Optical Characterizations of L-Alaninium Maleate Single Crystals

MATERIALS AND METHODS

In the present study, L-alanine (AR) is doped with maleic acid (AR) in different concentration and synthesized by Slow Evaporation method (Kumar et al., 2008; Chitra and Palaniswamy, 2010; Bright and Freeda, 2010; Lucia Rose et al., 2010). The grown crystals were subjected to Optical characterization studies such as single crystal XRD (Dhanuskodi and Vasantha, 2004) FTIR, optical absorbance were done (Prabha and Palaniswamy, 2010). Kurtz and Perry SHG test confirms the NLO property of the grown crystals (Franken et al., 1961; Bloembergen, 1965).

Pure L-alanine lattice parameter values and its pattern were compared with JCPDS (Joint Committee on Powder Diffraction Standards) file No. 28-1508. Pcpdfwin and National Bureau of Standards there by studied its dopant intrusion in the grown crystals.

Synthesis: L-alaninium maleate (LAMA) was synthesized from L-alanine (CH₃CHNH₂COOH) and maleic acid (C₄H₄O₄), both were mixed in different molar ratio, 1:0.25, 1:0.50, 1:0.75 and 1:1 (LAMA).

Instrumentation: The single crystal X-ray data were collected using an automatic X-ray diffractometer (MESSRS ENRAF NONIUS, The Netherlands) with MoK_α (λ = 0.717 Å) radiation. The freshly ground powder samples of LAMA crystals were subjected to Powder X-Ray Diffraction (PXRD) analysis, using an X-ray powder diffractometer, PANalytical with scintillation counter and monochromated Cu K_α (λ = 1.54056 Å) radiation. The FT-IR spectrum was recorded in the range of 4000-450 cm^–1 using BRUKER IFS 66V FT-IR SPECTROMETER. The optical absorption spectrum was recorded in the range of 190-800 nm using VARIAN CARY 5E UV-Vis-NIR SPECTROPHOTOMETER. The NLO efficiency of LAMA crystal were evaluated by Kurtz and Perry powder technique using Q-switched Nd:YAG laser.

RESULTS AND DISCUSSIONS

Single crystal XRD analysis: The single crystal XRD data of the LAMA crystals with different concentration of maleic acid are presented in Table 1. It is observed that the different concentration of maleic acid mixed in LAMA crystals have orthorhombic structure with a space group of P2₁2₁2₁, which is recognized as non-centrosymmetric, thus satisfying one the basic and essential material requirements for the SHG activity of the crystal (Zyss et al., 1984). The lattice parameters (a, b and c) and unit cell volume of the LAMA crystals increases with the increase of concentration of maleic acid in LAMA crystal (Raj and Madhavan, 2011).

Powder X-ray diffraction analysis: The XRD powder pattern has been indexed and the lattice parameters are evaluated. The Bragg’s diffraction peaks were indexed for the orthorhombic system with the space group P2₁2₁2₁.

Fig. 1:	Powder XRD pattern

Table 1:	Lattice parameters from single crystal XRD

Table 2:	Lattice parameters from powder XRD
#: Indicates lattice parameter values of LAMA crystal from literature (Balasubramanian et al., 2009)

The continuous peak shifting rule out the phase separation or separate nucleation of L-Alanine and maleic acid crystals (Fig. 1) with increasing dopant from a to d.

The uniform unit cell volume expansion reveals that concentration of maleic acid play a role in lattice expansion but do not modify the lattice structure of LAMA Table 2.

FTIR analysis: The FTIR absorption spectrum of LAMA crystals in the region 4000-450 cm^–1 is shown Fig. 2 (with increase in dopant from a to d). The NH stretching bond of aromatic compounds is generally observed in a range between 3200 and 3300 cm^–1. In the present study, it is observed at 3240 cm^–1. The vibration observed at 1361 cm^–1 was attributed to C-C stretching vibration. This analysis also indicates that the protonation of carboxyl group in Alanine takes place by maleic acid. The absorptions of LAMA have been compared with those of the L-alanine (Nakamoto, 1985).

Fig. 2:	FTIR Spectrum

Table 3:	FTIR band Assignments of LAMA crystals

The observed lower wave number and hence, the lower energy indicate the large hydrogen bonded association of LAMA in the crystal lattice. Table 3 shows the band assignments of LAMA crystals for various concentrations. The shifts in the positions of the characteristic peaks confirm the formation of the compound.

Optical absorption spectrum analysis: The optical absorption spectrum of LAMA (with different concentration) is shown in Fig. 3, in the wavelength range 190-800 nm. No absorption is observed in the visible region of the UV-V is spectra which is due to electronic transitions between the carboxylate anion (-COO^–) and the nitryl cation (NH₃⁺) (Shakir et al., 2009; Misoguti et al., 1996).

Fig. 3:	UV-Vis absorption spectrum

Fig. 4:	Plot for Optical band gap

A UV cut-off below 300 nm is sufficiently low for SHG laser radiation at 1064 nm or other applications in the blue region. Optical band gap has been calculated from the UV-V is absorbance data Table 4.

The absence of absorption bands in the visible region and the wide band gap of the grown crystal (Fig. 4) attest to the suitability of the grown crystal for photonic and optical applications (Rao and Smakula, 1965).

NLO studies: A high intensity Nd:YAG laser (λ = 1064 nm) with a pulse duration of 8 nsec was passed through the powdered sample. The SHG behavior was confirmed from the output of the laser beam having the green emission (λ = 532 nm).

Table 4:	Optical bandgap energy and cutoff wavelength of pure LA and LAMA crystal

Table 5:	SHG efficiency values of LAMA crystals

For a laser input pulse of 6.5 mJ, the second harmonic signal (532 nm) of 122.5 and 53 mV were obtained through LAMA and KDP samples, respectively (Table 5). The good second harmonic generation efficiency indicates that the LAMA crystals can be used for applications in nonlinear optical devices.

CONCLUSION

L-alaninium maleate (LAMA) single crystals were grown by mixing pure L-alanine with maleic acid in the molar concentration 1:0.25, 1:0.50, 1:0.75 and 1:1 and the crystals were grown using slow solvent evaporation technique. These crystals were characterized with single X-ray diffraction analysis and found to be orthorhombic crystal structure with the space group P2₁2₁2₁. The lattice parameters calculated for all the grown crystals were compared with the values with powder X-ray diffraction analysis and finds a perfect matching. The growth rate along the c-axis is large compared to other two crystallographic axes. It is found, among the well developed faces the (111) face is larger in area. It is verified that the increases in volume of the unit cell with the addition of maleic acid identifies its presence in the lattice construction. FTIR transmission spectrum is recorded in the range 4000-450 cm^–1. The observed lower wave number and lower energy indicates the large hydrogen bonded association of LAMA in the crystal lattice. The vibration observed at1361 cm^–1 was attributed to C-C stretching vibration. Optical absorption spectrum shows some significant blue shift in the shape and position of the absorption peaks but not affected the optical absorbance in the visible range and a small variation observed in a UV range is due to crystalline perfection of the grown crystals. The optical band gap energy is found to increase with increase of its molar concentration and UV cut off wavelength of these crystal decreases with increasing concentration revealing that the cut off wavelength can be tuned revealing as a potential material for frequency conversion. The nonlinear optical property was confirmed by Kurtz and Perry powder technique. The emission of green radiation confirms the existence of SHG.