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Growth, optical, thermal, mechanical and electrical properties of anhydrous sodium formate single crystals
Accepted Manuscript Growth, optical, thermal, mechanical and electrical properties of anhydrous sodium formate single crystals C. Amuthambigai, C.K. Mahadevan, X. Sahaya Shajan PII:
S1567-1739(16)30152-3
DOI:
10.1016/j.cap.2016.06.006
Reference:
CAP 4257
To appear in:
Current Applied Physics
Received Date: 13 January 2016 Revised Date:
13 April 2016
Accepted Date: 9 June 2016
Please cite this article as: C. Amuthambigai, C.K. Mahadevan, X. Sahaya Shajan, Growth, optical, thermal, mechanical and electrical properties of anhydrous sodium formate single crystals, Current Applied Physics (2016), doi: 10.1016/j.cap.2016.06.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Growth, optical, thermal, mechanical and electrical properties of anhydrous sodium formate single crystals C. Amuthambigai, C.K. Mahadevan, X. Sahaya Shajan* Centre for Scientific and Applied Research, PSN College of Engineering and
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Technology, Tirunelveli-626152, Tamilnadu, India Abstract
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Anhydrous sodium formate single crystals were grown by the solvent evaporation method at 40 ˚C. Etching study was carried out to assess the
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crystalline perfection. X-ray diffraction and FTIR spectral analyses were performed for the identification of the material and the crystal structure. Suitability of the crystal for photonic applications was studied by the optical transmittance, SHG efficiency and Z-scan measurements. The thermal and mechanical stabilities of the grown crystal were also investigated. Thermal
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decomposition and load dependence of various mechanical parameters viz. Hv, Kc, B, σv and C11 have been understood. AC (with various frequencies ranging from 20 Hz to 200 kHz) and DC electrical measurements were carried out at
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various temperatures ranging from 30 to 150 ˚C. Temperature and frequency dependences of important electrical parameters have been understood. Results
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obtained indicate that the grown crystal exhibits higher crystallinity, good optical transmittance, third order optical nonlinearity, good thermal stability (up to 318 ˚C), higher dielectric constant and normal mechanical behaviour. Keywords: Semi-organic crystal, Solution growth method, Spectral analysis, Optical properties, Thermal stability, Microhardness. *Corresponding author Phone: +919443128713 Email: [email protected], [email protected]
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1. Introduction The metal formate crystals have gained much interest due to their potential applications and interesting physical properties [1]. The formates of group I and II metals in periodic system exhibit strong nonlinear optical
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properties [2]. Crystal structures have been reported for several metal formates such as lithium formate [3], sodium formate [4], ammonium formate [5], strontium formate [6], barium formate [6], calcium formate [6], cupric formate [7] and gadolinium formate [8]. However, reports on physico-chemical
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properties are limited. Studies on amino acid formates such as l-threonine formate [9-12], l-alanine formate [13, 14], l-valinium formate [15] l-arginine
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formate [16] and l-argininium formate [17] have also been reported. Among the metal formate crystals, lithium formate [18] and strontium formate [19] crystals have been studied, to some extent. However, the literature shows very little about the other metal formate crystals.
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Among all the alkali metals, sodium has been widely used due to its higher charge density. In semi-organic crystals, sodium has the ability to combine with organic ligands. Sodium formate has been used in many
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industries and its aqueous solutions are used to absorb SO2 in thermoelectric power plants. The structure determination and vibrational spectral analysis for
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sodium formate crystal have already been reported in the literature [20-22]. Crystals of sodium formate have monoclinic-holohedral symmetry with the centro-symmetric space group C2/c. The crystal has four molecules in a unit cell and each sodium atom is linked to six oxygen atoms belonging to five different formate radicals. The formate anion lies on a twofold axis and has C2v symmetry. The density of this crystal is 1.91-1.93 g/cc. Sodium formate (hydrous/anhydrous) is a representative of hydrogen bonded crystals and the crystal is expected to be highly useful for device
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applications. In hydrous sodium formate, the water molecule may increase the elastic and NLO properties but reduces the thermal stability of the crystal. But, it is essential for the crystal to possess reasonably good thermal stability for their suitability in practical applications. So, we have made an attempt in the
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present study to grow single crystals of anhydrous sodium formate for device applications. In the present study, we have grown the anhydrous sodium formate (SF) single crystals by using the simple solvent evaporation method and characterized chemically, structurally, optically, thermally, mechanically and
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electrically. The results obtained are reported and discussed herein.
2.1.
Material synthesis
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2. Experimental
Sodium formate was synthesized by mixing high purity sodium hydroxide (obtained from Merck) and formic acid (obtained from Rankem) taken in equimolar ratio and dissolving completely in millipore water of
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resistivity 18.2 MΩcm at room temperature (30 ˚C) using a magnetic stirrer. The purity of the crystalline product was further improved by repeated recrystallization process. The sodium formate formation can be represented by
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the following chemical reaction:
2.2.
NaHCOO + H2O
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NaOH + CH2O2
Crystal growth
Solvent evaporation solution growth technique was employed to grow
single crystals of sodium formate. Saturated solution of sodium formate was prepared and allowed to equilibrate in a beaker (crystal growth cell). The cell was covered with a perforated sheet and kept undisturbed in a constant temperature bath maintained at 40 ˚C to avoid forming the dihydrate [22]. After a period of one month, evaporation of the solvent gives thin transparent platelike crystals with the maximum size of 14×9×1 mm3. The grown crystals were
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harvested and used for the characterization experiments. Two-dimensional nucleation may be considered as the mechanism behind the formation of single crystals. 2.3.
Characterization
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In order to understand the surface features of the grown crystals, good quality crystals (macroscopically free from defects) with flat and smooth surface of thickness 0.7 mm were selected and subjected to etching study with
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water as the etchant. The crystal was dipped in the etchant from 10 to 60 s with an interval of 10 s and then wiped with a tissue paper. The etch patterns were
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observed and photographed with an Olymbus-BX51M optical microscope in the reflection mode. Powder X-ray diffraction (PXRD) analysis of the crystal was carried out using a PANalytical EMPYREAN diffractometer with Cu Kα radiation of wavelength 1.5405 Å. The data were collected in the 2θ range of 5 to 80 ˚ with a step size of 0.026 ˚. The Fourier transform infrared (FTIR)
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spectrum was recorded for the as-grown crystal using a JASCO FT/IR-4100 spectrophotometer by the ATR technique at room temperature (30 ˚C) in the wave number region 4000-550 cm-1.
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The linear, second order nonlinear and third order nonlinear optical properties of the grown SF crystal were understood by measuring its optical
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transmittance in the UV-Vis-NIR region (200-1200 nm), second harmonic generation (SHG) efficiency and Z-scan measurements respectively. The optical transmittance of the crystal was measured using a Shimadzu UV-2600 UV-VisNIR spectrometer by placing the crystal of thickness 0.7 mm directly on the path of the source. The Kurtz and Perry powder method [23] was employed to measure the SHG efficiency. The input Q-switched Nd-YAG laser beam of wavelength 1064 nm with energy 1 mJ/pulse, pulse width 10 ns and repetition rate 10 Hz was made to fall on the powder form of the crystal filled in the micro
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capillary tube. The second harmonic output (emission of green light) generated by the crystal was detected by the photomultiplier tube. The third-order nonlinear refractive index (n2) and absorption coefficient (β) were evaluated by using the single beam Z–scan technique [24, 25]. The laser beam from 635 nm
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continuous wave (CW) diode laser was focused by a convex lens with focal length of 5 cm, which produces a beam waist ωo of 23.57 µm and the Raleigh length, ZR of 2.7 mm in the sample. A 1 mm thickness of cuvette containing the aqueous solution (0.4 g dissolved in 5 ml water) of SF crystal placed on a
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translational stage was moved across the focal region (+Z to –Z) along the axial direction, which is the direction of propagation of laser beam.
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The thermogravimetric (TG/DTG) analysis was performed on the SF crystal in powder form from 24 to 700 ˚C with a heating rate of 10 ˚C/min using a TGA Q500 V6.7 Build 203 instrument with pin-holed platinum crucible under nitrogen atmosphere. The mechanical stability analysis was done by a Shimadzu
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HMV-2T Vickers microhardness tester with diamond pyramidal indenter at room temperature. The crystals were selected by the same procedure followed for the etching studies and it was indented by applying a load varying from 25
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to 100 g with a dwell time of 5 s on the large surface area of the crystal. The AC electrical (dielectric) measurements were carried out by the
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parallel plate capacitor method with an accuracy of ± 2 % by varying the frequency from 200 Hz to 200 kHz at various temperatures ranging from 30 to 150 ˚C using a HIOKI IM3528 LCR meter. The DC electrical measurements were also made to an accuracy of ± 2 % at various temperatures ranging from 30 to 150 ˚C by the conventional two-probe method using the same LCR meter. In both the cases, the temperature was controlled to an accuracy of ± 0.01 ˚C and the measurements were made while cooling the sample crystal. An as grown (fresh) crystal was polished with an emery paper to get a perfect shape and the dimensions were measured using a travelling microscope (least count =
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0.001 mm). Opposite faces of the crystal were coated with good quality silver paste to obtain a good conductive surface layer. The measurements were carried out along the direction perpendicular to the large area surfaces of the crystal. Dielectric loss factor (tan δ) and capacitances with the crystal (Ccrys) and
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without the crystal (Cair) were considered in the case of dielectric measurements and resistance (R) was considered in the case of DC electrical measurement. The other AC and DC electrical parameters were determined using the above measured values.
General features
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3.1.
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3. Results and Discussion
Figure 1 shows the photograph of a sample SF crystal grown in the present study. The crystals grown are found to be well shaped, colourless and transparent. However, it exhibits some hygroscopic nature and it is not much stable in open atmosphere. This is in agreement with that reported by the earlier
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authors [20-22].
Etching analysis is the simplest technique to understand the growth mechanism and to access the quality of crystals. The etch patterns of the SF
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crystal grown in the present study on the large surface area are shown in Figure 2. When etched for 10 s, the image shows that there is a presence of tiny
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inclusions in random manner. For the period of 20 s, the presence of inclusions is small and there is a presence of hillocks. The density of inclusions is lower than the density of hillocks. By increasing the etchant time for 30 s, there is a formation of large size hillocks with lesser density. For the times of 40 and 50 s of etchant, the step pattern is formed. For the maximum time of 60 s, the surface of the crystal looks smooth with the decrease of inclusions and hillocks. This indicates that etching with water for 60 s is required to make the surface of the crystal smooth.
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3.2.
Chemical composition and structure The PXRD pattern observed in the present study is shown in Figure 3. As
this pattern is found to be in good agreement with the data available in the JCPDS file, the JCPDS data (card no. 14-0812) were used to index the present
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data. The PXRD analysis shows that the SF crystal system is monoclinic with the space group C2/c (Schoenflies notation C62h) and point group 2/m. The sharp peaks observed in the PXRD pattern show that the crystal is highly crystalline in nature. The lattice parameters were calculated using a Cell
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refinement software package and are provided in Table 1. They are found to
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match well with those reported in the literature [4].
The FTIR spectrum recorded in the present study is shown in Figure 4. The bands appearing around 2800 cm-1 can be attributed to the C-H vibrations. The bands at 2952 and 2826 cm-1 are respectively due to C-H asymmetric and symmetric vibrations. The C-H bending vibration appears at 2713 cm-1. The
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strong bands appearing at 1587 and 1352 cm-1 are due to C-O asymmetric and symmetric stretchings respectively. The peak at 767 cm-1 is attributed to C-O symmetric bending. Absence of any peak appearing at around 3400 cm-1
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indicates that the crystal grown is without any water molecules. This spectrum is found to be in good agreement with those reported in the literature [21, 22].
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Thus, the results obtained through PXRD and FTIR spectral analyses indicate that the SF crystal grown in the present study is of good crystalline nature and of anhydrous sodium formate. 3.3.
Optical properties The optical transmittance spectrum recorded in the present study is shown
in Figure. 5. The transmittance is found to be more than 54 % in the entire visible and NIR regions with an absorption edge at 225 nm. The absorption edges (AE) reported for various metal and amino acid formates are compared in
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Table 2. It can be seen that the AE observed for SF crystal is more than that reported for l-alanine formate and strontium formate but less than that reported for lithium formate, l-threonine formate, l-arginine formate and l-arginium formate.
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The SHG measurement shows no green light emission from the sample. It indicates that the grown SF crystal has zero second order susceptibility coefficient. This result is in good agreement with the PXRD, since the centrosymmetric space group crystals do not have SHG efficiency [26, 27]. The SHG
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efficiencies reported for various metal and amino acid formates are compared in Table 2. Among these, l-threonine and strontium formate exhibit respectively
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the maximum and minimum SHG efficiencies.
In closed aperture Z–scan, the intensity of the transmitted beam through an aperture (Z = 0.40) was measured by a photo detector fed to the digital laser power meter. When the sample is placed far away from the focus (–Z) the beam
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irradiance is low, while the sample is moved towards the focus, the irradiance of the beam increases leading to self-lensing effect. The normalized transmittance curve is characterized by a pre-focal transmittance maximum followed by a
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post-focal transmittance minimum as shown in Figure 6(a), which implies that the sign of the nonlinear refractive index of SF crystal is negative, i.e., self-
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defocusing. The self-defocusing effect is attributed to a thermal nonlinearity resulting from the absorption of laser radiation at 635 nm. The difference in peak and valley transmission (∆Tp-v) can be written in
terms of the on-axis phase shift ∆φ at the focus as [28] ∆Tp–v ≅ 0.406(1–S) 0.25 ∆φ Here ∆φ =
n I L
with L
=
(1)
and S = 1–exp(–2ra2/ωa2) is the
aperture linear transmittance with ra denoting the aperture radius and ωa
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denoting the beam radius at the aperture in the linear regime. Also, λ is the laser wavelength, Io is the intensity of the laser beam at focus Z = 0, α is the linear absorption coefficient and L is the thickness of the sample.
n =
∆
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Then nonlinear refractive index (n2) is given by [28] (2)
Figure 6(b) shows the open aperture (S = 1) Z–scan setup for SF crystal.
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The nonlinear absorption coefficient β can be measured where the aperture is removed and hence the entire transmittance of the beam is collected through a
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suitable lens. It is clearly seen in Figure 6(b) that the transmittance at the focus decreases which is the characteristic signature of Reverse Saturable Absorption (RSA).
The nonlinear absorption coefficient β can be estimated from the open
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aperture Z - scan data using the relation [28] β=
√∆
(3)
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The closed aperture transmittance is affected by the nonlinear refraction and absorption, the determination of n2 is less straight forward from the closed
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aperture scans. It is necessary to separate the effect of nonlinear refraction from nonlinear absorption. A simple and approximate method to obtain purely effective n2 is to divide the closed aperture transmittance by the corresponding open aperture transmittance and Figure 6(c) shows one such data. The real and imaginary parts of the third-order nonlinear optical susceptibility χ (3) are defined as Reχ(!) =
#$ (% &' (') (' ) !.+
(e. s. u)
(4)
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Imχ(!) =
#' (% &' (' 0) +'
(e. s. u)
χ(!) = 1(Reχ(!) ) + (Imχ(!) ) (e. s. u)
(5) (6)
and c is the velocity of light in vacuum.
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Here ε0 is the vacuum permittivity, no is the linear refractive index of the sample The calculated values of nonlinear refractive index n2, nonlinear absorption co-efficient β, and third-order nonlinear susceptibility χ (3) for the SF
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crystal are found to be – 0.55×10─6 (cm2/W), 2.06×10–2 (cm/W) and 4.6×10─6 (e.s.u) respectively. Thus, the SF crystal is found to exhibit third order optical
3.4.
Thermal properties
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nonlinearity and is expected to be useful in photonic devices.
The TG/DTG curves observed for the SF crystal is shown in Figure 7(a). It shows that the SF crystal is thermally stable up to 318 ˚C without any decomposition. The decomposition takes place in two steps. In the first step,
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with a weight loss of 18 % liberation of H2 molecules takes place in the temperature range 318.4 - 410 ˚C leading to the formation of sodium oxalate. In the second step, sodium oxalate decomposes in the temperature range 503 - 543
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˚C leading to the formation of sodium carbonate by the liberation of carbon monoxide with a weight loss of 8 %. In the corresponding DTG curve, during
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the first step, two peaks are formed: one for the elimination of hydrogen molecules and the other one for the phase transformation. During the second step, one peak is formed. So, the DTG curve confirms the above two decompositions. The decomposition steps are shown in Figure 7(b). It is also noticed that there is no adsorbed and absorbed water molecules present in the crystal. The TG and DTG curves show that the transformations are associated with mass changes and physical transformation which is independent of mass change or decomposition of the material. The thermal stabilities reported for
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various metal and amino acid formates are compared in Table 2. It can be seen that the thermal stability is maximum for sodium formate and minimum for lithium formate. 3.5.
Mechanical properties
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The micro-indentation test is a non-destructive method for studying the nature of plastic flow and its influence on the deformation of materials. It gives details about the Vickers hardness value, work hardening coefficient, crack length, yield strength, fracture toughness, brittle index and elastic stiffness
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constant. Hardness value depends on the size of the indenter impression. The
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Vickers hardness value was calculated using the formula [29]: H4 =
`
.56++×8 9'
:
;<
=='
>
(7)
Here, P is the applied load in kg, d is the average diagonal length of the
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indentation mark made by the Vickers indenter in mm. The dependence of microhardness with applied load does not show same type of behaviour for all materials. It may be (a) independent of load (b) function of load (c) complex
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variation with increasing load. The hardness value as a function of load observed for the SF crystal in the present study is displayed in Figure 8(a). The
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Hv value increases with the increase in load. The higher hardness value of the crystal indicates that greater stress is required to form dislocation. Both the PXRD and hardness data confirm that the grown SF crystal has greater crystalline perfection. The higher hardness value may be due to the strong interaction of C=O group in the SF crystal, since the TGA result shows that the C-H gets easily broken. The load-hardness variation can be interpreted using Meyer’s relation: P=K1dn, where P is the applied load, d is the diagonal length of the indentation, K1 is a constant and n is the Mayer’s constant or work hardening coefficient.
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From the best fitted slope of log P versus log d plot (shown in Figure 8(b)), n value was estimated. The value of n is found to be 2.68 which indicate that the grown SF crystal belongs to soft material category. Also, as per the theory, the results of microhardness measurement follow the normal indentation size effect
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trend. The n values reported for various metal and amino acid formates are compared in Table 2. It can be understood that l-arginine formate and larginium formate crystals belong to the hard material category whereas the others belong to the soft material category.
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Measurement of crack length (c) along with d leads to the estimation of various mechanical parameters like fracture toughness (Kc), brittleness (B),
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yield strength (σv) and elastic stiffness constant (C11).
Fracture toughness (Kc) is the ability of a material containing a crack to resist fracture and it is given by [29]:
8
A β@ B'
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K@ =
for c ≥
9
(8)
Here, β is the indenter constant (value is 7) and c is the crack length (distance between the center of indentation mark to crack tip). The mechanical contact
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between the indenter and the crystal surface produces radial cracks which can
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be determined from the crack length. Toughness determines how much fracture stress is applied under uniform
bending and it is the important parameter for the selection of materials in practical applications where the load exceeds the limit. The variation of crack length and fracture toughness on load can be attributed to the depth of penetration of indenter into surface. Brittleness (B) determines its fracture without any appreciable deformation. This property helps to understand its laser damage tolerance and it is resolved by the relation [29]:
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B=
(9)
KL
The yield strength (σv) is the stress at which the material begins to deform [30]: σ4 =
IJ
.M
N1 − (2 − n)R S
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plastically and it can be computed from the hardness value using the relation .6(() ( (()
T
(10)
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By the Wooster’s empirical formula, the elastic stiffness constant is the tightness of bonding between neighbouring atoms. It is the property of material by the equation C11=Hv7/4 [30].
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by virtue of which can absorb energy before fracture occurs and it is calculated
Using these relations, the above mechanical parameters of SF crystal have been estimated from the observed hardness data. The load dependences of
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these parameters are shown in Figure 9. The Kc, σv and C11 values increase whereas the B value decreases with the increase in load. This is considered to be a normal mechanical behaviour. Electrical properties
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3.6.
Measurement of AC electrical parameters (dielectric constant (ɛr),
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dielectric loss factor (tan δ) and AC electrical conductivity (σac)) as function of frequency and temperature of a crystal is of much interest to get knowledge about its electrical properties. Crystals with higher dielectric constants are considered to be useful in capacitor technology. The dielectric constant was determined, as the crystal surface area was smaller than the plate area of the cell, by using Mahadevan’s relation [31, 32]: εU =
\LVWX
&LVWX &YZV [ &YZV
\YZV
]
(
^_`a
^bacd
)
(11)
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Here, Acrys is the area of the crystal touching the electrode and Aair is the area of the electrode. The AC and DC electrical conductivities (σac and σdc respectively) were determined using the relations [31]: σac = ɛoɛrωtanδ f
h^bacd
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efg =
(12) (13)
Here, ɛo is the permittivity of free space, ω (=2πf) is the angular frequency of
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the applied field and d is the thickness of the sample.
The variation of ɛr, tan δ and σac with frequency and temperature are
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shown in Figures 10 - 12. Figure 13 shows the variation of σdc with temperature. It is found that the ɛr and tan δ values decrease whereas the σac value increases with the increase in frequency of the applied field (f). All the four parameters, viz. ɛr, tan δ, σac and σdc are found to increase with the increase in temperature. Also, it can be seen that the AC conductivities are more than the DC
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conductivities. These results indicate that the SF crystal grown in the present study exhibits a normal dielectric behaviour. The higher dielectric constants observed at low frequencies can be
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attributed to the presence of all types of polarizations, viz. electronic, ionic,
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orientational and space charge. In a dielectric crystal, the space charge contribution at lower temperature and higher frequency is expected to be small and negligible. The electronic and ionic displacements under an applied electric field contribute to the electronic and ionic polarizations respectively. In ionic crystals, due to the large forbidden energy gap, the charge transported by electrons is zero [32]. Moreover, the electronic polarizability practically remains constant [31]. It can be seen that the high frequency and low frequency permittivities significantly differ and the ionic polarization is found to be very large. In hydrogen bonded dielectric crystals like SF, large amount of proton
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transport (leading to large ionic polarization) is expected. Moreover, increase in temperature leads to the possibility of weakening the hydrogen bonding system due to rotation of the formate ions. This is expected to generate vacant hydrogen bonds (L-defects) at higher temperature. So, the increase in dielectric constant
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with temperature is essentially due to the temperature variation of ionic polarizability. It can be seen that the normal dielectric behaviour exhibited by the SF crystal, considered in the present study, can be explained by the above features.
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Electrical conduction in hydrogen bonded dielectric crystals like Sr(HCOO)2.2H2O, KH2PO4, NH4H2PO4 and NixZn1-xSO4.7H2O has been
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attributed to the proton transport within the framework of hydrogen bonds [19, 32, 33]. The electrical conduction in SF crystal considered in the present study can also be understood as protonic and mainly due to the formate ions. The temperature dependence of conductivity can be understood as due to the
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temperature dependence of the proton transport. It can also be seen that the conductivity increases smoothly through the temperature range considered. The AC and DC activation energies (Eac and Edc respectively) were
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relations [31]:
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determined by fitting the σac and σdc values into the respective Arrhenius
eig = ejig exp[
n_b
]
(14)
efg = ejfg exp[
nrb
]
(15)
op
op
Here, σoac and σodc are the constants depending on the material, k is the Boltzman constant and T is the absolute temperature. Plots between ln σac and 1000/T and ln σdc and 1000/T were made and they (not shown here) are found to be nearly linear. This indicates that the conductivity values are well fitting into the Arrhenius relation. The plots were best fitted using the least squares
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principle and the activation energies were estimated. The estimated Eac and Edc (for which f=0) values are shown in Figure 14. It can be seen that, as expected, the Eac values are less than the Edc value. Moreover, the Eac and Edc values are found to be low. This suggests that oxygen vacancies (consequently generating
region considered in the present study [34].
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L-defects) may be responsible for electrical conduction in the temperature
The observed ɛr value varies from 33 – 2537 for the frequencies and temperatures considered in the present study. This indicates that the SF crystal
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exhibits higher ɛr values and it may find an important place in the capacitor
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technology. 4. Conclusion
Optically transparent, colourless and good quality (assessed by the etching study) single crystals of anhydrous sodium formate (SF) have been grown by the solvent evaporation method at 40 ˚C. The sharp and high intensity
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peaks observed through PXRD analysis confirm the crystallinity of the crystal and it crystallizes in the monoclinic system with the centro-symmetric space group C2/c. The functional groups present in the SF crystal were identified and
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assigned by the FTIR spectral analysis. The SF crystal shows transmittance more than 54 % in the visible and NIR regions and a lower absorption edge
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(225 nm). The SF crystal shows third order optical nonlinearity and is expected to be useful in photonic devices. It shows no SHG efficiency which is in good agreement with the PXRD data, since the centro-symmetric crystal doesn’t show SHG efficiency. The thermal analysis shows that the SF crystal is thermally stable up to 318 ˚C. The microhardness measurement shows that the crystal follows normal indentation size effect and it belongs to the soft material category. Various mechanical parameters have been estimated which indicate that the SF crystal grown exhibit a normal mechanical behaviour. DC and AC
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electrical measurements indicate that the electrical conductivity is due to the proton transport. The observed higher ɛr values indicate that the SF crystal grown in the present study may find an important place in the capacitor technology.
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Acknowledgement
One of the authors (C. Amuthambigai) acknowledges the PSN management for the support under Institute Research Stipend (IRS) scheme and
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also thanks Research and Development Centre, Orchid Pharma Ltd, Chennai for the thermal studies. The authors are happy to thank Dr. S. Natarajan,
facilities. References
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Framework Solids Laboratory, IISc, Bangalore, for providing powder XRD
[1] Andres Goeta, Ricarda Baggio, Donka Stoilova, Synthesis, crystal structure and
infrared
spectroscopy
of
a
novel
copper
strontium
formate
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CuSr(HCOO)4.(H2O)2, Vibrational Spectroscopy, 34 (2004) 293-300. [2] T. Feliksinski, R. Grouhulski, W. Kolasinski, Z. Wawrzak, Growth and some properties of Barium Cadmium formate single crystals: (II) X-ray and
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Infrared studies, Materials Research Bulletin 18 (1983) 27-32.
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[3] A. Enders Beumer, S. Harkema, The crystal structure of lithium formate monohydrate, Acta Crystallographica B29 (1973) 682-685. [4] Peter L. Markila, Steven J. Rettig, James Trotter, Sodium formate, Acta Crystallographica B31 (1975) 2927-2928. [5] I. Nahringbauer, Hydrogen bond studies. XX. The crystal structure of ammonium formate, Acta Crystallographica B24 (1968) 565-570.
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[6] T. Watanabe, M. Matsui, The redetermination of the crystal structures of αcalcium formate, α-strontium formate and barium formate by X-ray analysis, Acta Crystallographica B34 (1978) 2731-2736. [7] R. Kiriyama, H. Ibamoto, K. Matsuo, The crystal structure of cupric formate
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tetrahydrate, Cu(HCO2)2.4H2O, Acta Crystallographica 7 (1954) 482-483.
[8] A. Pabst, Crystal structure of gadolinium formate, Gd(OOCH)3, The Journal of Chemical Physics 11 (1943) 145.
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[9] Redrothu Hanumantharoa, S. Kalainathan, Growth, spectroscopy, dielectric and nonlinear optical studies of novel organic NLO crystal: l-threonine formate,
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 94 (2012) 78-83.
[10] Redrothu Hanumantharoa, S. Kalainathan, Microhardness, chemical etching, SEM, AFM and SHG studies of novel nonlinear optical crystal-l-
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threonine formate, Materials Research Bulletin 47 (2012) 987-992. [11] Redrothu Hanumantharoa, S. Kalainathan, Growth, HR-XRD, optical, thermal, luminescence, nonlinear optical studies of novel organic nonlinear
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optical crystal: l-threonine formate, Optik 124 (2013) 3718-3722. [12] Xiaojing Liu, Yan Su, Haikun Zhang, Gang Chen, Theoretical calculations
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and surface morphology studies of l-threonine formate, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 389-393. [13] C. Justin Raj, S. Jerome Das, Growth and characterization of nonlinear optical active l-alanine formate crystal by modified SankaranarayananRamasamy (SR) method, Journal of Crystal Growth 304 (2007) 191-195. [14] C. Justin Raj, S. Dinakaran, S. Krishnan, B. Milton Boaz, R. Robert, S. Jerome Das, Studies on optical, mechanical and transport properties of NLO
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active l-alanine formate single crystals grown by modified SankaranarayananRamasamy (SR) method, Optics Communications 281 (2008) 2285-2290. [15] T. Joselin Beaula, D. Manimaran, I. Hubert Joe, V.K. Rastogi, V. Bena Jothy, Vibrational, spectroscopic studies and DFT computation of the nonlinear
and Biomolecular Spectroscopy 126 (2014) 170-177.
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optical molecule L-Valinium formate, Spectrochimica Acta Part A: Molecular
[16] S. Haussuhl, H.A. Karapetyan, R.P. Sukiasyan, A.M. Petrosyan, Structure
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and properties of orthorhombic L-arginine formate, Crystal Growth and Design 6 (2006) 2041-2046.
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[17] J. Packium Julius, A. Joseph Arul Pragasam, S.A. Rajasekar, S. Selva Kumar, A. Stephen, P. Sagayaraj, Studies on the growth and characterization of l-argininium formate single crystals, Journal of Crystal Growth 267 (2004) 619623.
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[18] D. Joseph Daniel, P. Ramasamy, Studies on some-semi-organic non linear optical single crystal: Lithium formate monohydrate (HCO2Li.H2O), Optical Materials 36 (2014) 971-976.
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[19] S. Muthupoongodi, S. Theodore David Manickam, C.K. Mahadevan, J. Angel Mary Greena, S. Balakumar, X. Sahaya Shajan, Effect of glycine
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addition on the structural, thermal, optical, mechanical and electrical properties of Sr(HCOO)2.2H2O crystals, Journal of Crystal Growth 428 (2015) 46-53. [20] W.H. Zachariasen, The crystal structure of sodium formate, NaHCO2, Journal of American Chemical Society 62 (1940) 1011-1013. [21] Roger Newman, Polarized infrared spectrum of sodium formate, The Journal of Chemical Physics 20 (1952) 1663-1664.
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[22] T.L. Charlton, K.B. Harvey, Infrared Absorption of single crystals of Anhydrous Sodium Formate, Canadian Journal of Chemistry 44 (1966) 27172727. [23] S. K. Kurtz, T.T. Perry, A Powder Technique for the Evaluation of
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Nonlinear Optical Materials, Journal of Applied Physics 39 (1968) 3798-3813. [24] M. Sheik- Bahae, A.A. Said, E.W. Van Stryland, High sensitivity single beam measurements, Optical Letters 14 (1989) 955-957.
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[25] M. Sheik- Bahae, A.A. Said, T. Wei, D.J. Hagan, E.W. Van Stryland, Sensitive measurement of optical nonlinearities using a single beam, IEEE
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Journal of Quantum Electronics QE 26 (1990) 760-769.
[26] A. Karolin, K. Jayakumari, C.K. Mahadevan, Growth and characterization of Pure and Ni2+ Added Crystals of Glycinepotassiumsulfate, International Journal of Engineering Research and Applications 3(12) (2013) 1906-1915.
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[27] Aram M. Petrosyan, Second harmonic generation in centrosymmetric crystal. Comment on the paper by K. Udaya Lakshmi et al., J. Cryst. Growth 311 (2009) 2484–2489, Journal of Crystal Growth 311 (2009) 4538.
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[28] M. Dhavamurthy, G. Peramaiyan, M. NizamMohideen, S. Kalainathan, R. Mohan, Structural, growth and optical characterizations of an organic third-
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order nonlinear crystal: Guanidinium trichloroacetate, Journal of Nonlinear Optical Physics and Materials 24 (2015) 1550045-1-1550045-14. [29] Mohd. Shakir, V. Ganesh, M.A. Wahab, G. Bhagavannarayana, K. Kishan Rao, Structural, optical and mechanical studies on pure and Mn2+ doped lasparagine monohydrate single crystals, Materials Science and Engineering B 172 (2010) 9-14.
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[30] K. Russel Raj, P. Murugakoothan, Growth and physical properties of a new crystal
for
NLO
applications:
Bisguanidinium
hydrogen
phosphate
monohydrate (G2HP), Journal of Crystal Growth 362 (2013) 130-134. [31] M. Priya, C.K. Mahadevan, Studies on multiphased mixed crystals of
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NaCl, KCl and KI, Crystal Research & Technology 44 (2009) 92-102.
[32] J.M. Kavitha, C.K. Mahadevan, Growth and characterization of NixZn1xSO4.7H2O
single crystals, Spectrochimica Acta Part A: Molecular and
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Biomolecular Spectroscopy 128 (2014) 342-350.
[33] M. Meena, C.K. Mahadevan, Growth and characterization of L-arginine
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added KDP and ADP single crystals, Crystal Research & Technology 43 (2008) 166-172.
[34] O.V. Mary Sheeja, C.K. Mahadevan, Effect of ZnS as an impurity on the physical properties of KDP single crystals, International Journal of Engineering
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Research and Applications 4(1:2) (2014) 55-65.
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Figure captions Fig. 1. Photograph of sample SF crystal grown Fig. 2. Etch patterns for the grown SF crystal on the large surface area
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Fig. 3. PXRD pattern of the SF crystal obtained in the present study Fig. 4. FTIR spectrum observed for the grown SF crystal
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Fig. 5. Optical transmittance spectrum observed for the grown SF crystal
Fig. 6 (a) Closed aperture Z-scan curve for aqueous solution of SF crystal
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Fig. 6 (b) Open aperture Z-scan curve for aqueous solution of SF crystal Fig. 6 (c) Pure nonlinearity curve for aqueous solution of SF crystal Fig. 7 (a). TG/DTG curves observed for the grown SF crystal
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Fig. 7 (b). Decomposition steps for the SF crystal
Fig. 8 (a). Variation of hardness as a function of load for the grown SF crystal Fig. 8 (b). Plot between log d and log P for the grown SF crystal
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Fig. 9. Variation of mechanical parameters with load: (a) Fracture toughness;
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(b) Brittleness; (c) Yield strength; and (d) Elastic stiffness constant Fig. 10. Variation of ɛr with frequency and temperature Fig. 11. Variation of tan δ with frequency and temperature Fig. 12. Variation of σac with frequency and temperature Fig. 13. Variation of σdc with temperature Fig. 14. The activation energies (Eac and Edc)
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Table captions Table 1. Lattice parameters for the SF crystal estimated from PXRD data compared with those available in the literature. Table 2. Absorption edge, SHG efficiency, thermal stability and work hardening
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coefficient of SF crystal compared with those available in the literature for some
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metal and amino acid formates.
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Table 1. Lattice parameters for the SF crystal estimated from PXRD data
Lattice
Present
From
parameters
work
JCPDS file literature [4]
a (Å)
6.252
b (Å)
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compared with those available in the literature.
From
6.257
6.259
6.743
6.753
6.757
6.166
6.166
6.172
90
90
90
β (˚)
116.2
116.2
116.14
γ (˚)
90
90
90
Volume (Å3)
233.2
-
-
System
Monoclinic Monoclinic Monoclinic
Space group
C2/c
c (Å)
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α (˚)
C2/c
C2/c
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Table 2. Absorption edge, SHG efficiency, thermal stability and work hardening coefficient of SF crystal compared with those available in the literature for some metal and amino acid formates. Work
efficiency
stability
hardening
(in KDP unit)
(˚C)
coefficient (n)
225
0
318
2.68
240
0.9
edge (nm)
Sodium formate
Strontium formate [19] l-threonine formate [9] l-alanine formate [13,14] l-arginine formate [16]
246
205
233
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l-arginium
220
formate [17]
307
70
2.6
0.48
72
2.73
1.21
234
2.108
0.75
234
1.7
-
180
1.09
-
220
1.17
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formate [18]
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Lithium
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Thermal
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Crystal
SHG
Absorption
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Highlights: • Anhydrous sodium formate single crystals with good quality grown. • Optical, thermal and mechanical properties reported for the first time. • Crystal
exhibits
centrosymmetric
and
good
optical
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transmittance.
structure
• Exhibits high thermal stability and decomposition steps understood.
• Mechanical parameters like Hv, Kc, B, σv and C11 determined and
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reported.
S1567-1739(16)30152-3
DOI:
10.1016/j.cap.2016.06.006
Reference:
CAP 4257
To appear in:
Current Applied Physics
Received Date: 13 January 2016 Revised Date:
13 April 2016
Accepted Date: 9 June 2016
Please cite this article as: C. Amuthambigai, C.K. Mahadevan, X. Sahaya Shajan, Growth, optical, thermal, mechanical and electrical properties of anhydrous sodium formate single crystals, Current Applied Physics (2016), doi: 10.1016/j.cap.2016.06.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Growth, optical, thermal, mechanical and electrical properties of anhydrous sodium formate single crystals C. Amuthambigai, C.K. Mahadevan, X. Sahaya Shajan* Centre for Scientific and Applied Research, PSN College of Engineering and
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Technology, Tirunelveli-626152, Tamilnadu, India Abstract
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Anhydrous sodium formate single crystals were grown by the solvent evaporation method at 40 ˚C. Etching study was carried out to assess the
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crystalline perfection. X-ray diffraction and FTIR spectral analyses were performed for the identification of the material and the crystal structure. Suitability of the crystal for photonic applications was studied by the optical transmittance, SHG efficiency and Z-scan measurements. The thermal and mechanical stabilities of the grown crystal were also investigated. Thermal
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decomposition and load dependence of various mechanical parameters viz. Hv, Kc, B, σv and C11 have been understood. AC (with various frequencies ranging from 20 Hz to 200 kHz) and DC electrical measurements were carried out at
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various temperatures ranging from 30 to 150 ˚C. Temperature and frequency dependences of important electrical parameters have been understood. Results
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obtained indicate that the grown crystal exhibits higher crystallinity, good optical transmittance, third order optical nonlinearity, good thermal stability (up to 318 ˚C), higher dielectric constant and normal mechanical behaviour. Keywords: Semi-organic crystal, Solution growth method, Spectral analysis, Optical properties, Thermal stability, Microhardness. *Corresponding author Phone: +919443128713 Email: [email protected], [email protected]
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1. Introduction The metal formate crystals have gained much interest due to their potential applications and interesting physical properties [1]. The formates of group I and II metals in periodic system exhibit strong nonlinear optical
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properties [2]. Crystal structures have been reported for several metal formates such as lithium formate [3], sodium formate [4], ammonium formate [5], strontium formate [6], barium formate [6], calcium formate [6], cupric formate [7] and gadolinium formate [8]. However, reports on physico-chemical
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properties are limited. Studies on amino acid formates such as l-threonine formate [9-12], l-alanine formate [13, 14], l-valinium formate [15] l-arginine
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formate [16] and l-argininium formate [17] have also been reported. Among the metal formate crystals, lithium formate [18] and strontium formate [19] crystals have been studied, to some extent. However, the literature shows very little about the other metal formate crystals.
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Among all the alkali metals, sodium has been widely used due to its higher charge density. In semi-organic crystals, sodium has the ability to combine with organic ligands. Sodium formate has been used in many
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industries and its aqueous solutions are used to absorb SO2 in thermoelectric power plants. The structure determination and vibrational spectral analysis for
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sodium formate crystal have already been reported in the literature [20-22]. Crystals of sodium formate have monoclinic-holohedral symmetry with the centro-symmetric space group C2/c. The crystal has four molecules in a unit cell and each sodium atom is linked to six oxygen atoms belonging to five different formate radicals. The formate anion lies on a twofold axis and has C2v symmetry. The density of this crystal is 1.91-1.93 g/cc. Sodium formate (hydrous/anhydrous) is a representative of hydrogen bonded crystals and the crystal is expected to be highly useful for device
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applications. In hydrous sodium formate, the water molecule may increase the elastic and NLO properties but reduces the thermal stability of the crystal. But, it is essential for the crystal to possess reasonably good thermal stability for their suitability in practical applications. So, we have made an attempt in the
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present study to grow single crystals of anhydrous sodium formate for device applications. In the present study, we have grown the anhydrous sodium formate (SF) single crystals by using the simple solvent evaporation method and characterized chemically, structurally, optically, thermally, mechanically and
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electrically. The results obtained are reported and discussed herein.
2.1.
Material synthesis
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2. Experimental
Sodium formate was synthesized by mixing high purity sodium hydroxide (obtained from Merck) and formic acid (obtained from Rankem) taken in equimolar ratio and dissolving completely in millipore water of
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resistivity 18.2 MΩcm at room temperature (30 ˚C) using a magnetic stirrer. The purity of the crystalline product was further improved by repeated recrystallization process. The sodium formate formation can be represented by
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the following chemical reaction:
2.2.
NaHCOO + H2O
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NaOH + CH2O2
Crystal growth
Solvent evaporation solution growth technique was employed to grow
single crystals of sodium formate. Saturated solution of sodium formate was prepared and allowed to equilibrate in a beaker (crystal growth cell). The cell was covered with a perforated sheet and kept undisturbed in a constant temperature bath maintained at 40 ˚C to avoid forming the dihydrate [22]. After a period of one month, evaporation of the solvent gives thin transparent platelike crystals with the maximum size of 14×9×1 mm3. The grown crystals were
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harvested and used for the characterization experiments. Two-dimensional nucleation may be considered as the mechanism behind the formation of single crystals. 2.3.
Characterization
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In order to understand the surface features of the grown crystals, good quality crystals (macroscopically free from defects) with flat and smooth surface of thickness 0.7 mm were selected and subjected to etching study with
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water as the etchant. The crystal was dipped in the etchant from 10 to 60 s with an interval of 10 s and then wiped with a tissue paper. The etch patterns were
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observed and photographed with an Olymbus-BX51M optical microscope in the reflection mode. Powder X-ray diffraction (PXRD) analysis of the crystal was carried out using a PANalytical EMPYREAN diffractometer with Cu Kα radiation of wavelength 1.5405 Å. The data were collected in the 2θ range of 5 to 80 ˚ with a step size of 0.026 ˚. The Fourier transform infrared (FTIR)
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spectrum was recorded for the as-grown crystal using a JASCO FT/IR-4100 spectrophotometer by the ATR technique at room temperature (30 ˚C) in the wave number region 4000-550 cm-1.
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The linear, second order nonlinear and third order nonlinear optical properties of the grown SF crystal were understood by measuring its optical
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transmittance in the UV-Vis-NIR region (200-1200 nm), second harmonic generation (SHG) efficiency and Z-scan measurements respectively. The optical transmittance of the crystal was measured using a Shimadzu UV-2600 UV-VisNIR spectrometer by placing the crystal of thickness 0.7 mm directly on the path of the source. The Kurtz and Perry powder method [23] was employed to measure the SHG efficiency. The input Q-switched Nd-YAG laser beam of wavelength 1064 nm with energy 1 mJ/pulse, pulse width 10 ns and repetition rate 10 Hz was made to fall on the powder form of the crystal filled in the micro
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capillary tube. The second harmonic output (emission of green light) generated by the crystal was detected by the photomultiplier tube. The third-order nonlinear refractive index (n2) and absorption coefficient (β) were evaluated by using the single beam Z–scan technique [24, 25]. The laser beam from 635 nm
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continuous wave (CW) diode laser was focused by a convex lens with focal length of 5 cm, which produces a beam waist ωo of 23.57 µm and the Raleigh length, ZR of 2.7 mm in the sample. A 1 mm thickness of cuvette containing the aqueous solution (0.4 g dissolved in 5 ml water) of SF crystal placed on a
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translational stage was moved across the focal region (+Z to –Z) along the axial direction, which is the direction of propagation of laser beam.
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The thermogravimetric (TG/DTG) analysis was performed on the SF crystal in powder form from 24 to 700 ˚C with a heating rate of 10 ˚C/min using a TGA Q500 V6.7 Build 203 instrument with pin-holed platinum crucible under nitrogen atmosphere. The mechanical stability analysis was done by a Shimadzu
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HMV-2T Vickers microhardness tester with diamond pyramidal indenter at room temperature. The crystals were selected by the same procedure followed for the etching studies and it was indented by applying a load varying from 25
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to 100 g with a dwell time of 5 s on the large surface area of the crystal. The AC electrical (dielectric) measurements were carried out by the
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parallel plate capacitor method with an accuracy of ± 2 % by varying the frequency from 200 Hz to 200 kHz at various temperatures ranging from 30 to 150 ˚C using a HIOKI IM3528 LCR meter. The DC electrical measurements were also made to an accuracy of ± 2 % at various temperatures ranging from 30 to 150 ˚C by the conventional two-probe method using the same LCR meter. In both the cases, the temperature was controlled to an accuracy of ± 0.01 ˚C and the measurements were made while cooling the sample crystal. An as grown (fresh) crystal was polished with an emery paper to get a perfect shape and the dimensions were measured using a travelling microscope (least count =
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0.001 mm). Opposite faces of the crystal were coated with good quality silver paste to obtain a good conductive surface layer. The measurements were carried out along the direction perpendicular to the large area surfaces of the crystal. Dielectric loss factor (tan δ) and capacitances with the crystal (Ccrys) and
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without the crystal (Cair) were considered in the case of dielectric measurements and resistance (R) was considered in the case of DC electrical measurement. The other AC and DC electrical parameters were determined using the above measured values.
General features
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3.1.
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3. Results and Discussion
Figure 1 shows the photograph of a sample SF crystal grown in the present study. The crystals grown are found to be well shaped, colourless and transparent. However, it exhibits some hygroscopic nature and it is not much stable in open atmosphere. This is in agreement with that reported by the earlier
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authors [20-22].
Etching analysis is the simplest technique to understand the growth mechanism and to access the quality of crystals. The etch patterns of the SF
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crystal grown in the present study on the large surface area are shown in Figure 2. When etched for 10 s, the image shows that there is a presence of tiny
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inclusions in random manner. For the period of 20 s, the presence of inclusions is small and there is a presence of hillocks. The density of inclusions is lower than the density of hillocks. By increasing the etchant time for 30 s, there is a formation of large size hillocks with lesser density. For the times of 40 and 50 s of etchant, the step pattern is formed. For the maximum time of 60 s, the surface of the crystal looks smooth with the decrease of inclusions and hillocks. This indicates that etching with water for 60 s is required to make the surface of the crystal smooth.
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3.2.
Chemical composition and structure The PXRD pattern observed in the present study is shown in Figure 3. As
this pattern is found to be in good agreement with the data available in the JCPDS file, the JCPDS data (card no. 14-0812) were used to index the present
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data. The PXRD analysis shows that the SF crystal system is monoclinic with the space group C2/c (Schoenflies notation C62h) and point group 2/m. The sharp peaks observed in the PXRD pattern show that the crystal is highly crystalline in nature. The lattice parameters were calculated using a Cell
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refinement software package and are provided in Table 1. They are found to
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match well with those reported in the literature [4].
The FTIR spectrum recorded in the present study is shown in Figure 4. The bands appearing around 2800 cm-1 can be attributed to the C-H vibrations. The bands at 2952 and 2826 cm-1 are respectively due to C-H asymmetric and symmetric vibrations. The C-H bending vibration appears at 2713 cm-1. The
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strong bands appearing at 1587 and 1352 cm-1 are due to C-O asymmetric and symmetric stretchings respectively. The peak at 767 cm-1 is attributed to C-O symmetric bending. Absence of any peak appearing at around 3400 cm-1
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indicates that the crystal grown is without any water molecules. This spectrum is found to be in good agreement with those reported in the literature [21, 22].
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Thus, the results obtained through PXRD and FTIR spectral analyses indicate that the SF crystal grown in the present study is of good crystalline nature and of anhydrous sodium formate. 3.3.
Optical properties The optical transmittance spectrum recorded in the present study is shown
in Figure. 5. The transmittance is found to be more than 54 % in the entire visible and NIR regions with an absorption edge at 225 nm. The absorption edges (AE) reported for various metal and amino acid formates are compared in
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Table 2. It can be seen that the AE observed for SF crystal is more than that reported for l-alanine formate and strontium formate but less than that reported for lithium formate, l-threonine formate, l-arginine formate and l-arginium formate.
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The SHG measurement shows no green light emission from the sample. It indicates that the grown SF crystal has zero second order susceptibility coefficient. This result is in good agreement with the PXRD, since the centrosymmetric space group crystals do not have SHG efficiency [26, 27]. The SHG
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efficiencies reported for various metal and amino acid formates are compared in Table 2. Among these, l-threonine and strontium formate exhibit respectively
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the maximum and minimum SHG efficiencies.
In closed aperture Z–scan, the intensity of the transmitted beam through an aperture (Z = 0.40) was measured by a photo detector fed to the digital laser power meter. When the sample is placed far away from the focus (–Z) the beam
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irradiance is low, while the sample is moved towards the focus, the irradiance of the beam increases leading to self-lensing effect. The normalized transmittance curve is characterized by a pre-focal transmittance maximum followed by a
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post-focal transmittance minimum as shown in Figure 6(a), which implies that the sign of the nonlinear refractive index of SF crystal is negative, i.e., self-
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defocusing. The self-defocusing effect is attributed to a thermal nonlinearity resulting from the absorption of laser radiation at 635 nm. The difference in peak and valley transmission (∆Tp-v) can be written in
terms of the on-axis phase shift ∆φ at the focus as [28] ∆Tp–v ≅ 0.406(1–S) 0.25 ∆φ Here ∆φ =
n I L
with L
=
(1)
and S = 1–exp(–2ra2/ωa2) is the
aperture linear transmittance with ra denoting the aperture radius and ωa
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denoting the beam radius at the aperture in the linear regime. Also, λ is the laser wavelength, Io is the intensity of the laser beam at focus Z = 0, α is the linear absorption coefficient and L is the thickness of the sample.
n =
∆
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Then nonlinear refractive index (n2) is given by [28] (2)
Figure 6(b) shows the open aperture (S = 1) Z–scan setup for SF crystal.
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The nonlinear absorption coefficient β can be measured where the aperture is removed and hence the entire transmittance of the beam is collected through a
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suitable lens. It is clearly seen in Figure 6(b) that the transmittance at the focus decreases which is the characteristic signature of Reverse Saturable Absorption (RSA).
The nonlinear absorption coefficient β can be estimated from the open
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√∆
(3)
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The closed aperture transmittance is affected by the nonlinear refraction and absorption, the determination of n2 is less straight forward from the closed
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aperture scans. It is necessary to separate the effect of nonlinear refraction from nonlinear absorption. A simple and approximate method to obtain purely effective n2 is to divide the closed aperture transmittance by the corresponding open aperture transmittance and Figure 6(c) shows one such data. The real and imaginary parts of the third-order nonlinear optical susceptibility χ (3) are defined as Reχ(!) =
#$ (% &' (') (' ) !.+
(e. s. u)
(4)
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Imχ(!) =
#' (% &' (' 0) +'
(e. s. u)
χ(!) = 1(Reχ(!) ) + (Imχ(!) ) (e. s. u)
(5) (6)
and c is the velocity of light in vacuum.
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Here ε0 is the vacuum permittivity, no is the linear refractive index of the sample The calculated values of nonlinear refractive index n2, nonlinear absorption co-efficient β, and third-order nonlinear susceptibility χ (3) for the SF
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crystal are found to be – 0.55×10─6 (cm2/W), 2.06×10–2 (cm/W) and 4.6×10─6 (e.s.u) respectively. Thus, the SF crystal is found to exhibit third order optical
3.4.
Thermal properties
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nonlinearity and is expected to be useful in photonic devices.
The TG/DTG curves observed for the SF crystal is shown in Figure 7(a). It shows that the SF crystal is thermally stable up to 318 ˚C without any decomposition. The decomposition takes place in two steps. In the first step,
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with a weight loss of 18 % liberation of H2 molecules takes place in the temperature range 318.4 - 410 ˚C leading to the formation of sodium oxalate. In the second step, sodium oxalate decomposes in the temperature range 503 - 543
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˚C leading to the formation of sodium carbonate by the liberation of carbon monoxide with a weight loss of 8 %. In the corresponding DTG curve, during
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the first step, two peaks are formed: one for the elimination of hydrogen molecules and the other one for the phase transformation. During the second step, one peak is formed. So, the DTG curve confirms the above two decompositions. The decomposition steps are shown in Figure 7(b). It is also noticed that there is no adsorbed and absorbed water molecules present in the crystal. The TG and DTG curves show that the transformations are associated with mass changes and physical transformation which is independent of mass change or decomposition of the material. The thermal stabilities reported for
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various metal and amino acid formates are compared in Table 2. It can be seen that the thermal stability is maximum for sodium formate and minimum for lithium formate. 3.5.
Mechanical properties
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The micro-indentation test is a non-destructive method for studying the nature of plastic flow and its influence on the deformation of materials. It gives details about the Vickers hardness value, work hardening coefficient, crack length, yield strength, fracture toughness, brittle index and elastic stiffness
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constant. Hardness value depends on the size of the indenter impression. The
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Vickers hardness value was calculated using the formula [29]: H4 =
`
.56++×8 9'
:
;<
=='
>
(7)
Here, P is the applied load in kg, d is the average diagonal length of the
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indentation mark made by the Vickers indenter in mm. The dependence of microhardness with applied load does not show same type of behaviour for all materials. It may be (a) independent of load (b) function of load (c) complex
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variation with increasing load. The hardness value as a function of load observed for the SF crystal in the present study is displayed in Figure 8(a). The
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Hv value increases with the increase in load. The higher hardness value of the crystal indicates that greater stress is required to form dislocation. Both the PXRD and hardness data confirm that the grown SF crystal has greater crystalline perfection. The higher hardness value may be due to the strong interaction of C=O group in the SF crystal, since the TGA result shows that the C-H gets easily broken. The load-hardness variation can be interpreted using Meyer’s relation: P=K1dn, where P is the applied load, d is the diagonal length of the indentation, K1 is a constant and n is the Mayer’s constant or work hardening coefficient.
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From the best fitted slope of log P versus log d plot (shown in Figure 8(b)), n value was estimated. The value of n is found to be 2.68 which indicate that the grown SF crystal belongs to soft material category. Also, as per the theory, the results of microhardness measurement follow the normal indentation size effect
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trend. The n values reported for various metal and amino acid formates are compared in Table 2. It can be understood that l-arginine formate and larginium formate crystals belong to the hard material category whereas the others belong to the soft material category.
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Measurement of crack length (c) along with d leads to the estimation of various mechanical parameters like fracture toughness (Kc), brittleness (B),
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yield strength (σv) and elastic stiffness constant (C11).
Fracture toughness (Kc) is the ability of a material containing a crack to resist fracture and it is given by [29]:
8
A β@ B'
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K@ =
for c ≥
9
(8)
Here, β is the indenter constant (value is 7) and c is the crack length (distance between the center of indentation mark to crack tip). The mechanical contact
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between the indenter and the crystal surface produces radial cracks which can
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be determined from the crack length. Toughness determines how much fracture stress is applied under uniform
bending and it is the important parameter for the selection of materials in practical applications where the load exceeds the limit. The variation of crack length and fracture toughness on load can be attributed to the depth of penetration of indenter into surface. Brittleness (B) determines its fracture without any appreciable deformation. This property helps to understand its laser damage tolerance and it is resolved by the relation [29]:
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B=
(9)
KL
The yield strength (σv) is the stress at which the material begins to deform [30]: σ4 =
IJ
.M
N1 − (2 − n)R S
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plastically and it can be computed from the hardness value using the relation .6(() ( (()
T
(10)
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By the Wooster’s empirical formula, the elastic stiffness constant is the tightness of bonding between neighbouring atoms. It is the property of material by the equation C11=Hv7/4 [30].
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by virtue of which can absorb energy before fracture occurs and it is calculated
Using these relations, the above mechanical parameters of SF crystal have been estimated from the observed hardness data. The load dependences of
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these parameters are shown in Figure 9. The Kc, σv and C11 values increase whereas the B value decreases with the increase in load. This is considered to be a normal mechanical behaviour. Electrical properties
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3.6.
Measurement of AC electrical parameters (dielectric constant (ɛr),
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dielectric loss factor (tan δ) and AC electrical conductivity (σac)) as function of frequency and temperature of a crystal is of much interest to get knowledge about its electrical properties. Crystals with higher dielectric constants are considered to be useful in capacitor technology. The dielectric constant was determined, as the crystal surface area was smaller than the plate area of the cell, by using Mahadevan’s relation [31, 32]: εU =
\LVWX
&LVWX &YZV [ &YZV
\YZV
]
(
^_`a
^bacd
)
(11)
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Here, Acrys is the area of the crystal touching the electrode and Aair is the area of the electrode. The AC and DC electrical conductivities (σac and σdc respectively) were determined using the relations [31]: σac = ɛoɛrωtanδ f
h^bacd
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efg =
(12) (13)
Here, ɛo is the permittivity of free space, ω (=2πf) is the angular frequency of
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the applied field and d is the thickness of the sample.
The variation of ɛr, tan δ and σac with frequency and temperature are
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shown in Figures 10 - 12. Figure 13 shows the variation of σdc with temperature. It is found that the ɛr and tan δ values decrease whereas the σac value increases with the increase in frequency of the applied field (f). All the four parameters, viz. ɛr, tan δ, σac and σdc are found to increase with the increase in temperature. Also, it can be seen that the AC conductivities are more than the DC
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conductivities. These results indicate that the SF crystal grown in the present study exhibits a normal dielectric behaviour. The higher dielectric constants observed at low frequencies can be
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attributed to the presence of all types of polarizations, viz. electronic, ionic,
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orientational and space charge. In a dielectric crystal, the space charge contribution at lower temperature and higher frequency is expected to be small and negligible. The electronic and ionic displacements under an applied electric field contribute to the electronic and ionic polarizations respectively. In ionic crystals, due to the large forbidden energy gap, the charge transported by electrons is zero [32]. Moreover, the electronic polarizability practically remains constant [31]. It can be seen that the high frequency and low frequency permittivities significantly differ and the ionic polarization is found to be very large. In hydrogen bonded dielectric crystals like SF, large amount of proton
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transport (leading to large ionic polarization) is expected. Moreover, increase in temperature leads to the possibility of weakening the hydrogen bonding system due to rotation of the formate ions. This is expected to generate vacant hydrogen bonds (L-defects) at higher temperature. So, the increase in dielectric constant
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with temperature is essentially due to the temperature variation of ionic polarizability. It can be seen that the normal dielectric behaviour exhibited by the SF crystal, considered in the present study, can be explained by the above features.
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Electrical conduction in hydrogen bonded dielectric crystals like Sr(HCOO)2.2H2O, KH2PO4, NH4H2PO4 and NixZn1-xSO4.7H2O has been
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attributed to the proton transport within the framework of hydrogen bonds [19, 32, 33]. The electrical conduction in SF crystal considered in the present study can also be understood as protonic and mainly due to the formate ions. The temperature dependence of conductivity can be understood as due to the
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temperature dependence of the proton transport. It can also be seen that the conductivity increases smoothly through the temperature range considered. The AC and DC activation energies (Eac and Edc respectively) were
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relations [31]:
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determined by fitting the σac and σdc values into the respective Arrhenius
eig = ejig exp[
n_b
]
(14)
efg = ejfg exp[
nrb
]
(15)
op
op
Here, σoac and σodc are the constants depending on the material, k is the Boltzman constant and T is the absolute temperature. Plots between ln σac and 1000/T and ln σdc and 1000/T were made and they (not shown here) are found to be nearly linear. This indicates that the conductivity values are well fitting into the Arrhenius relation. The plots were best fitted using the least squares
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principle and the activation energies were estimated. The estimated Eac and Edc (for which f=0) values are shown in Figure 14. It can be seen that, as expected, the Eac values are less than the Edc value. Moreover, the Eac and Edc values are found to be low. This suggests that oxygen vacancies (consequently generating
region considered in the present study [34].
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L-defects) may be responsible for electrical conduction in the temperature
The observed ɛr value varies from 33 – 2537 for the frequencies and temperatures considered in the present study. This indicates that the SF crystal
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exhibits higher ɛr values and it may find an important place in the capacitor
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technology. 4. Conclusion
Optically transparent, colourless and good quality (assessed by the etching study) single crystals of anhydrous sodium formate (SF) have been grown by the solvent evaporation method at 40 ˚C. The sharp and high intensity
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peaks observed through PXRD analysis confirm the crystallinity of the crystal and it crystallizes in the monoclinic system with the centro-symmetric space group C2/c. The functional groups present in the SF crystal were identified and
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assigned by the FTIR spectral analysis. The SF crystal shows transmittance more than 54 % in the visible and NIR regions and a lower absorption edge
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(225 nm). The SF crystal shows third order optical nonlinearity and is expected to be useful in photonic devices. It shows no SHG efficiency which is in good agreement with the PXRD data, since the centro-symmetric crystal doesn’t show SHG efficiency. The thermal analysis shows that the SF crystal is thermally stable up to 318 ˚C. The microhardness measurement shows that the crystal follows normal indentation size effect and it belongs to the soft material category. Various mechanical parameters have been estimated which indicate that the SF crystal grown exhibit a normal mechanical behaviour. DC and AC
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electrical measurements indicate that the electrical conductivity is due to the proton transport. The observed higher ɛr values indicate that the SF crystal grown in the present study may find an important place in the capacitor technology.
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Acknowledgement
One of the authors (C. Amuthambigai) acknowledges the PSN management for the support under Institute Research Stipend (IRS) scheme and
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also thanks Research and Development Centre, Orchid Pharma Ltd, Chennai for the thermal studies. The authors are happy to thank Dr. S. Natarajan,
facilities. References
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Framework Solids Laboratory, IISc, Bangalore, for providing powder XRD
[1] Andres Goeta, Ricarda Baggio, Donka Stoilova, Synthesis, crystal structure and
infrared
spectroscopy
of
a
novel
copper
strontium
formate
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CuSr(HCOO)4.(H2O)2, Vibrational Spectroscopy, 34 (2004) 293-300. [2] T. Feliksinski, R. Grouhulski, W. Kolasinski, Z. Wawrzak, Growth and some properties of Barium Cadmium formate single crystals: (II) X-ray and
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Infrared studies, Materials Research Bulletin 18 (1983) 27-32.
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[3] A. Enders Beumer, S. Harkema, The crystal structure of lithium formate monohydrate, Acta Crystallographica B29 (1973) 682-685. [4] Peter L. Markila, Steven J. Rettig, James Trotter, Sodium formate, Acta Crystallographica B31 (1975) 2927-2928. [5] I. Nahringbauer, Hydrogen bond studies. XX. The crystal structure of ammonium formate, Acta Crystallographica B24 (1968) 565-570.
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[6] T. Watanabe, M. Matsui, The redetermination of the crystal structures of αcalcium formate, α-strontium formate and barium formate by X-ray analysis, Acta Crystallographica B34 (1978) 2731-2736. [7] R. Kiriyama, H. Ibamoto, K. Matsuo, The crystal structure of cupric formate
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tetrahydrate, Cu(HCO2)2.4H2O, Acta Crystallographica 7 (1954) 482-483.
[8] A. Pabst, Crystal structure of gadolinium formate, Gd(OOCH)3, The Journal of Chemical Physics 11 (1943) 145.
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[9] Redrothu Hanumantharoa, S. Kalainathan, Growth, spectroscopy, dielectric and nonlinear optical studies of novel organic NLO crystal: l-threonine formate,
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 94 (2012) 78-83.
[10] Redrothu Hanumantharoa, S. Kalainathan, Microhardness, chemical etching, SEM, AFM and SHG studies of novel nonlinear optical crystal-l-
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threonine formate, Materials Research Bulletin 47 (2012) 987-992. [11] Redrothu Hanumantharoa, S. Kalainathan, Growth, HR-XRD, optical, thermal, luminescence, nonlinear optical studies of novel organic nonlinear
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optical crystal: l-threonine formate, Optik 124 (2013) 3718-3722. [12] Xiaojing Liu, Yan Su, Haikun Zhang, Gang Chen, Theoretical calculations
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and surface morphology studies of l-threonine formate, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 389-393. [13] C. Justin Raj, S. Jerome Das, Growth and characterization of nonlinear optical active l-alanine formate crystal by modified SankaranarayananRamasamy (SR) method, Journal of Crystal Growth 304 (2007) 191-195. [14] C. Justin Raj, S. Dinakaran, S. Krishnan, B. Milton Boaz, R. Robert, S. Jerome Das, Studies on optical, mechanical and transport properties of NLO
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active l-alanine formate single crystals grown by modified SankaranarayananRamasamy (SR) method, Optics Communications 281 (2008) 2285-2290. [15] T. Joselin Beaula, D. Manimaran, I. Hubert Joe, V.K. Rastogi, V. Bena Jothy, Vibrational, spectroscopic studies and DFT computation of the nonlinear
and Biomolecular Spectroscopy 126 (2014) 170-177.
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optical molecule L-Valinium formate, Spectrochimica Acta Part A: Molecular
[16] S. Haussuhl, H.A. Karapetyan, R.P. Sukiasyan, A.M. Petrosyan, Structure
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and properties of orthorhombic L-arginine formate, Crystal Growth and Design 6 (2006) 2041-2046.
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[17] J. Packium Julius, A. Joseph Arul Pragasam, S.A. Rajasekar, S. Selva Kumar, A. Stephen, P. Sagayaraj, Studies on the growth and characterization of l-argininium formate single crystals, Journal of Crystal Growth 267 (2004) 619623.
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[18] D. Joseph Daniel, P. Ramasamy, Studies on some-semi-organic non linear optical single crystal: Lithium formate monohydrate (HCO2Li.H2O), Optical Materials 36 (2014) 971-976.
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[19] S. Muthupoongodi, S. Theodore David Manickam, C.K. Mahadevan, J. Angel Mary Greena, S. Balakumar, X. Sahaya Shajan, Effect of glycine
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addition on the structural, thermal, optical, mechanical and electrical properties of Sr(HCOO)2.2H2O crystals, Journal of Crystal Growth 428 (2015) 46-53. [20] W.H. Zachariasen, The crystal structure of sodium formate, NaHCO2, Journal of American Chemical Society 62 (1940) 1011-1013. [21] Roger Newman, Polarized infrared spectrum of sodium formate, The Journal of Chemical Physics 20 (1952) 1663-1664.
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[22] T.L. Charlton, K.B. Harvey, Infrared Absorption of single crystals of Anhydrous Sodium Formate, Canadian Journal of Chemistry 44 (1966) 27172727. [23] S. K. Kurtz, T.T. Perry, A Powder Technique for the Evaluation of
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Nonlinear Optical Materials, Journal of Applied Physics 39 (1968) 3798-3813. [24] M. Sheik- Bahae, A.A. Said, E.W. Van Stryland, High sensitivity single beam measurements, Optical Letters 14 (1989) 955-957.
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[25] M. Sheik- Bahae, A.A. Said, T. Wei, D.J. Hagan, E.W. Van Stryland, Sensitive measurement of optical nonlinearities using a single beam, IEEE
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Journal of Quantum Electronics QE 26 (1990) 760-769.
[26] A. Karolin, K. Jayakumari, C.K. Mahadevan, Growth and characterization of Pure and Ni2+ Added Crystals of Glycinepotassiumsulfate, International Journal of Engineering Research and Applications 3(12) (2013) 1906-1915.
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[27] Aram M. Petrosyan, Second harmonic generation in centrosymmetric crystal. Comment on the paper by K. Udaya Lakshmi et al., J. Cryst. Growth 311 (2009) 2484–2489, Journal of Crystal Growth 311 (2009) 4538.
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[28] M. Dhavamurthy, G. Peramaiyan, M. NizamMohideen, S. Kalainathan, R. Mohan, Structural, growth and optical characterizations of an organic third-
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order nonlinear crystal: Guanidinium trichloroacetate, Journal of Nonlinear Optical Physics and Materials 24 (2015) 1550045-1-1550045-14. [29] Mohd. Shakir, V. Ganesh, M.A. Wahab, G. Bhagavannarayana, K. Kishan Rao, Structural, optical and mechanical studies on pure and Mn2+ doped lasparagine monohydrate single crystals, Materials Science and Engineering B 172 (2010) 9-14.
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[30] K. Russel Raj, P. Murugakoothan, Growth and physical properties of a new crystal
for
NLO
applications:
Bisguanidinium
hydrogen
phosphate
monohydrate (G2HP), Journal of Crystal Growth 362 (2013) 130-134. [31] M. Priya, C.K. Mahadevan, Studies on multiphased mixed crystals of
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NaCl, KCl and KI, Crystal Research & Technology 44 (2009) 92-102.
[32] J.M. Kavitha, C.K. Mahadevan, Growth and characterization of NixZn1xSO4.7H2O
single crystals, Spectrochimica Acta Part A: Molecular and
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Biomolecular Spectroscopy 128 (2014) 342-350.
[33] M. Meena, C.K. Mahadevan, Growth and characterization of L-arginine
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added KDP and ADP single crystals, Crystal Research & Technology 43 (2008) 166-172.
[34] O.V. Mary Sheeja, C.K. Mahadevan, Effect of ZnS as an impurity on the physical properties of KDP single crystals, International Journal of Engineering
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Research and Applications 4(1:2) (2014) 55-65.
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Figure captions Fig. 1. Photograph of sample SF crystal grown Fig. 2. Etch patterns for the grown SF crystal on the large surface area
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Fig. 3. PXRD pattern of the SF crystal obtained in the present study Fig. 4. FTIR spectrum observed for the grown SF crystal
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Fig. 5. Optical transmittance spectrum observed for the grown SF crystal
Fig. 6 (a) Closed aperture Z-scan curve for aqueous solution of SF crystal
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Fig. 6 (b) Open aperture Z-scan curve for aqueous solution of SF crystal Fig. 6 (c) Pure nonlinearity curve for aqueous solution of SF crystal Fig. 7 (a). TG/DTG curves observed for the grown SF crystal
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Fig. 7 (b). Decomposition steps for the SF crystal
Fig. 8 (a). Variation of hardness as a function of load for the grown SF crystal Fig. 8 (b). Plot between log d and log P for the grown SF crystal
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Fig. 9. Variation of mechanical parameters with load: (a) Fracture toughness;
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(b) Brittleness; (c) Yield strength; and (d) Elastic stiffness constant Fig. 10. Variation of ɛr with frequency and temperature Fig. 11. Variation of tan δ with frequency and temperature Fig. 12. Variation of σac with frequency and temperature Fig. 13. Variation of σdc with temperature Fig. 14. The activation energies (Eac and Edc)
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Table captions Table 1. Lattice parameters for the SF crystal estimated from PXRD data compared with those available in the literature. Table 2. Absorption edge, SHG efficiency, thermal stability and work hardening
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coefficient of SF crystal compared with those available in the literature for some
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metal and amino acid formates.
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Table 1. Lattice parameters for the SF crystal estimated from PXRD data
Lattice
Present
From
parameters
work
JCPDS file literature [4]
a (Å)
6.252
b (Å)
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compared with those available in the literature.
From
6.257
6.259
6.743
6.753
6.757
6.166
6.166
6.172
90
90
90
β (˚)
116.2
116.2
116.14
γ (˚)
90
90
90
Volume (Å3)
233.2
-
-
System
Monoclinic Monoclinic Monoclinic
Space group
C2/c
c (Å)
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α (˚)
C2/c
C2/c
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Table 2. Absorption edge, SHG efficiency, thermal stability and work hardening coefficient of SF crystal compared with those available in the literature for some metal and amino acid formates. Work
efficiency
stability
hardening
(in KDP unit)
(˚C)
coefficient (n)
225
0
318
2.68
240
0.9
edge (nm)
Sodium formate
Strontium formate [19] l-threonine formate [9] l-alanine formate [13,14] l-arginine formate [16]
246
205
233
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l-arginium
220
formate [17]
307
70
2.6
0.48
72
2.73
1.21
234
2.108
0.75
234
1.7
-
180
1.09
-
220
1.17
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formate [18]
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Lithium
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SHG
Absorption
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Highlights: • Anhydrous sodium formate single crystals with good quality grown. • Optical, thermal and mechanical properties reported for the first time. • Crystal
exhibits
centrosymmetric
and
good
optical
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transmittance.
structure
• Exhibits high thermal stability and decomposition steps understood.
• Mechanical parameters like Hv, Kc, B, σv and C11 determined and
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reported.