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Growth and properties of UV nonlinear optical crystal ZnCd(SCN)4
Materials Research Bulletin 38 (2003) 1269–1280
Growth and properties of UV nonlinear optical crystal ZnCd(SCN)4$ Xinqiang Wanga,*, Dong Xua, Mengkai Lua, Dourong Yuana, Guanghui Zhanga, Shouxi Xub, Shiyi Guoa, Xuening Jianga, Jiurong Liua, Chunfeng Songa, Quan Renc, Ji Huangd, Yupeng Tiane a
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Science and Technology Department, Shandong College of Public Security, Jinan 250014, PR China c Optics Department, Shandong University, Jinan 250100, PR China d Experimental Center, Shandong University, Jinan 250100, PR China e Department of Chemistry, Anhui University, Hefei 210093, PR China
b
Abstract A second-order nonlinear optical coordination crystal, zinc cadmium thiocyanate, ZnCd(SCN)4 (ZCTC) was grown as a frequency doubler, emitting UV light. A large typical single crystal with dimensions up to 15 7 7 mm3 has been obtained by slow solvent-evaporation method for the first time. The infrared (IR) spectroscopy and X-ray powder diffraction (XRPD) of single crystals were performed at room temperature. The specific heat of the crystal has been measured to be 367.9 J/mol K at 300 K. The thermal expansion coefficients a- and c-oriented, have been measured to be 1:69 105 and 1:95 104 K1, respectively. The second harmonic generation (SHG) efficiency of ZCTC crystal is 51.6 times as high as that of urea reference, and the measured transmittance spectra from 190 to 3200 nm showed that the UV transparency cutoff occurs at 290 nm and the transmission is 73.22% at 380 nm. UV laser light of wavelength 380 nm has been achieved by the frequency doubling of a 760 nm laser diode at room temperature. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: B. Crystal growth; C. X-ray diffraction; D. Specific heat; D. Thermal expansion; D. Optical properties
1. Introduction Lately, second-order nonlinear optical (NLO) materials capable of efficient frequency conversion of infrared or visible laser radiation to visible or ultraviolet (UV) wavelengths are of considerable interest $
PII of original article S0025-5408(01)00598-0. Corresponding author. Tel.: þ86-531-856-4451-8002; fax: þ86-531-8565403. E-mail address: [email protected] (X. Wang). *
0025-5408/03/$ – see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-5408(03)00111-9
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in the fields of telecommunications, high-density optical recording, laser remote sensing, color displays, and medical diagnostics et al. Materials with large second-order optical nonlinearities, transparency at all wavelengths involved and stable physicochemical performance are needed in order to realize many of these applications. During recent years, the group IIB divalent d10 ions, Zn2þ, Cd2þ, and Hg2þ complexes have attracted a great deal of attention for their unique characteristics: the pale color and high thermal stability [1–3]. As nonlinear optical materials, their bimetallic thiocyanates, ZnCd(SCN)4, ZnHg(SCN)4, and CdHg(SCN)4, (abbreviated as ZCTC, ZMTC, CMTC, respectively) exhibit efficient second-harmonic generation (SHG) at short wavelengths. CMTC and ZMTC are well known NLO materials for the SHG of 1064 nm radiation [4]. Recently, the growth and NLO properties of CMTC crystal were reported [5,6], which may generate blue-violet light when GaAlAs laser diodes are used. More recently, we reported the synthesis, crystal structure and some properties of ZCTC [7]. It belongs to tetragonal system, space group ˚ , V ¼ 542:6(2) A ˚ 3, Z ¼ 2, Dc ¼ 2:510 g/cm3. The properties is I4 with a ¼ 11:135(2), c ¼ 4:3760(10) A of ZCTC crystal have some better characters than that of CMTC, such as tolerance to hydrolysis, thermal stability, UV transparency cutoff, et al. In the present work, large and highly optical quality ZCTC single crystals were grown from a mixed solvent of water-NH4X (X ¼ NO3 , Cl) reaction mother solution (RMS) by means of solvent-evaporation method. Some structural, thermal and optical properties of ZCTC were investigated for the first time.
2. Experimental All the starting materials were analytical reagent grade and used as purchased, and all the synthetic and growth processes were carried out in aqueous solutions. The infrared (IR) spectroscopic measurement of ZCTC single crystal was recorded in the range 400–4000 cm1 using a Nicolet 750 FTIR spectrometer at room temperature. The X-ray powder diffraction (XRPD) pattern of ZCTC was registered with a Rigaku D/Max-gA diffractometer, operated at 40 KV and 60 mA, using a Cu-target tube and a graphite monochrometer. Fixed scatter and divergence slits of 18 and a 0.15 mm receiving slit were used. The intensity data were recorded by continuous scan in a 2y/y mode from 108 to 608 with a step size of 0.028 and a scan speed of 48/min. The specific heat of ZCTC crystal has been measured in the range from 318.15 to 448.15 K by DSC 7 made by Perkin-Elmer. The thermal expansion coefficients of ZCTC crystal were obtained by measuring the thermal expansion of the crystal samples by a thermome-chanical analyzer (TMA) made by PerkinElmer. The samples were a- and c-oriented crystals with a thickness of 3.70 and 4.02 mm, respectively. The second order nonlinear optical intensities were estimated by measuring powder crystal with 76–154 mm diameter in the form of pellet. The thickness of a pellet was about 0.8 mm. The experimental arrangement for measuring the nonlinear optical properties was by using a M200 high power Mode-Locked Nd:YAG laser with 200 ps pulse at a repetition rate of 5 HZ. The selected wavelength was 1064 nm. After the selection of the wavelength, the laser beam was split into two parts, one to generate the second harmonic signal in the sample and the other to generate the second harmonic signal in the reference (urea pellet). The optical transmission spectra of ZCTC crystal from UV to NIR with the wave length from 190 to 3200 nm were recorded by means of the Hitachi model U-3500 recording spectrophotometer. The light path direction were normal to (1 0 0) and (0 0 1) plane and the thicknesses of the samples used were 1.47, 1.46 mm, respectively.
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3. Results and discussion 3.1. Single-crystal growth All the raw materials were analytical reagent grade and used as purchased (Purity 98:0%). Preparations were accomplished by two continuous reactions, where A ¼ K, Na, NH4; X ¼ NO3 , or Cl: 4ASCN þ CdX2 ¼ A2 CdðSCNÞ4 þ 2AX A2 CdðSCNÞ4 þ ZnX2 ¼ ZnCdðSCNÞ4 þ 2AX The success of growing large and high-quality single crystals with low defect density is highly dependent on the purity of the starting materials. ZCTC should be also purified by recrystallization. Fig. 1 shows solubility variation of ZCTC with the change in the proportion of the mixtures of water and acetone, water and ethanol at room temperature, respectively. ZCTC exhibits very high solubility in the mixture of water and acetone at a certain ratio. Since acetone is very volatile, ZCTC can be easily purified by this mixed solvent. Because ZCTC easily decomposes in its molten state and it is not sublimable, the entire crystallization process is performed in solution. Crystallization tendency was checked by solvent evaporation from several solutions because crystallization solvents have effects on the crystal habit and solubility. The crystalline habits and solubilities of the ZCTC crystal in several solutions, e.g. pure or mixed solutions were observed. The results show that ZCTC crystals can dissolve in water and many organic solvents. However, during the crystal growth process, the metastable state and the supersaturation of ZCTC solutions are very difficult to control for methanol, ethanol, acetone, butanone and other easy volatile organic solvents; ZCTC crystals are also very difficult to obtain from dimethyl sulfoxide (DMSO), N,N-Dimethyl formamide (DMF) and other high-viscosity and high-boiling-point organic solvents.
Fig. 1. Solubility variation of ZCTC with the change in the proportion of the two mixture of water and acetone, water and ethanol at room temperature, respectively.
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Furthermore, ZCTC shows different crystal habits and solubilities from solution to solution. For example, needle-like crystals or branched dendrites and low solubilities are obtained from water, ethanol or acetone pure solvent, and byproduct crystals: Cd(SCN)2(DMSO)2 [8] are obtained instead of ZCTC from pure DMSO or water/DMSO mixed solvent. Moreover, the experiments indicate that the colors of the solutions easily turn slightly yellow-red and form white and yellow precipitates as byproducts at a relatively high temperature, and the crystals of ZCTC are not transparent in organic solvents. Among the different solvents tested, the reaction-mother-solution (RMS): H2O/NH4X (X ¼ NO3 , Cl) solvent gave the best results. The RMS was prepared by reacting amounts of ammonia thiocyanate (NH4SCN) with the appropriate either zinc chloride (ZnCl2) and cadmium nitrate (Cd(NO3)2) or zinc nitrate (Zn(NO3)2) and cadmium chloride (CdCl)2 in deionized water, the pH value of which was adjusted by adding amounts of quantitative hydrochloric acid (HCl). Fig. 2 shows the solubility of ZCTC in pure water and RMS as a function of temperature, respectively. Fig. 3 shows the typical optical microscope crystallization photograph of ZCTC crystals in RMS. From these two figures, one can see that ZCTC crystal exhibits fine crystal habit, relatively high solubility and a relatively large positive temperature coefficient in RMS, so we can carry out the crystal growth by using both temperature-lowering and solventevaporation methods, and temperature-lowering method is better for both gain rather large single crystals and avoiding an undesirable change of RMS compared with solvent-evaporation method. However, at the present time, we have only obtained fairly large single crystals from RMS by the solvent-evaporation method, the temperature-lowering method is presently on the march. The seed crystal was obtained by spontaneous nucleation from the saturated RMS and was verified to be a single crystal with a polarized microscope. A typical growth procedure is as follows: A saturated growth solution is kept at around 50.00 8C, which is ca 10.00 8C above the saturation temperature for at least 1 h. A seed crystal whose growth direction is [1 0 0] fixed by a seed rod is introduced into the solution for half an hour at which time slight dissolution should be observed. The solution is then cooled to a temperature of 40.00 8C and
Fig. 2. Solubility of ZCTC crystal in H2O and reaction mother solution (RMS) as functions of temperature.
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Fig. 3. Photograph of ZCTC crystal habit grown in reaction mother solution (RMS).
maintained at this temperature until neither growth nor dissolution takes place on the seed crystal for at least 24 h. After this procedure the solution is evaporated at this constant temperature slowly. The temperature controller is FP21, a programmable temperature controller with high accuracy of is 0.01 8C. Under the above growth conditions, and in over three months growth duration, large single crystal of ZCTC can be grown. The crystal was colorless and transparent, tested by polarizing microscopy, the crystal exhibited uniform extinction when the polarization vector was parallel to a dielectric axis, which indicates its high optical quality. A typical single crystal with dimensions of 15 7 7 mm3 shown in Fig. 4 was successfully grown by this procedure. Fig. 4 also shows another ZCTC crystal, with dimensions of the transparent area of 7 7 6 mm3. To grow higher-quality and larger single crystals of ZCTC from the seeded and saturated RMS, the temperature-lowering method is better than the solvent-evaporation method. We are now managing to adopt such a growth procedure. The crystal habit of ZCTC grown in solutions with different pH values were investigated by using an optical microscope. At a lower pH values, a large number of crystals were grown, but they were found to stick one upon another. This is because the solubility of ZCTC is high at lower pH values, which results in a large number of nucleation centers. On the other hand, at higher pH values, the number of nucleation centers is limited, but the crystals grown lack well developed faces. Therefore, we optimized the pH values of ZCTC solution at 2–4 to get well developed and good single crystals. At the same time, whole growth process of the ZCTC crystal is affected by the pH value of the solution. The reason is probably attributed to the complex equilibrium existing in the solution as follows, where x ¼ 4: Zn2þ þ xSCN $ ZnðSCNÞx ðx2Þ Cd2þ þ xSCN $ CdðSCNÞx ð2xÞþ Zn2þ þ Cd2þ þ xSCN $ ZnCdðSCNÞx ðx4Þ Hþ þ SCN $ HSCN The equilibrium concentrations of the various ions and the crystal growth radically change with the variation of the pH value and supersaturation, i.e. change of the growth rate of the ZCTC crystal and its morphology. Moreover, thiocyanic acid (HSCN) is much more unstable than its thiocyanates, which is
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Fig. 4. ZCTC single crystals grown by the solvent-evaporation method.
easy to decompose. Although the aqueous solution of ZCTC can retain in stable state without decomposition and hydrolysis at atmospheric pressure and relatively low temperature for a long time, whose aqueous solution will be decomposed and hydrosised if it is kept at high temperature for several days. The color of the solution slowly turn slightly red and form white and yellow precipitates as byproducts. Several possible chemical reactions may occur as shown by the following equations: 2ZnCdðSCNÞ4 þ 18H2 O $ ZnS # þ CdS # þ ZnCO3 # þ CdCO3 # þ 6CO2 þ 6H2 S þ 8NH3 ; ZnCdðSCNÞ4 $ Zn2þ þ Cd2þ þ 4SCN ; 2SCN þ 2H2 O þ 3O2 ! 2Hþ þ 2SO4 2 þ 2HCN; Zn2þ þ 2H2 O $ ZnðOHÞ2 þ 2Hþ ; Cd2þ þ 2H2 O $ CdðOHÞ2 þ 2Hþ : During growth process, one of the raw materials we used is ammonium thiocyanate (NH4SCN), not sodium thiocyanate (NaSCN) or potassic thiocyanate (KSCN), because the crystal habit of ZCTC in NH4X/H2O RMS system is somewhat better than that in NaX/H2O or KX/H2O system (X ¼ NO3 , Cl). The other two raw materials we used are ZnX2 and CdX2 (X ¼ NO3 , Cl) not ZnSO4, and CdSO4, because ZnSO4 (NH4)2SO4 6H2O and CdSO4 (NH4)2SO4 6H2O would be observed as byproducts if the latter are used as raw materials.
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Fig. 5. The IR spectrum of ZCTC crystal.
Several foreign metallic cations, such as Al3þ, Fe3þ, Co2þ, Ni2þ, Cu2þ and etc. will affect the whole growth process. Their effects are related with ionic radii, electric charge, complex formation in the bulk and at the surface, and frequency of solvent exchange. The presence of these impurities will badly influence the quality of ZCTC crystals, especially their transparency. Exploration of eliminating these ions are presently under way. 3.2. Structural characterization Fig. 5 shows the IR spectra of ZCTC and its corresponding vibrations. The assignments of the main characteristic IR band frequencies (cm1) observed for KSCN [9] and ZCTC are listed in Table 1. It is known that nCN often lies higher than 2100 cm1, nCS lies ca 860 780 cm1 (N-bonding) or 720 690 cm1 (S-bonding) and dSCN, lies near 480 cm1 (N-bonding) or 420 cm1 (S-bonding) [10]. From Fig. 3 and Table 1, one can see that the sharp increase in frequencies of nCN stretching and nCS stretching and decrease of dSCN bending in ZCTC compared with the corresponding bands in the free thiocyanate radical of KSCN. This confirms the metal-nitrogen and metal-sulfur coordination in the their structures. Table 1 Assignments of the main characteristic IR band frequencies (cm1) observed for KSCN and ZCTC Assignment
KSCN
ZCTC
nCN nCS dNCS 2dNCS
2063 746 488 940
2113.9, 2162.1 785.0 447.5, 472.5 896.9, 945.1
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Fig. 6. The XRPD pattern and diffraction indices of ZCTC.
The XRPD experiments showed that the synthesized material and the as-grown crystals are the single phase of ZCTC. The XRPD pattern and diffraction indices of the crystal were shown in Fig. 6. The tetragonal unit-cell parameters calculated by TREOR program [11] according to the values of 2y in ˚ , V ¼ 540:96 A ˚ 3, which are comparable with the results XRPD pattern are a ¼ 11:1141, c ¼ 4:3794 A determined by a R3m/E four-circle X-ray diffractometer [7]. The XRPD data for ZCTC are tabulated in Table 2. 3.3. Thermal properties The specific heat is one of the important factors that influence the damage threshold of crystal. The specific heat data of solids are generally described by the Debye theory in terms of the harmonic frequency spectrum for each crystalline lattice. However, it is very difficult to calculate the specific heat of a ZCTC crystal. Fig. 7 shows the temperature dependence of the specific heat of ZCTC crystal. From this figure, we can see that the specific heat of ZCTC crystal is linear with temperature in the measured temperature range. The specific heat of ZCTC crystal at room temperature is about 0.8972 J/g K, and is equivalent to 367.9 J/mol K. One can expect that upon irradiation by plused laser beam, a ZCTC crystal will absorb more energy while maintaining a smaller temperature gradient. A large gradient disturbs the phase-matching properties of a NLO material when compared to crystals with low specific heat. The thermal expansion coefficients of a crystal are important factors for crystal growth and applications. The laser absorption of a crystal will cause thermal gradient in the crystal, which will lead to crystal fracture if the laser power is high enough. For a tetragonal crystal, there are only two independent principal thermal expansion components, a1 and a3, respectively. Fig. 8 shows the temperature variation of the thermal expansions along the two crystallographic directions. No anomaly within the temperature range shown was observed. The average linear thermal expansion
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Table 2 XRPD data for ZCTC ˚) dobs (A
2yobs (8)
2ycalc (8)
I/I0 Intensity (%)
hkl
7.86000 5.55600 4.07600 3.93000 3.51400 3.28600 2.82800 2.77800 2.62000 2.52000 2.48400 2.29500 2.17900 2.10900 2.03800 1.98200 1.96500 1.91100 1.86800 1.85900 1.85200 1.75700 1.72000 1.68600 1.64300 1.61400 1.54500
11.25696 15.95094 21.80392 22.62443 25.34486 27.13589 31.63712 32.22189 34.22308 35.62569 36.15971 39.25452 41.43757 42.87966 44.45165 45.77780 46.19667 47.58132 48.74685 48.99833 49.19578 52.04876 53.25547 54.41703 55.96432 57.06107 59.86216
11.25866 15.94798 21.81194 22.62787 25.34067 27.13681 31.63229 32.21579 34.22838 35.61551 36.14198 39.24438 41.42476 42.87181 44.46867 45.77500 46.20401 47.53458 48.77715 49.01200 49.18610 52.03967 53.26118 54.40778 55.96634 57.07613 59.87109
9 100 94 13 8 31 34 4 8 23 18 22 3 8 4 11 5 4 10 25 15 5 3 5 3 5 13
110 200 101 220 310 211 301 400 330 321 420 411 510 112 202 501 440 222 521 312 600 620 402 611 422 541 512
coefficient of the a-oriented ZCTC crystal measured from 303.15 to 433.15 K is 1:69 105 K1 and that of the c-oriented crystal is 1:95 104 K1. These results show that the thermal expansion coefficients of ZCTC crystal are anisotropic along the different crystallographic axes. When a small temperature variation DT takes place throughout the ZCTC crystal, it shows both thermal expansion and contraction. 3.4. Optical properties According to the powder technique [12], the SHG efficiency can be estimated as about 51.6 times as that of urea crystal powders for ZCTC crystal powders. Fig. 9 shows transmission spectra of ZCTC along the two directions. The UV cutoff wavelengths are the same in the two directions which indicate that the UV cutoff wavelength of ZCTC crystal is 290 nm. The first IR absorption band occurs at 2320 cm1. The transmission normal to (1 0 0) plane is superior to that normal to (0 0 1) plane. In the transmission normal to (1 0 0) plane, the average transmittance of 70% in the transmission band between 330 and 2280 nm, and the transparent percentage at standard violet light of 404 and 380 nm are 74.16% and 73.22%, respectively, which is much higher than that of CMTC.
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Fig. 7. The temperature dependence of the specific heat of ZCTC crystal.
During crystal device processing, ZCTC is much more easy to process compared with CMTC. This result shows that ZCTC crystal has a high mechanical strength for device processing. Very recently, the UV SHG at 380 nm by direct frequency doubling of a laser-diode at 760 nm using a ZCTC crystal at room temperature. The 380 nm light output power of 20 mW is measured for a 110 mW input power of
Fig. 8. The temperature variation of the thermal expansions along the two crystallographic directions.
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Fig. 9. Transmission spectra of ZCTC normal to (1 0 0) and (0 0 1) plane and the thicknesses of the samples used were 1.47, 1.46 mm, respectively.
the laser-diode. The details of the experiment will be described elsewhere. Now, we are managing to improve the SHG output efficiency along with exploring better crystal growth conditions and crystal processing techniques such as cutting, polishing and antireflection coating.
4. Conclusions A novel coordination nonlinear-optical crystal material, ZCTC has been developed for UV SHG and grown into single crystals from the aqueous solution by slow solvent-evaporation method and characterized by various ways, and the slow temperature-lowering method is presently under way. The physicochemical changes of their crystal-growth solutions at high temperatures and different pH values show the difficulty of its single crystal growth. IR spectroscopy and XPRD studies confirmed the identity of the grown single crystals. ZCTC crystal exhibits large anisotropy in thermal expansion and a relatively large specific heat. The SHG intensities of ZCTC crystal powders are superior to that of CMTC crystal powders and over one order of magnitude higher than that of Urea crystal powders and ZCTC has excellent transparency in the UV regain. The UV light output by direct laser-diode frequency doubling using ZCTC crystal was realized at room temperature through preliminary experiment. In summary, being a coordination crystal, ZCTC emerges as a promising candidate for frequency doubling to violet and UV wavelengths, ZCTC possesses excellent performance compared to CMTC. Its large
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SHG intensities, high optical transparency, and excellent processability make it attractive for devices applications. Further investigations of crystal growth and device applications are now in progress.
Acknowledgements This work is supported by grants from the ‘‘863’’, the National Natural Science Foundation of China (69890230 and 69778023), and the Opening Foundation of the Institute of Crystal Materials, Shandong University (060115). The authors are indebted to Professor Yang Zhaohe for the transmission spectra determinations and TMA measurements, Engineer Sun Suoying for expert crystal cutting and polishing and last but not least Dr. Zhao Xian for helpful comments in the research project.
References [1] N. Zhang, M.H. Jiang, D.R. Yuan, D. Xu, X.T. Tao, Z.S. Shao, J. Cryst. Growth. 102 (1990) 581. [2] X.T. Tao, N. Zhang, D.R. Yuan, W.T. Yu, M.H. Jiang, J. Synth. Cryst. 20 (1991) 113 (in Chinese). [3] W.B. Hou, D.R. Yuan, D. Xu, N. Zhang, W.T. Yu, M.G. Liu, X.T. Tao, S.S. Ying, M.H. Jiang, Mater. Res. Bull. 28 (1993) 645. [4] J.G. Bergman, J.J. Mcfee, G.R. Crane, Mater. Res. Bull. 5 (1970) 913. [5] D.R. Yuan, Z.W. Zhong, M.G. Liu, D. Xu, Q. Fang, Y.H. Bing, S.Y. Sun, M.H. Jiang, J. Cryst. Growth 186 (1998) 240. [6] D.R. Yuan, D. Xu, M.G. Liu, Q. Fang, W.T. Yu, W.B. Hou, Y.H. Bing, S.Y. Sun, M.H. Jiang, Appl. Phys. Lett. 70 (1997) 544. [7] X.Q. Wang, D. Xu, D.R. Yuan, Y.P. Tian, W.T. Yu, S.S. Ying, Z.H. Yang, Q. Fang, L.M. Kai, Y.Y. Xin, F.Q. Meng, S.Y. Guo, G.H. Zhang, M.H. Jiang, Mater. Res. Bull. 12/13 (1999) 2003. [8] X.Q. Wang, W.T. Yu, D. Xu, D.R. Yuan, M.K. Lu, Y.P. Tian, P. Yang, F.Q. Meng, S.Y. Guo, M.H. Jiang, Z. Fur. Krist. 215 (2000) 91. [9] P.O. Kinell, B. Strandberc, Acta Chem. Stand. 13 (1959) 1607. [10] J. Lewis, R.S. Nyholm, P.W. Smith, J. Chem. Soc. (1961) 4590. [11] P.E. Werner, J. Appl. Cryst. 18 (1985) 367. [12] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798.
Growth and properties of UV nonlinear optical crystal ZnCd(SCN)4$ Xinqiang Wanga,*, Dong Xua, Mengkai Lua, Dourong Yuana, Guanghui Zhanga, Shouxi Xub, Shiyi Guoa, Xuening Jianga, Jiurong Liua, Chunfeng Songa, Quan Renc, Ji Huangd, Yupeng Tiane a
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Science and Technology Department, Shandong College of Public Security, Jinan 250014, PR China c Optics Department, Shandong University, Jinan 250100, PR China d Experimental Center, Shandong University, Jinan 250100, PR China e Department of Chemistry, Anhui University, Hefei 210093, PR China
b
Abstract A second-order nonlinear optical coordination crystal, zinc cadmium thiocyanate, ZnCd(SCN)4 (ZCTC) was grown as a frequency doubler, emitting UV light. A large typical single crystal with dimensions up to 15 7 7 mm3 has been obtained by slow solvent-evaporation method for the first time. The infrared (IR) spectroscopy and X-ray powder diffraction (XRPD) of single crystals were performed at room temperature. The specific heat of the crystal has been measured to be 367.9 J/mol K at 300 K. The thermal expansion coefficients a- and c-oriented, have been measured to be 1:69 105 and 1:95 104 K1, respectively. The second harmonic generation (SHG) efficiency of ZCTC crystal is 51.6 times as high as that of urea reference, and the measured transmittance spectra from 190 to 3200 nm showed that the UV transparency cutoff occurs at 290 nm and the transmission is 73.22% at 380 nm. UV laser light of wavelength 380 nm has been achieved by the frequency doubling of a 760 nm laser diode at room temperature. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: B. Crystal growth; C. X-ray diffraction; D. Specific heat; D. Thermal expansion; D. Optical properties
1. Introduction Lately, second-order nonlinear optical (NLO) materials capable of efficient frequency conversion of infrared or visible laser radiation to visible or ultraviolet (UV) wavelengths are of considerable interest $
PII of original article S0025-5408(01)00598-0. Corresponding author. Tel.: þ86-531-856-4451-8002; fax: þ86-531-8565403. E-mail address: [email protected] (X. Wang). *
0025-5408/03/$ – see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-5408(03)00111-9
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in the fields of telecommunications, high-density optical recording, laser remote sensing, color displays, and medical diagnostics et al. Materials with large second-order optical nonlinearities, transparency at all wavelengths involved and stable physicochemical performance are needed in order to realize many of these applications. During recent years, the group IIB divalent d10 ions, Zn2þ, Cd2þ, and Hg2þ complexes have attracted a great deal of attention for their unique characteristics: the pale color and high thermal stability [1–3]. As nonlinear optical materials, their bimetallic thiocyanates, ZnCd(SCN)4, ZnHg(SCN)4, and CdHg(SCN)4, (abbreviated as ZCTC, ZMTC, CMTC, respectively) exhibit efficient second-harmonic generation (SHG) at short wavelengths. CMTC and ZMTC are well known NLO materials for the SHG of 1064 nm radiation [4]. Recently, the growth and NLO properties of CMTC crystal were reported [5,6], which may generate blue-violet light when GaAlAs laser diodes are used. More recently, we reported the synthesis, crystal structure and some properties of ZCTC [7]. It belongs to tetragonal system, space group ˚ , V ¼ 542:6(2) A ˚ 3, Z ¼ 2, Dc ¼ 2:510 g/cm3. The properties is I4 with a ¼ 11:135(2), c ¼ 4:3760(10) A of ZCTC crystal have some better characters than that of CMTC, such as tolerance to hydrolysis, thermal stability, UV transparency cutoff, et al. In the present work, large and highly optical quality ZCTC single crystals were grown from a mixed solvent of water-NH4X (X ¼ NO3 , Cl) reaction mother solution (RMS) by means of solvent-evaporation method. Some structural, thermal and optical properties of ZCTC were investigated for the first time.
2. Experimental All the starting materials were analytical reagent grade and used as purchased, and all the synthetic and growth processes were carried out in aqueous solutions. The infrared (IR) spectroscopic measurement of ZCTC single crystal was recorded in the range 400–4000 cm1 using a Nicolet 750 FTIR spectrometer at room temperature. The X-ray powder diffraction (XRPD) pattern of ZCTC was registered with a Rigaku D/Max-gA diffractometer, operated at 40 KV and 60 mA, using a Cu-target tube and a graphite monochrometer. Fixed scatter and divergence slits of 18 and a 0.15 mm receiving slit were used. The intensity data were recorded by continuous scan in a 2y/y mode from 108 to 608 with a step size of 0.028 and a scan speed of 48/min. The specific heat of ZCTC crystal has been measured in the range from 318.15 to 448.15 K by DSC 7 made by Perkin-Elmer. The thermal expansion coefficients of ZCTC crystal were obtained by measuring the thermal expansion of the crystal samples by a thermome-chanical analyzer (TMA) made by PerkinElmer. The samples were a- and c-oriented crystals with a thickness of 3.70 and 4.02 mm, respectively. The second order nonlinear optical intensities were estimated by measuring powder crystal with 76–154 mm diameter in the form of pellet. The thickness of a pellet was about 0.8 mm. The experimental arrangement for measuring the nonlinear optical properties was by using a M200 high power Mode-Locked Nd:YAG laser with 200 ps pulse at a repetition rate of 5 HZ. The selected wavelength was 1064 nm. After the selection of the wavelength, the laser beam was split into two parts, one to generate the second harmonic signal in the sample and the other to generate the second harmonic signal in the reference (urea pellet). The optical transmission spectra of ZCTC crystal from UV to NIR with the wave length from 190 to 3200 nm were recorded by means of the Hitachi model U-3500 recording spectrophotometer. The light path direction were normal to (1 0 0) and (0 0 1) plane and the thicknesses of the samples used were 1.47, 1.46 mm, respectively.
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3. Results and discussion 3.1. Single-crystal growth All the raw materials were analytical reagent grade and used as purchased (Purity 98:0%). Preparations were accomplished by two continuous reactions, where A ¼ K, Na, NH4; X ¼ NO3 , or Cl: 4ASCN þ CdX2 ¼ A2 CdðSCNÞ4 þ 2AX A2 CdðSCNÞ4 þ ZnX2 ¼ ZnCdðSCNÞ4 þ 2AX The success of growing large and high-quality single crystals with low defect density is highly dependent on the purity of the starting materials. ZCTC should be also purified by recrystallization. Fig. 1 shows solubility variation of ZCTC with the change in the proportion of the mixtures of water and acetone, water and ethanol at room temperature, respectively. ZCTC exhibits very high solubility in the mixture of water and acetone at a certain ratio. Since acetone is very volatile, ZCTC can be easily purified by this mixed solvent. Because ZCTC easily decomposes in its molten state and it is not sublimable, the entire crystallization process is performed in solution. Crystallization tendency was checked by solvent evaporation from several solutions because crystallization solvents have effects on the crystal habit and solubility. The crystalline habits and solubilities of the ZCTC crystal in several solutions, e.g. pure or mixed solutions were observed. The results show that ZCTC crystals can dissolve in water and many organic solvents. However, during the crystal growth process, the metastable state and the supersaturation of ZCTC solutions are very difficult to control for methanol, ethanol, acetone, butanone and other easy volatile organic solvents; ZCTC crystals are also very difficult to obtain from dimethyl sulfoxide (DMSO), N,N-Dimethyl formamide (DMF) and other high-viscosity and high-boiling-point organic solvents.
Fig. 1. Solubility variation of ZCTC with the change in the proportion of the two mixture of water and acetone, water and ethanol at room temperature, respectively.
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Furthermore, ZCTC shows different crystal habits and solubilities from solution to solution. For example, needle-like crystals or branched dendrites and low solubilities are obtained from water, ethanol or acetone pure solvent, and byproduct crystals: Cd(SCN)2(DMSO)2 [8] are obtained instead of ZCTC from pure DMSO or water/DMSO mixed solvent. Moreover, the experiments indicate that the colors of the solutions easily turn slightly yellow-red and form white and yellow precipitates as byproducts at a relatively high temperature, and the crystals of ZCTC are not transparent in organic solvents. Among the different solvents tested, the reaction-mother-solution (RMS): H2O/NH4X (X ¼ NO3 , Cl) solvent gave the best results. The RMS was prepared by reacting amounts of ammonia thiocyanate (NH4SCN) with the appropriate either zinc chloride (ZnCl2) and cadmium nitrate (Cd(NO3)2) or zinc nitrate (Zn(NO3)2) and cadmium chloride (CdCl)2 in deionized water, the pH value of which was adjusted by adding amounts of quantitative hydrochloric acid (HCl). Fig. 2 shows the solubility of ZCTC in pure water and RMS as a function of temperature, respectively. Fig. 3 shows the typical optical microscope crystallization photograph of ZCTC crystals in RMS. From these two figures, one can see that ZCTC crystal exhibits fine crystal habit, relatively high solubility and a relatively large positive temperature coefficient in RMS, so we can carry out the crystal growth by using both temperature-lowering and solventevaporation methods, and temperature-lowering method is better for both gain rather large single crystals and avoiding an undesirable change of RMS compared with solvent-evaporation method. However, at the present time, we have only obtained fairly large single crystals from RMS by the solvent-evaporation method, the temperature-lowering method is presently on the march. The seed crystal was obtained by spontaneous nucleation from the saturated RMS and was verified to be a single crystal with a polarized microscope. A typical growth procedure is as follows: A saturated growth solution is kept at around 50.00 8C, which is ca 10.00 8C above the saturation temperature for at least 1 h. A seed crystal whose growth direction is [1 0 0] fixed by a seed rod is introduced into the solution for half an hour at which time slight dissolution should be observed. The solution is then cooled to a temperature of 40.00 8C and
Fig. 2. Solubility of ZCTC crystal in H2O and reaction mother solution (RMS) as functions of temperature.
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Fig. 3. Photograph of ZCTC crystal habit grown in reaction mother solution (RMS).
maintained at this temperature until neither growth nor dissolution takes place on the seed crystal for at least 24 h. After this procedure the solution is evaporated at this constant temperature slowly. The temperature controller is FP21, a programmable temperature controller with high accuracy of is 0.01 8C. Under the above growth conditions, and in over three months growth duration, large single crystal of ZCTC can be grown. The crystal was colorless and transparent, tested by polarizing microscopy, the crystal exhibited uniform extinction when the polarization vector was parallel to a dielectric axis, which indicates its high optical quality. A typical single crystal with dimensions of 15 7 7 mm3 shown in Fig. 4 was successfully grown by this procedure. Fig. 4 also shows another ZCTC crystal, with dimensions of the transparent area of 7 7 6 mm3. To grow higher-quality and larger single crystals of ZCTC from the seeded and saturated RMS, the temperature-lowering method is better than the solvent-evaporation method. We are now managing to adopt such a growth procedure. The crystal habit of ZCTC grown in solutions with different pH values were investigated by using an optical microscope. At a lower pH values, a large number of crystals were grown, but they were found to stick one upon another. This is because the solubility of ZCTC is high at lower pH values, which results in a large number of nucleation centers. On the other hand, at higher pH values, the number of nucleation centers is limited, but the crystals grown lack well developed faces. Therefore, we optimized the pH values of ZCTC solution at 2–4 to get well developed and good single crystals. At the same time, whole growth process of the ZCTC crystal is affected by the pH value of the solution. The reason is probably attributed to the complex equilibrium existing in the solution as follows, where x ¼ 4: Zn2þ þ xSCN $ ZnðSCNÞx ðx2Þ Cd2þ þ xSCN $ CdðSCNÞx ð2xÞþ Zn2þ þ Cd2þ þ xSCN $ ZnCdðSCNÞx ðx4Þ Hþ þ SCN $ HSCN The equilibrium concentrations of the various ions and the crystal growth radically change with the variation of the pH value and supersaturation, i.e. change of the growth rate of the ZCTC crystal and its morphology. Moreover, thiocyanic acid (HSCN) is much more unstable than its thiocyanates, which is
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Fig. 4. ZCTC single crystals grown by the solvent-evaporation method.
easy to decompose. Although the aqueous solution of ZCTC can retain in stable state without decomposition and hydrolysis at atmospheric pressure and relatively low temperature for a long time, whose aqueous solution will be decomposed and hydrosised if it is kept at high temperature for several days. The color of the solution slowly turn slightly red and form white and yellow precipitates as byproducts. Several possible chemical reactions may occur as shown by the following equations: 2ZnCdðSCNÞ4 þ 18H2 O $ ZnS # þ CdS # þ ZnCO3 # þ CdCO3 # þ 6CO2 þ 6H2 S þ 8NH3 ; ZnCdðSCNÞ4 $ Zn2þ þ Cd2þ þ 4SCN ; 2SCN þ 2H2 O þ 3O2 ! 2Hþ þ 2SO4 2 þ 2HCN; Zn2þ þ 2H2 O $ ZnðOHÞ2 þ 2Hþ ; Cd2þ þ 2H2 O $ CdðOHÞ2 þ 2Hþ : During growth process, one of the raw materials we used is ammonium thiocyanate (NH4SCN), not sodium thiocyanate (NaSCN) or potassic thiocyanate (KSCN), because the crystal habit of ZCTC in NH4X/H2O RMS system is somewhat better than that in NaX/H2O or KX/H2O system (X ¼ NO3 , Cl). The other two raw materials we used are ZnX2 and CdX2 (X ¼ NO3 , Cl) not ZnSO4, and CdSO4, because ZnSO4 (NH4)2SO4 6H2O and CdSO4 (NH4)2SO4 6H2O would be observed as byproducts if the latter are used as raw materials.
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Fig. 5. The IR spectrum of ZCTC crystal.
Several foreign metallic cations, such as Al3þ, Fe3þ, Co2þ, Ni2þ, Cu2þ and etc. will affect the whole growth process. Their effects are related with ionic radii, electric charge, complex formation in the bulk and at the surface, and frequency of solvent exchange. The presence of these impurities will badly influence the quality of ZCTC crystals, especially their transparency. Exploration of eliminating these ions are presently under way. 3.2. Structural characterization Fig. 5 shows the IR spectra of ZCTC and its corresponding vibrations. The assignments of the main characteristic IR band frequencies (cm1) observed for KSCN [9] and ZCTC are listed in Table 1. It is known that nCN often lies higher than 2100 cm1, nCS lies ca 860 780 cm1 (N-bonding) or 720 690 cm1 (S-bonding) and dSCN, lies near 480 cm1 (N-bonding) or 420 cm1 (S-bonding) [10]. From Fig. 3 and Table 1, one can see that the sharp increase in frequencies of nCN stretching and nCS stretching and decrease of dSCN bending in ZCTC compared with the corresponding bands in the free thiocyanate radical of KSCN. This confirms the metal-nitrogen and metal-sulfur coordination in the their structures. Table 1 Assignments of the main characteristic IR band frequencies (cm1) observed for KSCN and ZCTC Assignment
KSCN
ZCTC
nCN nCS dNCS 2dNCS
2063 746 488 940
2113.9, 2162.1 785.0 447.5, 472.5 896.9, 945.1
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Fig. 6. The XRPD pattern and diffraction indices of ZCTC.
The XRPD experiments showed that the synthesized material and the as-grown crystals are the single phase of ZCTC. The XRPD pattern and diffraction indices of the crystal were shown in Fig. 6. The tetragonal unit-cell parameters calculated by TREOR program [11] according to the values of 2y in ˚ , V ¼ 540:96 A ˚ 3, which are comparable with the results XRPD pattern are a ¼ 11:1141, c ¼ 4:3794 A determined by a R3m/E four-circle X-ray diffractometer [7]. The XRPD data for ZCTC are tabulated in Table 2. 3.3. Thermal properties The specific heat is one of the important factors that influence the damage threshold of crystal. The specific heat data of solids are generally described by the Debye theory in terms of the harmonic frequency spectrum for each crystalline lattice. However, it is very difficult to calculate the specific heat of a ZCTC crystal. Fig. 7 shows the temperature dependence of the specific heat of ZCTC crystal. From this figure, we can see that the specific heat of ZCTC crystal is linear with temperature in the measured temperature range. The specific heat of ZCTC crystal at room temperature is about 0.8972 J/g K, and is equivalent to 367.9 J/mol K. One can expect that upon irradiation by plused laser beam, a ZCTC crystal will absorb more energy while maintaining a smaller temperature gradient. A large gradient disturbs the phase-matching properties of a NLO material when compared to crystals with low specific heat. The thermal expansion coefficients of a crystal are important factors for crystal growth and applications. The laser absorption of a crystal will cause thermal gradient in the crystal, which will lead to crystal fracture if the laser power is high enough. For a tetragonal crystal, there are only two independent principal thermal expansion components, a1 and a3, respectively. Fig. 8 shows the temperature variation of the thermal expansions along the two crystallographic directions. No anomaly within the temperature range shown was observed. The average linear thermal expansion
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Table 2 XRPD data for ZCTC ˚) dobs (A
2yobs (8)
2ycalc (8)
I/I0 Intensity (%)
hkl
7.86000 5.55600 4.07600 3.93000 3.51400 3.28600 2.82800 2.77800 2.62000 2.52000 2.48400 2.29500 2.17900 2.10900 2.03800 1.98200 1.96500 1.91100 1.86800 1.85900 1.85200 1.75700 1.72000 1.68600 1.64300 1.61400 1.54500
11.25696 15.95094 21.80392 22.62443 25.34486 27.13589 31.63712 32.22189 34.22308 35.62569 36.15971 39.25452 41.43757 42.87966 44.45165 45.77780 46.19667 47.58132 48.74685 48.99833 49.19578 52.04876 53.25547 54.41703 55.96432 57.06107 59.86216
11.25866 15.94798 21.81194 22.62787 25.34067 27.13681 31.63229 32.21579 34.22838 35.61551 36.14198 39.24438 41.42476 42.87181 44.46867 45.77500 46.20401 47.53458 48.77715 49.01200 49.18610 52.03967 53.26118 54.40778 55.96634 57.07613 59.87109
9 100 94 13 8 31 34 4 8 23 18 22 3 8 4 11 5 4 10 25 15 5 3 5 3 5 13
110 200 101 220 310 211 301 400 330 321 420 411 510 112 202 501 440 222 521 312 600 620 402 611 422 541 512
coefficient of the a-oriented ZCTC crystal measured from 303.15 to 433.15 K is 1:69 105 K1 and that of the c-oriented crystal is 1:95 104 K1. These results show that the thermal expansion coefficients of ZCTC crystal are anisotropic along the different crystallographic axes. When a small temperature variation DT takes place throughout the ZCTC crystal, it shows both thermal expansion and contraction. 3.4. Optical properties According to the powder technique [12], the SHG efficiency can be estimated as about 51.6 times as that of urea crystal powders for ZCTC crystal powders. Fig. 9 shows transmission spectra of ZCTC along the two directions. The UV cutoff wavelengths are the same in the two directions which indicate that the UV cutoff wavelength of ZCTC crystal is 290 nm. The first IR absorption band occurs at 2320 cm1. The transmission normal to (1 0 0) plane is superior to that normal to (0 0 1) plane. In the transmission normal to (1 0 0) plane, the average transmittance of 70% in the transmission band between 330 and 2280 nm, and the transparent percentage at standard violet light of 404 and 380 nm are 74.16% and 73.22%, respectively, which is much higher than that of CMTC.
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Fig. 7. The temperature dependence of the specific heat of ZCTC crystal.
During crystal device processing, ZCTC is much more easy to process compared with CMTC. This result shows that ZCTC crystal has a high mechanical strength for device processing. Very recently, the UV SHG at 380 nm by direct frequency doubling of a laser-diode at 760 nm using a ZCTC crystal at room temperature. The 380 nm light output power of 20 mW is measured for a 110 mW input power of
Fig. 8. The temperature variation of the thermal expansions along the two crystallographic directions.
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Fig. 9. Transmission spectra of ZCTC normal to (1 0 0) and (0 0 1) plane and the thicknesses of the samples used were 1.47, 1.46 mm, respectively.
the laser-diode. The details of the experiment will be described elsewhere. Now, we are managing to improve the SHG output efficiency along with exploring better crystal growth conditions and crystal processing techniques such as cutting, polishing and antireflection coating.
4. Conclusions A novel coordination nonlinear-optical crystal material, ZCTC has been developed for UV SHG and grown into single crystals from the aqueous solution by slow solvent-evaporation method and characterized by various ways, and the slow temperature-lowering method is presently under way. The physicochemical changes of their crystal-growth solutions at high temperatures and different pH values show the difficulty of its single crystal growth. IR spectroscopy and XPRD studies confirmed the identity of the grown single crystals. ZCTC crystal exhibits large anisotropy in thermal expansion and a relatively large specific heat. The SHG intensities of ZCTC crystal powders are superior to that of CMTC crystal powders and over one order of magnitude higher than that of Urea crystal powders and ZCTC has excellent transparency in the UV regain. The UV light output by direct laser-diode frequency doubling using ZCTC crystal was realized at room temperature through preliminary experiment. In summary, being a coordination crystal, ZCTC emerges as a promising candidate for frequency doubling to violet and UV wavelengths, ZCTC possesses excellent performance compared to CMTC. Its large
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SHG intensities, high optical transparency, and excellent processability make it attractive for devices applications. Further investigations of crystal growth and device applications are now in progress.
Acknowledgements This work is supported by grants from the ‘‘863’’, the National Natural Science Foundation of China (69890230 and 69778023), and the Opening Foundation of the Institute of Crystal Materials, Shandong University (060115). The authors are indebted to Professor Yang Zhaohe for the transmission spectra determinations and TMA measurements, Engineer Sun Suoying for expert crystal cutting and polishing and last but not least Dr. Zhao Xian for helpful comments in the research project.
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