Studies onMechanical and Dielectric Properties of L-Phenylalanine Benzoic Acid Single Crystal for NLO Applications

Nonlinear optics (NLO)materials have a signi�cant impact on laser technology, optical communication, optical storage technology, and electrooptic modulation. Nonlinear optical single crystal of L-phenylalanine benzoic acid has been grown by slow evaporation. e �RD analysis con�rms that the crystal belongs to the monoclinic system with noncentrosymmetric space group P21. Microhardness investigations are conducted on the grown crystals. e dielectric response of the sample is studied as a function of different frequencies and different temperatures. e photoconducting studies con�rm that the title compound has negative photoconducting nature.


Introduction
In recent years, great efforts have been made to develop new organic, inorganic, and semiorganic nonlinear optical (NLO) crystals due to their widespread applications such as frequency conversion, high-speed information processing, optical communications, and optical data storage [1][2][3].In the �eld of nonlinear optical crystal growth, amino acids play a vital role.Amino acids exhibit natural chiral properties and crystallize in the noncentrosymmetric space groups, which are an essential criterion for NLO applications.Optically active amino acids show high efficient optical second harmonic generation (SHG) and are promising candidates for laser and optical communication technology.Since isometrically pure and optically active substances always meet the symmetry requirements for optical second harmonic generation, there is considerable variation in efficiency for the generation of SHG, ranging from almost zero to greater than that of potassium dihydrogen phosphate crystals.Crystals capable of generating second harmonics must have a unit cell with no centre of inversion and this requirement is met by the crystals of pure amino acids because these molecules themselves are asymmetric [4][5][6].A number of Lhistidine compounds exhibiting the NLO behaviour, namely, L-histidine acetate [7], L-histidine chloride monohydrate [8], L-histidine tetra�uoroborate [9], L-histidine hydrochloride monohydrate [10], L-histidine hydro�uoride dihydrate [11], L-histidine bromide [12], and L-histidinium trichloroacetate [13], were reported earlier.e crystal growth and characterization of L-histidinium tri�uoroacetate and Lhistidine nitrate were reported from this laboratory [14,15], recently.In the present paper, attention is focused on the mechanical, dielectric, and photoconductivity properties of L-phenylalanine benzoic acid single crystals.

Experimental Procedure
L-Phenylalanine benzoic acid (LPBA) was synthesized from L-phenylalanine and benzoic acid taken in equimolar ratio 1 : 1. e calculated amount of the reactants was thoroughly dissolved in deionized water and stirred well using magnetic stirrer to ensure uniform temperature and concentration throughout the entire volume of the solution.e solution was �ltered and transferred to crystal growth vessels and crystallization was allowed to take place by slow evaporation under room temperature.A good colourless single crystal of LPBA was obtained in 30 days.

Single X-Ray Diffraction
From the XRD data, it is found that LPBA belongs to the monoclinic system with    Å,    Å and   180 Å, the space group being P 1 , and the volume of the system   1 Å 3 .e results are very close in agreement with that of reported values [16].

Mechanical Properties
To �nd surface hardness of the as grown LPBA crystal, microhardness was measured from 5 to 25 gram load using HMV Microhardness tester.e Vickers hardness number (  ) was calculated using the standard formula: where  is the applied load and d is the mean diagonal length of the indentation.e trace is shown in Figure 1, which shows that the hardness increases with the increase of load.Meyer's index number was calculated from Meyer's law, which relates the load and indentation diagonal length as the following: where  is the material constant and "" is Meyer's index.
In order to �nd the value of ", " a graph is plotted for log  against log  (Figure 2) which gives a straight line.From the slope of the line Meyer's index number "" was calculated to be 2.63.According to Onitsch [17], "" lies between 1 and 1.6 for hard materials and is greater than 1.6 for so materials [18].e "" value observed in the present studies is around 2.63 suggesting that the grown LPBA crystal is a relatively so material.

Dielectric Studies
e dielectric constant and the dielectric loss of the LPBA crystals were studied at different temperatures using HIOKI 3532 LCR HITESTER in the frequency region from 50 Hz to 5 MHz.e dielectric constant was measured as a function of frequency at different temperatures ranging from 35 ∘ C, 50 ∘ C, and 75 ∘ C and is shown in Figure 3, while the corresponding dielectric losses are depicted in Figure 4. Figure 3 shows the plot of dielectric constant () versus log frequency for 35 ∘ C, 50 ∘ C, and 75 ∘ C. It is seen that the value of dielectric constant is high in the lower-frequency region for all the temperatures and then it decreases with increase in frequency.e high value of dielectric constant at low-frequency region is attributed to space charge polarization due to charged lattice defects [19].A graph is drawn between dielectric loss and log frequency for various temperatures (35 ∘ C, 50 ∘ C, and 75 ∘ C) and is shown in Figure 4. e low value of dielectric loss at high frequency suggests that the grown crystals possess good optical quality.is parameter is of vital importance for nonlinear optical materials in their applications [20].

Photoconductivity Studies
Figure 5 shows the plot of dark current and photocurrent against electric �eld.It is observed from the graph that both dark current and photocurrent increase linearly with the applied electric �eld, but the photocurrent is less than the dark current which is termed as negative photoconductivity.e negative photoconductivity in a solid is due to the reduction in the number of charge carriers or their lifetime in the presence of radiation [21].e decrease in mobile charge carriers during negative photoconductivity can be explained by the Stockman model [22].e photocurrent was less than the dark current, signifying negative photoconducting nature.

Conclusion
Single crystals of LPBA are grown by slow evaporation technique in a period of 30 days.e structure of the grown crystal was con�rmed by single crystal �RD analysis.e grown LPBA crystal is a relatively so material from the investigations of microhardness.e dielectric studies were studied as a function of frequency.e variation of dielectric constant and dielectric loss was studied with varying frequency at different temperatures.Photoconductivity studies con�rm that the crystal possesses a negative photoconducting nature.

F 1 :F 2 :
Vickers hardness versus load.Plot of log  versus log .

F 3 :
Variation of dielectric constant with frequency.