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Investigation on growth and macro-defects of a UV nonlinear optical crystal: ZnCd(SCN)4
Journal of Crystal Growth 235 (2002) 340–346
Investigation on growth and macro-defects of a UV nonlinear optical crystal: ZnCd(SCN)4 X.Q. Wanga,*, J.G. Zhanga, D. Xua, M.K. Lu. a, D.R. Yuana, S.X. Xub, J. Huangc, G.H. Zhanga, S.Y. Guoa, S.L. Wanga, X.L. Duana, Q. Rend, G.T. Lu. a a
State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, China b Institute of Electronics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China c College of Environmental Science and Engineering, Institute of Modern Analysis, Shandong University, Jinan 250100, People’s Republic of China d Optics Department, Shandong University, Jinan 250100, People’s Republic of China Received 20 August 2001; accepted 18 October 2001 Communicated by M. Roth
Abstract Large single crystals of the coordination complex nonlinear optical material zinc cadmium thiocyanate, ZnCd(SCN)4 (abbreviated as ZCTC), were grown from aqueous solutions by the solvent evaporation method. The morphology of the crystals was indexed. The grown crystals were characterized by the powder X-ray diffraction analysis allowing to identify the diffraction planes. Six kinds of macro-defects were found in ZCTC large crystals. These defects include cracks, inclusions, negative crystals, growth striations, sector boundaries and straight pipes. The morphologies and distribution regularities of these defects were observed and analyzed using optical microscopy. Their formation mechanisms and the methods of eliminating these defects are discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 42.65; 61.72; 07.85.N Keywords: A1. Crystal morphology; A1. Defects; A1. Growth model; A2. Growth from solutions; B1. Cadmium compounds; B2. Nonlinear optic materials
1. Introduction Versatile and efficient sources of blue–violet or ultraviolet (UV) light are of fundamental importance for many applications, such as high-density optical data storage, medical diagnosis, photo*Corresponding author. Tel.: +86-531-856-4451; fax: +86531-856-5403. E-mail address: [email protected] (X.Q. Wang).
lithography, underwater communications, laser displays, etc. In spite of the rapid development of blue laser diodes and concurrent physical principles, like optical upconversion, optical second harmonic generation (SHG) is still one of the most important methods to achieve intense coherent blue light. Lately, second-order nonlinear optical (SONLO) materials capable of efficient frequency conversion of infrared or visible laser radiation
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 8 1 1 - 5
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
to visible or UV wavelengths are of considerable interest in these fields. Materials with large secondorder optical nonlinearities, transparency at all wavelengths involved and stable physicochemical performance are needed in order to realize many of these applications [1–3]. As SONLO materials, bimetallic thiocyanates: ZnCd(SCN)4, ZnHg(SCN)4, CdHg(SCN)4 and MnHg(SCN)4 (abbreviated as ZCTC, ZMTC, CMTC and MMTC, respectively), exhibit efficient SHG at short wavelengths [4]. Among them, the ZCTC crystal exhibits the largest powder second harmonic efficiency, the shortest transparency cutoff and the widest transparent wave band. It belongs to the tetragonal crystallographic system, space group I4% ; with cell parameters: a ¼ ( V ¼ 542:6ð2Þ A ( 3, Z ¼ 11:135ð2Þ; c ¼ 4:3760ð10Þ A, 3 2; Dc ¼ 2:510 g/cm [5]. Recently, the growth and properties of ZCTC crystals has been reported [6,7]. In addition, violet light at 404 nm and UV light at 380 nm have been realized by the use of ZCTC crystals through direct frequency doubling of a diode laser (emitting at 808 nm) and a continuous-wave Ti:sapphire laser (at 760 nm), respectively [8]. In the present work, large and highly optical quality ZCTC single crystals have been grown by means of the solvent-evaporation method. The crystal defects are analyzed from the viewpoint of the crystal growth mechanisms and growth conditions, and the approach to abate and even eliminate these defects is presented.
341
aqueous solution by controlled evaporation at 401C. The quality of the crystal can be improved by adding HX (X=Cl, NO3, CH3COO, etc) during crystal growth. Two typical crystals with dimensions of 24 11 12 and 23 12 14 mm3, respectively, are shown in Fig. 1(a). The X-ray powder diffraction (XRPD) data of a ZCTC crystal powder sample were collected on a Rigaku D/Max-gA diffractometer using a Ni-filtered Cutarget tube and a graphite monochromator (40 kV, 60 mA, fixed scatter and divergence slits of 11 and a receiving slit of 0.15 mm). Intensities for the diffraction peaks were recorded in the 10–701 (2y) range with a step size of 0.021 and a scan speed of 41/min. The tetragonal unit-cell parameters calculated by the TEROR program [9], according to the values of 2y in the XRPD pattern, are: ( and V ¼ 542:83 A ( 3. a ¼ 11:1261; c ¼ 4:3851 A These parameters are more consistent with the results obtained by a R3m/E four-circle X-ray
2. Crystal growth ZCTC can be prepared in deionized water according to the following reaction: ZnX2 þ CdX2 þ 4ASCN-ZnCdðSCNÞ4 þ 4AX; ð1Þ where A=K, Na, NH4; X=Cl, NO3, CH3COO. ZCTC exhibits a very high solubility in the water/acetone solution mixed in a certain ratio [6]. Since acetone is very volatile, ZCTC can be easily recrystallized and purified by this mixed solvent, and single crystals of ZCTC are grown from seeds cut perpendicular to the c-axis in a saturated
Fig. 1. As-grown ZCTC single crystals (a) and the schematic drawing of crystal morphology (b).
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
342
diffractometer [5] than those reported by us previously [6]. Table 1 gives the relative experimental intensities, d-spacings (observed), 2y (observed and calculated) and their indices for ZCTC. The crystal morphology reported previously [7] was incorrect. The morphology of the ZCTC crystal was indexed measuring the interfacial angles in the habit. Fig. 1(b) shows the typical crystal habit formed by a tetragonal prism with {1 0 0} facets and a tetragonal disphenoid with {3 0 1} facets. The crystal exhibits a 4% morphological symmetry. In addition to the major {1 0 0}
Table 1 Relative experimental intensities, d-spacings (observed), 2y (observed and calculated) and their indices for ZCTC ( dobs (A)
2yobs (1)
2ycalc (1)
I=I0 Intensity h k l (%)
7.86738 5.56298 4.07980 3.93360 3.51801 3.28958 2.83045 2.78001 2.71473 2.62246 2.52265 2.48697 2.29733 2.18139 2.11110 2.03990 1.98371 1.96680 1.91511 1.86990 1.86104 1.85433 1.75900 1.72223 1.68827 1.64479 1.61545 1.54650 1.44921 1.44352 1.43952 1.40471 1.35463
11.24637 15.93081 21.78334 22.60348 25.31549 27.10576 31.60900 32.19800 32.99400 34.19000 35.58700 36.11500 39.21300 41.39000 42.83500 44.40800 45.73600 46.15200 47.47300 48.69400 48.94101 49.13000 51.98500 53.18100 54.33801 55.89800 57.00500 59.79800 64.27299 64.55700 64.75800 66.56800 69.37201
11.24643 15.93060 21.78401 22.60306 25.31279 27.10359 31.59429 32.17998
9 100 94 13 8 31 34 4
110 200 101 220 310 211 301 400
34.19019 35.57310 36.10152 39.19779 41.37786 42.81430 44.40942 45.72055 46.15111 47.47183 48.71894 48.94753 49.12934 51.97913 53.19151 54.34219 55.89321 57.00691 59.79275 64.27100 64.57543 64.76582 66.57706 69.35896
8 23 18 22 3 8 4 11 5 4 10 25 15 5 3 5 3 5 13 6 8 6 3 4
330 321 420 411 510 112 202 501 440 222 521 312 600 620 402 611 422 541 512 103 721 532 612 651
and {3 0 1} facets, some small facets, such as {1 1 0}, {3 1 0} and {1 0 1} are present occasionally.
3. Defects The crystal samples were mostly wafers parallel to the tetragonal prisms formed by (1 0 0) or (0 1 0) facets or to the tetragonal disphenoid (3 0 1) or (0 3% 1) facets. They were all cut and optically polished before use. The experimental equipment comprised the MeF3 metalloscope microscope and the Opton polarizing microscope, which were used to observe the cracks, inclusions, growth striations, growth boundaries and straight pipe defects in the crystals. All pictures were taken by a video camera. 3.1. Cracks The crack is one of the growth features of ZCTC crystals. Some microcracks often appear inside the crystal, as shown in the Fig. 2. The directions of the cracks are mostly parallel to the c-axis. These cracks usually appear in the crystal whose rate of pyramid formation is faster and in which there are
Fig. 2. Cracks in the ZCTC crystal.
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
343
Fig. 3. Diffuse bubble-shaped solution inclusions (300 ).
large quantities of prismatic solution inclusions along the c-direction after the formation of the pyramid. The existence of these inclusions greatly damages the crystalline structure, forms large-scale imperfect areas, and gives rise to appreciable mechanical stresses. The thermal expansion coefficients of the inclusions are different from those of the crystal, which results in cracks along the c-direction of the crystal. 3.2. Inclusions ZCTC crystals contain various kinds of inclusions. Fig. 3 shows a diffuse bubble-shaped solution inclusion observed in a (0 3 1) wafer. As it has been mentioned above, when the pyramid formation rate is faster, large-scale imperfect areas are created in the crystal. There are also large quantities of solution inclusions near the capping region, as shown in Fig. 4. Most of these inclusions may induce negative growth in the crystal and result in ‘‘negative crystals’’. Rectangular negative crystals are observed on the prismatic facets, and trigonal negative crystals are observed on the pyramidal facets, which are shown in Figs. 5(a) and (b), respectively. These kinds of inclusions possess the same structural morphology as that of the crystal. Insoluble solid inclusions shown in Fig. 6 are found in the middle of (1 0 0) and (1% 0 0) facets in the crystal, i.e. they are entrapped during the bipyramid growth at the junction of the (0 3 1) and (0 3% 1) facets. The multiform inclusions described above are symbiotic in the crystal during crystal growth, and
Fig. 4. Solution inclusions near the capping region (300 ).
they are greatly influenced by the growth conditions and environment. Their incorporation is also related to the crystal growth mechanisms. According to the preliminary observations, the main growth mechanism is a two-dimensional layer-bylayer growth. The growth rate of the prismatic {1 0 0} faces is very slow, and crystal growth relies mainly on the growth of bipyramidal {0 3 1} faces. Layer growth requires a high degree of supersaturation that, in turn, may result in the instability of the growth interface. When the growth conditions are not appropriate, for example when the degree of supersaturation becomes considerably low, the growth may halt temporarily
344
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
Fig. 5. Solution inclusions near to the capping region and negative crystals (300 ). Negative crystal morphology: (a) the (1 0 0) face; (b) the (0 3 1) face.
and produce negative crystallites inside the main crystal. 3.3. Growth striations and sector boundaries
Fig. 6. Insoluble solid inclusions.
We have indicated above that the crystal growth relies mainly on the growth of bipyramidal (0 3 1) and (0 3% 1) faces. The growth kinetics factors and the undulation of the growth conditions result in growth striations. The growth rates of these two faces are different. This results in structure mismatch and lattice distortion between the two growth pyramids and forms sector boundaries. Impurities can be easily captured on the sector boundaries, which results in the formation of inclusions. Fig. 7 shows the growth striations, sector boundaries and insoluble solid inclusions that are observed on the (1 0 0) face. 3.4. Straight pipes
or completely. Subsequently, growth may restart in response to an opposite action, e.g. an increase in the supersaturation. Under these conditions, the growth units cannot arrange regularly but rather overspread one layer quickly, which facilitates the incorporation of numerous inclusions into the crystal. Some inclusions induce negative growth
There is a new kind of defect, namely straight pipes observed on the (1 0 0) face of the ZCTC crystal, which can be seen in Fig. 8. These straight pipes are parallel to the c-direction, and some of them are interrupted by curved pipes along the a-direction. Microscopic chemical analysis shows
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
345
Fig. 7. Growth striations, sector boundaries and insoluble solid inclusions observed on the (1 0 0) face: (a) sector boundaries; (b) inclusions and striations within marked area in (a) after 300 magnification.
may easily breakdown. The crystal growth occurs mainly along the c-direction, i.e. the growth units arrange along the c-direction. We suggest that in course the crystal growth process the S ¼ C chains break down, while growth halts and solution clusters fill in the breakdown areas, which results in the formation of the straight pipe defeat parallel to the c-direction.
Fig. 8. Straight pipes.
that the cross-section area contains a certain amount of the solution. Therefore, the curved pipe defects are considered as solution inclusions. We presume that the straight pipe is closely related to the crystal structure of ZCTC. The crystal is a coordination complex material, whose structure is composed of two distorted tetrahedra, ZnN4 and CdS4. The aN ¼ C ¼ Sa bridges connect the central atoms of these flattened tetrahedra forming infinite three-dimensional aZnaN ¼ C ¼ SaCda networks. However, the S ¼ C bonds in the three-dimensional bridges
4. Conclusions Large and high optical quality single crystals of ZCTC can be easily obtained by controlled evaporation from solutions at 401C. Six kinds of macroscopic defects in the large single crystals of ZCTC have been identified and described. They include cracks, inclusions, negative crystals, growth striations, sector boundaries and straight pipes. The cracks in ZCTC crystals are mainly caused by the thermal expansion coefficients difference between the inclusions and the crystal. Therefore, cracks in ZCTC crystals can be avoided if suitable growth conditions are selected and the amount of inclusions is minimized.
346
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
Multiform inclusions, negative crystals, growth striations and sector boundaries are caused by the instability of the growth conditions and the layer growth mechanism. Therefore, strict control of the rate of crystal growth is particularly essential for the homogeneous growth for ZCTC crystals. The straight pipes are considered to be caused by the breakdown of aN ¼ C ¼ Sa bridges along the c-direction, which can be abated and even eliminated through controlling the growth conditions, such as temperature, pH value, etc.
Acknowledgements This work is supported by grants from the National Natural Science Foundation of China (69890230 and 60178029) and the Opening Foundation of the Institute of Crystal Materials, Shandong University (060115).
References [1] [2] [3] [4]
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I. Ledoux, Synth. Metals 54 (1993) 123. N.J. Long, Angew. Chem. Int. Ed. Engl. 34 (1995) 21. M.H. Jiang, Q. Fang, Adv. Mater. 11 (1999) 1147. X.Q. Wang, D. Xu, M.K. Lu, . D.R. Yuan, G.H. Zhang, F.Q. Meng, S.Y. Guo, M. Zhou, J.R. Liu, X.R. Li, Cryst. Res. Technol. 36 (2001) 73. X.Q. Wang, D. Xu, D.R. Yuan, Y.P. Tian, W.T. Yu, S.S. Ying, Z.H. Yang, Q. Fang, M.K. Lu, . Y.X. Yan, F.Q. Meng, S.Y. Guo, G.H. Zhang, M.H. Jiang, Mater. Res. Bull. 34 (1999) 2003. X.Q. Wang, D. Xu, M.K. Lu, . D.R. Yuan, G.H. Zhang, S.X. Xu, S.Y. Guo, X.N. Jiang, J.R. Liu, C.F. Song, Q. Ren, J. Huang, Y.P. Tian, Mater. Res. Bull. 36 (2001) 1287. X.N. Jiang, D. Xu, D.R. Yuan, M.K. Lu, . S.Y. Guo, G.H. Zhang, X.Q. Wang, Q. Fang, J. Crystal Growth 222 (2001) 755. G.H. Zhang, D. Xu, M.K. Lu, . D.R. Yuan, X.Q. Wang, F.Q. Meng, S.Y. Guo, Q. Ren, M.H. Jiang, Optics Laser Technol. 33 (2001) 121. P.E. Werner, L. Eriksson, Westdahi, J. Appl. Cryst. 18 (1985) 367.
Investigation on growth and macro-defects of a UV nonlinear optical crystal: ZnCd(SCN)4 X.Q. Wanga,*, J.G. Zhanga, D. Xua, M.K. Lu. a, D.R. Yuana, S.X. Xub, J. Huangc, G.H. Zhanga, S.Y. Guoa, S.L. Wanga, X.L. Duana, Q. Rend, G.T. Lu. a a
State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, China b Institute of Electronics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China c College of Environmental Science and Engineering, Institute of Modern Analysis, Shandong University, Jinan 250100, People’s Republic of China d Optics Department, Shandong University, Jinan 250100, People’s Republic of China Received 20 August 2001; accepted 18 October 2001 Communicated by M. Roth
Abstract Large single crystals of the coordination complex nonlinear optical material zinc cadmium thiocyanate, ZnCd(SCN)4 (abbreviated as ZCTC), were grown from aqueous solutions by the solvent evaporation method. The morphology of the crystals was indexed. The grown crystals were characterized by the powder X-ray diffraction analysis allowing to identify the diffraction planes. Six kinds of macro-defects were found in ZCTC large crystals. These defects include cracks, inclusions, negative crystals, growth striations, sector boundaries and straight pipes. The morphologies and distribution regularities of these defects were observed and analyzed using optical microscopy. Their formation mechanisms and the methods of eliminating these defects are discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 42.65; 61.72; 07.85.N Keywords: A1. Crystal morphology; A1. Defects; A1. Growth model; A2. Growth from solutions; B1. Cadmium compounds; B2. Nonlinear optic materials
1. Introduction Versatile and efficient sources of blue–violet or ultraviolet (UV) light are of fundamental importance for many applications, such as high-density optical data storage, medical diagnosis, photo*Corresponding author. Tel.: +86-531-856-4451; fax: +86531-856-5403. E-mail address: [email protected] (X.Q. Wang).
lithography, underwater communications, laser displays, etc. In spite of the rapid development of blue laser diodes and concurrent physical principles, like optical upconversion, optical second harmonic generation (SHG) is still one of the most important methods to achieve intense coherent blue light. Lately, second-order nonlinear optical (SONLO) materials capable of efficient frequency conversion of infrared or visible laser radiation
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 8 1 1 - 5
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
to visible or UV wavelengths are of considerable interest in these fields. Materials with large secondorder optical nonlinearities, transparency at all wavelengths involved and stable physicochemical performance are needed in order to realize many of these applications [1–3]. As SONLO materials, bimetallic thiocyanates: ZnCd(SCN)4, ZnHg(SCN)4, CdHg(SCN)4 and MnHg(SCN)4 (abbreviated as ZCTC, ZMTC, CMTC and MMTC, respectively), exhibit efficient SHG at short wavelengths [4]. Among them, the ZCTC crystal exhibits the largest powder second harmonic efficiency, the shortest transparency cutoff and the widest transparent wave band. It belongs to the tetragonal crystallographic system, space group I4% ; with cell parameters: a ¼ ( V ¼ 542:6ð2Þ A ( 3, Z ¼ 11:135ð2Þ; c ¼ 4:3760ð10Þ A, 3 2; Dc ¼ 2:510 g/cm [5]. Recently, the growth and properties of ZCTC crystals has been reported [6,7]. In addition, violet light at 404 nm and UV light at 380 nm have been realized by the use of ZCTC crystals through direct frequency doubling of a diode laser (emitting at 808 nm) and a continuous-wave Ti:sapphire laser (at 760 nm), respectively [8]. In the present work, large and highly optical quality ZCTC single crystals have been grown by means of the solvent-evaporation method. The crystal defects are analyzed from the viewpoint of the crystal growth mechanisms and growth conditions, and the approach to abate and even eliminate these defects is presented.
341
aqueous solution by controlled evaporation at 401C. The quality of the crystal can be improved by adding HX (X=Cl, NO3, CH3COO, etc) during crystal growth. Two typical crystals with dimensions of 24 11 12 and 23 12 14 mm3, respectively, are shown in Fig. 1(a). The X-ray powder diffraction (XRPD) data of a ZCTC crystal powder sample were collected on a Rigaku D/Max-gA diffractometer using a Ni-filtered Cutarget tube and a graphite monochromator (40 kV, 60 mA, fixed scatter and divergence slits of 11 and a receiving slit of 0.15 mm). Intensities for the diffraction peaks were recorded in the 10–701 (2y) range with a step size of 0.021 and a scan speed of 41/min. The tetragonal unit-cell parameters calculated by the TEROR program [9], according to the values of 2y in the XRPD pattern, are: ( and V ¼ 542:83 A ( 3. a ¼ 11:1261; c ¼ 4:3851 A These parameters are more consistent with the results obtained by a R3m/E four-circle X-ray
2. Crystal growth ZCTC can be prepared in deionized water according to the following reaction: ZnX2 þ CdX2 þ 4ASCN-ZnCdðSCNÞ4 þ 4AX; ð1Þ where A=K, Na, NH4; X=Cl, NO3, CH3COO. ZCTC exhibits a very high solubility in the water/acetone solution mixed in a certain ratio [6]. Since acetone is very volatile, ZCTC can be easily recrystallized and purified by this mixed solvent, and single crystals of ZCTC are grown from seeds cut perpendicular to the c-axis in a saturated
Fig. 1. As-grown ZCTC single crystals (a) and the schematic drawing of crystal morphology (b).
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
342
diffractometer [5] than those reported by us previously [6]. Table 1 gives the relative experimental intensities, d-spacings (observed), 2y (observed and calculated) and their indices for ZCTC. The crystal morphology reported previously [7] was incorrect. The morphology of the ZCTC crystal was indexed measuring the interfacial angles in the habit. Fig. 1(b) shows the typical crystal habit formed by a tetragonal prism with {1 0 0} facets and a tetragonal disphenoid with {3 0 1} facets. The crystal exhibits a 4% morphological symmetry. In addition to the major {1 0 0}
Table 1 Relative experimental intensities, d-spacings (observed), 2y (observed and calculated) and their indices for ZCTC ( dobs (A)
2yobs (1)
2ycalc (1)
I=I0 Intensity h k l (%)
7.86738 5.56298 4.07980 3.93360 3.51801 3.28958 2.83045 2.78001 2.71473 2.62246 2.52265 2.48697 2.29733 2.18139 2.11110 2.03990 1.98371 1.96680 1.91511 1.86990 1.86104 1.85433 1.75900 1.72223 1.68827 1.64479 1.61545 1.54650 1.44921 1.44352 1.43952 1.40471 1.35463
11.24637 15.93081 21.78334 22.60348 25.31549 27.10576 31.60900 32.19800 32.99400 34.19000 35.58700 36.11500 39.21300 41.39000 42.83500 44.40800 45.73600 46.15200 47.47300 48.69400 48.94101 49.13000 51.98500 53.18100 54.33801 55.89800 57.00500 59.79800 64.27299 64.55700 64.75800 66.56800 69.37201
11.24643 15.93060 21.78401 22.60306 25.31279 27.10359 31.59429 32.17998
9 100 94 13 8 31 34 4
110 200 101 220 310 211 301 400
34.19019 35.57310 36.10152 39.19779 41.37786 42.81430 44.40942 45.72055 46.15111 47.47183 48.71894 48.94753 49.12934 51.97913 53.19151 54.34219 55.89321 57.00691 59.79275 64.27100 64.57543 64.76582 66.57706 69.35896
8 23 18 22 3 8 4 11 5 4 10 25 15 5 3 5 3 5 13 6 8 6 3 4
330 321 420 411 510 112 202 501 440 222 521 312 600 620 402 611 422 541 512 103 721 532 612 651
and {3 0 1} facets, some small facets, such as {1 1 0}, {3 1 0} and {1 0 1} are present occasionally.
3. Defects The crystal samples were mostly wafers parallel to the tetragonal prisms formed by (1 0 0) or (0 1 0) facets or to the tetragonal disphenoid (3 0 1) or (0 3% 1) facets. They were all cut and optically polished before use. The experimental equipment comprised the MeF3 metalloscope microscope and the Opton polarizing microscope, which were used to observe the cracks, inclusions, growth striations, growth boundaries and straight pipe defects in the crystals. All pictures were taken by a video camera. 3.1. Cracks The crack is one of the growth features of ZCTC crystals. Some microcracks often appear inside the crystal, as shown in the Fig. 2. The directions of the cracks are mostly parallel to the c-axis. These cracks usually appear in the crystal whose rate of pyramid formation is faster and in which there are
Fig. 2. Cracks in the ZCTC crystal.
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
343
Fig. 3. Diffuse bubble-shaped solution inclusions (300 ).
large quantities of prismatic solution inclusions along the c-direction after the formation of the pyramid. The existence of these inclusions greatly damages the crystalline structure, forms large-scale imperfect areas, and gives rise to appreciable mechanical stresses. The thermal expansion coefficients of the inclusions are different from those of the crystal, which results in cracks along the c-direction of the crystal. 3.2. Inclusions ZCTC crystals contain various kinds of inclusions. Fig. 3 shows a diffuse bubble-shaped solution inclusion observed in a (0 3 1) wafer. As it has been mentioned above, when the pyramid formation rate is faster, large-scale imperfect areas are created in the crystal. There are also large quantities of solution inclusions near the capping region, as shown in Fig. 4. Most of these inclusions may induce negative growth in the crystal and result in ‘‘negative crystals’’. Rectangular negative crystals are observed on the prismatic facets, and trigonal negative crystals are observed on the pyramidal facets, which are shown in Figs. 5(a) and (b), respectively. These kinds of inclusions possess the same structural morphology as that of the crystal. Insoluble solid inclusions shown in Fig. 6 are found in the middle of (1 0 0) and (1% 0 0) facets in the crystal, i.e. they are entrapped during the bipyramid growth at the junction of the (0 3 1) and (0 3% 1) facets. The multiform inclusions described above are symbiotic in the crystal during crystal growth, and
Fig. 4. Solution inclusions near the capping region (300 ).
they are greatly influenced by the growth conditions and environment. Their incorporation is also related to the crystal growth mechanisms. According to the preliminary observations, the main growth mechanism is a two-dimensional layer-bylayer growth. The growth rate of the prismatic {1 0 0} faces is very slow, and crystal growth relies mainly on the growth of bipyramidal {0 3 1} faces. Layer growth requires a high degree of supersaturation that, in turn, may result in the instability of the growth interface. When the growth conditions are not appropriate, for example when the degree of supersaturation becomes considerably low, the growth may halt temporarily
344
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
Fig. 5. Solution inclusions near to the capping region and negative crystals (300 ). Negative crystal morphology: (a) the (1 0 0) face; (b) the (0 3 1) face.
and produce negative crystallites inside the main crystal. 3.3. Growth striations and sector boundaries
Fig. 6. Insoluble solid inclusions.
We have indicated above that the crystal growth relies mainly on the growth of bipyramidal (0 3 1) and (0 3% 1) faces. The growth kinetics factors and the undulation of the growth conditions result in growth striations. The growth rates of these two faces are different. This results in structure mismatch and lattice distortion between the two growth pyramids and forms sector boundaries. Impurities can be easily captured on the sector boundaries, which results in the formation of inclusions. Fig. 7 shows the growth striations, sector boundaries and insoluble solid inclusions that are observed on the (1 0 0) face. 3.4. Straight pipes
or completely. Subsequently, growth may restart in response to an opposite action, e.g. an increase in the supersaturation. Under these conditions, the growth units cannot arrange regularly but rather overspread one layer quickly, which facilitates the incorporation of numerous inclusions into the crystal. Some inclusions induce negative growth
There is a new kind of defect, namely straight pipes observed on the (1 0 0) face of the ZCTC crystal, which can be seen in Fig. 8. These straight pipes are parallel to the c-direction, and some of them are interrupted by curved pipes along the a-direction. Microscopic chemical analysis shows
X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
345
Fig. 7. Growth striations, sector boundaries and insoluble solid inclusions observed on the (1 0 0) face: (a) sector boundaries; (b) inclusions and striations within marked area in (a) after 300 magnification.
may easily breakdown. The crystal growth occurs mainly along the c-direction, i.e. the growth units arrange along the c-direction. We suggest that in course the crystal growth process the S ¼ C chains break down, while growth halts and solution clusters fill in the breakdown areas, which results in the formation of the straight pipe defeat parallel to the c-direction.
Fig. 8. Straight pipes.
that the cross-section area contains a certain amount of the solution. Therefore, the curved pipe defects are considered as solution inclusions. We presume that the straight pipe is closely related to the crystal structure of ZCTC. The crystal is a coordination complex material, whose structure is composed of two distorted tetrahedra, ZnN4 and CdS4. The aN ¼ C ¼ Sa bridges connect the central atoms of these flattened tetrahedra forming infinite three-dimensional aZnaN ¼ C ¼ SaCda networks. However, the S ¼ C bonds in the three-dimensional bridges
4. Conclusions Large and high optical quality single crystals of ZCTC can be easily obtained by controlled evaporation from solutions at 401C. Six kinds of macroscopic defects in the large single crystals of ZCTC have been identified and described. They include cracks, inclusions, negative crystals, growth striations, sector boundaries and straight pipes. The cracks in ZCTC crystals are mainly caused by the thermal expansion coefficients difference between the inclusions and the crystal. Therefore, cracks in ZCTC crystals can be avoided if suitable growth conditions are selected and the amount of inclusions is minimized.
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X.Q. Wang et al. / Journal of Crystal Growth 235 (2002) 340–346
Multiform inclusions, negative crystals, growth striations and sector boundaries are caused by the instability of the growth conditions and the layer growth mechanism. Therefore, strict control of the rate of crystal growth is particularly essential for the homogeneous growth for ZCTC crystals. The straight pipes are considered to be caused by the breakdown of aN ¼ C ¼ Sa bridges along the c-direction, which can be abated and even eliminated through controlling the growth conditions, such as temperature, pH value, etc.
Acknowledgements This work is supported by grants from the National Natural Science Foundation of China (69890230 and 60178029) and the Opening Foundation of the Institute of Crystal Materials, Shandong University (060115).
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