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Synthesis, growth and third order nonlinear optical studies of a rhombohedral crystal: Sodium tetraborate pentahydrate
Accepted Manuscript
Synthesis, growth and third order nonlinear optical studies of a rhombohedral crystal: sodium tetraborate pentahydrate S. Sudha , M. Peer Mohamed , G. Vinitha , C. Rathika Thaya Kumari , P. Sangeetha , M. Lydia Caroline PII: DOI: Reference:
S0577-9073(18)30812-8 https://doi.org/10.1016/j.cjph.2018.11.014 CJPH 708
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
11 June 2018 22 November 2018 27 November 2018
Please cite this article as: S. Sudha , M. Peer Mohamed , G. Vinitha , C. Rathika Thaya Kumari , P. Sangeetha , M. Lydia Caroline , Synthesis, growth and third order nonlinear optical studies of a rhombohedral crystal: sodium tetraborate pentahydrate, Chinese Journal of Physics (2018), doi: https://doi.org/10.1016/j.cjph.2018.11.014
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ACCEPTED MANUSCRIPT Revised Highlights The UV-Visible spectrum of STBPH revealed lower cutoff wavelength of 192 nm. Microhardness study revealed that the crystal belongs to soft material category. The dielectric properties attest its suitability in electro-optic devices. STBPH possess negative nonlinearity proved by Z-scan technique.
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Synthesis, growth and third order nonlinear optical studies of a rhombohedral crystal: sodium tetraborate pentahydrate S. Sudhaa, M. Peer Mohameda,c, G.Vinithad, C. Rathika Thaya Kumaria, P. Sangeethaa, M. Lydia Caroline a,b* aPG & Research Department of Physics, Arignar Anna Govt. Arts College,
b
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Cheyyar - 604 407, Tamil Nadu, India
Department of Physics, Dr. Ambedkar Govt. Arts College, Vyasarpadi, Chennai-600039, Tamil Nadu, India c
Department of Physics, C. Abdul Hakeem College, Melvisharam-632509, Tamil Nadu, India
d
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Department of Physics, School of Advanced Sciences, VIT, Chennai – 600127, Tamil Nadu, India
Abstract
An apt and a futuristic nonlinear optical material sodium tetraborate pentahydrate
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(STBPH) was synthesized and grown by the technique of slow evaporation. The structural
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parameters of the crystal was confirmed by single crystal XRD revealed rhombohedral crystal system. Crystal's broad optical transparency was revealed by UV-Vis absorption
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spectrum. From the absorption spectrum, theoretical calculations were made to determine absorption coefficient, band gap, Urbach energy and reflectance. The functional groups
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were identified by FT-IR and FT-Raman spectra. Vicker's microhardness measurement
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disclosed that the grown crystal belongs to soft material category. Photoluminescence (PL) spectral study of STBPH indicated violet and blue emission peaks respectively at 343.06 nm and at 453.11 nm, 490.19 nm. Dielectric studies reveal the dielectric nature of the grown crystal. TGA-DTA studies showed that the crystal holds good thermal stability. The third order nonlinearities of STBPH was investigated by Z scan technique manifest its suitability in nonlinear optical applications.
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ACCEPTED MANUSCRIPT Key words: Nonlinear optics; Crystal growth; UV-Visible studies; Dielectric studies; ZScan technique. Corresponding author Email address and mobile no: [email protected](M.Lydia Caroline); Tel: 044-25520151: Fax: 044 - 2552 1852:+91 9841720216 Introduction
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1.
There has been an appreciable interest towards the synthesis and development of many borate crystals for the past few years due to their wide use in optical communication, laser medicine and signal processing [1,2]. Borate crystals occupy a crucial position in
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frequency conversion in view of demand for a powerful or tunable laser source in the deep ultraviolet region [3]. Borates show excellent physical properties, such as short growth period, large effective nonlinear coefficient, high laser damage tolerance, piezoelectric,
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luminescence and other useful properties [4]. Comparing to other commonly used NLO materials such as potassium dihydrogen phosphate (KDP) or lithium niobate (LN), borate
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crystals are found to be illustrious to UV studies as their transmittance start from the
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wavelength of 155 nm [5]. Meantime, influenced versatile deep UV nonlinear optical materials have been synthesized by We et al and Yu et al [6, 7] proved them as novel
series
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crystals applicable in laser applications. Also much effort has been made on developing of
remarkable
borate
NLO
crystals
such
as
Potassium
aluminum
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borate(KAB),Potassium pentaborate, strontium tetraborate (SrB4O7), Lanthanaum calcium borate (La2CaB10O19) and barium strontium borate where the studies exhibited high transmittance and hence successfully handled in conversion of laser radiation into the UV wavelength region and vacuum [8-12]. Some borates based candidates also were studied for fourth harmonic generation and fifth harmonic generation of the Nd: YAG lasers (Zhang et al 2002). Possessing chemical stability, high damage threshold and high optical quality as
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ACCEPTED MANUSCRIPT well as wide range of transparency in the ultraviolet region influences a rather large difference in the electro negativity of B and O atoms stand as the basic characteristics of borates [13]. In borate family of crystals, the boron atom usually coordinate with either three or four oxygen atoms forming [BO3] or [BO4] groups [14]. Consequently, the electronic orbitals are hybridized to a planar sp2 or a three-dimensional sp3 structure. Borates which
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hold water molecules in it possess polyanions in their crystal structure [14,15]. Every boron centre has a negative charge that is balanced by various metal cations. One of the basic cation of the borate minerals is Sodium. Eventually Sodium, Potassium and Ammonium Borates are brilliant non-linear optical (NLO) materials and are useful in modern short-
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wavelength laser techniques [16]. The synthesis of title inorganic borate system has been already reported by Estud Geol [17]. But its characterization studies has not been reported yet. Here we detail the synthesis of nonlinear optical crystal sodium tetraborate
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pentahydrate, and its structural characterization by XRD, FT-IR, FT-Raman, microhardness, dielectric and thermogravimetric analysis. The STBPH crystal was confirmed as third
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harmonic crystal by closed and open aperture Z-scan signatures from which optical
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nonlinearities were evaluated. 2. Experimental
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2.1. Material Synthesis
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Sodium tetraborate pentahydrate (STBPH) was synthesized from sodium hydroxide and boric acid in molar ratio (1:1) using water as solvent. The chemical reaction is given below.
2NaOH +4H3BO3→Na2B4O7.5H2O+2H2O
(A1)
The materials were thoroughly dissolved in solvent of deionised water. After continuous stirring was done for about 8 hours using a magnetic stirrer homogeneous clear
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The photograph of harvested crystals is shown in Fig. 1a.
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Fig. 1a. Photograph of as grown Sodium tetraborate pentahydrate crystals 2.1.1. Solubility study
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Solubility of a material in a perfect solvent and at perfect temperature is important as
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it can nucleate to give bulk sized pure crystals. Solubility curve of STBPH is presented in Fig.1b. The solubility of STBPH was measured using water solvent at different temperatures ranging from 35 to 60°C. Gravimetric method was used to estimate the amount of solvent required to achieve saturation. From the solubility curve, it was observed that STBPH exhibit high solubility in water and positive temperature gradient henceforth adopting slow evaporation technique, good quality STBPH crystals have been grown. Solubility of the
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31°C which was near to the room temperature.
Fig.1b. Solubility curve of STBPH in water
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3. Results and Discussion
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3.1. Single crystal X-ray diffraction
The single crystal XRD examination revealed that Sodium tetra borate pentahydrate
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̅̅̅ and the obtained lattice corresponds to the rhombohedral system having space group R3
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parameter values are shown in the Table 1 are in conformity with the values reported already in the literature [17]. Table 1: Single crystal XRD data of STBPH Crystal
Crystal system Space group Present Work
̅̅̅̅ 𝑅3
a (Å)
b(Å)
c (Å)
11.17
11.17
21.28
Rhombohedral
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Volume(Å3) 2301
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Reported Work
11.13
11.13
21.01
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[14]Rhombohedral 3.2. FT-IR and FT-Raman spectral analyses FTIR investigation of STBPH was accomplished using PERKIN ELMER
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Spectrophotometer in 4000–500 cm−1 wave number range and is shown in Fig 2a. The molecular bonding of the crystals determines its absorption characteristics. Due to their intermolecular hydrogen bond forces, a strong molecular binding prevail between the molecules of Sodium tetraborate pentahydrate [18]. The OH stretch of water is noticed
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at 3360 cm-1. Water molecules which are free and those bonded to sodium ion show H-O-H bending mode vibration at 1662 cm-1. The ring B-O asymmetric stretching vibrations present in the compound appear at wavenumber1079 cm-1. The very strong peak at 1345 cmin the IR spectra has been attributed to B-O terminal symmetric stretching vibrations. The
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bands at 946 cm-1 and 824cm-1 are attributed to symmetric stretching of B–O in BO3 and
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BO4 respectively. In the three IR spectral regions, the borate crystals exhibit molecular vibrations .They are (a) B-O stretching of trigonal B-O in BO3 which occurs between 1500-
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1200 cm-1(b) B-O stretching of trigonal B-O in BO4 which occurs between 1200-850 cm-1
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and (c) bending vibrations of various borate series occurs in between 800-650 cm-1[19]. BRUKER RFS 27 spectrometer was used to record the FT-Raman spectrum of
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STBPH and it is depicted in Fig. 2b. At 1345 cm-1 in IR and at 1356.85 cm-1 in Raman spectrum, B-O symmetric stretching is observed. The Ring B-O asymmetric stretching vibrations is observed at 1079 cm-1 in IR and at 1067.77 cm-1 in Raman counterpart respectively [19]. The observed absorption bands and the vibrational assignments in FTIR and FT-RAMAN spectra are encapsulated in Table 2.
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Fig.2(a,b) FTIR and FT-RAMAN Spectra of STBPH
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Table 2: Assignments for FTIR and FT-RAMAN FT-IR (cm-1 )
3350.94
OH Stretch of water Overtones
1690.08
H-O-H Bending mode of the water molecules
1468
1345
and combination
OH in plane bending mode
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2615
1662
Assignments
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3360
FT-Raman (cm-1)
B-O
Stretching
of
B-O-B
bridge 1362.02
B-O
terminal
symmetric
stretching vibrations 1259 1079
B-O-H Bending mode 1067.77
Ring
B-O
asymmetric
stretching vibrations 8
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941.65
Symmetric stretching of B–O
824
765.32
Symmetric stretching of B–O
712
OBO ring asymmetric bending
520
OBO terminal bending
3.3. UV-Visible Studies
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In photonics and optoelectronic field, crystals with good transmittance is the key requirement [20] can be studied by performing UV-Vis analysis employing LAMBDA-35 UV-visible spectrophotometer operated in the range 200-1200 nm and corresponding UV-
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Vis profile is depicted in Fig. 3. It is evinced from the spectrum of STBPH crystal that very low absorbance and high transmittance in the entire visible region is the significant factor for the materials possessing NLO properties. The lower cut-off wavelength is noticed at 192 nm. The spectrum gives structural information from σ and π electronic transitions in the
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UV-Visible region of the material through the photon absorption process [20].
Fig. 3.UV-Vis spectrum of STBPH
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ACCEPTED MANUSCRIPT 3.3.1. Absorption Edge The absorption coefficient (α) of the crystal in terms of transmittance (T) and thickness (t) [21] are related through,
𝛼=
2.3026 log(1⁄𝑇 )
(B.1)
𝑡
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The first derivative of the optical transmittance gives the knowledge of absorption edge. A graph is plotted between dT/dλ versus wavelength and is presented in the Fig 4.The highest peak position in this figure (Fig 4) is used to calculate the absorption band edge. The
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highest peak value for the crystal of 200.92 nm suggests that the absorption band edge of
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title compound is 6.187 eV.
Fig.4. The Plot of dT/dλ versus wavelength
3.3.2. Band Gap determination Useful knowledge about the electronic and atomic band structures of materials can be concluded from the optical band gap (Eg) of the material [22]. The optical band gap was estimated using the following relation, 10
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(C1)
where, h is the Planck's constant,ν is the frequency of the incident photons ,Eg is the optical band gap energy and A is a constant. Tauc's plot of (αhν)2 versus hν was used to estimate the band gap energy of crystal and by extrapolating the straight line portion of the graph to hν
(x-axis)(i.e) at (αhν)2 = 0 (Fig. 5),the band gap of the crystal was estimated to be 6.52
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eV. Also the larger energy band gap value infers a less deep UV-absorption edge that opens a wide transmission window in the visible region making it suitable for photonic and optical applications [23]. The large transmittance in the visible region makes the crystal fit for
Fig. 5. Tauc's Plot of STBPH
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nonlinear optical applications.
3.3.3. Estimation of optical constants From the absorption spectrum (Fig.3), the exact optical parameters of crystals such
as transmittance, absorption, reflectance, absorption coefficient, optical energy gap and refractive index were evaluated. In processing, tuning, calibrating and design of technology devices, the thorough analysis of optical constants is necessary as it gives the idea about optical quality of crystals [24].Thus the impact of extinction coefficient (K), refractive index 11
ACCEPTED MANUSCRIPT (n0) and reflectance(R) of crystal has been probed using the measured transmittance data. The internal efficiency of the device also depends on absorption coefficient and extinction coefficient (K) denotes partial loss of light due to scattering and absorption per unit distance in a participating medium. The extinction coefficient is obtained from the following relation:
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K
(D1)
The plot between extinction coefficient and photon energy infer that the extinction
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coefficient decreases with energy [25] is represented in Fig. 6(a). The reflectance (R) in terms of optical absorption coefficient and refractive index (n0) is written as √1−exp(−𝛼𝑡)+exp(𝛼𝑡) 1+exp(−𝛼𝑡)
(𝑅+1)±√3R2 +10𝑅−3
no=-{
}(6)
(D3)
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2(𝑅−1)
(D2)
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R=1±
Fig. 6(b) shows the twain reflectance and extinction coefficient dependence on absorption
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coefficient. The distinguishing property of material to modify the path of the light when
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traverses through the medium termed as refractive index is illustrated in Fig.6(c) showing the variation of refractive index (n0) with photon energy (hν).The materials with low
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refractive index could be utilized in holographic data storage facilities. For antireflection coating in solar thermal devices these materials find huge application [24]. The optical conductivity (σop) of the crystal was acquired using the following relation op
n0 c 4
(D4)
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e
2 op
(D5)
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The existence of high photo response nature of the material is affirmed by Fig 6(d) which shows that the optical conductivity ( σop) increases with photon energy (hν) having high magnitude . From Fig 6(b) and Fig 6(d) it is noticed that extinction value and electrical conductivity rely on photon energy [26].
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The electric susceptibility χc is closely related to optical constants given by 𝜀𝑟 = 𝜀0 + 4𝜋𝜒𝑐 = 𝑛02 − 𝐾 2
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(D6)
n K 2 0 c 0 4
(D7)
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The observed in electric susceptibility with photon energy are shown in the Fig. 6(e). Thus
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crystal with enhanced linear optical performance is suggested as prospective material to be
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utilized in NLO applications.
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Fig. 6(a)Plot of
Extinction coefficient (K) with photon energy (b) Variation of
reflectance (R) and extinction coefficient(K) with absorption coeffficient(α) (c)Plot of refractive index (n0) with photon energy (d)Variation of electrical conductivity and optical conductivity with photon energy (e)Plot of electric susceptibility vs. photon energy.
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h E u
(h ) 0 exp
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(E1)
where α0 is a constant, Eu is Urbach's Energy which characterizes the slope of the exponential region. Eu is the width of the localized states associated with amorphous structures in the band gap of the material [28]. A plot is made between ln(α) and hν and it is
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presented in the Fig.7. Urbach energy Eu was calculated by taking the reciprocal of the slope of linear portion of the plot drawn between ln(α) and hυ . If the obtained value of Eu is higher, there may be high topological or structural disorder in the crystal [29].Whereas the lower Eu
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value can be assigned to a decrease in edge broadening. The observed value of the slope is found to be 4.12. The value of Eu is 0.2211 eV which is obtained from the Fig. 7, indicate
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that the grown crystal has minimum structural defects and high crystallinity.
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Fig. 7. Plot ln(α) with hν
3.4. Dielectric studies
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In order to recognize the lattice dynamics in the crystal, dielectric studies are
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performed. Capacitance of crystal was calculated for the temperatures 313K, 333K, 353K and 373K in the frequency
range 5 MHz-50MHz and
the dielectric constant was
Cd A 0
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determined from the formula,
(F1)
where C is the capacitance, d is the thickness of the crystal. ε0 is the vacuum dielectric constant. [30]. The variations of dielectric constant and dielectric loss in terms of frequency for selected temperatures are represented in the Fig. 8(a,b). It is discerned that dielectric
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ACCEPTED MANUSCRIPT constant and dielectric loss decrease with increase in frequency. The large value of dielectric constant at small frequencies is due to the polarizations such as space charge, orientational, electronic and ionic polarization and its low value at higher frequencies may be due to the loss of these polarizations. Space charge polarization is significant whereas electronic and ionic polarizations are somewhat submissive in low frequency region [31]. The crystals with
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high dielectric constant will have power dissipation [30]. The crystals possessing low dielectric constant will have smaller amount of dipoles per unit volume. Thus it will have minimum losses as individuated to the material having high dielectric constant [32]. Low dielectric loss in the high frequency region is due to the superior optical quality of the
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crystal with lesser defect and this factor plays an imperative role for the production of nonlinear optical devices [33]. Crystal expansion, electronic and ionic polarizations and crystal defects are the major reason for the raise of dielectric constant with respect to
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temperature.
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3.4.1. Activation Energy Using the following relation [34], activation energy can be obtained.
Ea k BT
0 exp
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(G1)
Where, σac is the conductivity at temperature T, Ea the activation energy for the electrical process and KB is the Boltzmann constant (1.38 x 10-23 J/K). The plot of log σac versus
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1000/T is shown in Fig.9. This is almost linear in behaviour and the slope of this graph is used to determine activation energy using the formula, Ea = - Slope x 1000 x KB
(G2)
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The values of the activation energy was found to be 0.02539, 0.01658, 0.0033 and 0.0023 eV at frequencies of 400 Hz, 20 KHz,80 KHz and 4 MHz respectively. The low value of
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activation energies attribute to the ordered state of the sample. Since the activation energy is
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low, charge carriers in the crystal need more activation energy to jump between energy states. The crystals which are having less number of defects will have low activation energy.
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For device fabrications, defect less crystals are in demand.
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Fig.9. Plot of 1000/T verses Log σac
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3.5. Microhardness studies
Hardness is elucidated as the ability of the material to resist lattice deformation or
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the resistance offered by the material to permanent deformation or damage [35].
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Microhardness testing is made to perceive the mechanical properties such as elastic stiffness constant and Knoop's hardness number HK. Vickers indentor is engaged to learn
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microhardness for varying loads. By varying the load from 25g to 100g, the indentation marks were made on the surfaces. Diagonal length (d) of the indentation impression was
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measured using a microscope. The Vickers hardness number (Hv) is calculated using the relation:
Hv(Kg/mm2) = 1.8544x
P
(H1)
d2
where P is the applied load in Kg , d is the diagonal length in mm. Fig 10 (a) shows the variation of hardness number (Hv) as a function of applied load. From the Fig 10(a), it is 19
ACCEPTED MANUSCRIPT discerned that the hardness number (Hv) increases with the increase of load which is a reverse indentation effect. As specified by Meyer's law, P = adn where P is the load, d is the diagonal length of the indentation, a and n are constants, where n named as Meyer's index or work hardening coefficient is evaluated to be 3.14 obtained from the graph between Log P and Log d (Fig 10(b)). As stated by Onitsch [36] the material is termed as hard material if
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the value of n lies in between 1.0 and 1.6 or else the material is termed as a soft material if the value of n is greater than 1.6. The n value acquired here is greater than 1.6, so it falls under the category of soft material.
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From the Wooster's empirical formula C11= Hv 7/4, the elastic stiffness constant C11 is determined [37]. A graph is drawn between load P and elastic stiffness constant C11 and is displayed in the Fig 10(c).The greater value of stiffness constant endorse that sodium borate crystal can be utilized for NLO modulators and it also gives an idea about the tightness of
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bonding between neighbouring atoms.
HK(Kg/mm2)=14.229
P
(H2)
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d2
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The Knoop's hardness number (HK) is calculated using the relation,
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A graph is drawn between load P and Knoop's hardness number HK and is shown in the Fig
The calculated mechanical parameters are detailed in the Table 3.
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Hv(Kg/mm2)
N
HK(Kg/mm2)
C11(Gpa)
25
26.5
3.14
20.37
3.0951
50
35.8
3.14
27.52
5.2395
100
44.05
3.14
33.83
7.5319
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load P(g)
Fig.10.(a)Microhardness(b)Meyers plot(c)Stiffness constant(d) Knoop's hardness number
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ACCEPTED MANUSCRIPT 3.6. Thermal Analysis In the temperature range 30 to 600˚C under the atmosphere of nitrogen, thermogram was recorded for STBPH using TG/DTA model 6200 SII thermal analyzer operated at heating rate of 20˚C/min. The thermogram is represented in Fig. 7 (tiny endothermic peaks) outline that adsorbed surface water prevails in the compound, which loses at 60 °C and 100
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°C respectively. At 142 ˚C, presence of a sharp endothermic peak is due to the loss of water of crystallization. The sample is thermally stable up to 142 ˚C as it is shown in the graph. Up to 662 ˚C, a weight loss of 36.53% is observed, i.e. due to the presence of water molecules in the compound [8,19]. Below 142 oC, there is no weight loss observed in the
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compound. The endothermic peaks of the DTA trace concur with the decomposition of the TGA trace. The noticed decomposition temperature (142oC) serves as a good advantage for
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the material usage for NLO applications.
Fig.11. TG/DTA curve for STBPH single crystal 22
ACCEPTED MANUSCRIPT 3.7. Photoluminescence studies Using the technique of Photoluminescence (PL) spectroscopy, the electronic structure of compound is studied employing PERKIN ELMER LS 45 spectrofluorimeter. The emission spectra of STBPH single crystal was recorded at the excitation wavelength 220 nm and the observed emission spectra of STBPH seems to lie in the range 240–900 nm
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(4.969–1.552 eV) is sketched in Fig 12. In PL emission spectra of STBPH,one violet fluorescence peak at 343.06 nm is seen . For borate crystals, PL intrinsic emission at 343.06 nm is characteristic. This may be due to the disappearance of self-trapped excitons
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associated with band gap excitations or molecular transitions within the BO33- group [38]. Emission spectra exhibits strong blue fluorescence peak at 453.11 nm and 490.19 nm. Blue fluorescence emission of crystal ensures that the material can be used in the production of blue light emitting diodes [38]. The crystal also shows one red fluorescence peak at 682.59
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Fig.12.Emission spectrum of STBPH 23
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3.8. Z-Scan measurements By Z-scan technique, the third order nonlinear response of the STBPH has been explored. This technique was carried out using He-Ne laser of intensity 5 mW [39]. In this Z-scan technique self-focussing or self-defocussing of laser beam is made in a thin
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nonlinear medium with sample thickness (L) [40]. The sample was translated across the +Z to -Z axial direction by using stepper motor to change the incident intensity falling on the sample. Normal transmittance for closed aperture, open aperture and ratio of closed to open
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as a function of sample position has been done for the nonlinear sample. In open aperture, the nonlinear transmission of sample without aperture was calculated in the far field as the sample was shifted through the focal point. This allows us to determine the nonlinear absorption coefficient β. In closed aperture (i.e. aperture in place in the far field) the
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transmittance of the sample through the aperture is monitored in the far field as a function of
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the position, Z of the nonlinear sample. It is done in the vicinity of the linear optics focal position [41] to measure nonlinear refraction. The ratio of closed to open normal
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transmittance function of sample position Z have been made for the crystal and it is portrayed in the Fig. 13(c).The normalized transmittance of the sample through the aperture
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is monitored in the far field as a function of the position Z. The measurable quantity (∆TP-V ) can be defined as the difference between the transmittance change of peak and valley Tp-TV
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and on axis phase shift at the focus |∆φ | is given by:
TPV 0.406(1 S ) 0.25 0
(I 1)
S is the linear transmittance aperture and is estimated by,
2ra 2 S 1 exp 2 a .
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ACCEPTED MANUSCRIPT Here ra is the radius of the aperture and ωa is the beam radius at the aperture. The nonlinear refractive index (n2) can be calculated by using the relation:
n2
0 KI 0 Leff
(I 3)
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where |φ| is the on axis phase shift, where I0 is the on-axis irradiance at focus (Z=0) (I0 = 26.31 MWm-2) Leff is the effective thickness of the sample which can be calculated using the relation
Leff [1 exp(L)] /
. Here L denotes the sample length, α represents the linear
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absorption coefficient, I0 is the laser beam intensity at focus Z = 0 and k represents the wave number (k = 2π/λ).
The nonlinear absorption coefficient can be evaluated from the knowledge of open aperture
2 2T I 0 Leff
(I 4)
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trace, using the relation
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In normalized transmittance curve obtained from the closed aperture Z scan data, we can observe that peak is followed by valley. This result suggests that the sign of refraction
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nonlinearity is negative i.e., self-defocussing. The self-defocussing effect is due to the local
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variation of refractive index with temperature. The defocussing nature of the material is due to the negative refractive index and this property is used in the shielding of optical sensor such as night vision devices [26]. For the negative refractive index laser damage threshold value of the sample will also be higher [40].
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Fig. 13.(a) Closed aperture spectrum of STBPH (b)Open aperture spectrum of STBPH
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(c) ratio of the closed to open Z scan traces of STBPH The real and imaginary parts of the third order nonlinear optical susceptibility were
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estimated using the formula:
10 4 ( 0C 2 no n2 ) cm2 W
(I 5)
10 2 ( 0 C 2 n0 ) cm2 (esu) 4 2 W
(I 6)
2
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Re (3) (esu)
Im
( 3)
Where, ε0 is the vacuum permittivity, c is the velocity of light in vacuum, no is the linear refractive index of the sample and λ is the wavelength of laser beam. The third order nonlinear optical susceptibility of the crystal can be evaluated using the relation: 26
ACCEPTED MANUSCRIPT (3) (Re( (3) ))2 (Im( (3) ))2
(I 7)
Table 4 gives the experimental details and the results of Z-scan technique for STBPH. The nonlinear absorption coefficient (β) value is 0.03 x 10-4cm2W-1and its positive value stipulates that two photon absorption have been taken place [42].The nonlinear optical
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properties of STBPH make it as a resourceful candidate in the field of optical limiting application.
Table 4: Obtained nonlinear optical parameters from Z-scan measurements of STBPH
Measured values
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Third-order nonlinear properties
1.63 x 10-8cm2W-1
Nonlinear refractive index (n2)
0.03 x 10-4cm2W-1
Real susceptibility (χ3 ) Imaginary susceptibility (χ3 )
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Nonlinear absorption coefficient (β)
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4. Conclusion
0.16 x 10-6 esu 1.21 x 10-6 esu
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Absolute susceptibility (χ3)
1.20 x 10-6 esu
Optically transparent single crystals of STBPH have been successfully grown by
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slow aqueous evaporation process. The lattice parameters of STBPH was affirmed by XRD
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analysis and the crystal system corresponds to Rhombohedral with the space group ̅̅̅̅ R3. The vibrational spectral analysis established the different functional groups that contribute the grown crystal. The measurable optical factors such as Urbach energy, band gap and extinction coefficient of transparent STBPH crystal suggest that the material can be concentrated in optoelectronics and fabrication of devices. The thermal analyses reveal that the sample is thermally stable up to 142 ˚C. PL spectra exhibit a sharp blue emission peak at 453.11 nm suggest its application for fabricating lasers. The Vickers microhardness test 27
ACCEPTED MANUSCRIPT concludes that the STBPH belong to the category of soft NLO material. The nonlinear refractive index (n2), absorption coefficient (β) and third-order nonlinear susceptibility (χ3) resolved using the technique of Z-scan substantiate that STBPH could be an admirable contender for the future optoelectronic, photonic and nonlinear optical applications.
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Acknowledgement
The scientific supports rendered by sophisticated analytical instrument facility and Department of Chemistry, IITM Chennai for support in single crystal XRD,
FTIR,
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FT-RAMAN is gratefully acknowledged. Also the authors thank for the facilities arranged by Department of Physics, St. Joseph College, Trichy for providing microhardness and PL measurement. References
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ACCEPTED MANUSCRIPT studies of sulphanilic acid monohydrate: A promising third-order nonlinear optical material J. Nonlinear Opt. Phys. Mat. 26 (2017) 1750020-40. [35] A. Darlin Mary, K. Jayakumari ,C.K. Mahadevan, Growth and Characterization of Zinc Magnesium Tris (Thiourea) Sulphate (ZMTS) Single Crystals, Int. J. Engineering Research and Applications 3 (2013) 1183-1197. [36] B. Deepa, P. Philominathan, Investigation on the optical, mechanical and magnetic 127 (2016)
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[38] Neeti Goel , Nidhi Sinha , Binay Kumar, Growth and properties of sodium tetraborate decahydrate single crystals, Mater. Res. Bull. 48 (2013) 1632–1636.
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potassium dichromate single crystals by Z-scan technique, Optik 124 (2013) 4716–4720. [41] M. G. Kuzyk and C. W. Dirk, Eds., Characterization Techniques and Tabulations for
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Organic Nonlinear Materials, Marcel Dekkar Inc. 1998 655-692. [42] M. Nageshwari, C. Rathika Thaya Kumari, G. Vinitha, M. Peer Mohamed , S. Sudha,
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Table caption
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Figure shows the Z-scan signature of STBPH in (a) Closed aperture and (b) Open aperture
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Table 1: Single crystal XRD data of STBPH Crystal
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Table 2: Assignments for FTIR and FT-RAMAN Table 3: Mechanical parameters of STBPH Table 4: Obtained nonlinear optical parameters from Z-scan measurements for STBPH
Figure caption
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ACCEPTED MANUSCRIPT Fig. 1a. Photograph of as grown Sodium tetraborate pentahydrate crystals Fig.1b. Solubility curve of STBPH in water Fig. 2(a,b). FTIR and FT-RAMAN Spectra of STBPH Fig. 3.UV-Vis spectrum of STBPH Fig.4. The plot of dT/dλ versus wavelength
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Fig 5. Tauc's Plot of STBPH
Fig. 6(a)Plot of Extinction coefficient (K) with photon energy (b) Variation of reflectance (R) and extinction coefficient(K) with absorption coeffficient(α) (c)Plot of refractive index (n0) with photon energy (d) Variation of electrical conductivity and optical conductivity
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with photon energy (e) Plot of electric susceptibility vs. photon energy. Fig. 7. Plot of ln(α) with hν
Fig.8 (a) Plot of Log f vs dielectric constant and (b) Plot of Log f vs dielectric loss
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Fig.9. Plot of 1000/T verses Log σac
Fig.10. (a) Microhardness (b) Meyers plot (c) Stiffness constant (d) Knoop's hardness
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number
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Fig.11. TG/DTA curve for STBPH single crystal Fig.12.Emission spectrum of STBPH
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Fig. 13 (a) Closed aperture spectrum of STBPH (b) Open aperture spectrum of STBPH
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(c) ratio of the closed to open Z-scan traces of STBPH
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Synthesis, growth and third order nonlinear optical studies of a rhombohedral crystal: sodium tetraborate pentahydrate S. Sudha , M. Peer Mohamed , G. Vinitha , C. Rathika Thaya Kumari , P. Sangeetha , M. Lydia Caroline PII: DOI: Reference:
S0577-9073(18)30812-8 https://doi.org/10.1016/j.cjph.2018.11.014 CJPH 708
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
11 June 2018 22 November 2018 27 November 2018
Please cite this article as: S. Sudha , M. Peer Mohamed , G. Vinitha , C. Rathika Thaya Kumari , P. Sangeetha , M. Lydia Caroline , Synthesis, growth and third order nonlinear optical studies of a rhombohedral crystal: sodium tetraborate pentahydrate, Chinese Journal of Physics (2018), doi: https://doi.org/10.1016/j.cjph.2018.11.014
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ACCEPTED MANUSCRIPT Revised Highlights The UV-Visible spectrum of STBPH revealed lower cutoff wavelength of 192 nm. Microhardness study revealed that the crystal belongs to soft material category. The dielectric properties attest its suitability in electro-optic devices. STBPH possess negative nonlinearity proved by Z-scan technique.
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Synthesis, growth and third order nonlinear optical studies of a rhombohedral crystal: sodium tetraborate pentahydrate S. Sudhaa, M. Peer Mohameda,c, G.Vinithad, C. Rathika Thaya Kumaria, P. Sangeethaa, M. Lydia Caroline a,b* aPG & Research Department of Physics, Arignar Anna Govt. Arts College,
b
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Cheyyar - 604 407, Tamil Nadu, India
Department of Physics, Dr. Ambedkar Govt. Arts College, Vyasarpadi, Chennai-600039, Tamil Nadu, India c
Department of Physics, C. Abdul Hakeem College, Melvisharam-632509, Tamil Nadu, India
d
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Department of Physics, School of Advanced Sciences, VIT, Chennai – 600127, Tamil Nadu, India
Abstract
An apt and a futuristic nonlinear optical material sodium tetraborate pentahydrate
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(STBPH) was synthesized and grown by the technique of slow evaporation. The structural
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parameters of the crystal was confirmed by single crystal XRD revealed rhombohedral crystal system. Crystal's broad optical transparency was revealed by UV-Vis absorption
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spectrum. From the absorption spectrum, theoretical calculations were made to determine absorption coefficient, band gap, Urbach energy and reflectance. The functional groups
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were identified by FT-IR and FT-Raman spectra. Vicker's microhardness measurement
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disclosed that the grown crystal belongs to soft material category. Photoluminescence (PL) spectral study of STBPH indicated violet and blue emission peaks respectively at 343.06 nm and at 453.11 nm, 490.19 nm. Dielectric studies reveal the dielectric nature of the grown crystal. TGA-DTA studies showed that the crystal holds good thermal stability. The third order nonlinearities of STBPH was investigated by Z scan technique manifest its suitability in nonlinear optical applications.
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ACCEPTED MANUSCRIPT Key words: Nonlinear optics; Crystal growth; UV-Visible studies; Dielectric studies; ZScan technique. Corresponding author Email address and mobile no: [email protected](M.Lydia Caroline); Tel: 044-25520151: Fax: 044 - 2552 1852:+91 9841720216 Introduction
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1.
There has been an appreciable interest towards the synthesis and development of many borate crystals for the past few years due to their wide use in optical communication, laser medicine and signal processing [1,2]. Borate crystals occupy a crucial position in
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frequency conversion in view of demand for a powerful or tunable laser source in the deep ultraviolet region [3]. Borates show excellent physical properties, such as short growth period, large effective nonlinear coefficient, high laser damage tolerance, piezoelectric,
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luminescence and other useful properties [4]. Comparing to other commonly used NLO materials such as potassium dihydrogen phosphate (KDP) or lithium niobate (LN), borate
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crystals are found to be illustrious to UV studies as their transmittance start from the
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wavelength of 155 nm [5]. Meantime, influenced versatile deep UV nonlinear optical materials have been synthesized by We et al and Yu et al [6, 7] proved them as novel
series
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crystals applicable in laser applications. Also much effort has been made on developing of
remarkable
borate
NLO
crystals
such
as
Potassium
aluminum
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borate(KAB),Potassium pentaborate, strontium tetraborate (SrB4O7), Lanthanaum calcium borate (La2CaB10O19) and barium strontium borate where the studies exhibited high transmittance and hence successfully handled in conversion of laser radiation into the UV wavelength region and vacuum [8-12]. Some borates based candidates also were studied for fourth harmonic generation and fifth harmonic generation of the Nd: YAG lasers (Zhang et al 2002). Possessing chemical stability, high damage threshold and high optical quality as
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ACCEPTED MANUSCRIPT well as wide range of transparency in the ultraviolet region influences a rather large difference in the electro negativity of B and O atoms stand as the basic characteristics of borates [13]. In borate family of crystals, the boron atom usually coordinate with either three or four oxygen atoms forming [BO3] or [BO4] groups [14]. Consequently, the electronic orbitals are hybridized to a planar sp2 or a three-dimensional sp3 structure. Borates which
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hold water molecules in it possess polyanions in their crystal structure [14,15]. Every boron centre has a negative charge that is balanced by various metal cations. One of the basic cation of the borate minerals is Sodium. Eventually Sodium, Potassium and Ammonium Borates are brilliant non-linear optical (NLO) materials and are useful in modern short-
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wavelength laser techniques [16]. The synthesis of title inorganic borate system has been already reported by Estud Geol [17]. But its characterization studies has not been reported yet. Here we detail the synthesis of nonlinear optical crystal sodium tetraborate
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pentahydrate, and its structural characterization by XRD, FT-IR, FT-Raman, microhardness, dielectric and thermogravimetric analysis. The STBPH crystal was confirmed as third
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harmonic crystal by closed and open aperture Z-scan signatures from which optical
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nonlinearities were evaluated. 2. Experimental
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2.1. Material Synthesis
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Sodium tetraborate pentahydrate (STBPH) was synthesized from sodium hydroxide and boric acid in molar ratio (1:1) using water as solvent. The chemical reaction is given below.
2NaOH +4H3BO3→Na2B4O7.5H2O+2H2O
(A1)
The materials were thoroughly dissolved in solvent of deionised water. After continuous stirring was done for about 8 hours using a magnetic stirrer homogeneous clear
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The photograph of harvested crystals is shown in Fig. 1a.
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Fig. 1a. Photograph of as grown Sodium tetraborate pentahydrate crystals 2.1.1. Solubility study
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Solubility of a material in a perfect solvent and at perfect temperature is important as
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it can nucleate to give bulk sized pure crystals. Solubility curve of STBPH is presented in Fig.1b. The solubility of STBPH was measured using water solvent at different temperatures ranging from 35 to 60°C. Gravimetric method was used to estimate the amount of solvent required to achieve saturation. From the solubility curve, it was observed that STBPH exhibit high solubility in water and positive temperature gradient henceforth adopting slow evaporation technique, good quality STBPH crystals have been grown. Solubility of the
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31°C which was near to the room temperature.
Fig.1b. Solubility curve of STBPH in water
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3. Results and Discussion
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3.1. Single crystal X-ray diffraction
The single crystal XRD examination revealed that Sodium tetra borate pentahydrate
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̅̅̅ and the obtained lattice corresponds to the rhombohedral system having space group R3
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parameter values are shown in the Table 1 are in conformity with the values reported already in the literature [17]. Table 1: Single crystal XRD data of STBPH Crystal
Crystal system Space group Present Work
̅̅̅̅ 𝑅3
a (Å)
b(Å)
c (Å)
11.17
11.17
21.28
Rhombohedral
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Volume(Å3) 2301
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Reported Work
11.13
11.13
21.01
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[14]Rhombohedral 3.2. FT-IR and FT-Raman spectral analyses FTIR investigation of STBPH was accomplished using PERKIN ELMER
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Spectrophotometer in 4000–500 cm−1 wave number range and is shown in Fig 2a. The molecular bonding of the crystals determines its absorption characteristics. Due to their intermolecular hydrogen bond forces, a strong molecular binding prevail between the molecules of Sodium tetraborate pentahydrate [18]. The OH stretch of water is noticed
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at 3360 cm-1. Water molecules which are free and those bonded to sodium ion show H-O-H bending mode vibration at 1662 cm-1. The ring B-O asymmetric stretching vibrations present in the compound appear at wavenumber1079 cm-1. The very strong peak at 1345 cmin the IR spectra has been attributed to B-O terminal symmetric stretching vibrations. The
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bands at 946 cm-1 and 824cm-1 are attributed to symmetric stretching of B–O in BO3 and
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BO4 respectively. In the three IR spectral regions, the borate crystals exhibit molecular vibrations .They are (a) B-O stretching of trigonal B-O in BO3 which occurs between 1500-
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1200 cm-1(b) B-O stretching of trigonal B-O in BO4 which occurs between 1200-850 cm-1
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and (c) bending vibrations of various borate series occurs in between 800-650 cm-1[19]. BRUKER RFS 27 spectrometer was used to record the FT-Raman spectrum of
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STBPH and it is depicted in Fig. 2b. At 1345 cm-1 in IR and at 1356.85 cm-1 in Raman spectrum, B-O symmetric stretching is observed. The Ring B-O asymmetric stretching vibrations is observed at 1079 cm-1 in IR and at 1067.77 cm-1 in Raman counterpart respectively [19]. The observed absorption bands and the vibrational assignments in FTIR and FT-RAMAN spectra are encapsulated in Table 2.
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Fig.2(a,b) FTIR and FT-RAMAN Spectra of STBPH
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Table 2: Assignments for FTIR and FT-RAMAN FT-IR (cm-1 )
3350.94
OH Stretch of water Overtones
1690.08
H-O-H Bending mode of the water molecules
1468
1345
and combination
OH in plane bending mode
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2615
1662
Assignments
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3360
FT-Raman (cm-1)
B-O
Stretching
of
B-O-B
bridge 1362.02
B-O
terminal
symmetric
stretching vibrations 1259 1079
B-O-H Bending mode 1067.77
Ring
B-O
asymmetric
stretching vibrations 8
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941.65
Symmetric stretching of B–O
824
765.32
Symmetric stretching of B–O
712
OBO ring asymmetric bending
520
OBO terminal bending
3.3. UV-Visible Studies
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In photonics and optoelectronic field, crystals with good transmittance is the key requirement [20] can be studied by performing UV-Vis analysis employing LAMBDA-35 UV-visible spectrophotometer operated in the range 200-1200 nm and corresponding UV-
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Vis profile is depicted in Fig. 3. It is evinced from the spectrum of STBPH crystal that very low absorbance and high transmittance in the entire visible region is the significant factor for the materials possessing NLO properties. The lower cut-off wavelength is noticed at 192 nm. The spectrum gives structural information from σ and π electronic transitions in the
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UV-Visible region of the material through the photon absorption process [20].
Fig. 3.UV-Vis spectrum of STBPH
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ACCEPTED MANUSCRIPT 3.3.1. Absorption Edge The absorption coefficient (α) of the crystal in terms of transmittance (T) and thickness (t) [21] are related through,
𝛼=
2.3026 log(1⁄𝑇 )
(B.1)
𝑡
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The first derivative of the optical transmittance gives the knowledge of absorption edge. A graph is plotted between dT/dλ versus wavelength and is presented in the Fig 4.The highest peak position in this figure (Fig 4) is used to calculate the absorption band edge. The
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highest peak value for the crystal of 200.92 nm suggests that the absorption band edge of
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title compound is 6.187 eV.
Fig.4. The Plot of dT/dλ versus wavelength
3.3.2. Band Gap determination Useful knowledge about the electronic and atomic band structures of materials can be concluded from the optical band gap (Eg) of the material [22]. The optical band gap was estimated using the following relation, 10
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(C1)
where, h is the Planck's constant,ν is the frequency of the incident photons ,Eg is the optical band gap energy and A is a constant. Tauc's plot of (αhν)2 versus hν was used to estimate the band gap energy of crystal and by extrapolating the straight line portion of the graph to hν
(x-axis)(i.e) at (αhν)2 = 0 (Fig. 5),the band gap of the crystal was estimated to be 6.52
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eV. Also the larger energy band gap value infers a less deep UV-absorption edge that opens a wide transmission window in the visible region making it suitable for photonic and optical applications [23]. The large transmittance in the visible region makes the crystal fit for
Fig. 5. Tauc's Plot of STBPH
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nonlinear optical applications.
3.3.3. Estimation of optical constants From the absorption spectrum (Fig.3), the exact optical parameters of crystals such
as transmittance, absorption, reflectance, absorption coefficient, optical energy gap and refractive index were evaluated. In processing, tuning, calibrating and design of technology devices, the thorough analysis of optical constants is necessary as it gives the idea about optical quality of crystals [24].Thus the impact of extinction coefficient (K), refractive index 11
ACCEPTED MANUSCRIPT (n0) and reflectance(R) of crystal has been probed using the measured transmittance data. The internal efficiency of the device also depends on absorption coefficient and extinction coefficient (K) denotes partial loss of light due to scattering and absorption per unit distance in a participating medium. The extinction coefficient is obtained from the following relation:
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4
K
(D1)
The plot between extinction coefficient and photon energy infer that the extinction
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coefficient decreases with energy [25] is represented in Fig. 6(a). The reflectance (R) in terms of optical absorption coefficient and refractive index (n0) is written as √1−exp(−𝛼𝑡)+exp(𝛼𝑡) 1+exp(−𝛼𝑡)
(𝑅+1)±√3R2 +10𝑅−3
no=-{
}(6)
(D3)
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2(𝑅−1)
(D2)
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R=1±
Fig. 6(b) shows the twain reflectance and extinction coefficient dependence on absorption
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coefficient. The distinguishing property of material to modify the path of the light when
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traverses through the medium termed as refractive index is illustrated in Fig.6(c) showing the variation of refractive index (n0) with photon energy (hν).The materials with low
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refractive index could be utilized in holographic data storage facilities. For antireflection coating in solar thermal devices these materials find huge application [24]. The optical conductivity (σop) of the crystal was acquired using the following relation op
n0 c 4
(D4)
12
ACCEPTED MANUSCRIPT and also the electrical conductivity is associated to the optical conductivity of the crystal as follows,
e
2 op
(D5)
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The existence of high photo response nature of the material is affirmed by Fig 6(d) which shows that the optical conductivity ( σop) increases with photon energy (hν) having high magnitude . From Fig 6(b) and Fig 6(d) it is noticed that extinction value and electrical conductivity rely on photon energy [26].
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The electric susceptibility χc is closely related to optical constants given by 𝜀𝑟 = 𝜀0 + 4𝜋𝜒𝑐 = 𝑛02 − 𝐾 2
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(D6)
n K 2 0 c 0 4
(D7)
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2
The observed in electric susceptibility with photon energy are shown in the Fig. 6(e). Thus
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crystal with enhanced linear optical performance is suggested as prospective material to be
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utilized in NLO applications.
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Fig. 6(a)Plot of
Extinction coefficient (K) with photon energy (b) Variation of
reflectance (R) and extinction coefficient(K) with absorption coeffficient(α) (c)Plot of refractive index (n0) with photon energy (d)Variation of electrical conductivity and optical conductivity with photon energy (e)Plot of electric susceptibility vs. photon energy.
14
ACCEPTED MANUSCRIPT 3.3.4. Urbach's Tail Near the fundamental absorption edge, α shows an exponential dependence on the incident photon energy and obey the empirical Urbach relation [27] represented as:
h E u
(h ) 0 exp
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(E1)
where α0 is a constant, Eu is Urbach's Energy which characterizes the slope of the exponential region. Eu is the width of the localized states associated with amorphous structures in the band gap of the material [28]. A plot is made between ln(α) and hν and it is
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presented in the Fig.7. Urbach energy Eu was calculated by taking the reciprocal of the slope of linear portion of the plot drawn between ln(α) and hυ . If the obtained value of Eu is higher, there may be high topological or structural disorder in the crystal [29].Whereas the lower Eu
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value can be assigned to a decrease in edge broadening. The observed value of the slope is found to be 4.12. The value of Eu is 0.2211 eV which is obtained from the Fig. 7, indicate
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that the grown crystal has minimum structural defects and high crystallinity.
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Fig. 7. Plot ln(α) with hν
3.4. Dielectric studies
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In order to recognize the lattice dynamics in the crystal, dielectric studies are
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performed. Capacitance of crystal was calculated for the temperatures 313K, 333K, 353K and 373K in the frequency
range 5 MHz-50MHz and
the dielectric constant was
Cd A 0
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determined from the formula,
(F1)
where C is the capacitance, d is the thickness of the crystal. ε0 is the vacuum dielectric constant. [30]. The variations of dielectric constant and dielectric loss in terms of frequency for selected temperatures are represented in the Fig. 8(a,b). It is discerned that dielectric
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ACCEPTED MANUSCRIPT constant and dielectric loss decrease with increase in frequency. The large value of dielectric constant at small frequencies is due to the polarizations such as space charge, orientational, electronic and ionic polarization and its low value at higher frequencies may be due to the loss of these polarizations. Space charge polarization is significant whereas electronic and ionic polarizations are somewhat submissive in low frequency region [31]. The crystals with
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high dielectric constant will have power dissipation [30]. The crystals possessing low dielectric constant will have smaller amount of dipoles per unit volume. Thus it will have minimum losses as individuated to the material having high dielectric constant [32]. Low dielectric loss in the high frequency region is due to the superior optical quality of the
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crystal with lesser defect and this factor plays an imperative role for the production of nonlinear optical devices [33]. Crystal expansion, electronic and ionic polarizations and crystal defects are the major reason for the raise of dielectric constant with respect to
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temperature.
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ACCEPTED MANUSCRIPT Fig.8.(a)Plot of Log f vs dielectric constant and(b)Plot of Log f vs dielectric loss
3.4.1. Activation Energy Using the following relation [34], activation energy can be obtained.
Ea k BT
0 exp
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(G1)
Where, σac is the conductivity at temperature T, Ea the activation energy for the electrical process and KB is the Boltzmann constant (1.38 x 10-23 J/K). The plot of log σac versus
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1000/T is shown in Fig.9. This is almost linear in behaviour and the slope of this graph is used to determine activation energy using the formula, Ea = - Slope x 1000 x KB
(G2)
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The values of the activation energy was found to be 0.02539, 0.01658, 0.0033 and 0.0023 eV at frequencies of 400 Hz, 20 KHz,80 KHz and 4 MHz respectively. The low value of
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activation energies attribute to the ordered state of the sample. Since the activation energy is
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low, charge carriers in the crystal need more activation energy to jump between energy states. The crystals which are having less number of defects will have low activation energy.
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Fig.9. Plot of 1000/T verses Log σac
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3.5. Microhardness studies
Hardness is elucidated as the ability of the material to resist lattice deformation or
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the resistance offered by the material to permanent deformation or damage [35].
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Microhardness testing is made to perceive the mechanical properties such as elastic stiffness constant and Knoop's hardness number HK. Vickers indentor is engaged to learn
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microhardness for varying loads. By varying the load from 25g to 100g, the indentation marks were made on the surfaces. Diagonal length (d) of the indentation impression was
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measured using a microscope. The Vickers hardness number (Hv) is calculated using the relation:
Hv(Kg/mm2) = 1.8544x
P
(H1)
d2
where P is the applied load in Kg , d is the diagonal length in mm. Fig 10 (a) shows the variation of hardness number (Hv) as a function of applied load. From the Fig 10(a), it is 19
ACCEPTED MANUSCRIPT discerned that the hardness number (Hv) increases with the increase of load which is a reverse indentation effect. As specified by Meyer's law, P = adn where P is the load, d is the diagonal length of the indentation, a and n are constants, where n named as Meyer's index or work hardening coefficient is evaluated to be 3.14 obtained from the graph between Log P and Log d (Fig 10(b)). As stated by Onitsch [36] the material is termed as hard material if
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the value of n lies in between 1.0 and 1.6 or else the material is termed as a soft material if the value of n is greater than 1.6. The n value acquired here is greater than 1.6, so it falls under the category of soft material.
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From the Wooster's empirical formula C11= Hv 7/4, the elastic stiffness constant C11 is determined [37]. A graph is drawn between load P and elastic stiffness constant C11 and is displayed in the Fig 10(c).The greater value of stiffness constant endorse that sodium borate crystal can be utilized for NLO modulators and it also gives an idea about the tightness of
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bonding between neighbouring atoms.
HK(Kg/mm2)=14.229
P
(H2)
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d2
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The Knoop's hardness number (HK) is calculated using the relation,
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A graph is drawn between load P and Knoop's hardness number HK and is shown in the Fig
The calculated mechanical parameters are detailed in the Table 3.
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ACCEPTED MANUSCRIPT Table 3: Mechanical parameters of STBPH
Hv(Kg/mm2)
N
HK(Kg/mm2)
C11(Gpa)
25
26.5
3.14
20.37
3.0951
50
35.8
3.14
27.52
5.2395
100
44.05
3.14
33.83
7.5319
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load P(g)
Fig.10.(a)Microhardness(b)Meyers plot(c)Stiffness constant(d) Knoop's hardness number
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ACCEPTED MANUSCRIPT 3.6. Thermal Analysis In the temperature range 30 to 600˚C under the atmosphere of nitrogen, thermogram was recorded for STBPH using TG/DTA model 6200 SII thermal analyzer operated at heating rate of 20˚C/min. The thermogram is represented in Fig. 7 (tiny endothermic peaks) outline that adsorbed surface water prevails in the compound, which loses at 60 °C and 100
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°C respectively. At 142 ˚C, presence of a sharp endothermic peak is due to the loss of water of crystallization. The sample is thermally stable up to 142 ˚C as it is shown in the graph. Up to 662 ˚C, a weight loss of 36.53% is observed, i.e. due to the presence of water molecules in the compound [8,19]. Below 142 oC, there is no weight loss observed in the
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compound. The endothermic peaks of the DTA trace concur with the decomposition of the TGA trace. The noticed decomposition temperature (142oC) serves as a good advantage for
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the material usage for NLO applications.
Fig.11. TG/DTA curve for STBPH single crystal 22
ACCEPTED MANUSCRIPT 3.7. Photoluminescence studies Using the technique of Photoluminescence (PL) spectroscopy, the electronic structure of compound is studied employing PERKIN ELMER LS 45 spectrofluorimeter. The emission spectra of STBPH single crystal was recorded at the excitation wavelength 220 nm and the observed emission spectra of STBPH seems to lie in the range 240–900 nm
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(4.969–1.552 eV) is sketched in Fig 12. In PL emission spectra of STBPH,one violet fluorescence peak at 343.06 nm is seen . For borate crystals, PL intrinsic emission at 343.06 nm is characteristic. This may be due to the disappearance of self-trapped excitons
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associated with band gap excitations or molecular transitions within the BO33- group [38]. Emission spectra exhibits strong blue fluorescence peak at 453.11 nm and 490.19 nm. Blue fluorescence emission of crystal ensures that the material can be used in the production of blue light emitting diodes [38]. The crystal also shows one red fluorescence peak at 682.59
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nm.
Fig.12.Emission spectrum of STBPH 23
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3.8. Z-Scan measurements By Z-scan technique, the third order nonlinear response of the STBPH has been explored. This technique was carried out using He-Ne laser of intensity 5 mW [39]. In this Z-scan technique self-focussing or self-defocussing of laser beam is made in a thin
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nonlinear medium with sample thickness (L) [40]. The sample was translated across the +Z to -Z axial direction by using stepper motor to change the incident intensity falling on the sample. Normal transmittance for closed aperture, open aperture and ratio of closed to open
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as a function of sample position has been done for the nonlinear sample. In open aperture, the nonlinear transmission of sample without aperture was calculated in the far field as the sample was shifted through the focal point. This allows us to determine the nonlinear absorption coefficient β. In closed aperture (i.e. aperture in place in the far field) the
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transmittance of the sample through the aperture is monitored in the far field as a function of
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the position, Z of the nonlinear sample. It is done in the vicinity of the linear optics focal position [41] to measure nonlinear refraction. The ratio of closed to open normal
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transmittance function of sample position Z have been made for the crystal and it is portrayed in the Fig. 13(c).The normalized transmittance of the sample through the aperture
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is monitored in the far field as a function of the position Z. The measurable quantity (∆TP-V ) can be defined as the difference between the transmittance change of peak and valley Tp-TV
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and on axis phase shift at the focus |∆φ | is given by:
TPV 0.406(1 S ) 0.25 0
(I 1)
S is the linear transmittance aperture and is estimated by,
2ra 2 S 1 exp 2 a .
(I 2) 24
ACCEPTED MANUSCRIPT Here ra is the radius of the aperture and ωa is the beam radius at the aperture. The nonlinear refractive index (n2) can be calculated by using the relation:
n2
0 KI 0 Leff
(I 3)
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where |φ| is the on axis phase shift, where I0 is the on-axis irradiance at focus (Z=0) (I0 = 26.31 MWm-2) Leff is the effective thickness of the sample which can be calculated using the relation
Leff [1 exp(L)] /
. Here L denotes the sample length, α represents the linear
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absorption coefficient, I0 is the laser beam intensity at focus Z = 0 and k represents the wave number (k = 2π/λ).
The nonlinear absorption coefficient can be evaluated from the knowledge of open aperture
2 2T I 0 Leff
(I 4)
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trace, using the relation
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In normalized transmittance curve obtained from the closed aperture Z scan data, we can observe that peak is followed by valley. This result suggests that the sign of refraction
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nonlinearity is negative i.e., self-defocussing. The self-defocussing effect is due to the local
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variation of refractive index with temperature. The defocussing nature of the material is due to the negative refractive index and this property is used in the shielding of optical sensor such as night vision devices [26]. For the negative refractive index laser damage threshold value of the sample will also be higher [40].
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Fig. 13.(a) Closed aperture spectrum of STBPH (b)Open aperture spectrum of STBPH
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(c) ratio of the closed to open Z scan traces of STBPH The real and imaginary parts of the third order nonlinear optical susceptibility were
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estimated using the formula:
10 4 ( 0C 2 no n2 ) cm2 W
(I 5)
10 2 ( 0 C 2 n0 ) cm2 (esu) 4 2 W
(I 6)
2
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Re (3) (esu)
Im
( 3)
Where, ε0 is the vacuum permittivity, c is the velocity of light in vacuum, no is the linear refractive index of the sample and λ is the wavelength of laser beam. The third order nonlinear optical susceptibility of the crystal can be evaluated using the relation: 26
ACCEPTED MANUSCRIPT (3) (Re( (3) ))2 (Im( (3) ))2
(I 7)
Table 4 gives the experimental details and the results of Z-scan technique for STBPH. The nonlinear absorption coefficient (β) value is 0.03 x 10-4cm2W-1and its positive value stipulates that two photon absorption have been taken place [42].The nonlinear optical
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properties of STBPH make it as a resourceful candidate in the field of optical limiting application.
Table 4: Obtained nonlinear optical parameters from Z-scan measurements of STBPH
Measured values
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Third-order nonlinear properties
1.63 x 10-8cm2W-1
Nonlinear refractive index (n2)
0.03 x 10-4cm2W-1
Real susceptibility (χ3 ) Imaginary susceptibility (χ3 )
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Nonlinear absorption coefficient (β)
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4. Conclusion
0.16 x 10-6 esu 1.21 x 10-6 esu
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Absolute susceptibility (χ3)
1.20 x 10-6 esu
Optically transparent single crystals of STBPH have been successfully grown by
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slow aqueous evaporation process. The lattice parameters of STBPH was affirmed by XRD
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analysis and the crystal system corresponds to Rhombohedral with the space group ̅̅̅̅ R3. The vibrational spectral analysis established the different functional groups that contribute the grown crystal. The measurable optical factors such as Urbach energy, band gap and extinction coefficient of transparent STBPH crystal suggest that the material can be concentrated in optoelectronics and fabrication of devices. The thermal analyses reveal that the sample is thermally stable up to 142 ˚C. PL spectra exhibit a sharp blue emission peak at 453.11 nm suggest its application for fabricating lasers. The Vickers microhardness test 27
ACCEPTED MANUSCRIPT concludes that the STBPH belong to the category of soft NLO material. The nonlinear refractive index (n2), absorption coefficient (β) and third-order nonlinear susceptibility (χ3) resolved using the technique of Z-scan substantiate that STBPH could be an admirable contender for the future optoelectronic, photonic and nonlinear optical applications.
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Acknowledgement
The scientific supports rendered by sophisticated analytical instrument facility and Department of Chemistry, IITM Chennai for support in single crystal XRD,
FTIR,
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FT-RAMAN is gratefully acknowledged. Also the authors thank for the facilities arranged by Department of Physics, St. Joseph College, Trichy for providing microhardness and PL measurement. References
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cadmium dizinc diborate, CdZn2(BO3)2, Z. Kristallogr. NCS 223 (2008) 3-4. [2] Becker P “Borate materials in nonlinear optics”, Adv. Mater. 10 (1998) 979-992. [3] Zhang-Gui Hu, Naoki Ushiyama, Yoke Khin Yap, Masashi Yoshimura, Takatomo
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potassium dichromate single crystals by Z-scan technique, Optik 124 (2013) 4716–4720. [41] M. G. Kuzyk and C. W. Dirk, Eds., Characterization Techniques and Tabulations for
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Organic Nonlinear Materials, Marcel Dekkar Inc. 1998 655-692. [42] M. Nageshwari, C. Rathika Thaya Kumari, G. Vinitha, M. Peer Mohamed , S. Sudha,
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M. Lydia Caroline, Crystal growth, structural, spectral, thermal, dielectric, linear and nonlinear optical characteristics of a new organic acentric material: L-Methionine-Succinic acid (2/1), J. Mol. Struct. 1155 (2018) 101-109.
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Figure shows the Z-scan signature of STBPH in (a) Closed aperture and (b) Open aperture
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Table 1: Single crystal XRD data of STBPH Crystal
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Table 2: Assignments for FTIR and FT-RAMAN Table 3: Mechanical parameters of STBPH Table 4: Obtained nonlinear optical parameters from Z-scan measurements for STBPH
Figure caption
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ACCEPTED MANUSCRIPT Fig. 1a. Photograph of as grown Sodium tetraborate pentahydrate crystals Fig.1b. Solubility curve of STBPH in water Fig. 2(a,b). FTIR and FT-RAMAN Spectra of STBPH Fig. 3.UV-Vis spectrum of STBPH Fig.4. The plot of dT/dλ versus wavelength
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Fig 5. Tauc's Plot of STBPH
Fig. 6(a)Plot of Extinction coefficient (K) with photon energy (b) Variation of reflectance (R) and extinction coefficient(K) with absorption coeffficient(α) (c)Plot of refractive index (n0) with photon energy (d) Variation of electrical conductivity and optical conductivity
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with photon energy (e) Plot of electric susceptibility vs. photon energy. Fig. 7. Plot of ln(α) with hν
Fig.8 (a) Plot of Log f vs dielectric constant and (b) Plot of Log f vs dielectric loss
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Fig.9. Plot of 1000/T verses Log σac
Fig.10. (a) Microhardness (b) Meyers plot (c) Stiffness constant (d) Knoop's hardness
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number
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Fig.11. TG/DTA curve for STBPH single crystal Fig.12.Emission spectrum of STBPH
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Fig. 13 (a) Closed aperture spectrum of STBPH (b) Open aperture spectrum of STBPH
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(c) ratio of the closed to open Z-scan traces of STBPH
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