Flux growth of CsLiB6O10 crystals

Flux growth of CsLiB6O10 crystals

ARTICLE IN PRESS Journal of Crystal Growth 312 (2010) 2415–2418 Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage...

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ARTICLE IN PRESS Journal of Crystal Growth 312 (2010) 2415–2418

Contents lists available at ScienceDirect

Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Flux growth of CsLiB6O10 crystals Xuesong Yu a,b,c, Zhang-Gui Hu a,b,n a

Key Laboratory of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China Beijing Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China c Graduate School of Chinese Academy of Sciences, Beijing 100049, China b

a r t i c l e in fo

abstract

Article history: Received 6 October 2009 Received in revised form 1 March 2010 Accepted 5 May 2010 Communicated by A.G. Ostrogorsky Available online 12 May 2010

To obtain high quality CsLiB6O10 (CLBO) crystal, we used NaF as flux to grow CLBO crystals for the first time. The solubility of CLBO showed that the favorable mol concentration of CLBO in the NaF flux system was in the range of 65–85%. The viscosity of solutions was investigated. Using the top-seeded solution growth (TSSG) method, a CsLiB6O10 (CLBO) crystal with the dimensions of 54  51  32 mm3 was grown from the NaF flux system. A 5  5  5 mm3 crystal sample cut from the grown CLBO crystal was etched by an etchant that consisted of 15 ml glycerol and 15 ml water at room temperature, for comparison with another CLBO crystal sample got from the self-flux system was used. From the surface etching patterns, it was indicated that CLBO crystal grown from NaF flux system may be more stable than that from self-flux system. & 2010 Elsevier B.V. All rights reserved.

Keywords: A1. CsLiB6O10 (CLBO) A2. Top-seeded solution growth (TSSG) B1. Flux B2. Nonlinear optical materials (NLO)

1. Introduction CsLiB6O10 (CLBO), which possesses excellent nonlinear optical (NLO) properties in the UV range, was discovered in 1995 [1]. Since then, some continued research was also reported [2–7]. The greatest merit of CLBO is that it can be grown from congruent or nearcongruent melts at very fast growth rate [2–4]. Mori et al. reported that they gained a large CLBO crystal with the dimension of 140 mm  110 mm  110 mm from a self-flux system [2] and another large CLBO crystal with the dimension of 146 mm  132 mm  118 mm was obtained from a congruent melt [3]. Because of its good NLO properties, such as sufficient large nonlinear coefficient, short absorption edge, large values of angular, spectral, and temperature bandwidths, CLBO crystal is well suited for second and higher-order harmonic generation and an excellent material to produce UV and deep UV all-solid-state lasers. Unfortunately, CLBO crystals easily crack in the air at room temperature, which has strongly limited their use in devices [3,5–7]. Some reports mentioned that the water vapor from the air may cause cracking of CLBO crystals [5,6], and the cracking mechanism of the CLBO crystal was discussed [3,7]. The CLBO crystal was grown from the self-flux system or congruent melt system by the former authors [1–3]. Because of the high viscosity of mass B2O3 at high temperature [8], the viscosities

n Corresponding author at: Key Laboratory of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel.: +86 10 82543721; fax: + 86 10 82543709. E-mail address: [email protected] (Z.-G. Hu).

0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.05.008

of the two grown systems were both very high and the growth defects were easily generated during the growth of CLBO crystal [9]. In order to grow high quality CLBO crystals, one method is to decrease the viscosity of the CLBO crystal growth system. Since the addition of alkali halide could reduce solution viscosity [8], we tried to use NaF as flux to grow CLBO crystal and succeeded in decreasing the viscosity of the CLBO crystal growth system. In this paper, we report the growth of high quality CLBO crystal with NaF as a new flux for the first time. The solubility of CLBO and the viscosities of different solutions for CLBO crystal growth were measured. A 54  51  32 mm3 CLBO crystal was obtained from the NaF flux system by the top-seeded solution growth (TSSG) technique. To investigate the hygroscopic nature of CLBO crystal, the etching experiment was carried out.

2. Crystal growth The raw materials were prepared as follows. Cs2CO3 (Xin-Jiang Metallurgy Inst.), Li2CO3 (Xin-Jiang Metallurgy Inst.), H3BO3 (Tianjing Chem. Co.) and NaF (Tianjing Chem. Co.) were weighed in the appropriate proportions, mixed homogeneously in an agate mortar, and then melted quickly in a f100  70 mm platinum crucible in a preheated (900 1C) muffle furnace. This process was repeated until there was enough amount of charge in the crucible for crystal growth. A homemade three-zone resistive heated furnace was used for crystal growth as shown in Fig. 1. The power fed to the furnace was controlled by three separate programmable Eurotherm (model 818) temperature controllers. When the crucible was enough charged, it was placed in the three-zone resistive heated

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Fig. 2. CLBO crystal grown by NaF flux system with the dimensions of 54  51  32 mm3.

Fig. 1. Schematic diagram of three-zone resistive heated furnace (1) seed rotator; (2) lip of furnace; (3) seed rod; (4) resistance-heater; (5) thermal couple; (6) Pt wire; (7) seed; (8) Pt crucible; (9) Al2O3 tube.

furnace and the temperature was raised to 850 1C. The solution was stirred with a platinum stirrer for 24 h. The top-seeded solution growth (TSSG) technique was used. A [0 0 1] seed was slowly introduced into the crucible. The growing crystal was rotated at 20–40 rpm rates, with the rotation direction inverted every 2 min. The growth rate was controlled to be very slow at the beginning with the cooling rate being in the range of 0.1–0.6 1C/day. When the growth finished, the crystal was drawn slowly out of the solution surface and the furnace was cooled to room temperature at a rate of 10–15 1C/h. A CLBO crystal with the dimension of 54  51  32 mm3 and the weight of 122 g was obtained from melt of Cs2CO3:Li2CO3:H3BO3: NaF¼1:1:12:0.5 (all ratios listed in this paper were molar ratios). As shown in Fig. 2, the CLBO crystal was colorless and free from voids, inclusions, and tubular defects. The growth period was 20 days. The impurity of the CLBO crystal was measured by inductivecoupled plasma (ICP) atomic emission spectrometry. The result was as follows: Impurity (molar ratio): CsLiB6O10:Na + ¼1:0.0067. This result showed that 0.67 mol% of Na + cation was doped into the CLBO crystal.

3. Results and discussions 3.1. The solubility of CLBO The solubility of CLBO in NaF flux system over the temperature range of 700–860 1C was shown in Fig. 3. The data was gained by determining the temperature of saturation point of the solutions. The saturation temperature of the CLBO growth system decreased evidently (from 844 to 712 1C) with the increase of NaF mol concentration (from 10 to 50 mol%). The data indicated that NaF had a high solubility of CLBO and suggested that NaF may be a good flux for the CLBO crystal growth, in the light of the choice of solvents for high temperature solution [8]. Further research

CLBO composition (mol%)

100 90 80 70 60 50 700

720

740

760

780

800

820

840

860

Saturation Temperature (°C) Fig. 3. Solubility versus saturation temperature of CLBO in NaF flux system.

showed that when the NaF molar concentration was more than 60%, CLBO crystal could not be obtained from the solution. The growth experiments demonstrated that the favorable molar concentration of CLBO in solution was in the range of 65–85%.

3.2. The viscosities of solutions The high temperature solution of borate usually has a high viscosity and the viscosity of the solution could be decreased by the addition of alkali and halide ions [8]. For the growth of CLBO crystal, addition of fluoride ions broke extended –O–B–O– chains in the solution and reduced the solution viscosity. The viscosity of self-flux melt (Cs2CO3:Li2CO3:H3BO3 ¼1:1:11) was even higher than that of the congruent melt (Cs2CO3:Li2CO3:H3BO3 ¼1:1:12), as shown in Fig. 4. The melt viscosity of different systems were measured by DV-II+(America Brookfield Co) viscosity meter. In our experiment, NaF was chosen as flux to grow CLBO crystals. When the ratio of NaF was increased from Cs2CO3:Li2CO3:H3BO3:NaF¼ 1:1:12:0.5 up to Cs2CO3:Li2CO3:H3BO3:NaF¼1:1:12:2, the viscosity decreased tremendously from 720 cP down to 293 cP at 850 1C. Compared with the self-flux melt (Cs2CO3:Li2CO3:H3BO3 ¼1:1:11) and the congruent melt (Cs2CO3:Li2CO3:H3BO3 ¼ 1:1:12), the viscosity of NaF flux melts was lower at the same temperature. For example, at 850 1C, the viscosity

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2417

2500

Viscosity/cP

2000 A B C D E

1500

1000

500

0 920

900

880

860

840

820

800

780

Temperature/°C Fig. 4. Temperature dependence of the viscosity of CLBO solutions: (A) Cs2CO3:Li2CO3: H3BO3 ¼ 1:1:12; (B) Cs2CO3:Li2CO3:H3BO3 ¼ 1:1:11; (C) Cs2CO3:Li2CO3:H3BO3:NaF¼ 1:1:12:0.5; (D) Cs2CO3:Li2CO3:H3BO3:NaF¼ 1:1:12:0.667; (E) Cs2CO3:Li2CO3:H3BO3: NaF¼1:1:12:2.

of the NaF flux system with Cs2CO3:Li2CO3:H3BO3:NaF¼1:1:12:0.5 was 720cP, but those of the self-flux melt (Cs2CO3:Li2CO3:H3BO3 ¼ 1:1:11) and the congruent melt (Cs2CO3:Li2CO3:H3BO3 ¼1:1:12) were 1284 and 1062 cP, respectively. As shown in Fig. 4, in the range of 780–920 1C, the viscosity of the solutions increased sharply with the decrease of temperature, except for the solution (Cs2CO3:Li2CO3: H3BO3:NaF¼ 1:1:12:2). The result proved that the NaF content had a strong effect on the solution viscosity. When the NaF content was increased, the solution viscosity would decrease and the growth temperature would also decrease. The decrease of the growth temperature could reduce the volatilization of Cs in the solution but on the other hand increase the solution viscosity. To get a suitable concentration of NaF, the solubility of CLBO, the growth temperature, and the viscosity of the solution should be considered together. The best combination of growth temperature and viscosity was found to be Cs2CO3:Li2CO3: H3BO3:NaF of 1:1:12:0.5 to 1:1:12:0.667. The saturation temperatures of 815–825 1C proved suitable for growth of CLBO crystal. The advantages of this NaF molar concentration range were as follows. First, the solution viscosity was decreased enough while the decrease of the growth temperature was just 20–30 1C. For example, for Cs2CO3:Li2CO3:H3BO3:NaF¼1:1:12:0.5, compared with the selfflux system, the growth temperature of the solution was lowered to 822 1C from 845 1C but the solution viscosity decreased by about 23% (from about 1420 to 1100 cP). Second, the decrease of the growth temperature could reduce the volatilization of Cs and make the solution more stable. Third, there was a larger coolingtemperature range during the CLBO crystal growth in the NaF flux system, compared with the self-flux system and the congruent melt. The optical homogeneity of the as-grown CLBO crystal was measured on a Wyko RTI400 laser interferometer with a 5.00  5.00  5.03 mm3 sample, which was cut from the as-grown crystal and polished. The optical source in the instrument was a He–Ne laser with a wavelength of 633 nm and the incident beam laser was parallel to the crystal optical axis. The homogeneity plot of the CLBO crystal was shown in Fig. 5. The result indicated that the optical homogeneity characterized by the root-mean-square of the gradient of refractive index was about 1.674  10  5 cm  1. 3.3. Surface etching and its observation To investigate the hygroscopic nature of the CLBO crystal, the surface etching method was used. A glycerol/water solution was

Fig. 5. The homogeneity plot of the CLBO crystal sample with the size of 5.00  5.00  5.03 mm3. The root-mean-square of the gradient of refractive index was about 1.674  10  5 cm  1.

Fig. 6. Microphotographs of (0 0 1) plane in different CLBO crystal samples immerged in the etchant for different times. The left was sample A and the right was sample B.

used as the etchant [3] and the details were as follows. Sample A with the dimension of 5  5  5 mm3 along [0 0 1] direction was cut from the as-grown CLBO crystal, Sample B with the same size and direction as sample A was cut from a CLBO crystal grown from the self-flux system in similar growth condition as sample A. After mechanically polished and then rinsed in ethanol, the samples were submerged into an etchant that consisted of 15 ml glycerol and

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15 ml water at room temperature. The appearances of the (0 0 1) plane of the samples for different immersion time were shown in Fig. 6 (under an optical microscope with  100 magnification). In the two samples, cracks began from both (1 0 0) and (0 1 0) planes, and they generated new cracks when going deeper into the crystal bulk with the increase of etching time. At last, the surfaces of the samples were full of cracks. This result was similar to that mentioned in Ref. [3]. However, other than cracks parallel to the (1 0 0) and (0 1 0) planes in water etchant [3], the cracks was developed into a network finally in this research. As shown in Fig. 6, at 90 and 180 min, sample A had fewer cracks than sample B. This result indicated that sample A was more stable than sample B when immersed in the etchant. The main reason was that the quality of sample A was better than that of sample B. This observation clearly proved the advantage of the NaF flux system for the CLBO crystal growth. As mentioned before, the viscosity of the NaF flux system was lower than that of the self-flux system, and the lower viscosity made the mass transfer of the solution more quickly [10], so the mass transfer condition of the NaF flux system was better than that of the self-flux system. Considering that good mass transfer can reduce the defects of the crystal and improve the integrity of the crystal, and thus improve the quality of the crystal [10,11]. The quality of the CLBO crystal obtained from the NaF flux system should be better than that from the self-flux system. That was in accordance with the result of the etching experiment.

4. Conclusions A high quality CLBO crystal with the size up to 54  51  32 mm3 was grown with the use of the top-seeded solution growth

technique from the NaF flux system for the first time. The solubility of CLBO in NaF flux system was measured and the favorable mol concentration of CLBO in the flux system was in the range of 65–85%. The viscosity of the NaF flux system was investigated between 780 and 920 1C, and the viscosity was found to increase sharply with the decrease of temperature. The suitable NaF molar concentrations were decided and the advantages were discussed. Using samples with the dimension of 5  5  5 mm3 along [0 0 1] direction, the etching experiment was carried out and the result indicated that CLBO sample from the NaF flux system may be more stable than that from self-flux system. References [1] T. Sasaki, I. Kuroda, S. Nakajima, S. Watanabe, Y. Mori, S. Nakai, Proceedings of Advanced Solid-State Laser Conference, Memphis, TN, January 30–February 2 1995, Paper WD3. [2] Y. Mori, I. Kuroda, S. Nakajinma, T. Sasaki, S. Nakai, Appl. Phys. Lett. 67 (1995) 1818–1820. [3] Xin Yuan, Guangqiu Shen, Xiaoqing Wang, Dezhong Shen, Guiling Wang, Zuyan Xu, J. Cryst. Growth 293 (2006) 97–101. [4] A.K. Karnal, Indranil Bhaumik, S. Ganesamoorthy, R. Bhatt, A. Saxena, V.K. Wadhawan, H.L. Bhat, Mater. Lett. 62 (2007) 600–604. [5] G. Ryu, C.S. Yoon, T.P.J. Han, H.G. Gallagher, J. Cryst. Growth 191 (1998) 492–500. [6] N.G. Kononora, A.E. Kokh, P.P. Fedorov, M.S. Feraponova, R.M. Zakalyukin, E.A. Tkachenko, Inorg. Mater. 38 (2002) 1264. [7] F. Pan, X.Q. Wang, G.Q. Shen, D.Z. Shen, J. Cryst. Growth 241 (2002) 129–134. [8] D. Elwell, H.J. Scheel, Crystal Growth from High Temperature Solutions, Academic Press, London, 1975. [9] X. Zhang, S. Zhang, Y. Chai, J. Synth. Cryst. 31 (2002) 104–106. [10] J.C. Brice, The Growth of Crystals from Liquids, North-Holland Publishing Company, London, 1973. [11] H.J. Scheel, T. Fukuda, Crystal Growth Technology, John Wiley & Sons Ltd, West Sussex, 2003.