CRYSTAL GROWTH & DESIGN
Phase Relations around the Pyrosilicate Phase in the Gd2O3-Ce2O3-SiO2 System Sohan Kawamura,*,† Mikio Higuchi,† Junichi H. Kaneko,† Shusuke Nishiyama,† Jun Haruna,† Shohei Saeki,† Shunsuke Ueda,‡ Kazuhisa Kurashige,‡ Hiroyuki Ishibashi,‡ and Michihiro Furusaka†
2009 VOL. 9, NO. 3 1470–1473
DiVision of Quantum Science and Engineering, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan, and Yamazaki Works, Hitachi Chemical Co. Ltd, 1380-1 Tarazaki, Hitachinaka-city, Ibaraki, 312-0003, Japan ReceiVed August 24, 2008; ReVised Manuscript ReceiVed December 1, 2008
ABSTRACT: The melting and solidification behavior of the (Gd0.9Ce0.1)2Si2O7 (Ce:GPS) which shows attractive scintillation performance was investigated by means of the slow cooling floating zone (SCFZ) method. Although Ce:GPS melts incongruently, the peritectic composition is estimated to be very close to the GPS composition, less than 67.0 mol % of SiO2 in the (Gd,Ce)2O3-SiO2 system. Constitutional supercooling occurred when approximately 70% volume of the melt was solidified, consequently, the lamellar structure consists of Ce:GPS and the amorphous phase was obtained. On the basis of our study, it is estimated that a volume of 70% of the starting material, whose composition is SiO2 ) 67 mol % in the (Gd,Ce)2O3-SiO2 system at Ce 10 mol %, can be solidified as the GPS phase using top-seeded solution growth (TSSG) with the self-flux of SiO2. Introduction Tl:NaI, Ce:Gd2SiO5 (Ce:GSO), Bi4Ge3O12 (BGO), and Ce: Lu2SiO5 (Ce:LSO) are widely used as scintillator materials for gamma-ray detection because of high light yield, short decay time, and high density.1-4 However, recent progress in nuclear diagnosis instruments has stimulated the development of new scintillator materials that show both higher light output and shorter decay time. Ce:Lu2Si2O7 (Ce:LPS),5-7 Ce:LaBr3,8,9 and Pr:Lu3Al5O12 (Pr:LuAG)10,11 are recent examples having desired performance for those devices. In previous studies, we examined the potential of Ce-doped Gd2Si2O7 (Ce:GPS) as a scintillator on the basis of Yagi’s report.12 As a first step, we revealed that the GPS powder doped with Ce of 10% exhibited 1.2 times greater light output than Ce:GSO for alpha-particles from 241Am.13 Then, we successfully prepared a Ce:GPS single crystal by the floating zone method, and it showed 2.5 times greater light output than Ce:GSO single crystals and decay time of 46 ns for 662 keV gamma-rays.14 According to Toropov’s report,15 the pyrosilicate phase melts incongruently in the phase diagram of the Gd2O3-SiO2 system; nevertheless, Ce:GPS single crystal was grown by the floating zone (FZ) method. Either of following two reasons is acceptable to explain this result: (i) The composition of the molten zone spontaneously changed to a SiO2-rich one that coexists with the GPS phase, and the traveling solvent floating zone (TSFZ) manner consequently resulted. (ii) The pyrosilicate phase melted congruently because of heavy Ce-doping of 10 mol %. Although the floating zone method makes it possible to grow a single crystal pyrosilicate, it is generally not appropriate for industrial production especially for scintillator materials because of the limitation of the crystal size. The Czochralski (CZ) method is one of the most suitable techniques to grow largesize single crystals; however, the crystal to be grown by this technique should melt congruently, and incongruent melting materials are usually grown by the top-seeded solution growth * Corresponding author. E-mail:
[email protected]. Tel/Fax: +81(0)11-706-6678. † Hokkaido University. ‡ Hitachi Chemical Co. Ltd.
(TSSG) method. Therefore, the knowledge of melting behavior of Ce:GPS is indispensable to determine the crystal growth technique. The slow cooling floating zone (SCFZ) method developed by Shindo16 is simple and convenient to evaluate the melting behavior of a compound and also enables semiquantitative discussion of the phase relation. This paper deals with the phase relation around the pyrosilicate phase in the Gd2O3Ce2O3-SiO2 system using the SCFZ method. Our previous study proved that the segregation coefficient of Ce3+ in Ce: GPS, was close to unity;14 therefore, it is practically possible to identify Gd3+ and Ce3+ as a RE3+ ion and the phase diagram can be treated as a (Gd,Ce)2O3-SiO2 system at a fixed Ceconcentration. Experimental Procedures Gd2O3 (5N), SiO2 (5N), and CeO2 (4N) powders were used as starting materials. They were mixed at a desired composition of Gd1.8Ce0.2Si2O7. The mixed powder was put into a rubber bag and pressed under a hydrostatic pressure of 70 MPa to form a rod. The rod was sintered at 1650 °C for 8 h in air, and then the rod was attached to an upper shaft of an image furnace (FZ-T-10000-H-III-TK; Crystal Systems Inc.). After an appropriate amount of the lower end of the rod was melted, the rod was slowly pulled up at 0.5 mm/h. The atmosphere was N2 flow of 0.30 L/min. Quenching of the molten zone during the steady growth of a Ce:GPS single crystal was also done to evaluate the composition of the zone. The solidified specimen was cut parallel to the pulling direction and polished to be mirror finish. Chemical compositions in the specimen were analyzed with a wavelength dispersive type electron probe microanalyzer (EPMA). Crystalline phases in the specimen were identified using X-ray powder diffraction (XRD).
Experimental Results 1. Slow Cooling of the Melt of Pyrosilicate Composition. The observed solidification process was the following. The solidification occurred at the interface between the melt and the original sintered rod, and then continuous solidification was observed until 60% of the melt solidified. After the stable solidification, the residual melt froze rapidly. Consequently, a zonal structure was formed in the solidified specimen. Figure 1
10.1021/cg800932b CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
Phase Relations in the Gd2O3-Ce2O3-SiO2 System
Figure 1. Longitudinal cross section of the solidified specimen, which can be divided into three regions, that is, region 1: next to the original rod, region 2: a transparent part, and region 3: an opaque part. Growth direction was indicated by the arrow.
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Figure 4. XRD pattern (left) and a back scattered electron image (right) of region 3. XRD showed pyrosilicate phase and halo pattern at lower angles. Lamella structures were observed in the image. The bright region was identified to be pyrosilicate phase. The dark region was amorphous phase comprising Gd2O3-SiO2-Ce2O3. Table 1. Composition of Each Region Analyzed by EPMA region Region 1 Region 2 Region 3 quenching
Figure 2. XRD pattern (left) and a back scattered electron image (right) of region 1. XRD patterns of apatite and pyrosilicate phase were observed. Bright and dark regions in the image were identified to be apatite and pyrosilicate phases, respectively.
Figure 3. XRD pattern (left) and a back scattered electron image (right) of region 2. Single phase of pyrosilicate was obtained.
shows a longitudinal cross section of the specimen, which can be divided into three regions, that is, region 1: next to the original sintered rod, region 2: a transparent part, and region 3: an opaque part. Figure 2 shows a back scattered electron (BSE) image and XRD pattern of boundary between the region 1 and the region 2. XRD pattern of this region showed the existence of both apatite and pyrosilicate phases. Brighter region of the image indicates the phase of relatively heavier composition. The crystalline phases of the bright and dark regions correspond to apatite and pyrosilicate, respectively. There was a sintered part between the apatite phase and the original rod which contained both the apatite and the GPS phase due to the light irradiation during the cooling process. No contrast was observed in the BSE image of the region 2 as shown in Figure 3 and the composition of the phase corresponded to that of pyrosilicate. The XRD pattern of the region 2 proved that this region comprised only the single phase of pyrosilicate. Figure 4 displays the results of region 3. A lamella pattern was observed in the BSE image. Composition of the bright region was pyrosilicate, while the atomic ratio in the dark region
Gd/Si/O/Ce bright dark bright dark bright dark
22.3: 13.6: 62.5: 1.6 17.1: 17.0: 64.1: 1.8 17.1: 16.9: 64.2: 1.9 17.1: 16.9: 64.1: 1.9 14.4: 18.8: 65.0: 1.8 16.9: 17.7: 63.5: 1.9 12.4: 21.6: 64.0: 2.0
was determined to be Gd:Si:O:Ce ) 14.4:18.8:65.0:1.8. The XRD pattern of region 3 demonstrated the existence of the pyrosilicate phase and the amorphous phase. Therefore, the dark region in the BSE image of this region may be the amorphous phase of the Gd-Si-O system. The volume ratio of GPS phase to amorphous phase was approximately 1; thus, the bulk composition of the lamellar part was estimated as RE2O3:SiO2 ) 31.5:68.5. Compositions of all the regions analyzed by EPMA are summarized in Table 1. Uniform distribution of Ce3+ was confirmed in all regions. According to Toropov,15 there is a eutectic point between pyrosilicate and silica phases and the composition of the eutectic point is silica-richer than the approximate bulk composition of region 3. Our result of solid-state reaction also proved that the existence of pyrosilicate and crystalline silica phase in the subsodius region. Thus, the lamella structure of region 3 was formed under a nonequilibrium process, in which the viscosity of the residual melt increased as the melt composition became silica-rich and constitutional supercooling occurred, as a result. On the basis of these results, the solidification process of the melt of GPS composition is described as follows. (1) The apatite phase crystallizes as the primary one. (2) The amount of crystallized apatite is small; thus, the melt composition rapidly reaches the peritectic point. (3) At a lower temperature than the peritectic point, GPS crystallizes as a single phase. (4) Because of the synergetic effect of the increase in the silica component and the decrease in the liquidus temperature, the viscosity of the residual melt increases. Thus, constitutional supercooling was likely to occur, and consequently solidification completes rapidly before the melt composition reaches the eutectic point. (5) As a result, mixed phases of GPS and amorphous one are obtained at the end of the process. These results indicate that pyroslicate phase melts incongruently to form the apatite and liquid phases in the Gd2O3-Ce2O3-SiO2 system. 2. Quenching of the Molten Zone. By the slow cooling method, determination of the peritectic composition or the melt composition during the crystal growth was difficult. Therefore,
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Figure 5. Longitudinal cross section of the quenched specimen. An opaque part in the figure is the quenched molten zone. Growth direction is indicated by the arrow.
Figure 7. The phase diagram around the GPS phase in the (Gd,Ce)2O3-SiO2 system at Ce 10 mol %.
Figure 6. A back scattered electron image of the region C in Figure 5. The gray part was GPS phase and darker black lines were an amorphous phase. The composition of the amorphous phase is shown in Table 1.
quenching of the molten zone was carried out to investigate the melt composition during the growth process. Figure 5 shows a longitudinal cross section of the molten zone quenched during the steady growth process of the GPS by turning off the lamp power. The lower part is the polycrystalline seed, a transparent part is the GPS phase and an opaque part is the quenched melt. A BSE image of the quenched part is shown in Figure 6. The brighter part and dark lines in the image were determined as the GPS phase and a SiO2-rich amorphous phase, respectively, as summarized in Table 1. The melt composition during the growth process was estimated as RE2O3:SiO2 ) 33.0: 67.0 from the comparison of the area of the GPS and the amorphous phase; thus, the peritectic point composition exists in between 66.7 and 67.0 mol % of SiO2. Discussion On the basis of results on the SCFZ and the quenching experiments, the phase diagram around the GPS phase in the (Gd,Ce)2O3-SiO2 system is estimated as Figure 7. The bulk compositions of the lamellar part and the melt composition during the growth process were determined as SiO2 ) 68.5 and 67.0 mol %, respectively; however, the compositions of the peritectic point and the eutectic point were not revealed. According to Toropov’s report, the GPS phase melts incongruently in the Gd2O3-SiO2 system; however, the peritectic composition is different from our result; the composition was SiO2 ) 73.7 mol %.15 It is considered that heavy Ce-doping affected the melting behavior of the GPS phase; consequently, the different peritectic composition was obtained in our experiments. Therefore, it is considered that the Ce:GPS single crystal, which melts incongruently in the Gd2O3-SiO2 system, could be grown by the FZ method due to the following reason; the
composition of the molten zone spontaneously changed to a SiO2-rich one that coexists with the GPS phase, and the TSFZ manner consequently resulted in our previous work. According to Masubuchi et al., Nd2Si2O7 also melts incongruently in the phase diagram of the Nd2O3-SiO2 system and the peritectic composition at which Nd9.33(SiO4)6O2, Nd2Si2O7 and liquid coexist may be close to Nd2Si2O7.17 Ion radius of both Nd3+ and Ce3+ are larger than that of Gd3+; therefore, the average ion radius of RE3+ of Ce:GPS is considered larger than that of Gd3+. On the basis of these results, it might be considered that the ion radius of RE3+ affects the peritectic composition at which RE9.33(SiO4)6O2, RE2Si2O7 and liquid coexist; the peritectic composition is close to RE2Si2O7 with an increase of the ion radius of RE3+. From the compositions of the melt and the lamellar part, it is considered that approximately 70% volume of the starting material, whose composition is SiO2 ) 67.0 mol % in the (Gd,Ce)2O3-SiO2 system at Ce 10 mol %, can be solidified as the GPS phase using TSSG growth with the self-flux of SiO2. This estimation corresponds well to the result from the volume comparison of the GPS phase and the lamellar part of the solidified specimen; VGPS/Vlameller ) 3:1. Conclusion Investigation of the phase diagram around the GPS phase in the (Gd,Ce)2O3-SiO2 system at Ce 10 mol % using the SCFZ method was carried out aiming at large-size single crystal growth of Ce:GPS. Although our results revealed the incongruent melting of the GPS phase, the peritectic point was very close to GPS composition; the peritectic composition was estimated SiO2 ) 66.7 - 67.0 mol %. Therefore, in the case of the Ce:GPS composition used as starting material, it is estimated that the composition of the molten zone spontaneously changes to a SiO2-rich one that coexists with the GPS phase during the growth process; consequently, Ce:GPS single crystal can be grown by the TSSG method which consequently results. The possible solidification volume ratio of the GPS phase, in the case of the TSSG with the self-flux of SiO2, was estimated to be approximately 70% of the starting material whose composition is SiO2 ) 67.0 mol % in the (Gd,Ce)2O3-SiO2 system at Ce 10 mol %. Acknowledgment. This study was supported by Research Fellowships of the Japan Society for the Promotion of Science
Phase Relations in the Gd2O3-Ce2O3-SiO2 System
for Young Scientists and the Industrial Technology Research Grant Program in 2006 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
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