ABSTRACT

Significant betterments have been accounted recently in the field of nonlinear optics in the area of materials engineering and the associated optoelectronic device technologies which experiences a revival of both practical and conceptual nature. The requirements of nonlinear optical materials are primarily regularized by the nonlinear figure of merit, and by the equally relevant optical quality and robustness of the materials towards different applications. Recent advances in non-linear optical materials (NLO) have invoked a large revival of interest in this area of research on account of their widespread industrial requirements. The noetic construction of structurally controlled supramolecular assemblies (e.g., acentric and chiral solids) remains a great challenge even though the art of chemical synthesis of discrete molecules has significantly advanced in recent years. The relevance of organic materials in this percolating context is due to the fact that the delocalized electronic structure of Π-bonded organic compound offers a number of tantalizing opportunities in applications as non-linear optical materials. Organic crystals in terms of non-linear optical property possess advantages, when compared to their inorganic counterparts. Acentric molecules consisting of highly delocalized Π electron systems interacting with suitably substituted electron donor and acceptor groups exhibit a high value second order polarizability (β). L- Asparaginium picrate (LASP) is one such p donor acceptor molecular compounds in which l- Aasparagine acts as donor and the picric acid as electron acceptor.

LASP crystals were grown by slow evaporation solution growth technique. Picric acid is less soluble in water, whereas L-asparagine shows more affinity towards solubility in water only. Equimolar quantities of the parent compounds picric acid (Loba Chemie, 99%) and L-asparagine (Loba Chemie, 99%) were dissolved in a mixture of acetone and water (1:1). On cooling of the solution, the salt was obtained by crystallization at low temperature (25 °C). The material was purified over again from aqueous solution by the recrystallization process. Single crystals of LASP have been grown from saturated solution (pH = 2.01) of the synthesized salt of LASP by the slow evaporation solution growth technique at 30 °C using a constant-temperature bath having the control accuracy of +0.01 °C. Yellowish crystals of size 1.5 x 0.8 x 0.4 cm ³ have been obtained .

X-ray diffraction data were collected at room temperature using a single crystal X-ray diffractometer (ENRAF NONIUS CAD4, MoKa, l=0.71069. The high resolution X-ray diffraction analysis was carried out to study the structural perfection of LASP. A multicrystal X-ray diffractometer (MCD) developed at NPL(National Physical Laboratory, New Delhi), set in (+,-,-,+) configuration has been employed using MoKa₁ radiation originated from a fine focus sealed tube X-ray generator ( Philips, 1743; 2 kW). The rocking curve of LASP crystal recorded for (101) bears two additional peaks, with adjacent angles 40 and 12 arc sec away from the main peak. From the HRXRD results we elucidate that the incorporation of solvent into the growing crystal which is common in solution grown crystal is responsible for the very low angle boundaries and we conclude that the quality of the crystal is good.

Carefully selected samples of LASP were cut and polished on a soft tissue paper with fine grade alumina powder (0.1mm) dispersed in a mixture of acetone, water and DMFO(1:1:4). Each sample was covered on both sides with silver paste (air drying) to make the sample to behave as a parallel plate capacitor.A HIOKI 3635 Model LCR meter was used to measure the capacitance, dielectric loss (tan d) and resistance of the sample as a function of frequency ( range 30 – 95°C). A small cylindrical furnace (20 cm x 20 cm x 20 cm), whose temperature was controlled by a Eurotherm temperature controller ( +0.01 °C) was used to house the sample. The dielectric constant was calculated by using the relation

ε′ = C t /ε_oA

where ε_o is the vacuum permitivitty, t is the thickness of the sample, C is the capacitance and A is the area of cross section. The conductivity of the crystal was calculated using the relation

σ_ac = 2 Π f ε_oε′ tanδ

The results of dielectric behaviour are show that the dielectric constant (ε′ ) and dielectric loss (tanδ) decreases with increase of frequency. The large values of dielectric constant at low frequency suggest that there is a contribution from all four known sources of polarization namely, electronic, ionic, dipolar and space charge polarization. The dielectric constant and dielectric loss decrease with increasing frequencies . The low value of dielectric loss (tanδ) indicates that the grown crystals of LASP are of reasonably good quality. The high dielectric constant makes the title crystal a better newbie for electro- optic modulators.

Qualitative measurement of relative SHG efficiency of LASP with respect to well known SHG materials KDP and Urea was made by Kurtz and Perry powder technique. The relative SHG efficiency elicits that the material is 66.5 times greater than that of KDP and 10 times that of Urea.

The hardness measurements were made on the prominent (301) plane of the crystal of thickness of 0.3 cm using Leitz –Wetzler Vicker’s hardness tester fitted with a Vicker’s diamond pyramidal indenter and attached to an incident light microscope (Neophot-2 of Carl-Zeiss, Germany). Loads ranging from 0.5 g to 5 g were used for making indentations, keeping the time of indentation constant at 10 s for all the cases. The fracture toughness K_c was calculated using the relation.

K_c = k P / a l^1/2.

where the constant k=1/7 for the Vickers indentor, l =C-a is the mean palmquist crack length.The value of Brittleness index Bi was calculated using

Bi = H_v/ K_c

The hardness study projects the crystal is moderately hard. The fracture toughness, brittleness index and the yield strength of LASP have been enunciated . The results will be presented in detail.

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