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Growth of NaBi(WO4)2 crystal by modified-Bridgman method
Journal of Crystal Growth 240 (2002) 459–462
Growth of NaBi(WO4)2 crystal by modified-Bridgman method Hongsheng Shi*, Dingzhong Shen, Guohao Ren, Haibing Zhang, Bo Gong, Qun Deng Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received 23 December 2001; accepted 22 January 2002 Communicated by M. Schieber
Abstract NaBi(WO4)2 (NBW) crystals have been grown for the first time by modified-Bridgman method. Influences of some factors on the crystal growth process are discussed. X-ray powder diffraction experiments show that the unit cell parameters of NBW crystal are a ¼ b ¼ 0:5284 nm, c ¼ 1:1517 nm, and V ¼ 0:3215 nm3. The differential thermal analysis shows that the NBW crystal melts at 9231C. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 81.10.Aj Keywords: A1. Anisotropy; A2. Bridgman technique; A2. Growth from melt; B1. Double tungstate oxide; B2. Cherenkov crystal
1. Introduction Double tungstates MIMIII(WO4)2, where M =Li, Na, K; MIII=Bi, Cr, Gd have attracted investigators attention for a long time. In these compounds, NaBi(WO4)2 (NBW) is a promising material both as luminescent and laser media [1–6]. The NBW crystal is suggested as a potential candidate Cherenkov crystal for the electromagnetic calorimeters (EMC) because of its high density (r ¼ 7:58 g/cm3), high radiation resistance (107 Grad), small radiation length (X0 ¼ 1:04 cm), small Moliere radius (Rm ¼ 2:4 cm), and low cost. The experiment result based on a small model shows that the NBW crystal EMC could exhibit a fast, satisfactory energy resolution and a good I
*Corresponding author. E-mail address: [email protected] (H. Shi).
space resolution. The NBW crystal melts congruently, and no phase transitions occur during cooling to room temperate. Therefore, this crystal can be grown by the Czochralski method and Bridgman method. But up to now, we have not found any article reporting the growth of NBW crystal by Bridgman method [7]. In this article, the growth of NBW crystal by modified-Bridgman method is reported in detail.
2. Experiment and discussion The Na2CO3(99.9%), Bi203(99.9%) and WO3(99.99%) are mixed according to the 1:1:4 molar ratio and placed into the platinum crucible. The platinum crucible is put into the refractory Al2O3 tube vertically. Al2O3 powders are filled in the refractory Al2O3 tube to cram around the
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 2 ) 0 0 9 1 5 - 6
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H. Shi et al. / Journal of Crystal Growth 240 (2002) 459–462
platinum crucible. The refractory Al2O3 tube is raised to a certain height in the furnace, and it ensures that the bottom of the platinum crucible is in the temperature gradient zone. Then the furnace is heated to melt the raw material. After the reaction of the raw material having been finished and the raw material being wholly melted, the refractory Al2O3 tube is lowered down at a special speed. After the solidification has been finished, the refractory Al2O3 tube is stopped being lowered and the furnace is cooled down to room temperature. Then the platinum crucible is taken out, and the crystal is obtained. All the process is carried out in the air atmosphere. When the refractory Al2O3 tube is lowered, the temperature at the bottom of the platinum crucible is about 9501C. With the lowering of the refractory Al2O3 tube, the temperate field is changed gradually. So, at the beginning, the temperature is controlled at 11001C in the high temperature zone, and it is gradually raised to 12001C till the end. The longitudinal temperature gradient is 40– 501C/cm and the transverse temperature gradient is about 101C/cm. The 1 and 0.5 mm/h lowering rates have been tried, but these different lowering rates have almost the same effect on the crystal growth. The shape of the crucible is of great importance. Crystals obtained from crucibles of different shapes are very different. Three different crucibles are adopted in the crystal growth, and these crucibles are shown in Fig. 1. In this paper, they are named, respectively, as the flat bottom crucible, conical bottom crucible and capillary crucible according to their respective bottom shape. From the flat bottom crucible, only some small crystals are obtained, and these small crystals are
Fig. 1. Crucibles of three different shapes.
dispersed in the process of cleaning the platinum crucible. Some relatively large crystal blocks are obtained from the conical bottom crucible, but from the capillary crucible large crystal ingots without macroscopical defect can be obtained. As is known, crystal nuclei are formed through spontaneous crystallization in the Bridgman method, and the shape of the crucible is important for the competition among nuclei to occupy the melt. In these three different crucibles, the number of the crystal nuclei spontaneously formed in the flat bottom crucible is the largest because its cross section is also the largest, so the process of competition among nuclei to occupy the melt takes the longest time. In the capillary crucible, the cross-sectional area is very small, just few nuclei spontaneously formed and it is easy for the nucleus to occupy the whole cross section of the crucible. Therefore, the melt solidifies into a single crystal. In the conical bottom crucible, though the bottom section is very small, the diameter of the section will soon become larger and larger and new nuclei will spontaneously be formed, so that the state of the crystallization is between the round bottom crucible and the capillary crucible. One of the large crystal ingots with size 22 22 80 mm is shown in Fig. 2. These crystals have a slightly yellow color. They are machined into some experimental samples. These samples include some crystal ingots and some sectional crystal slices. They are also shown in Fig. 2. The thickness of these crystal slices varies from 0.4 to 2 mm. These crystal slices were observed under the polarization microscope, and they were moved and rotated when observed. Most slices can become transparent or opaque simultaneously under the polarization microscope. These slices can also be oriented by X-rays, and it is found that the two facets of these slices deviate from the [0 0 1] facet by about 81. The structure of the NBW crystal is of the scheelite type, space group I41/a. In most of the typical scheelite crystals, the (0 0 1) direction grows the fastest among all the directions. If the artificial factor in the crystal machining is considered, the 81 departure is small and can be neglected. So, it can be concluded that the (0 0 1) direction of the NBW crystal grows the fastest among all the directions like other typical scheelite crystals.
H. Shi et al. / Journal of Crystal Growth 240 (2002) 459–462
461
Fig. 2. Crystals obtained from the capillary crucible.
Fig. 3. The XRD of the NBW crystal.
The result of the X-ray powder diffraction (XRD) is shown in Fig. 3. Every diffraction line is indexed. It is the first time that the XRD data of the NBW crystal is reported according to the investigation of the related papers. The unit cell parameters are calculated as a ¼ b ¼ 0:5284 nm, c ¼ 1:1517 nm, and V ¼ 0:3215 nm3 by using a powder diffraction index program PTRUM. The result of the X-ray fluorescence analysis is listed in Table 1, and it shows that the composition of the NBW single crystal obtained is close to the expected stoichiometric composition. Different thermal analysis (DTA) studies of NBW crystals obtained show that the melting point of the NBW crystal is 9231C. It is lower than many other double tungstate oxides.
Table 1 The Na/Bi/W/O molar ratio of the NBW crystal Sample
Lowering rate (mm/h)
[Na]/[Bi]/[W]/[O]
1 2 3 4
1 1 0.5 0.5
1/1.00/1.98/7.96 1/1.01/2.00/7.97 1/1.02/1.95/7.86 1/1.01/1.99/7.85
3. Conclusion NBW crystals have been grown by modifiedBridgman method for the first time. Large single NBW crystals without macroscopical defects are obtained. The influence of the shape of the crucible on the crystal growth is discussed. It is found that
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H. Shi et al. / Journal of Crystal Growth 240 (2002) 459–462
the (0 0 1) direction grows the fastest among all the directions of NBW crystal like other typical scheelite crystals. Some basic parameters of the NBW crystal are obtained.
References [1] V.G. Baryshevsky, M.V. Korzhik, V.A. Kachanov, B.I. Zadneprovsky, L.L. Nagornaja, Nucl. Instrum. Methods A 322 (1992) 231.
[2] A.A. Kamlnskii, A. Kholov, P.V. Klevtsov, S.Kh. Khafizov, Phys. Status Solidi A 114 (1989) 713. [3] K. Nitsch, M. Nikl, D. Schultze, R. Uecker, Phys. Status Solidi A 118 (1990) K133. [4] A.A. Kaminskii, S.N. Bagayev, H. Nishioka, Y. Kubota, X. Chen, A. Kholov, Jpn. J. Appl. Phys. 34 (1995) L1461. [5] B.N. Kalinin, Yu.F. Krechetov, A.M. Kolchuzhkin, D.E. Kustov, A.V. Moiseenko, G.A. Naumenko, M.N. Strikhanov, Nucl. Instrum. Methods A 361 (1995) 157. [6] J. Hanuza, M. Maczka, J.H. Van der mass, J. Solid State Chem. 117 (1995) 177. [7] V. Volkov, C. Zaldo, J. Crystal Growth 206 (1999) 60.
Growth of NaBi(WO4)2 crystal by modified-Bridgman method Hongsheng Shi*, Dingzhong Shen, Guohao Ren, Haibing Zhang, Bo Gong, Qun Deng Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received 23 December 2001; accepted 22 January 2002 Communicated by M. Schieber
Abstract NaBi(WO4)2 (NBW) crystals have been grown for the first time by modified-Bridgman method. Influences of some factors on the crystal growth process are discussed. X-ray powder diffraction experiments show that the unit cell parameters of NBW crystal are a ¼ b ¼ 0:5284 nm, c ¼ 1:1517 nm, and V ¼ 0:3215 nm3. The differential thermal analysis shows that the NBW crystal melts at 9231C. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 81.10.Aj Keywords: A1. Anisotropy; A2. Bridgman technique; A2. Growth from melt; B1. Double tungstate oxide; B2. Cherenkov crystal
1. Introduction Double tungstates MIMIII(WO4)2, where M =Li, Na, K; MIII=Bi, Cr, Gd have attracted investigators attention for a long time. In these compounds, NaBi(WO4)2 (NBW) is a promising material both as luminescent and laser media [1–6]. The NBW crystal is suggested as a potential candidate Cherenkov crystal for the electromagnetic calorimeters (EMC) because of its high density (r ¼ 7:58 g/cm3), high radiation resistance (107 Grad), small radiation length (X0 ¼ 1:04 cm), small Moliere radius (Rm ¼ 2:4 cm), and low cost. The experiment result based on a small model shows that the NBW crystal EMC could exhibit a fast, satisfactory energy resolution and a good I
*Corresponding author. E-mail address: [email protected] (H. Shi).
space resolution. The NBW crystal melts congruently, and no phase transitions occur during cooling to room temperate. Therefore, this crystal can be grown by the Czochralski method and Bridgman method. But up to now, we have not found any article reporting the growth of NBW crystal by Bridgman method [7]. In this article, the growth of NBW crystal by modified-Bridgman method is reported in detail.
2. Experiment and discussion The Na2CO3(99.9%), Bi203(99.9%) and WO3(99.99%) are mixed according to the 1:1:4 molar ratio and placed into the platinum crucible. The platinum crucible is put into the refractory Al2O3 tube vertically. Al2O3 powders are filled in the refractory Al2O3 tube to cram around the
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 2 ) 0 0 9 1 5 - 6
460
H. Shi et al. / Journal of Crystal Growth 240 (2002) 459–462
platinum crucible. The refractory Al2O3 tube is raised to a certain height in the furnace, and it ensures that the bottom of the platinum crucible is in the temperature gradient zone. Then the furnace is heated to melt the raw material. After the reaction of the raw material having been finished and the raw material being wholly melted, the refractory Al2O3 tube is lowered down at a special speed. After the solidification has been finished, the refractory Al2O3 tube is stopped being lowered and the furnace is cooled down to room temperature. Then the platinum crucible is taken out, and the crystal is obtained. All the process is carried out in the air atmosphere. When the refractory Al2O3 tube is lowered, the temperature at the bottom of the platinum crucible is about 9501C. With the lowering of the refractory Al2O3 tube, the temperate field is changed gradually. So, at the beginning, the temperature is controlled at 11001C in the high temperature zone, and it is gradually raised to 12001C till the end. The longitudinal temperature gradient is 40– 501C/cm and the transverse temperature gradient is about 101C/cm. The 1 and 0.5 mm/h lowering rates have been tried, but these different lowering rates have almost the same effect on the crystal growth. The shape of the crucible is of great importance. Crystals obtained from crucibles of different shapes are very different. Three different crucibles are adopted in the crystal growth, and these crucibles are shown in Fig. 1. In this paper, they are named, respectively, as the flat bottom crucible, conical bottom crucible and capillary crucible according to their respective bottom shape. From the flat bottom crucible, only some small crystals are obtained, and these small crystals are
Fig. 1. Crucibles of three different shapes.
dispersed in the process of cleaning the platinum crucible. Some relatively large crystal blocks are obtained from the conical bottom crucible, but from the capillary crucible large crystal ingots without macroscopical defect can be obtained. As is known, crystal nuclei are formed through spontaneous crystallization in the Bridgman method, and the shape of the crucible is important for the competition among nuclei to occupy the melt. In these three different crucibles, the number of the crystal nuclei spontaneously formed in the flat bottom crucible is the largest because its cross section is also the largest, so the process of competition among nuclei to occupy the melt takes the longest time. In the capillary crucible, the cross-sectional area is very small, just few nuclei spontaneously formed and it is easy for the nucleus to occupy the whole cross section of the crucible. Therefore, the melt solidifies into a single crystal. In the conical bottom crucible, though the bottom section is very small, the diameter of the section will soon become larger and larger and new nuclei will spontaneously be formed, so that the state of the crystallization is between the round bottom crucible and the capillary crucible. One of the large crystal ingots with size 22 22 80 mm is shown in Fig. 2. These crystals have a slightly yellow color. They are machined into some experimental samples. These samples include some crystal ingots and some sectional crystal slices. They are also shown in Fig. 2. The thickness of these crystal slices varies from 0.4 to 2 mm. These crystal slices were observed under the polarization microscope, and they were moved and rotated when observed. Most slices can become transparent or opaque simultaneously under the polarization microscope. These slices can also be oriented by X-rays, and it is found that the two facets of these slices deviate from the [0 0 1] facet by about 81. The structure of the NBW crystal is of the scheelite type, space group I41/a. In most of the typical scheelite crystals, the (0 0 1) direction grows the fastest among all the directions. If the artificial factor in the crystal machining is considered, the 81 departure is small and can be neglected. So, it can be concluded that the (0 0 1) direction of the NBW crystal grows the fastest among all the directions like other typical scheelite crystals.
H. Shi et al. / Journal of Crystal Growth 240 (2002) 459–462
461
Fig. 2. Crystals obtained from the capillary crucible.
Fig. 3. The XRD of the NBW crystal.
The result of the X-ray powder diffraction (XRD) is shown in Fig. 3. Every diffraction line is indexed. It is the first time that the XRD data of the NBW crystal is reported according to the investigation of the related papers. The unit cell parameters are calculated as a ¼ b ¼ 0:5284 nm, c ¼ 1:1517 nm, and V ¼ 0:3215 nm3 by using a powder diffraction index program PTRUM. The result of the X-ray fluorescence analysis is listed in Table 1, and it shows that the composition of the NBW single crystal obtained is close to the expected stoichiometric composition. Different thermal analysis (DTA) studies of NBW crystals obtained show that the melting point of the NBW crystal is 9231C. It is lower than many other double tungstate oxides.
Table 1 The Na/Bi/W/O molar ratio of the NBW crystal Sample
Lowering rate (mm/h)
[Na]/[Bi]/[W]/[O]
1 2 3 4
1 1 0.5 0.5
1/1.00/1.98/7.96 1/1.01/2.00/7.97 1/1.02/1.95/7.86 1/1.01/1.99/7.85
3. Conclusion NBW crystals have been grown by modifiedBridgman method for the first time. Large single NBW crystals without macroscopical defects are obtained. The influence of the shape of the crucible on the crystal growth is discussed. It is found that
462
H. Shi et al. / Journal of Crystal Growth 240 (2002) 459–462
the (0 0 1) direction grows the fastest among all the directions of NBW crystal like other typical scheelite crystals. Some basic parameters of the NBW crystal are obtained.
References [1] V.G. Baryshevsky, M.V. Korzhik, V.A. Kachanov, B.I. Zadneprovsky, L.L. Nagornaja, Nucl. Instrum. Methods A 322 (1992) 231.
[2] A.A. Kamlnskii, A. Kholov, P.V. Klevtsov, S.Kh. Khafizov, Phys. Status Solidi A 114 (1989) 713. [3] K. Nitsch, M. Nikl, D. Schultze, R. Uecker, Phys. Status Solidi A 118 (1990) K133. [4] A.A. Kaminskii, S.N. Bagayev, H. Nishioka, Y. Kubota, X. Chen, A. Kholov, Jpn. J. Appl. Phys. 34 (1995) L1461. [5] B.N. Kalinin, Yu.F. Krechetov, A.M. Kolchuzhkin, D.E. Kustov, A.V. Moiseenko, G.A. Naumenko, M.N. Strikhanov, Nucl. Instrum. Methods A 361 (1995) 157. [6] J. Hanuza, M. Maczka, J.H. Van der mass, J. Solid State Chem. 117 (1995) 177. [7] V. Volkov, C. Zaldo, J. Crystal Growth 206 (1999) 60.