Mechanical, thermal, linear and nonlinear optical

Mater. Res. Express 4 (2017) 016502

doi:10.1088/2053-1591/aa56cc

PAPER

RECEIVED

29 November 2016

Mechanical, thermal, linear and nonlinear optical properties of barium L-tartrate single crystal

ACCEP TED FOR PUBLICATION

4 January 2017 PUBLISHED

30 January 2017

K Rajesh1 and P Praveen Kumar2 1 2

Department of Physics, AMET University, Eeast Coast Road, Kanathur, Chennai 603112, Tamilnadu, India Department of Physics, Presidency College, Chennai 600005, Tamilnadu, India

E-mail: [email protected] Keywords: organometallic, hardness, nonlinear optics, dielectric

Abstract A potential semiorganic nonlinear optical (NLO) single crystal of barium L-tartrate (BaTr) was grown by slow evaporation technique. Single and powder x-ray diffraction study was carried out for the grown crystal. The hardness of the material was carried out by a Vickers micro hardness tester. Thermal behavior of the crystal was studied by TG-DTA thermal analyzer. Optical and electrical conductivity of the crystal was measured by photo conductivity and dielectric studies. NLO property of the crystal is confirmed by Kurt–Perry powder technique. Laser damage threshold (LDT) value of the grown crystal has been carried out using a Q-switched Nd:YAG laser beam.

1. Introduction Crystals are very important in the field of research and technology. Many crystals have been grown with the aim of identifying new materials for practical and industrial application purposes [1–3]. Organic crystals are a suitable candidate for nonlinear optical (NLO) application for their chemical properties [4, 5]. However most organic NLO crystals have poor mechanical and thermal properties [6]. Semi-organic NLO crystals have been proposed as a new approach. They have the combined properties of both inorganic and organic crystals, which make them a suitable candidate for applications [7]. Single crystals of tartrate compounds exhibit ferroelectric, piezoelectric, dielectric, NLO, and spectral characteristics [8, 9]. Tartrate crystals are very useful in transducer, linear and nonlinear mechanic devices, and the fabrication of crystal oscillators and resonators [10, 11]. Barium is a metallic alkaline earth metal. It can be used in vacuum tubes, and high temperature superconductors. Among the semi organic NLO materials, metal complexes of Barium which have low UV cut off wavelengths, applicable for high power frequency conversion, have received significant attention. These materials can be used as better alternatives for KDP crystals in frequency doubling and Laser fusion experiments due to their high values of laser damage threshold (LDT) and mechanical strength. BaTr is a promising semi organic NLO material for second harmonic generation (SHG). The crystal structure of Barium Tartrate (BaTr) was reported by Gonzalez et al [12]. The crystal was crystallizing in the orthorhombic crystal system with the space group P212121. The growth of BaTr crystal in Gel medium and its growth kinetics was reported [13, 14]. In the present work, investigations have been made on electrical, mechanical, thermal and NLO characteristics of BaTr single crystal.

2. Materials and methods 2.1. Solubility test Solubility is one the most important parameters for the growth of good quality crystals in a low temperature solution growth method. The solubility of a BaTr crystal was determined at different temperatures by dissolving it in millipore water in an airtight container maintained at a constant temperature with continuous stirring. After saturation was attained, the equilibrium concentration of the solute was analyzed gravimetrically.

© 2017 IOP Publishing Ltd


K Rajesh and P P Kumar

Figure 1. Solubility curve of BaTr.

Figure 2. Photograph of as grown BaTr crystal.

The same procedure was repeated to estimate the solubility for different temperatures. Figure 1 shows the solubility curve of BaTr in water and acetone solutions. From the figure it is found that the BaTr crystal is more soluble in water than acetone. 2.2. Synthesis and growth Barium chloride (Merck, 99% of purity) and L-tartaric acid (Merck, 98 % of purity) were taken in the 1:1 equimolar ratio to synthesis BaTr single crystal. Water has been used as the solvent. The reactants were thoroughly dissolved separately in deionized water at room temperature. Further the parent solutions are mixed together and continuously stirred for 8 h for obtaining the saturated solution. The saturated solution is transferred into evaporating dish. The solution is allowed to evaporate in room temperature. Seed crystals of BaTr were formed in 3 d from the saturated solution prepared. Further seed crystals were collected carefully from the parent solution and recrystallization was carried out to grow bulk and transparent single crystal. Colorless single crystal was obtained in a period of 22 d. Figure 2 shows the photograph of as grown BaTr single crystal.

3. Results and discussion 3.1. XRD analysis The cell parameter values of BaTr crystal were confirmed by both single and powder x-ray diffraction studies. Single crystal x-ray diffraction studies of BaTr crystal was carried out using an Enraf Nonius CAD-4/MACH 3 diffractometer, with MoKα radiation. Single crystal XRD studies shows that the lattice parameter values of the grown crystal are a = 8.129 Å, b = 9.107 Å and c = 8.426 Å and its crystallized in the orthorhombic crystal system

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Figure 3. Powder x-ray diffraction pattern of BaTr Crystal.

Table 1. Powder x-ray diffraction data for BaTr. 2θ

D

hkl

14.594

6.064

110

18.032

4.9154

111

19.633

4.518 00

020

24.185

3.6769

201

24.868

3.5775

121

31.431

2.8518

221

35.998

2.4956

311

38.366

2.3445

132

with the space group P212121.These values are in good agreement with those values reported by Gonzalez-Silgo et al [12]. Powder x-ray diffraction pattern of BaTr crystal was recorded on a Rich Seifert diffractometer with Cukα (λ = 1.540 Å) radiation. The powder sample was scanned over the range 10–70° at a scan rate of 1° min−1. The PXRD pattern of the crystal is shown in figure 3. The PXRD data has been carried out and tabulated in table 1. The PXRD peaks indexed and the cell parameter of the crystal were calculated. The cell parameter values of the grown BaTr crystal found from single crystal XRD is compared with the reported values and is shown in table 2. 3.2. Dielectric studies The dielectric study of BaTr single crystal was carried out using the HIOKI 3532-50 LCRHITESTER instrument. The capacitance values for grown BaTr crystal are found for frequencies varying from 50 Hz to 5 MHz at different temperature range. The rectangular shaped crystal of thickness 2.74 mm area is used for dielectric studies. The dielectric constant is calculated using the formula ε′ =

Ct ε0A

where C is the capacitance value, t is the thickness of the crystal, A is the area of the crystal, and ε0 is the absolute permittivity in the free space. The dependence of dielectric constant, dielectric loss on log frequency of the applied AC voltage was studied in different temperature range. Figures 4 and 5 shows the variation of dielectric constant (ε′) and dielectric loss with log frequency at different temperature ranges. The dielectric behavior of the material is described in two frequency intervals, first in lower frequency range and second in higher frequency range. Both dielectric constant and dielectric loss are decreasing with increasing frequency. It is also found that the dielectric constant and dielectric loss changes with temperature at low frequencies. It shows that both dielectric constant and loss are dependent on temperature in lower frequency range [15] In the higher frequency region, the value of dielectric constant almost attain saturation at all temperatures. In higher frequency range it is clear that the dielectric constant and loss of the material is strongly independent on temperatures. The dependence of temperature is almost exists on higher frequency.

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Table 2. Unit cell parameters of BaTr. Cell parameters

Present work

Reported values

a

8.129 Å

8.181 Å

b

9.107 Å

9.036 Å

c

8.426 Å

8.392 Å

Crystal system

Orthorhombic

Orthorhombic

Volume

623.7853 Å3

620.3661 Å3

α = β = γ

90°

90°

Space group

P212121

P212121

Figure 4. Log frequency verses dielectric constant.

Figure 5. Log frequency verses dielectric loss.

3.3. Microhardness studies The hardness of the crystal carries information about the mechanical strength, molecular binding between atoms, and yield strength and elastic constants of the materials [16]. In the present study, micro hardness measurements were carried out on BaTr single crystals. Vickers micro hardness number was then evaluated from the relation ⎛ (1.8544 P ) ⎞ ⎟ (kg mm−2) HV = ⎜ ⎝ ⎠ d2

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Figure 6. Load verses hardness number plot.

Figure 7. Plot of log d verses lop P.

where, ‘Hv’ is the Vickers microhardness number, ‘P’ is the applied load and ‘d’ is the diagonal length of the indentation impression. To evaluate the Vickers micro hardness, several indentations were made on the face of the crystal. The diagonal length of the indentations was measured using a micrometer eyepiece. Dependence of the microhardness on the load for BaTr crystal has been evaluated. Loads of different magnitude (25 g, 50 g, 75 g and 100 g) were applied on the crystal surface for the fixed interval of time. Figure 6 shows the variation of load (P) with hardness number of BaTr crystal. Hardness is found to increase gradually up to the load value of 100 g. At lower loads there is an increase in the hardness number which can be attributed to the electrostatic attraction between the zwitter ions present in the molecule [17]. Above the load of 75 g multiple cracks were developed on the crystal surface due to the release of internal stresses generated locally by indentation. The hardness coefficient ‘n’ of the material was calculated from the slope drown between log P and log d (figure 7) and it is found to be 1.55. According to Onitsch, n lies between 1 and 1.6 for hard materials and n is greater than 1.6 for soft materials [18]. Thus, BaTr belongs to hard material category. This implies that BaTr single crystal is a good engineering material for device fabrications. 3.4. Thermal analysis Differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) give information regarding phase transition, crystallization and different stages of decomposition of the crystal system [19].The TGA of BaTr crystals were carried out between 0 °C and 600 °C using TGA Q500 instrument in the nitrogen atmosphere at a heating rate of 10 K min−1. 5



Figure 8. TG and DTA curve of BaTr crystal.

Figure 9. Photoconductivity plot of BaTr crystal.

Figure 8 shows the TGA&DTA curve of BaTr crystal. The sharp weight loss of the material starts around 79.5 °C. The crystal is completely free of any water of crystallization or any physically adsorbed water on the surface. 9.71 percentage of weight loss is occurred in the first stage. Heating the crystal above 140 °C resulting the liberation of volatile substances, probably carbon dioxide and ammonia. The sharp exothermic peak absorbed at 240 °C matches with the decomposition point of the material. The crystal was fully decomposed at 600 °C. 3.5. Photo conductivity studies Photoconductivity studies were carried out for the BaTr crystal using keithley 485 picoammeter at room temperature. The dark current (Id) of the sample was measured using DC power supply and picoammeter. The light from a halogen lamp is focused on the material using convex and the photo current is measured. DC supply is increased step by step from 10 V to 100Vand the photo current (Ip) was measured. Figure 9 shows the variation of photocurrent and dark current as a function of applied field. It is observed from the plot that dark current (Id) and photo current (Ip) of the sample increase linearly with the applied field and the dark current is always higher than the photo current. This phenomenon is known as negative photoconductivity which may be due to the reduction in the number of charge carriers of their life time in the presence of radiation. When the sample is kept under exposure to light, the recombination of electrons and holes takes place, resulting in decrease in the number of mobile charge carriers, giving rise to negative photoconductivity [20]. 6



Table 3. LDT value of the BaTr with KDP and Urea (single shot method). Compound

LDT (GW cm−2)

BaTr

4.3

Tartaric acid

5.4 [21]

KDP

0.20 [22]

Urea

1.50 [22]

3.6. Laser damage threshold study The LDT of an optical crystal is an important factor affecting its applications. It depends on the specific heat, thermal conductivity and optical absorption of the crystal. LDT measurement of the BaTr crystal has been carried out using a Q-witched Nd:YAG laser beam of wavelength 1064 nm with the pulse width of 10 ns. The repetition rate of the measurement is 10 Hz. The laser beam of focal length 1 mm is focused on the sample of 0.4 mm. The LDT value of the grown crystal is found to be 4.3 GW cm−2. The laser damage threshold value of BaTr crystal with KDP and urea and tartaric acid crystals are shown in table 3. 3.7. Second harmonic efficiency The NLO property of the grown crystal is studied by the Kurtz–Perry powder technique [23]. The standard KDP crystal has been used as the reference material for the grown crystal. A finely crushed and powdered sample was obtained from grown BaTr crystal. The powder sample is further made as pellets and used for laser interactions. The fundamental laser beam was split into two components using a beam splitter. One was sent to the sample through the centre hole concave mirror and the other half component was sent to photodiode for calibration. The SHG output wave at 532 nm was collected by the mirror, collimated and focused on to an photomultiplier for detection. The output SHG signal of 29.7 mV for the BaTr crystal was obtained for an input energy of 5 mJ/ pulse, whereas, the KDP crystal gave an output of 15.3 mV for the same input signal. Thus it is evident that the SHG efficiency of the grown BaTr crystal is 1.94 times that the KDP crystal and the crystal is suitable for device fabrication.

4. Conclusion A potential semi organic NLO single crystal of BaTr was grown by slow evaporation technique. Solubility studies show the crystal is more soluble in water compare to acetone. Single crystal x-ray diffraction study confirmed that the grown crystal belongs to the orthorhombic system with the space group P212121. The hardness of the material indicates that the material belongs to hard material category. This implies that BaTr single crystal is a good engineering material for device fabrications. Thermal studies reveal that BaTr crystal thermally stable up to 79 °C. Photo conductivity studies shows that BaTr is possess negative photo conductivity. The laser damage threshold value of the grown crystal is found to be 4.3 GW cm−2. The SHG efficiency of the Grown BaTr crystal is 1.94 times of that the KDP crystal and the crystal is suitable for device fabrication.

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