# Growth and Etching Studies of Cadmium Mercury Thiocyanate Single Crystals Grown by Gel Technique

P Nisha Santhakumari, S Kalainathan

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P Nisha Santhakumari, S Kalainathan. Growth and Etching Studies of Cadmium Mercury Thiocyanate Single Crystals Grown by Gel Technique. Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, 10.2412/mmse.94.21.874. hal-01503826

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# Growth and Etching Studies of Cadmium Mercury Thiocyanate Single Crystals Grown by Gel Technique

P. Nisha Santhakumari¹, S. Kalainathan², a

1. Department of Physics, Auxilium College, Vellore, India

2. Centre for Crystal Growth, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India

a. skalainathan@gmail.com

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DOI 10.2412/mmse.94.21.874 provided by Seo4U.link

**Keywords:** diffusion, X-ray diffraction, crystal structure, second harmonic generation.

**ABSTRACT.** Single crystals of Cadmium mercury thiocyanate CdHg(SCN)₄, a bimetallic thiocyanate complex material have been grown in silica gel using gel technique by the process of diffusion. Colourless transparent crystals of size 10mm x 3.1mm x 3.2mm have been obtained. The grown crystals were subjected to single crystal X-ray diffraction and high resolution X-ray diffraction studies. The crystal structure belongs to tetragonal system. Etching studies were made on the grown crystal to analyze the structural imperfection of the crystal. The Second harmonic generation efficiency of the grown crystal has been determined using Kurtz powder technique in comparison with Urea. Its efficiency is found to be 6.2 times greater than that of Urea.

**Introduction.** Recent studies show that bimetallic thiocyanate complexes of type AB(SCN)₄ exhibit excellent nonlinear optical properties. Zinc mercury thiocyanate (ZMTC), cadmium mercury thiocyanate (CMTC), zinc cadmium thiocyanate (ZCTC) and manganese mercury thiocyanate (MMTC) are some of the crystals belonging to this category. ZMTC and CMTC are found to have all the good characteristics such as crystallizing in a noncentrosymmetric space group I4, colourless and high thermal stability [1]. ZMTC and CMTC are excellent nonlinear materials capable of second harmonic generation of Nd: YAG laser radiation of wavelength 1064 nm. Intracavity frequency doubling of a 946 nm Nd: YAG laser with CMTC crystal was reported by Changqiang Wang et al. [2] and generation of blue-violet light at room temperature with GaAlAs diode laser was reported by Yuan et al. [3]. These crystals cannot be grown from melt as they undergo decomposition before melting and can be obtained only from solution. Some of the doped crystals of the bimetallic thiocyanate complexes are also found to exhibit excellent nonlinear optical properties [4]. CMTC single crystals of considerably large size have been obtained from silica gel in our laboratory.

**Growth procedure.** Single diffusion technique and analytical grade reagents of mercury (II) chloride, ammonium thiocyanate and cadmium chloride were used for the growth CMTC single crystals in silica gel. Mercury (II) chloride and ammonium thiocyanate together were taken as the inner reagent and cadmium chloride was used as the outer reagent. Stock solution of sodium meta silicate was prepared by adding 244 grams of sodium meta silicate (Na₂SiO₃ 9H₂O) to 500 ml of distilled water. Stock solution (7.5 ml) was diluted with equal quantity of distilled water (7.5 ml) and then its pH was adjusted to 3.4 using ExcelaR grade glacial acetic acid of purity 99.8. This stock solution of pH 3.4 was mixed with a mixture of 32 ml aqueous solution of 0.2M HgCl₂ and 16 ml aqueous solution of 4M NH₄SCN and then allowed to gel in test tubes of length 15 cm and diameter 1.5 cm. After gelation, it was left as such for 48 hours for gel ageing and then the outer reagent, which is a mixture of 90% aqueous solution of 3M CdCl₂, was added on to the top of the gel using a pipette.
The outer reagent diffused into the gel medium and due to its reaction with the inner reagent, crystals started growing. The harvested pure CMTC crystals are colorless. The experiment was carried out at an ambient temperature of about 30°C. Fig. 1 shows the photograph of CMTC single crystals inside and outside the gel medium respectively.

<figure><img src="image_3.png" /><figcaption>Fig. 1. CMTC Crystal.</figcaption></figure>

**Characterizations.** The characterizations carried out for the gel-grown pure and Mn doped CMTC single crystals are briefly described below. The powder X-ray diffractometry analysis was performed with a graphite monochromated CuKα radiation of wavelength 1.5418 Å. Single crystal X-ray diffraction analysis was performed on ENRAF NONIUS FR590 diffractometer with Mo Kα radiation of wave length 0.7170 Å. The crystalline perfection of the grown crystal was analyzed with a multicrystal X-ray diffractometer developed at National Physical Laboratory (NPL). Etching studies were made on the grown crystal to analyze the structural imperfection of the crystal.

**Results and Discussion.**

**X-ray diffraction analysis.** The powder X-ray diffraction pattern of CMTC crystals are shown in Fig. 2. The diffraction planes are identified and indexed. Single crystal X-ray diffraction study was performed using ENRAF NONIUS FR590 diffractometer with Mo Kα radiation of wavelength 0.7170 Å to confirm the crystallographic system. The crystal structure is found to be tetragonal with the lattice parameters as a=b=11.473 Å, c=4.250 Å, α=β=γ = 90° and V=549Å³ [5].

**X-ray rocking curve studies.** The specimen surfaces were lapped and polished and then chemically etched by a non-preferential chemical mixed with water and acetone in 1:2 ratios before recording the diffraction curves. Fig. 3 shows the high-resolution diffraction curves recorded for (2 0 2) plane with a multicrystal X-ray diffractometer in symmetrical Bragg geometry. The diffraction curve recorded contains two additional peaks which are 16 and 37 arc sec away from the main peak. These two additional peaks correspond to two internal structural very low angle (tilt angle ≤ 1 arc min) boundaries [6] whose tilt angles (misorientation angle between the two crystalline regions on both sides of the structural grain boundary) are 16 and 21 arc sec from their adjoining regions. The FWHM (full width at half maximum) of the main peak and the low angle boundaries are respectively 20, 17 and 21 arc sec. Though the specimen contains very low angle boundaries, the relatively low angular spread of around 150 arc sec of the diffraction curve and the low FWHM values show that the crystalline perfection is quite good.
HRXRD analysis pure CMTC

<figure><img src="image_4.png" /><figcaption>Fig. 2. X-ray rocking curve recorded on the grown sample.</figcaption></figure>

**Etching studies.** The surface photograph of the CMTC crystal before etching and the etch patterns produced in the (0 1 1) plane of the crystal are shown in the figure. Etching study was carried out for different timings 5s, 10s and 15s using ethanol as etching agent. It is observed clearly that the etch pit formed in the surface is rectangular in shape. When the etching time is 5sec, many small rectangular shaped etch pits were formed in the crystal. As the etching time increases there is no change in the shape; but the size of the etch pattern was found to be decreased; there is an enormous increase in the etch pit density which suggests that the etch pits are due to dislocations.

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3. (a) Surface of the as grown CMTC crystal (b) Etch pattern produced for 5s (c) 10s (d) 15s (e) 20s.</figcaption></figure>

**Powder Kurtz method.** A preliminary study on the second harmonic generation efficiency of the crystal was carried out by Powder Kurtz method. In this experiment Q-switched pulses were obtained from a Q-switched Nd-YAG laser and urea was used as a reference material. Using an incident beam of power 5 mJ/ P, SHG signal of intensity 930 mV was recorded for our sample while that for urea was 150 mV. Its second harmonic generation efficiency is found to be 6.2 times greater than that of urea.

**Summary.** CMTC single crystals can be obtained from silica gel using gel technique by the process of diffusion in an acidic medium. It belongs to tetragonal system. The crystalline perfection is reasonably good. Etching studies reveals the presence of dislocation. Its second harmonic generation efficiency is found to be greater than that of urea.

**References**

[1] X.Q.Wang et al. Spectroscopic and thermal behavior of ZnHg(SCN)4, Materials Research Bulletin 37 (2002) 1859-1871, Article in Materials Research Bulletin 37(11):1859-1871 · September 2002 DOI: 10.1016/S0025-5408(02)00859-0

[2] Changqiang Q. Wang, Y.T.Chow, W.A.Gambling, Duorong Yuan, Dong Xu, Guanghui Zhang, Mingguo Liu, Minhua Jiang, Optics & Laser Technology 30 (1998) 291-293.
[3] D.R.Yuan, M.G.Liu, D.Xu, Q.Fang, W.T.Yu, W.B.Hou, Y.H.Bing, S.Y.Sun, M.H.Jiang, Appl. Phys. Lett. 70 (1997) 544.

[4] G.P.Joseph, I.Korah, K.Raja Rajan, P.C.Thomas, M.Vimalan, J.Madhavan and P.Sagayaraj, Cryst. Res.Technol. 42, No.3, (2007) 295-299.

[5] P Nisha Santha kumari, M.Margaret Beatrice, S. Kalainathan. A comparative study of pure and potassium doped cadmium mercury thiocyanate single crystal grown in silica gel Cryst Res Technol, (2009) 44, 177-183.

[6] G. Bhagavannarayana, R.V. Ananthamurthy, G.C. Budakoti, B. Kumar and K.S.Bartwal, J. Appl. Cryst. 38, (2005) 768-771.

Cite the paper

P. Nisha Santhakumari, S. Kalainathan (2017). *Growth and Etching Studies of Cadmium Mercury Thiocyanate Single Crystals Grown by Gel Technique*. *Mechanics, Materials Science & Engineering, Vol 9.*
Doi [10.2412/mmse.94.21.874](https://www.tandfonline.com/doi/10.2412/mmse.94.21.874)