The Effect of Cobalt Mixing on Pure Copper Mercury Thiocyanate Nonlinear Optical Crystal

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 691-694

Published Online July 2012 (http://www.SciRP.org/journal/jmmce)

The Effect of Cobalt Mixing on P u re Copper Me rcury

Thiocyanate Nonlinear Optical Crystal

B. Vijayabhaskaran1, C. Ramachandra Raja2*

1Department of Physics, Anjalai Ammal Mahalingam Engineering College, Kovilvenni, India

2Department of Physics, Government Arts College (Autonomous), Kumbakonam, India

Email: *[email protected]

Received March 3, 2012; revised April 13, 2012; accepted May 9, 2012

ABSTRACT

The nonlinear optical crystals of cobalt (Co2+) mixed copper mercury thiocyanate have been grown by slow evaporation

method using water and ethanol as solvents. The grown crystals have been subjected to different characterization

analyses and the results were compared with pure copper mercury thiocyanate crystal (CMTC), which has been already

reported. The single crystal X-ray diffraction shows that the addition of metallic impurity does not alter the basic struc-

ture of the parent crystal, but increases the cell volume markedly. The presence of functional groups has been identified

using FT-IR analysis. Further the grown crystal is characterized by optical transmission analysis and thermal analysis.

The thermal stability of the grown crystal is high, compared to pure CMTC crystal. The optical transparency of the

grown crystal is studied by UV-Vis-NIR analysis. This study reveals that Co2+ mixed CMTC crystal has wider trans-

parent waveband than pure CMTC crystal. The relative second harmonic generation efficiency of the Co2+ mixed

CMTC crystal has been tested by Kurtz-Perry powder technique.

Keywords: Crystal Growth; Slow Evaporation Method; X-Ray Technique; FT-IR; Nonlinear Optical Material; Thermal

Analysis

1. Introduction

Bimetallic thiocyanate complexes of type AB(SCN)4 and

their derivatives are much potentially useful among the

inorganic systems because all of them contain -S=C=N-

bridges, which connect A and B atoms, forming infinite

two dimensional or three dimensional networks. The

infinite networks produce a relatively large polarization

which induces relatively large macroscopic nonlinearities

in the materials [1]. Compared to organic crystals, the

inorganic crystals have good physicochemical stabilities

and larger second order nonlinearities. Due to these rea-

sons, the inorganic crystals are gaining popularity in the

field of nonlinear optics. Inorganic complex crystals have

wide range of application in the field of optical disk data

storage, laser remote sensing, optical information proc-

essing, optical computing, laser driven fusion, colour

display and medical diagnostics [2-4]. Some of the doped

crystals of the bimetallic thiocyanate complexes are also

found to exhibit nonlinear optical properties [5,6]. In the

present work an attempt has been made by substituting

certain amount of Hg2+ by Co2+ in the already reported

crystal copper mercury thiocyanate (CMTC) [7]. The

characterization of the new crystal was compared with

respect to pure CMTC crystal.

2. Experimental Details

The synthesis, growth and characterization of pure CMTC

crystal has been already reported [7]. According to the

above literature, the raw materials were taken in the

proper stoichiometric ratios and then dissolved in de-

ionized water and ethanol using the following reaction.



22 4

CuClHgCl4KSCNCuHg SCN4KCl 

The solution was then filtered twice to remove any in-

soluble impurities. Then, the purity of the compound was

increased by successive recrystallization processes. By

using the slow evaporation technique, large crystals of

CMTC were successfully grown from supersaturated

solution at a temperature 35˚C in a constant temperature

bath of accuracy ±0.01˚C [7]. The same procedure was

repeated for the growth of Co2+ mixed CMTC crystal by

substituting 75% of Hg2+ by Co2+. Within 22 days many

tiny crystals were formed by spontaneous nucleation.

After a period of 4 weeks, the grown crystals were har-

vested and subjected to different characterization meth-

ods. Due to the addition of metallic impurity in the pure

CMTC crystal, the colour of the crystal is changed from

*Corresponding author.

B. VIJAYABHASKARAN, C. R. RAJA

692

clear white to bluish white. The photograph of the grown

single crystal is shown in Figure 1.

3. Characterization Techniques

The lattice parameters of the grown crystal have been

determined from the single crystal X-ray diffraction

analysis using Bruker AXS kappa APEX II CCD dif-

fractometer equipped with graphite-monochromated Mo

(Kα) (λ = 0.7107 Å) radiation. The powder form of Co2+

mixed CMTC was mixed with KBr to form pellets for

obtaining FT-IR spectrum in the mid IR range (400 -

4000 cm−1) using Perkin-Elmer IFS 66 spectrometer. In

the present study, the transmission spectrum of Co2+

mixed CMTC crystal was recorded using Lambda 35

spectrophotometer. The combined thermogravimetric (TG)

and differential thermal analysis (DTA) of Co2+ mixed

CMTC crystal was recorded in the range from room

temperature to 1000˚C using SDT Q600 V8.3 Build 101,

under nitrogen atmosphere, with a heating rate of 20˚C/

min. The second harmonic generation conversion effi-

ciency test has been carried out using modified setup of

Kurtz and Perry.

4. Results and Discussion

4.1. Single Crystal XRD

The observed results indicate that both the pure and Co2+

mixed CMTC crystals belong to monoclinic crystal sys-

tem. The lattice parameters of pure [7] and cobalt mixed

CMTC crystals are given below:

Crystals a (Å) b (Å) c (Å)α β γ V (Å3)

Pure CMTC 11.09 4.10 11.3490˚ 115.13˚ 90˚467

Cobalt mixed

CMTC 6.14 12.18 9.0490˚ 104.99˚ 90˚653

The variation of lattice parameters and the increase in

cell volume are attributed to the incorporation of cobalt

in the pure CMTC crystal.

Figure 1. Photograph of Co2+ mixed CMTC crystal.

4.2. FT-IR Spectral Analysis

FT-IR (Fourier Transform Infrared) spectroscopy is one

of the most reliable methods for identification of func-

tional groups in organic, inorganic and polymeric mate-

rials. The recorded spectrum has been compared with the

available literatures [8-11]. The FT-IR absorption spec-

trum of Co2+ mixed CMTC crystals is shown in Figure 2.

The comparative study of absorption peaks and their as-

signments of frequencies are given below:

Crystals ν (OH)

(cm−1)

ν (CN)

(cm−1)

ν (CN)

(cm−1)

ν (CS)

(cm−1)

ν (SCN)

(cm−1)

Pure CMTC3543 2111 1115 718 618

Cobalt mixed

CMTC 3423 2077 1109 750 620

The symmetric stretching of OH gives rise to the ab-

sorption bands at 3543 and 3423 cm−1. In both the cases

C-N vibration is observed around 2100 and 1100 cm−1. It

is also well known that the peaks at 2111, 2077, 1115

and 1109 cm−1 correspond to C-N stretching vibrations. It

is well known that the peaks at 718 and 750 cm−1 corre-

spond to C-S stretching vibration and the peaks at 618

and 620 cm−1 correspond to SCN stretching vibration

respectively. The differences in the frequencies of func-

tional groups may be due to the addition of cobalt in the

pure crystal of CMTC.

4.3. Optical Transmission Spectral Analysis

The recorded spectrum is shown in Figure 3. The optical

transmission range and transparency cut-off wavelength

of Co2+ mixed CMTC crystal have been compared with

the reported literature [7] of pure CMTC crystal.

Crystals UV cut-off

wavelength (nm)

Transparent wave

band region (nm)

Pure CMTC 390 390 - 973

Cobalt mixed CMTC245 245 - 1100

From this analysis, it is understood that the UV cut-off

wavelength of Co2+ doped CMTC is low and it has wider

transparent wave band compared to pure CMTC crystal.

This transparent nature in the UV-Vis-NIR region can be

used for various nonlinear optical applications [12].

4.4. Thermal Analysis

The recorded TG/DTA curve of Co2+ mixed CMTC

crystal is shown in Figure 4. The material exhibits single

stage weight loss starting at 360˚C. But below this tem-

perature no weight loss is observed. In DTA analysis,

there is a broad endothermic peak at around 345˚C,

B. VIJAYABHASKARAN, C. R. RAJA 693

Figure 2. FT-IR spectral pattern of Co2+ mixed CMTC

crystal.

Figure 3. UV-Vis-NIR spectrum of Co2+ mixed CMTC

crystal.

Figure 4. TGA/DTA curves of Co2+ mixed CMTC crystal.

which is assigned as the melting point of the specimen,

followed by a sharp exothermic peak around 772˚C

which corresponds to the decomposition of Co2+ mixed

CMTC compound. It may be worth the mention here that

this value coincides well with the recorded TGA value.

Crystals Weight loss starting at Melting point

Pure CMTC 300˚C 253˚C

Cobalt mixed CMTC360˚C 345˚C

Due to the inclusion of Co2+ in pure CMTC crystal the

thermal stability of the material is increased. The ob-

served results are better than that of pure CuHg(SCN)4,

ZnHg(SCN)4 and (Cd(SCN)2(DMSO)2) [13,14] crystals.

4.5. Kurtz Powder Technique

The second harmonic generation conversion efficiency

test has been carried out using modified setup of Kurtz

and Perry [15] at the Indian Institute of Science, Banga-

lore. A Q-switched Nd:YAG laser beam of wavelength

1064 nm, with an input power of 4.5 mJ/pulse, and pulse

width of 10 ns with a repetition rate of 10 Hz was used.

The grown crystals were crushed into a fine powder and

then packed in a micro-capillary of uniform bore and

exposed to laser radiations. The 532 nm radiation was

collected by a monochromater after separating the 1064

nm pump beam with an infra-red blocking filter. The

second harmonic radiation generated by the randomly

oriented micro-crystals was focused by a lens and de-

tected by a photomultiplier tube (Hamamatsu R2059).

The emission of green light confirms the second har-

monic generation. The output power of Co2+ mixed

CMTC is 6 mV. For the same input the output power of

pure CMTC crystal is 4.5 mV. It was found that the con-

version efficiency of Co2+ mixed CMTC crystal was

found to be marginally greater than that of pure CMTC

crystal.

5. Conclusion

Co2+ mixed CMTC crystal has been successfully synthe-

sized and the crystals have been grown by slow evapora-

tion method at 35˚C in a constant temperature bath. The

results of its characterization analyses were compared

with that of pure CMTC single crystal. Single crystal

X-ray diffraction analysis showed that both the crystals

belong to monoclinic system. Functional groups were

analyzed by using FT-IR analysis, which has revealed the

characteristic vibration modes of pure and Co2+ mixed

CMTC crystals. UV cut-off wavelength of the Co2+

mixed CMTC grown crystal was found to be 245 nm

which is better than the pure CMTC crystal. The TGA

and DTA analysis under nitrogen atmosphere reveals that

B. VIJAYABHASKARAN, C. R. RAJA

694

Co2+ mixed CMTC crystal has marginally better thermal

stability than the pure CMTC crystal. But the second

harmonic generation efficiency test by Kurtz-Perry pow-

der technique reveals that both the crystals are inferior to

that of standard potassium dihydrogen phosphate (KDP)

crystal.

6. Acknowledgements

The authors are thankful to Prof. P. K. Das, IISC, Ban-

galore, India for the SHG test. They also express their

gratitude to the authorities of SAIF, IIT, Chennai, India,

ACIC, St. Joseph’s College, Tirchirappalli, India and ICP,

CECRI, Karaikudi, India for providing spectral facili-

ties, to undertake this study. The authors are also thank-

ful to Prof. M. Arulanandasamy, Department of English,

AAMEC, Kovilvenni, for his careful revision and proof

reading of the text at every stage of its preparation.

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