morphology of three-dimensional shapes, shape index and curvedness map are needed. The shape index (S) can be expressed in terms of the principal curvatures:



  k þ k2 arctan 1 p k1  k2 2

(3)

The curvature surface shows the electron density surface curves around the molecular interactions. It can be shown as [24,25]:



2

p

ln

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi .ffi k21 þ k22 2

(4)

where k1 and k2 are principal curvatures. Fig. 4(c) depicts the shape index of the lithium hydrogen oxalate monohydrate. It contains the hollow red and blue bump region. The hollow red region shows the close attachment to other molecules. The front and rear sides of the shape index surface are quite different, which exhibits different intermolecular interactions with neighbouring molecules. The mapping of curvedness on Hirshfeld surface (Fig. 4 (d)) shows the green regions are separated by blue edges corresponding to curved regions. The low range of region and light colour on the Hirshfeld surface represent a weaker and longer interaction other than hydrogen bonds.

3.3. FTIR and FT-Raman spectral analyses The FTIR and FT-Raman spectra of lithium hydrogen oxalate monohydrate are shown in Figs. 7 and 8, respectively. The bands observed in FT-IR at 3294 cm1 and FT-Raman at 3329 cm1 are assigned as OeH stretching vibration. The broad peaks noted at 1772 cm1 and 1723 cm1 in both FTIR and FT-Raman are attributed to the C]O stretching vibration. The peaks at 1430 cm1 (IR) and 1436 cm1 (Raman) are due to the combination of the CeO stretching and OeH deformation. The broad peak in FTIR at 1087 cm1 is assigned to CeO stretching and 780 cm1 peak is due to the OeLieO stretching modes of overlapping [22]. The sharp peak at 719 cm1 is attributed to OeC]O in plane deformation and

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peak observed at 592 cm1 is assigned to the O]CeO rocking mode. The peak at 849 cm1 (Raman) is assigned to CeC stretching vibration. In IR and Raman spectra, peaks observed at 519 cm1 and 518 cm1 are due to the CeC]O in-plane bending vibrations [26,27]. 3.4. Optical analysis The optical absorption at 281 nm (Fig. 9) is due to non-bonding (n) to anti bonding (p*) electronic transition (n/ p*) in the material. The optical constants such as band gap energy, extinction coefficient and refractive index are crucial to analyze the optical material for the fabrication of optical devices. The electronic transitions occur in a material due to the electronic band structure, electrical properties and so on [28]. The absorption coefficient (a) can be demonstrated by Tauc's formula [29,30]:

 n ðahnÞ ¼ A hn  Eg

(5)

where a is the absorption co-efficient, h is Planck's constant, Eg is the optical band gap energy of the material, n is the frequency of the incident photons, A is the constant and n is the characteristics of transition. The above relation can be modified as [31]:

d½lnðahnÞ n  ¼ dðhnÞ hn  Eg

(6)

To determine type of optical transition in a material, graph has been plotted between d½lnðahnÞ=dðhgÞ and (hn). The graph (Fig. 10 (a)) shows discontinuity. It gives (particular maximum energy value) approximately band gap energy, Eg ¼ 4.09 eV. The ‘n’ value can be calculated from the plot drawn between lnðahnÞ and lnðhn  Eg Þ. The value of electron transitions (n) is 2 for allowed indirect transition, 3 for indirect forbidden transition, n ¼ 1/2 for allowed direct transition and 3/2 for direct forbidden transition [32]. The value of ‘n’ is computed to be 0.53 z 0.5 ¼ 1/2 from the slope of curve (Fig. 10 (b)). This asserts that electronic transition of lithium hydrogen oxalate monohydrate is allowed direct transition. The band gap energy can be calculated using the Tauc's plot relation [5]:

 1 ðahnÞ ¼ A hn  Eg 2



2:3026 A t

(7) (8)

where A is the absorbance, t is the thickness of the crystal (2 mm). The band gap energy of the crystal can be found out by plotting ðahnÞ2 versus ðhnÞ which is shown in Fig. 10(c) The band gap of the material was obtained by extrapolation of the linear portion of the curve to zero absorption. The energy band gap of lithium hydrogen oxalate monohydrate was estimated to be 4.15 eV. Its wide band gap, high transmittance and low cut-off value make it a suitable material for the optoelectronic device applications [33,34]. 3.5. Thermal properties TG/DTA analysis of lithium hydrogen oxalate monohydrate crystal was carried out between 40  C and 550  C at a heating rate of 10  C/min in nitrogen atmosphere. TG/DTA curves of lithium hydrogen oxalate monohydrate have been shown in Fig. 11. The TG curve shows that the material is stable upto 93  C as there is no weight loss below this temperature. After that, the weight loss occurs at different stages between 97  C and 550  C. It is noted that initial decomposition (endothermic peak) of sample starts at

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Fig. 5. Two dimensional fingerprint plots for lithium hydrogen oxalate monohydrate.

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Fig. 6. Relative contribution of various intermolecular interactions in lithium hydrogen oxalate monohydrate.

Fig. 7. FTIR spectrum of lithium hydrogen oxalate monohydrate.

Fig. 9. UV-Vis-NIR spectrum of lithium hydrogen oxalate monohydrate crystal.

112  C. The first weight loss is attributed to the decomposition of water molecule (16%) in the range between 99  C and 146  C. Then, a major weight loss (44%) happens in the temperature range between 170  C and 237  C which can be assigned to the decomposition of oxalic acid into carbon dioxide (CO2) in the material and corresponding small endothermic peak absorbed around 202  C. Further weight loss of the crystals is around 10% in range between 444  C and 530  C. This may be due to liberation of CO molecule. Finally, it leaves 30% of material as residue. 3.6. Laser damage threshold

Fig. 8. Raman spectrum of lithium hydrogen oxalate monohydrate.

The selection of a material for nonlinear optical (NLO) devices is crucial. Laser damage threshold of optical material is defined not only by fundamental physical damage mechanisms but also by the structural defects which are present in the crystals [35,36]. For nonlinear optical device applications the material must withstand high power laser intensities. Laser damage in non-metals may critically affect the functioning of high power laser systems and the efficiency of optical systems. Hence, extensive research has been

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Fig. 11. TG-DTA analysis of lithium hydrogen oxalate monohydrate single crystal.

laser beam was assessed by Knife-Edge method. LDT values for (0 0 1) plane crystal were recorded when the clear visible spot occurred on surface. Initially 5 mJ was enforced on the (0 01) surface and no damage was detected up to 20 s and the laser energy of the beam was increased from 5 mJ to 25 mJ in steps of 5 but no damage was noted. When the laser energy was increased to 30 mJ with time of 7 s, circular dot damage was seen on the surface. The energy was slowly increased to 35 mJ with the time of 10 s visible damage (Fig. 12 (a)) and cracks had been observed. The energy density of the crystal was determined using the relation [38]:

Power density ðPd Þ ¼

E

tpr2

(9)

where E is the energy (mJ), t is the pulse width (ns) and r is the radius of the spot (0.9 mm). The multiple-shot laser damage threshold was estimated to be 14.20 GW/cm2 using a 532 nm wavelength. The surface damage of lithium hydrogen oxalate monohydrate crystal is further confirmed by optical microscope (Fig. 12 (b)). The thermal behaviour of crystals is of basic importance, which is relevant for different applications. When crystal is irradiated with laser, portion of the light will be absorbed by the crystal and changed into heat energy. Temperature gradient is formed and a corresponding thermal expansion happens. If the thermal expansion coefficients of the crystal are anisotropic, the crystal will crack when it absorbs a high thermal energy [39,40]. This affects the stability of the device. The dark region of the damage spot occurs due to the thermal effects. Further, TG/DTA curves confirm (Fig. 11) that lithium hydrogen oxalate monohydrate crystal decomposes around at 112  C (endothermic peak). The blackening and decomposition of the crystal is characteristic of oxalate and it may be the cause for dark regions in and around the damaged spot. The similar behaviour was observed for lithium L-ascorbate dihydrate [41] and sodium D-isoascorbate monohydrate [42] crystals. Fig. 10. a. Plot of lnðahyÞ=hy vs hy b. Plot of lnðahyÞ vs lnðhy  Eg Þ c. Optical bandgap energy.

3.7. Dielectric studies

made in the last three decades. Multiple-shot laser damage provides information about uses of crystal in practical applications [37]. From this point of view, we carried out multiple-shot laser damage threshold measurements on the lithium hydrogen oxalate monohydrate crystal using Q-switched Nd:YAG laser (l ¼ 532 nm) laser of 7 ns pulse width and 10 Hz frequency. The spot size of the

The dielectric property of the material gives a clear insight into the lattice dynamics, molecular dynamics, molecular anisotropy and electro-optical properties [43]. The study of dielectric behaviour as a function of frequency and temperature displays various polarization and electrical processes in crystals. The dielectric constant of the material was determined using the following relation [17,44]:

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Fig. 12. The damage morphologies after laser irradiation (a) lithium hydrogen oxalate monohydrate crystal (b) optical microscope image.

Fig. 13. Dielectric constant of lithium hydrogen oxalate monohydrate single crystal.

εr ¼

Ccrys Cair

(10)

where Ccrys is the capacitance of the material and Cair is the capacitance of the same dimension of air. The dielectric studies were carried out at the frequency regions of 1 kHze1 MHz and different temperatures from 40  C to 80  C. Figs. 13 and 14 are the dielectric constant and dielectric loss of the grown crystal. From the plots, both dielectric constant and dielectric loss decrease with increasing frequency. This may be due to the charge collection at the interface among the sample and electrode ensuing in space charge polarization effects. The high values at low frequencies of the grown crystal is possibly bearing of all the four polarizations such as electronic, ionic, orientational and space charge polarization. At high frequency, decreasing of dielectric constant may be considered as a result of the periodic reversal of the electric field [43,45,46]. The dielectric losses fall into two classes, intrinsic and extrinsic. Intrinsic losses are dependent on the crystal structure. Intrinsic losses determine the lower limit of losses found in pure “defect free” crystals. Extrinsic losses are related with imperfections in the crystal. The low values of dielectric loss at higher frequencies indicates that the grown crystal has good quality and defect free nature and this is important for microelectronic and photonic applications [17,46,47]. The lithium hydrogen oxalate monohydrate crystal has low dielectric constant values which is important criteria for the possible SHG conversion efficiency and was in agreement with Miller rule [48].

Fig. 14. Dielectric loss of lithium hydrogen oxalate monohydrate single crystal.

4. Conclusion The lower cut-off wavelength of the material is 281 nm and optical bandgap value of the material is found to be 4.15 eV. The multiple shot laser damage threshold study was performed on (0 0 1) plane and the LDT value of lithium hydrogen oxalate monohydrate was found to be 14.20 GW cm2.The dielectric measurements of lithium hydrogen oxalate monohydrate crystal exhibit very low dielectric constant and dielectric loss which is important for microelectronic and photonic applications. The peak observed at 780 cm1 is due to the OeLieO stretching modes of overlapping. The Li/O/O/Li interactions comprise 22.9% of the total Hirshfeld surface. Major weight loss (44%) occurs in the temperature range from 170  C to 237  C which can be assigned to the decomposition of oxalic acid into carbon dioxide (CO2). Acknowledgments One of the authors, C. Senthilkumar acknowledges the JRF offered by SSN Institutions. References [1] S.A. Avila, S. Selvakumar, M. Francis, A.L. Rajesh, J. Mater. Sci. Mater. Electron. 28 (2017) 1051e1059. [2] N.R. Rajagopalan, P. Krishnamoorthy, J. Inorg. Organomet. Polym. 27 (2017) 296e312. [3] N. Ahmad, M.M. Ahmad, P.N. Kotru, J. Cryst. Growth 412 (2015) 72e79.

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