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
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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