Bulk Growth and Characterization of D-(–)-Alanine Single Crystals

doi:10.4236/jcpt.2013.34021

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Journal of Crystallization Process and Technology, 2013, 3, 130-135

http://dx.doi.org/10.4236/jcpt.2013.34021 Published Online October 2013 (http://www.scirp.org/journal/jcpt)

Bulk Growth and Characterization of D-(–)-Alanine Single

Crystals

Kalimuthu Moovendaran, Subramanian Natarajan*

School of Physics, Madurai Kamaraj University, Madurai, India.

Email: *[email protected]

Received July 2nd, 2013; revised August 2nd, 2013; accepted August 9th, 2013

Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original

work is properly cited.

ABSTRACT

Single crystal of D-(–)-alanine (DALA), a non-linear optical material from the amino acid family was grown using a

home-made crystal growth setup. The crystals of DALA were also grown by slow evaporation solution technique (SEST).

The grown crystals were characterized by using single crystal X-ray diffraction, high resolution X-ray diffraction (HRXRD)

and UV-vis-NIR and CD spectroscopy. Measurements of Vicker’s microhardness, laser damage threshold (LDT) value

and second harmonic generation (SHG) efficiency are also reported. Thermal and dielectric studies were also carried out.

Keywords: D-(–)-Alanine; NLO Material; MKN Setup; HRXRD; CD; SHG

1. Introduction

The search for new materials with high optical nonlin-

earities is an important field of research due to their prac-

tical applications in harmonic generation, amplitude and

phase modulation, switching and other signal processing

devices. Alanine is the simplest amino acid with an

asymmetric carbon atom. L-Alanine and DL-Alanine are

the usually available forms of alanine. D-Amino acids do

not occur in nature and are usually synthesized by manu-

facturers. The crystals of L-alanine [1] and DL-alanine [2]

were grown by slow evaporation method, characterized

and their NLO property established. Recently, the spec-

tral characterization, second harmonic generation (SHG)

and hyper Raleigh scattering studies of D-(–)-alanine

were reported from our laboratory [3]. The bulk growth

of the single crystal of D-(–)-alanine has not been re-

ported till date. In the present investigation, a bulk single

crystal of D-(–)-alanine (DALA) was grown using a

home-made crystal growth setup. The crystals of DALA

were also grown by slow evaporation solution technique

(SEST). The crystals were characterized by using XRD,

HRXRD and spectroscopic techniques. Several useful

properties of the crystal, such as microhardness, LDT

and SHG were evaluated. Thermo gravimetric and dif-

ferential thermal analysis curves were recorded and in-

terpreted. Measurements of dielectric constant and loss

were also made.

2. Crystal Growth Experiments

2.1. Growth of DALA Crystals Using Slow

Evaporation Solution Technique (SEST)

D-Alanine (C3H7NO2) is a white crystalline powder with

a molecular weight of 89.09. Commercially available

D-alanine (M/s. Loba Chemie Pvt. Ltd., purity: 99%)

was further purified by recrystallising it once using water

as the solvent. A saturated solution of D-alanine was

prepared and allowed to evaporate at a room temperature

(30˚C). Transparent colourless crystals of size: 10 × 5 × 3

mm were obtained within a period of one week.

2.2. Growth of a Bulk Crystal Using a Home

Made Setup

A new setup was designed in our laboratory to conduct

several crystal growth experiments simultaneously [4].

The home made setup (called as MKN setup) is made up

of two large tanks and it consists of several ampoules,

dimmerstat, temperature controller, heating coil and a

thermometer. Constant temperature and uniform tem-

perature gradient were maintained, to increase the growth

rate and quality of the crystal grown. The details of this

setup has been given elsewhere [4,5]. A suitable seed

crystal was chosen from the crystals obtained by the

*Corresponding author.

Bulk Growth and Characterization of D-(–)-Alanine Single Crystals

131

SEST experiment and mounted for the crystal growth

along the [020] direction. An aqueous saturated solution

was prepared and transferred to the growth ampoule. A

growth run for 20 days resulted in a crystal of sizes: 15

mm diameter and 45 mm length, the average growth rate

being about 2 mm per day. The photograph of the grown

crystal of DALA is shown in Figure 1.

3. Characterization

The grown crystal was subjected to single crystal X-ray

diffraction studies using an Enraf Nonius CAD-4/MACH

3 diffractometer, with MoKα radiation (0.71073 Å). The

accurate cell parameters of the grown crystals at room

temperature (25˚C) were obtained from the least-squares

refinement of the setting angles of 25 reflections. To

study the crystalline perfection of the grown crystals, a

multicrystal X-ray diffractometer developed at the Na-

tional Physical Laboratory, New Delhi [6] has been used

for recording the high-resolution diffraction curves (DCs)

for (0 2 0) diffracting plane in symmetrical Bragg ge-

ometry. The description of the above instrument has al-

ready been given elsewhere [7]. The transmittance of the

grown crystal (for a crystal of thickness: 2 mm) was

measured using a Perkin-Elmer Lambda-35 spectropho-

tometer in the wavelength range of 200 - 1100 nm with a

slit of width 2 nm and scan speed of 240 nm/min. CD

spectra were recorded in the wavelength range of 150 -

400 nm at room temperature, using a JASCO J-180 spec-

tropolarimeter. The aqueous solution (3 mM) of the sam-

ple was kept in a quartz cuvette of path length 0.1 cm.

Microhardness measurements were made on the cut and

polished plate of the crystal using a Shimadzu Micro-

hardness Tester (Model No. HMV2T) with a diamond

indenter. The well-polished crystal was mounted on the

platform of the microhardness tester and loads of differ-

ent magnitudes were applied over a fixed interval of time

(8 s). The laser damage threshold (LDT) measurement

has been carried out for the grown crystal using a

Q-swtiched Nd: YAG laser (λ = 1064 nm). The second

Figure 1. Photograph of the grown crystal.

harmonic generation behaviour of the powdered materi-

als was tested using the Kurtz and Perry method [8],

making use of the laser source described above. Simul-

taneous thermogravimetry (TG) and differential thermal

analyses (DTA) of powdered samples were performed in

the temperature range of 25˚C to 900˚C, using a Netzsch

STA 409 PC/PG thermal analyzer at a heating rate of

10˚C/min. An Al2O3 (alumina) crucible was used and it

served as a reference for the sample. Dielectric constant

and loss measurements were made using a HIOKI

352-50 LCR meter in the frequency range of 100 Hz - 1

MHz. Crystals of reasonable dimensions were cut from

the bulk crystal and polished using the solvent (water). A

silver coating was applied on the opposite sides of the

crystal and placed between two copper electrodes and

thus a parallel plate capacitor was formed.

4. Results and Discussion

4.1. Single Crystal X-Ray Diffraction

From the single crystal X-ray diffraction studies, the lat-

tice parameters were determined as: a = 6.043(3), b =

12.337(5) and c = 5.784(3) Å. These values are in agree-

ment with those reported by Sullivan et al. [9]. It is al-

ready known that this compound crystallizes in the or-

thorhombic system with the space group P212121. The

density of the single crystals of DALA was determined

as 1.37(2) gm/cm3 using the floatation method. The

melting point was found out as 290(2)˚C.

4.2. High-Resolution X-Ray Diffraction Analysis

The high-resolution diffraction curve (DC, Figure 2) was

recorded for the DALA single crystal using (0 2 0) dif-

fracting planes in symmetrical Bragg geometry by em-

ploying the multicrystal X-ray diffractometer described

above with MoKα1 radiation. On deconvolution of the

diffraction curve, it is clear that the curve contains an

additional peak which is 105 arc s away from the main

peak. The additional peak corresponds to an internal

structural low angle grain boundary. For better under-

standing, the schematic of a structural grain boundary is

given in the inset of Figure 2(a). As seen in the inset,

two regions of the crystal are misoriented by a finite an-

gle α also known as tilt angle. Tilt angle may be defined

as the misorientation angle between the two crystalline

regions on both sides of the structural grain boundary.

The two regions may be perfect. If the value of α is ≤1

arc min, it is considered as very low angle boundary. If α

> 1 arc min but less than a deg, it is considered as low

angle boundary. The tilt angle for the very low angle

boundary is 105 arc s with respect to the main crystal

block. The full width at half maximum (FWHM) of the

main peak and the boundary are respectively 23 and 30

arc s. The low FWHM values of main peak and the very

Bulk Growth and Characterization of D-(–)-Alanine Single Crystals

132

Figure 2. High resolution DC for the crystal.

low angle boundary indicate that the crystalline perfec-

tion of the specimen is reasonably good. It may be men-

tioned here that such minute defects could be detected

with well resolved peaks in the diffraction curve only

because of the high-resolution of the diffractometer,

characterized by very low values of wavelength spread

i.e. Δλ/λ and horizontal divergence for the exploring or

incident beam, which are respectively around 10−5 and

much less than 3 arc s of the multicrystal X-ray diffrac-

tometer used in the present studies.

4.3. UV-vis-NIR and CD Spectroscopic Studies

From the UV-vis-NIR transmission spectrum (Figure 3),

it is seen that the compound has no absorption in the re-

gion between 229 nm and 1100 nm. The optical trans-

mittance of the crystals grown using the MKN setup is

52%, which is 6% higher than that for the crystal grown

by SEST. The reason for the higher transparency of the

crystal grown using the MKN setup may be the absence

of inclusions, which reduces absorption of the UV-vis-

NIR radiation. The CD spectrum of DALA unambigu-

ously confirms its chiral purity and thus enables its char-

acterization as D-(–)-alanine (Figure 4). From the nega-

tive Cotton effect observed, it can be concluded that the

compound DALA is D-(–)-alanine.

4.4. Microhardness Studies

Microhardness is an important parameter which helps to

know about the mechanical properties of materials such

as fracture behavior, yield strength, brittleness index and

temperature of cracking. Figure 5 shows the plot be-

tween the hardness value (Hv) and load P. From the re-

sults, it is observed that the hardness value increases with

increase of load up to 50 g and with a further increase in

applied load, cracks have been observed on the crystal

surface due to the release of internal stress generated

locally by indentation. It is also observed that the me-

Figure 3. Transmission spectrum of DALA.

Figure 4. CD spectra of D-(–)-alanine.

Figure 5. Variation of Vickers hardness number with ap-

plied load.

chanical stability of the DALA crystal grown in the

MKN setup is higher compared to the crystals grown by

Bulk Growth and Characterization of D-(–)-Alanine Single Crystals

133

SEST. A plot obtained between log (P) and log (d) gives

a straight line (Figure 6). The relation connecting the

applied load (P) and diagonal length (d) of the indenter is

given by the Meyer’s law: P = adn, where, n is the Mey-

er’s index or work hardening coefficient and a, a constant

of proportionality. The work hardening coefficient value

(n) of the crystals grown using SEST and the MKN setup

were determined by the least-squares fit method and

found to have a values of 1.75 and 1.68, respectively.

Onitsch [10] has pointed out that n lies between 1.0 and

1.6 for moderately hard materials and it is more than 1.6

for soft materials. Hence, it is concluded that DALA sin-

gle crystal is a soft material.

4.5. Laser Damage Threshold (LDT)

Measurements

The laser damage threshold (LDT) measurement has

been carried out for the grown crystal using a Q-swtiched

Nd: YAG laser (λ = 1064 nm) with pulse width of 10 ns,

operated at the repetition rate of 10 Hz. The laser beam

(1 mm diameter) was focused on the sample using a lens

with focal length of 20 cm. The spot size of the focused

beam on the sample was 0.3 mm. The measured LDT

values of DALA single crystals grown using SEST and

the MKN setup are 6.8 GW/cm2 and 7.5 GW/cm2, re-

spectively. It is observed that the crystal grown in the

MKN setup has slightly higher LDT value than that of

the crystal grown by SEST.

4.6. Second Harmonic Generation (SHG)

Conversion Efficiency

Quantitative measurement of the SHG conversion effi-

ciency of the sample was made using the powder tech-

nique developed by Kurtz and Perry. The output from the

sample was monochromated to collect only the second

Figure 6. Plot of log d vs log P.

harmonic (λ = 532 nm), eliminating the fundamental and

the intensity was measured using a photomultiplier tube.

A SHG signal of 3 mV for the crystal was obtained for

an input energy of 3.9 mJ/pulse. The standard KDP crys-

tal gave a SHG signal of 13 mV for the same input en-

ergy. It is seen that the SHG efficiency of the sample

crystal is about 23% of that of the standard KDP crys-

tals.

4.7. Thermal Studies

In the TG-DTA (Figure 7), an endothermic peak ob-

served at 290˚C in DALA corresponds to the melting

point of the material. It is seen from the TG plot that the

melting and decomposition of the compound occur si-

multaneously. Hence, this compound may be useful for

its applications upto its melting point. The exothermic

peak observed at 466˚C is due to the phase transition

expected to happen in the carbon residue.

4.8. Dielectric Studies

The dielectric constant and dielectric loss of the DALA

crystal were studied as a function of frequency, at dif-

ferent temperatures (30˚C, 40˚C, 50˚C and 60˚C). How-

ever, the variation of dielectric constant and the dielectric

loss with frequency only at the room temperature (30˚C)

are shown in Figure 8. It is observed from the plots, that

both the dielectric constant (ε') and the dielectric loss (ε")

are decreasing rapidly and get saturated at high frequen-

cies. A comparison of the dielectric response of the crys-

tals suggests that the dielectric constant and dielectric

loss of the crystals grown using the MKN setup are less

compared to that for the crystal grown by SEST. The

low values of dielectric loss at high frequencies suggest

that the crystal grown using the MKN setup possesses

enhanced optical quality with low density of defects

[11].

Figure 7. TG-DTA plots of DALA.

Bulk Growth and Characterization of D-(–)-Alanine Single Crystals

134

(a)

(b)

Figure 8. (a) The variation of dielectric constant with fre-

quency; (b) The variation of dielec tri c loss wi th freque ncy.

5. Conclusion

Bulk NLO crystal of D-(–)-alanine was grown using a

home-made (MKN) crystal growth setup. The optical

transparency, the values of LDT, microhardness, dielec-

tric constant and dielectric loss are found to be better for

the crystal grown in the MKN setup compared to that

grown by SEST. The bulk crystal of DALA also pos-

sesses a high melting point and reasonably good crystal-

line perfection. The value of the SHG conversion effi-

ciency of the crystal was found to be about 23% of that

of the standard KDP crystal. Hence, the crystal of D-

(–)-alanine grown using the MKN setup may have some

applications as a NLO material.

6. Acknowledgements

The authors are thankful to Dr. G. Bhagavannarayana,

National Physical Laboratory, New Delhi, for the help in

recording the HRXRD curve. SN thanks the Council of

Scientific and Industrial Research, New Delhi for the

financial support under the Emeritus Scientist Scheme.

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