Jerald Vijay R et al Journal ofCrystalGrowth338(2012)170–176

8/13/2019 Jerald Vijay R et al Journal ofCrystalGrowth338(2012)170176

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Journal of Crystal Growth 338 (2012) 170176

Growth, structural, optical and thermal properties of potential THz

material: N, N-dimethylamino-N-methylstilbazolium 4-styrenesulphonate

R. Jerald Vijaya, N. Melikechib, Tina Thomasa, R. Gunaseelana, M. Antony Arockiaraja

and P. Sagayaraja*

aDepartment of Physics, Loyola Col lege, Chennai, I ndia

bDepartment of Physics and PreEngineering,

Centr e for Research and Education i n

Optical Sciences and Applications, Delaware State University,

Dover DE 19901, US.

Abstract

N, N-dimethylamino-N-methylstilbazolium 4-styrenesulphonate (DSSS), was synthesized by

metathesization of the N, N-dimethylamino-N-methylstilbazolium iodide (DASI) salt with

sodium 4-styrenesulfonate. The growth of DSSS single crystal was carried out by adopting

the slope nucleation coupled slow evaporation method. DSSS crystals with size 4x2x1mm3

were grown within a period of 15 days. The structure and composition of the crystal were

studied by single crystal X-ray diffraction, CHN and nuclear magnetic resonance (NMR)

analyses. The FTIR results reveal the existence of the vinyl groups and their corresponding

vibrational modes. The melting point and thermal behavior of DSSS were investigated using

differential scanning calorimetric (DSC) and thermogravimetric analyses (TGA).

Key Words: A1. Crystal structure;A2. Growth from solutions; B1. Organic compounds; B2.

Nonlinear optical materials; B3. Terahertz technology.

*Corresponding author

Dr. P. Sagayaraj, Associate Professor of Physics, Loyola College, Chennai600 034, India

Email:[email protected], Tel: +9144 28178200; Fax: +9144 28175566


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1. IntroductionOrganic crystals have been a recent source of interest as THz emitters as they have

been reported to generate stronger THz signals than commonly used semiconductor or

inorganic electro optic emitters owing to their large second-order nonlinear electric

susceptibility [1]. They offer vast design possibilities to tailor the linear and nonlinear

properties, and due to the almost completely electronic origin of the nonlinearity, they are

well suited for future high speed devices [2]. However, only very few organic materials could

so far be crystallized in reasonable crystal size and optical grade to realize various

applications. Organic crystals like; N, N-dimethylamino-N-methylstilbazolium

p-toluenesulphonate (DAST), have very high NLO coefficients and at the same time have a

low dielectric constant and thus making them a perfect choice for THz generation [3, 4].

By far, the generation of broadband high power THz waves from DAST single crystal

has been realized up to 30 THz by difference frequency generation (DFG) and, the generation

of sub-10 THz waves are achieved by optical rectification (OR) with appropriate lasers [5, 6].

However, a number of absorption features in the THz spectra of DAST limits its application,

for example; the strongest absorption at 1.1 THz is attributed to the TO phonon resonance

and it leads to a gap in the emitted spectrum at this frequency. Another major constraint for

the development of DAST crystal is; the formation of hydrated DAST co-crystals, which

destroys the noncentrosymmetric crystal structure [3]. The formation of hydrated DAST

prevents the use of water as a solvent for the growth of large size DAST crystal. Hence,

efforts have been made to find high quality THz materials which can serve as alternates for

DAST. Interestingly, these kinds of materials are searched within those materials which

exhibit DAST-like structure.

When DAST crystals are used as terahertz wave generators using difference

frequency generation (DFG), polarized direction of the two laser beams with different

wavelengths should be set parallel to crystallographic a-axis because the diagonal component

d11 of DAST crystals is used. However, in DAST crystals, the cations with a large first-

hyperpolarizability () align tilted from the polar a-axis by about 20 0[7], which means that

the maximum performance of the cation has not been brought out as macroscopic NLO

properties in DAST crystals. In this connection, the anion exchange of DAST is worth

investigating to find crystals with larger NLO properties [8, 9]. An important feature of anion

is its effectiveness in solubility control. For crystal growth from solution, appropriate

solubility of the crystal is required [10]. The studies on the property variation caused by anion


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exchange are considered as an interesting subject to find out new compounds not only with

large NLO properties but also with easy crystal-growing ability [11].

In an effort to develop new molecules with high molecular nonlinearities, three series

of stilbazolium derivatives with various sizes of the counter anions have been systematically

investigated by Yang et al to understand the effect of counteranions on the crystal structure

and SHG activity of these compounds [12]. The counter anions attempted in this experiment

included the methyl group as well as bulky counter anions of naphthalenesulfonate and

styrenesulfonate. The use of p-styrenesulfonate resulted in 4-N,N-Dimethylamino-4-N-

methyl stilbazolium 4-styrenesulfonate (DSSS); an interesting DAST derivative, which not

only exhibited the DAST-like structure but also possessed powder SHG efficiency almost

equal to that of DAST [12]. But unfortunately, only polycrystalline DSSS was obtained by

slow evaporation technique, even though the material was well-dissolved in DMF. In another

attempt, Ogawa et al have adopted a different approach to synthesize DSSS by the anion

exchange reaction of N,N-dimethylamino-N-methylstilbazolium iodide (DASI) with silver

arylsulfonate [11]. By using this procedure, tiny DSSS crystals of size up to 0.5 mm were

obtained. However; owing to the smaller size of the crystals, the studies were limited to the

preliminary analysis with powder XRD, elemental analysis, FT-IR and proton NMR.

In view of the DAST-like structure and the high SHG efficiency observed in DSSS, it

is expected that the crystal can act as a THz emitter. But the size of the crystal has to be

further improved to realize various applications including, the THz generation. In our earlier

work [4], we reported on the growth of DAST crystal by rapid evaporation, as a continuation

of our on-going research on organic THz materials, an attempt has been made in this article

to investigate the growth of bulk size crystals of DSSS. In the present work, DSSS was

prepared by metathesization of the N,N-dimethylamino-N-methylstilbazolium iodide (DASI)

salt with sodium 4-styrenesulfonate. Deviated from the earlier works, mixed solvent of

water:methanol (1:1) was used successfully to grow bulk size crystal of DSSS. The size of

the harvested crystal has been improved at least 8 times than the previously reported work

[11, 12]. The structure of the crystals has been solved by single crystal XRD analysis. As a

potential THz material, the DSSS crystal was subjected to CHN, NMR, TG-DTA, DSC and

FT-IR studies.

2. Experimental

2.1 Synthesis

DSSS was prepared by metathesization of the N,N-dimethylamino-N-methylstilbazolium iodide (DASI) salt with sodium 4-styrenesulfonate. DASI was


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synthesized by the condensation of 1,4-dimethyl pyridinium iodide (2.35 g, 10 mmol),

methanol (30 ml) and 4-N, N-dimethylamino-benzaldehyde (1.79 g, 10 mmol) in the

presence of piperidine (0.2 ml) [13]. The above mixture was refluxed for 20 hours and then

cooled to room temperature. The product was filtered and recrystallized from methanol at

least three times. The reaction scheme employed in the present synthesis of DSSS is

presented in Fig. 1. The metathesizaton reaction was carried out as follows: 1 g (2.73 mmol)

of DASI was dissolved in 100 ml of distilled water by heating and simultaneously, 0.563 g

(2.73 mmol) of sodium p-styrenesulfonate was dissolved in 20 ml of water by heating. The

two hot solutions were mixed and further heated for 30 minutes at 70 0C and then cooled to

room temperature. The resulting product (DSSS) appeared as a greenish precipitate due to the

anion exchange reaction. The aqueous sodium iodide was separated from the former by

vacuum filtration. The purity of DSSS was further improved by successive recrystallization.

Fig. 1 Synthesis scheme for DSSS


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2.2 Solubility

In order to investigate the crystalline habit of DSSS, several solvents in pure or mixed

forms were investigated to grow DSSS crystal. Initially, various combinations of solvents

were taken on a trial and error basis; solvents like methanol-acetonitrile, methanol-water,acetonitrile-water and methanol-acetonitrile-DMF were tried. Interestingly, only in the

methanol-water mixed solvent system, the crystal formation was observed and for other

systems, the crystallization was absent. Further, we changed the volume ratios of methanol

and water in different ratios and the study revealed the possibility of crystal growth only in

equal volume ratio of methanol-water mixed solvent system. The solubility diagram for

DSSS in methanol-water (taken in 1:1 ratio) at different temperatures of 30, 35, 40, 45 and

50

0

C is shown in Fig. 2. DSSS has a positive solubility coefficient in methanol:watersolvent, since its solubility increases with the temperature, the crystal can be conveniently

grown by the method of temperature lowering or slow solvent evaporation method.

Fig. 2 Solubility curve of DSSS in methanol-water

2.3 Crystal growth

Crystal growth was performed by employing slope nucleation coupled slow solvent

evaporation technique. The details of the experimental setup used for slope nucleation

method are discussed elsewhere [4]. Based on the solubility data, 1 g of DSSS was dissolved

in 150 ml of methanol-water solvent system (75 ml of methanol and 75 ml of water) at 45 oC.

The solution was prepared in a Teflon beaker with a Teflon plate with parallel grooves; it was


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then sealed with a perforated cap and kept in a constant temperature bath at 45 oC. After 15

days of evaporation, crystals with size up to 4 x 2 x 1 mm3were harvested. The size of the

crystal has increased at least 8 fold when compared to the 0.5 x 0.5 mm2plate-shaped crystals

reported by Ogawa et al [11]. The DSSS crystals appeared green in colour (Fig. 3). We

believe that the growth of the crystal with mixed solvent of water-methanol along with the

slope nucleation coupled slow evaporation method have favoured the growth of good quality

size crystals. In a similar manner, during the growth of N, N-dimethylamino-N-

methylstilbazolium p-chlorobenzenesulphonate (another DAST derivative); Matsukawa et al

have successfully used mixed solvents of methanol-acetonitrile to improve the size of the

crystal [5].

Fig. 3 Photograph of DSSS single crystals

3. Results and Discussion

3.1 Single crystal X-ray diffraction analysis

The crystallographic structure of DSSS was determined by single X-ray diffraction

analysis using a Bruker Kappa APEX II diffractometer. The structure was determined from

the single crystal XRD intensity data collected at 293 K. The absorption correction was given

using the semi-empirical from equivalents. The structure solution and refinement were

performed using SHELXL program. The structure was solved by direct methods and full

matrix least-squares refinements using F2taking all the unique reflections. The crystal data

for DSSS along with experimental conditions and structure refinements parameters are


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presented in Table 1. All the non-hydrogen atoms were refined with anisotropic thermal

parameters. The H-atoms which participated in the H-bonds were located from the different

Fourier and refined with isotropic thermal parameters. The hydrogen co-ordinates and

isotropic displacement parameters for DSSS are listed in Table 2. [CCDC 824517 contains

the supplementary crystallographic data for this paper. These data can be obtained free of

charge via www.ccdc.cam.ac.uk/data_request/cif, by e-mailing [email protected]

or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge

CB2 1EZ, UK; fax: +44 1223 336033]. It is evident from Table 1 that the DSSS crystal

belongs to monoclinic system with space group Cc and point group m with four ion-pairs per

unit cell. It is observed that the crystal system, space group and cell parameters of DSSS are

almost similar to that of DAST. Interestingly, there are a few other DAST derivatives like N,

N-dimethylamino-N-methylstilbazolium 2,4,6-trimethylbenzenesulphonate (DSTMS), N,N-

dimethylamino-N-methylstilbazolium p-trifluoromethylbenzenesulphonate (DSPFS) and

DASC which have DAST-like structure and thereby regarded as isomorphs of DAST [12, 14,

15]. The present XRD data of DSSS is almost matching with those crystal data for DAST

reported by Marder et al. and also for DSSS reported by Ogawa et al. where DSSS is referred

as material 2e [7, 11]. The ORTEP representation of one ion-pair of DSSS is shown in Fig. 4

and the crystal packing diagram is given in Fig. 5. For ion structure of DSSS, disorder was

observed in the anion part although the cation structure was determined unequivocally. The

vinyl group of DSSS is seen located right and left to the molecular long axis on the benzyl

ring plane. This is explained by -conjugation stabilization between the benzene ring and

vinyl group in DSSS. The complete parallel alignment for cation, as evident from the DSSS

crystal packing, is expected to give maximum second order NLO performance of the cation

in microscopic scale [16].


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Fig. 4 ORTEP representation of one ion-pair of DSSS

Fig. 5 Crystal packing diagram of DSSS


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Table 1

Crystal data and structure refinement for DSSS

Identification code shelxl

Empirical formula C24H26N2O3S

Formula weight 422.53

Temperature 293(2) K

Wavelength 0.71073

Crystal system, space group Monoclinic, Cc

Unit cell dimensions a = 10.6005(4) alpha = 90 deg.

b = 11.2150(4) beta = 92.533(2) deg.

c = 17.9452(7) gamma = 90 deg.Volume 2131.32(14) 3

Z, Calculated density 4, 1.317 Mg/m3

Absorption coefficient 0.180 mm-1

F(000) 896

Crystal size 0.30 x 0.25 x 0.20 mm

Theta range for data collection 2.27 to 23.79 deg.

Limiting indices -11


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Table 2

Hydrogen coordinates ( x 104) and isotropic displacement parameters

(2x 103) for DSSS.

________________________________________________________________x y z U(eq)

________________________________________________________________

H(1A) -3555 1702 3353 135

H(1B) -3119 2854 2941 135

H(1C) -3150 2837 3814 135

H(2) -1370 2118 2356 90

H(3) 646 1434 2413 87

H(5) 353 931 4600 96

H(6) -1623 1661 4513 97H(7) 2313 376 4090 92

H(8) 2514 698 2596 88

H(10) 4427 133 2045 87

H(11) 6412 -592 2078 84

H(13) 6308 -901 4302 97

H(14) 4313 -212 4247 98

H(15A) 7968 -1969 2202 131

H(15B) 9269 -1823 2642 131

H(15C) 8586 -704 2291 131

H(16A) 8483 -940 4242 141H(16B) 9301 -1865 3820 141

H(16C) 8007 -2255 4134 141

H(17) 4877 1147 5696 86

H(18) 2725 979 5843 107

H(20) 2411 4497 5912 99

H(21) 4547 4685 5802 79

H(23') 833 1624 6228 123

H(24A) 323 3898 5749 151

H(24B) -697 2949 6026 151H(23) 563 3595 5706 98

H(24C) 634 1371 6344 130

H(24D) -617 2154 6167 130

________________________________________________________________


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3.2 CHN Analysis

The elemental composition of the DSSS crystal was analyzed using Perkin-Elmer

series II 2400 CHNS/O elemental analyzer. The calculated values of C24H26N2O3S are;

C=68.22; H=6.2; N=6.63 and the values experimentally found are; C=67.96; H=6.18;N=6.68. Thus there is a close agreement between the calculated and experimental values of

CHN.

3.3 NMR Analysis

The proton NMR spectrum of DSSS was recorded by dissolving the sample in

deuterated methanol using Burker AVANCE III 500 MHz FT NMR spectrometer. NMR

spectrum is analysed in order to confirm the molecular structure of the synthesized

compounds by identifying the presence of Hydrogen nuclei. In the proton NMR spectrum

(Fig. 6) of DSSS, the singlets at 3.09 and 4.215 are assigned to three CCH3hydrogens and

six N-(CH3)2hydrogens. The doublets at 5.33 and 5.89 are due to two hydrogens of CH2 and

the multiplet at 6.79 is due to CH hydrogen of the vinyl group. The doublets at 7.63 and 7.51

are attributed to the four hydrogens of the N-(CH3)2-C6H4aromatic ring. The doublets at 7.79

and 7.86 are attributed to the two aromatic hydrogens ortho to SO3 and two aromatic

hydrogens ortho toCH3. The doublets at 7.98 and 8.52 are due the four hydrogens ortho to

the C5H4N aromatic ring. The doublet at 7.12 is due to the two oliphinic hydrogens

(HC=CH).

Fig. 6 NMR spectrum of DSSS


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3.4 Optical absorption spectral analysis

Figure 7 shows the absorption spectrum of DSSS recorded in solid phase using

SHIMADZU spectrophotometer in the wavelength region 200 to 900 nm. Since the DSSS

crystal is iso-structural to DAST one may not expect much deviation in the optical absorptionproperty. The optical absorption spectrum of DSSS dissolved in methanol was also recorded.

It is well known that when DAST and its derivatives are dissolved in methanol, it leads to

dissociated state generating free cations and anions [17]. Thus, the absorption behaviour of

DSSS in the visible region is not different from that of DAST in the solution phase. The

observed major peak at 476 nm represents the stilbazolium chromophore [18].

200 300 400 500 600 700 800 900

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Absorption(A.U.)

Wavelength (nm)

DSSS-Bulk

DSSS-methanol

Fig. 7 Optical absorption spectrum of DSSS

3.5FTIR spectral analysis

The sample was characterized by FT-IR spectroscopy in order to identify the

functional groups and detect the vibrational modes of molecules of the sample. The FTIR

spectrum recorded using BRUKER IFS 66V FT-IR spectrometer. The measurement was done

with KBr method for the wavelength range 400 to 4000 cm-1and the spectrum is shown in

Fig. 8. The characteristic frequencies observed between 500 and 700 cm-1 are due to the out

of plane ring bending modes and frequencies between 1100 and 1200 cm-1 are assigned to the

in plane ring deformation modes are observed. The CH stretching mode is observed at 3032


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cm-1 [4, 19]. The peaks at 3010, 1437, 1029 and 941 cm-1are assigned to CH2 symmetric

stretch, CH2scissoring CH2rocking and CH2wagging of the vinyl group. The peaks that are

observed at 1370 and 1342 cm-1 are attributed to the ring C-C stretching and ring C-H in-

plane bending, vinyl C-H rocking and CC stretching modes, respectively [20].

Fig. 8 FTIR spectrum of DSSS

3.6 Thermal Analysis

The thermal behavior of DSSS was investigated by DSC and TG-DTA techniques

using Perkin-Elmer DSC-7 and TGA-7 spectrometers respectively. The endothermic peak

seen at 277 oC in the DSC traces (Fig. 9) is attributed to the melting of the DSSS sample.

This result is in agreement with the thermal analysis report on DSSS by Ogawa et al [11].

The TG-DTA thermogram (Fig. 10) indicates that the DSSS crystal starts decomposing at

around 300 oC. There are three stages of weight loss in the sample, the first decomposition

occurs between 300 and 350 oC. In the second stage, a major weight loss of 39.59 % is

noticed which may be due to the release of SO3and the third stage of decomposition takes

place after 500 0C which corresponds to the removal of toluene. A comparison of melting

point of the sample with a few other stilbazolium derivatives with promising SHG activity is

listed in Table 3. It is evident that DSSS is thermally more stable than DAST as well as

DAST derivatives like DSTMS, DSDMS, DSANS, DSAS, DSMAS and DSMOS [11 - 13].

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 450.0

0.3

2

4

6

8

10

12

14

16

18

20

22

24

26

28

29.0

cm-1

%T

3435.65

3032.19

1644.55

1574.69

1527.43

1437.71

1370.47

1342.46

1208.26

1160.27

1029.04

997.05

941.67

888.56

855.07

834.98

813.80

672.98

559.39

530.89

499.04


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Fig. 9 DSC Traces of DSSS

Fig. 10 TG-DTA Thermogram of DSSS


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Table 3

Comparison of melting point of stilbazolium derivatives

Crystal Melting point (0C) References

DAST 2561 [12]

DSDMS 2671 [12]

DSTMS 2581 [12]

DSANS 2631 [12]

DSAS 2721 [13]

DSMAS 2561 [13]

DSMOS 2641 [13]

DASC 2811 [15]

DSSS 2771 Present work

4. Conclusion

The growth of a novel single crystal of DSSS was achieved by slope nucleation

coupled slow evaporation method. The present study demonstrated the enhancement in the

size of the crystal achieved through proper choice of solvent and growth conditions. The

improvement in the size of the crystal assumes much significance as the material is known

for its high SHG efficiency. Single crystal XRD analysis confirmed the DAST-like structure

of the grown crystal. The composition of the sample was verified by CHN, NMR and FT-IR

studies. The thermal analysis ascertains superior thermal stability of DSSS when compared to

DAST and a few other DAST derivatives. The development of DSSS crystal with its superior

SHG activity coupled with moderate thermal stability makes it a potential material for THz

generation. Further studies on the THz-TDS spectra of the crystal are under progress and it

will be reported soon.


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Acknowledgement

The authors acknowledge the University Grants Commision (UGC), India for funding

this research work (F.38-119/2009(SR)). The authors thank Dr. Babu Varghese,

Sophosticated Analyatical Instrumrntation Facility (SAIF), IIT Madras, for single crystalXRD analysis.

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Jerald Vijay R et al Journal ofCrystalGrowth338(2012)170–176

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