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Reinvestigation of phase relations around the oxyapatite phase in the Nd2O3–SiO2 system
Journal of Crystal Growth 247 (2003) 207–212
Reinvestigation of phase relations around the oxyapatite phase in the Nd2O3–SiO2 system Yuji Masubuchi*, Mikio Higuchi, Kohei Kodaira Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Received 27 June 2002; accepted 6 September 2002 Communicated by M.E. Glicksman
Abstract Phase relations around the oxyapatite phase in the Nd2O3–SiO2 system have been investigated by means of a slow cooling floating zone (SCFZ) method and solid-state reactions. The results on SCFZ proved that the oxyapatite phase melts congruently at the composition of Nd2O3:SiO2=7:9, corresponding to Nd9.33(SiO4)6O2. The 2:3 phase (Nd8(SiO4)6), which had been believed to have an apatite structure and to melt congruently, does not exist and consists of a mixture of Nd9.33(SiO4)6O2 and Nd2Si2O7 phases. The solid-state reaction revealed that a solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 66.30.Dn Keywords: A1. Phase diagrams; B1. Rare earth compounds; B1. Silicates
1. Introduction Oxyapatite-type lanthanide silicates are a new type of oxide ionic conductors found by Nakayama and coworkers [1,2]; their conductivities at relatively low temperatures below 6001C are superior to that of stabilized zirconia. We have already succeeded in growing oxyapatite-type neodymium silicate single crystals with high quality by the floating zone method [3], and revealed their intrinsic electrical properties which could not be clarified using sintered compacts [4,5]. *Corresponding author. Fax: +81-11-706-6573. E-mail address: [email protected] (Y. Masubuchi).
They have a definite anisotropy of conductivities on the basis of their hexagonal crystal structure (P63/m) whereas conventional oxide ionic conductors usually have a cubic fluorite or perovskite structure and accordingly no anisotropy of conductivities. Three types of phase diagrams for the Nd2O3– SiO2 system have so far been proposed by Toropov [6,7] and Miller [8]. In Refs. [6,8], which are essentially the same except subtle difference in eutectic and peritectic temperatures, the apatite phase is identified as Nd8(SiO4)6 that has a defective apatite structure and melts congruently. In a preliminary experiment, however, we obtained a poor-quality crystal having a trace of cellular growth by using a feed rod with a composition of
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Nd8(SiO4)6. High-quality crystals were grown only when a feed composition of Nd9.33(SiO4)6O2 or Nd9.20(SiO4)6O1.8 was used [9,10]. On the other hand, sub-solidus phase relations are investigated in Ref. [7], which indicates that there is an appreciably wide solid solution region of the oxyapatite phase with a composition of Nd9.33x(SiO4)6O21.5x (x ¼ 021:33). The general formula of the apatite is expressed as A10(BO4)6X2, in which cations A occupy 4f and 6h sites and anions X occupy 2a site. If an apatite phase having a composition of Nd8(SiO4)6 exists, it should contain a number of cation and anion vacancies: that is, two-tenth of the 4f and 6h sites are vacant and the occupancy of the 2a site is zero. Such a structure is expected to be very unstable and existence of the Nd8(SiO4)6 apatite phase is accordingly suspicious. Knowledge of correct phase diagrams is inevitable to grow high-quality crystals. In this study, the phase relations around the oxyapatite phase in the Nd2O3–SiO2 system have been reinvestigated using a slow-cooling floating zone (SCFZ) method [11] and conventional solid-state reactions. A new phase diagram for this system is proposed in the present report compared with previous ones.
appropriate length was formed, both upper and lower shafts were slowly pulled up and down, respectively, and the lamp power was slowly decreased so that solid–liquid interfaces moved at less than 1 mm/h. The solidified specimens were cut parallel to the pulling direction and polished. After observation of the solidified specimens with an optical microscope, chemical compositions in the specimens were analyzed with a wavelength dispersive-type electron probe microanalyzer (EPMA). Crystalline phases in the specimen were determined with X-ray powder diffraction (XRD). 2.2. Solid-state reaction In order to clarify the compositional region within which a single phase of oxyapatite can be obtained, a conventional solid-state reaction was carried out. Powders of Nd2O3 and SiO2 were mixed with desired compositions between Nd9.33(SiO4)6O2 and Nd8(SiO4)6. The mixed powders were molded into pellets under a uniaxial pressure of 100 MPa. The molded specimens were then fired at 16501C for 20 h. The fired specimens were ground and crystalline phases were identified with XRD. The cycle of grinding, firing and XRD measurement was repeated five times at least to complete the solid-state reaction in each specimen.
2. Experimental procedure 2.1. SCFZ method
3. Results and discussion
The principle and basic technique of the SCFZ method is detailed in Ref. [11]. Commercial Nd2O3 (99.9%) and SiO2 (99.9%) powders were used as raw materials. This study does not cover the whole compositional range but deals with the phase relations around the oxyapatite phase in the Nd2O3–SiO2 system, that is, compositions of Nd2O3:SiO2=7:9, 2:3, 1:2 and 5:6 were examined. The powders were mixed with desired compositions and molded into rods by a rubber press technique under 100 MPa, followed by sintering at 16501C for 20 h. They were fixed to the upper and lower shafts of an image furnace with double ellipsoidal mirrors (NEC Machinery: SC-N35HDL), in which 1.5 kW halogen lamps were used as a heating source. After a molten zone with an
3.1. Results on SCFZ 3.1.1. Starting composition Nd2O3:SiO2=2:3 According to Refs. [6,8], Nd8(SiO4)6 has been believed to melt congruently, and to have an apatite structure. However, a zonal structure is clearly seen in the cut and polished specimen solidified with a starting composition of Nd2O3: SiO2=2:3 as shown in Fig. 1. The first region corresponds to the original sintered portion. The second and third region are found to be single phases with the compositions of Nd:Si=9.33:6, corresponding to the oxyapatite phase, and Nd:Si=2:2, corresponding to Nd2Si2O7 phase, respectively. These results indicate that the 2:3 phase (Nd8(SiO4)6) does not melt congruently and
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Fig. 2. Solidified specimen with a starting composition Nd2O3: SiO2=7:9. Original sintered rod (a) and oxyapatite phase (b).
does not exist as a pure single phase in the Nd2O3– SiO2 system. Furthermore, the solidified boules are not cone shape, which do not reflect the shape of the meniscus. This is because the solidification occurred in the mode of solution growth due to a large difference in the compositions between liquid and solid phases.
original sintered rod. This result indicates that the oxyapatite phase melts congruently at a composition of Nd2O3:SiO2=7:9 in the Nd2O3–SiO2 system. Higuchi et al. reported that the congruent composition of the oxyapatite phase of the neodymium silicate was at Nd:Si=9.20:6 [12]. In principle, the congruent composition can be determined by using SCFZ. In this system, however, a little evaporation of SiO2 occurred during the SCFZ run, and the exact congruent composition could consequently not be determined.
3.1.2. Starting composition Nd2O3:SiO2=7:9 This composition corresponds to the formula Nd9.33(SiO4)6O2, for which single crystals with good quality were obtained [3]. In contrast with Nd2O3:SiO2=2:3, the SCFZ run for this composition gave a pair of boules having a simple coneshape portion recrystallized at each end as shown in Fig. 2. By means of EPMA and XRD, the solidified phase was identified to be a single phase of oxyapatite with a composition of approximately Nd:Si=9.33:6, which is the same as that of the
3.1.3. Starting composition Nd2O3:SiO2=1:2 Fig. 3 shows a solidified specimen which has a starting composition of Nd2O3:SiO2=1:2, indicating three regions with a zonal structure. The first region corresponds to the original sintered rod. According to the EPMA and XRD analyses, the second region consists of a single phase of Nd2Si2O7. In the third region, a typical eutectic lamellar structure is observed. The results of the analyses reveal that the lamellar structure consists of two phases, Nd2Si2O7 and SiO2. The eutectic
Fig. 1. Vertical cross section of the solidified specimen with a starting composition Nd2O3:SiO2=2:3. Original sintered rod (a), oxyapatite phase (b) and Nd2Si2O7 (c).
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Fig. 3. Solidified specimen with a starting composition Nd2O3: SiO2=1:2. Original sintered rod (a), Nd2Si2O7 (b) and eutectic lamellar structure of the Nd2Si2O7 and SiO2 (c).
compositional ratio of Nd2O3:SiO2 could not be determined, because the composition of whole region in the third portion consisting of the clear lamellar structure could not be measured by EPMA. According to Refs. [6,8], the eutectic compositional ratio Nd2O3:SiO2 was about 28:72 (mole ratio). The results on the starting composition of Nd2O3:SiO2=2:3 indicate that Nd2Si2O7 melts incongruently as suggested by Toropov et al. [6] and Miller et al. [8], since the single phase of Nd2Si2O7 precipitates next to oxyapatite. However, the single phase of oxyapatite did not precipitate as an initial phase when the starting composition of Nd2O3:SiO2=1:2 was used. Furthermore, the Nd2Si2O7 single crystals were easily grown using feed rods with the same composition. From these facts, the peritectic composition appears to be close to Nd2Si2O7. 3.1.4. Starting composition Nd2O3:SiO2=5:6 With sintered rods which had a composition of Nd2O3:SiO2=5:6, the melt infiltrated into the upper and lower sintered rods, and consequently
Fig. 4. Solidified specimen with a starting composition Nd2O3: SiO2=5:6. Original single crystal rod (a), oxyapatite phase (b) and eutectic mixture of the Nd2SiO5 and Nd2O3 (c).
slow cooling of the molten zone could not be carried out. In a preliminary experiment, it was revealed that the single phase of Nd9.33(SiO4)6O2 precipitated as the initial phase on an SCFZ run with the starting composition Nd2O3:SiO2=5:6. Thus, we used Nd9.33(SiO4)6O2 single crystals, instead of the sintered rods, as melt holders. These crystals were fixed to upper and lower shafts and a small sintered compact with the composition of Nd2O3:SiO2=5:6 was put onto the lower crystal. The sintered compact was then melted to form a stable molten zone with an appropriate length, and SCFZ run was performed without infiltration. Fig. 4 shows a vertical cross section of the solidified specimen with a starting composition of Nd2O3:SiO2=5:6. The first region (a) in Fig. 4 corresponds to the original single crystal rod. The second region (b) is the single phase of oxyapatite. The third region (c) is the eutectic mixture, which consists of the Nd2SiO5 and Nd2O3, and eutectic composition was found to be Nd2O3:SiO2=65:35
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(mole ratio). The composition of Nd2O3:SiO2=5:6 is between Nd9.33(SiO4)6O2 and Nd2SiO5 (Nd2O3: SiO2=1:1) which has been thought to melt congruently. Since eutectic composition was on an Nd-rich composition of about 65:35 (mole ratio), and most portion of the solidified specimen comprises the oxyapatite phase, the phase of Nd2O3:SiO2=1:1 is found to melt incongruently. Furthermore, the peritectic temperature of the Nd2SiO5 appears to be close to the eutectic temperature of the Nd2SiO5 and Nd2O3, since the Nd2SiO5 single phase does not precipitate between the oxyapatite phase and the eutectic mixture of the Nd2O3 and Nd2SiO5. Miller et al. reported the existence of the anomalously low eutectic temperature between Nd2SiO5 and Nd8(SiO4)6 phases. The liquid they found between those phases will correspond to the liquid phase which coexist with Nd9.33(SiO4)6O2 down to the peritectic temperature of Nd2SiO5.
the oxyapatite phase lies from Nd9.33(SiO4)6O2 to Nd8(SiO4)6 in Ref. [7], the specimens with compositions of Nd:Si=9.20:6 and 8:6 consist of the oxyapatite phase and Nd2Si2O7 phase, and the Nd2Si2O7 content increases with increasing Si content. These results indicate that the solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C. The oxyapatite single crystal without macroscopic defects was successfully grown from a feed rod with a composition of Nd9.20(SiO4)6O1.8, and it showed a higher conductivity than that of the Nd9.33(SiO4)6O2 single crystals [9,10]. The increase in the conductivity is attributable to the existence of oxygen vacancy. Therefore, the solid solution region of neodymium and oxygen-deficient-type oxyapatite may exist at a higher temperature.
3.2. Solid solution region
The results of SCFZ runs and solid-state reactions reveal the following facts:
Fig. 5 shows XRD patterns for the specimens with compositions of Nd:Si=9.33:6, 9.20:6 and 8:6, after the solid-state reaction at 16501C. Only the specimen with a composition of Nd:Si=9.33:6 is identified to comprise a single phase of oxyapatite. Although the solid solution region of
Fig. 5. X-ray diffraction patterns of x ¼ 0; 0.13, 1.33 in Nd9.33x(SiO4)6O21.5x. Oxyapatite phase (J) and Nd2Si2O7 (m).
3.3. Reconstruction of the phase diagram of the Nd2O3–SiO2 system
(1) The solid phases encountered were Nd2SiO5, Nd9.33(SiO4)6O2 and Nd2Si2O7. No other compounds were observed, except for the end members of Nd2O3 and SiO2. (2) Only the oxyapatite phase melted congruently at a composition of Nd2O3:SiO2=7:9, corresponding to Nd9.33(SiO4)6O2. Both Nd2SiO5 and Nd2Si2O7 melted incongruently. (3) The solidification occurs in the following sequence: Nd9.33(SiO4)6O2, Nd2Si2O7 and the mixture of Nd2Si2O7 and SiO2, towards Sirich from Nd9.33(SiO4)6O2. Towards Nd-rich from Nd9.33(SiO4)6O2, the solidification sequence is Nd9.33(SiO4)6O2, Nd2SiO5, and the mixture of Nd2SiO5 and Nd2O3. (4) The peritectic composition at which Nd9.33(SiO4)6O2, Nd2Si2O7 and liquid coexist may be close to Nd2Si2O7. (5) The peritectic temperature of Nd2SiO5 appeared to be close to the eutectic temperature between Nd2SiO5 and Nd2O3. (6) The solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C.
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Fig. 6. Modified phase diagram of the Nd2O3–SiO2 system.
On the basis of these results, a modified phase diagram around the oxyapatite phase in the Nd2O3–SiO2 system is obtained as shown in Fig. 6. One of the disadvantages of the SCFZ method is that every temperature, such as melting point, cannot be measured, and thus we referred to Refs. [6,8] for the temperature scale in Fig. 6. Although the exact temperature scale is important from the viewpoint of physical chemistry, it is not so important in crystal growth, especially in the floating zone method. In crystal growth, the most important knowledge is whether a desired phase melts congruently or what kind of liquid coexists with a desired phase. From this viewpoint, the phase diagram obtained on this study is satisfactory for the crystal growth of oxyapatite.
4. Conclusion The present phase diagram of the Nd2O3–SiO2 system determined by using the SCFZ method and
solid-state reactions shows some different features from Refs. [6–8]. The first point is the congruent composition of the oxyapatite phase. The oxyapatite phase melts congruently at a composition of Nd2O3:SiO2=7:9, corresponding to Nd9.33(SiO4)6O2. In the second point, the 1:1 phase (Nd2SiO5), which has been thought to melt congruently, melts incongruently. The third point is that the solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C. With the aid of the phase diagram of the Nd2O3–SiO2 system modified in this study, high-quality single crystal of oxyapatite-type neodymium silicate, which is a new type of oxide ionic conductor, is expected to be successfully grown.
References [1] S. Nakayama, T. Kageyama, H. Aono, Y. Sadaoka, J. Mater. Chem. 5 (1995) 1801. [2] S. Nakayama, M. Sakamoto, J. Eur. Ceram. Soc. 18 (1998) 1413. [3] M. Higuchi, K. Kodaira, S. Nakayama, J. Crystal Growth 207 (1999) 298. [4] S. Nakayama, M. Sakamoto, M. Higuchi, K. Kodaira, M. Sato, S. Kakita, T. Suzuki, K. Itoh, J. Eur. Ceram. Soc. 19 (1995) 507. [5] M. Higuchi, H. Katase, K. Kodaira, S. Nakayama, J. Crystal Growth 218 (2000) 282. [6] N.A. Toropov, Transactions of the Seventh International Ceramic Congress, London, 1960, p. 440. [7] N.A. Toropov, M.V. Kougiya, Izv. Akad. Nauk SSSR, Neorg. Mater. 7 (7) (1971) 1220. [8] R.O. Miller, D.E. Rase, J. Am. Ceram. Soc. 47 (12) (1964) 653. [9] S. Nakayama, M. Sakamoto, H. Higuchi, K. Kodaira, J. Mater. Sci. Lett. 19 (2000) 91. [10] M. Higuchi, K. Kodaira, S. Nakayama, J. Crystal Growth 216 (2000) 317. [11] I. Sindo, J. Crystal Growth 50 (1980) 839. [12] M. Higuchi, K. kodaira, S. Nakayama, J. Crystal Growth 216 (2000) 317.
Reinvestigation of phase relations around the oxyapatite phase in the Nd2O3–SiO2 system Yuji Masubuchi*, Mikio Higuchi, Kohei Kodaira Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Received 27 June 2002; accepted 6 September 2002 Communicated by M.E. Glicksman
Abstract Phase relations around the oxyapatite phase in the Nd2O3–SiO2 system have been investigated by means of a slow cooling floating zone (SCFZ) method and solid-state reactions. The results on SCFZ proved that the oxyapatite phase melts congruently at the composition of Nd2O3:SiO2=7:9, corresponding to Nd9.33(SiO4)6O2. The 2:3 phase (Nd8(SiO4)6), which had been believed to have an apatite structure and to melt congruently, does not exist and consists of a mixture of Nd9.33(SiO4)6O2 and Nd2Si2O7 phases. The solid-state reaction revealed that a solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 66.30.Dn Keywords: A1. Phase diagrams; B1. Rare earth compounds; B1. Silicates
1. Introduction Oxyapatite-type lanthanide silicates are a new type of oxide ionic conductors found by Nakayama and coworkers [1,2]; their conductivities at relatively low temperatures below 6001C are superior to that of stabilized zirconia. We have already succeeded in growing oxyapatite-type neodymium silicate single crystals with high quality by the floating zone method [3], and revealed their intrinsic electrical properties which could not be clarified using sintered compacts [4,5]. *Corresponding author. Fax: +81-11-706-6573. E-mail address: [email protected] (Y. Masubuchi).
They have a definite anisotropy of conductivities on the basis of their hexagonal crystal structure (P63/m) whereas conventional oxide ionic conductors usually have a cubic fluorite or perovskite structure and accordingly no anisotropy of conductivities. Three types of phase diagrams for the Nd2O3– SiO2 system have so far been proposed by Toropov [6,7] and Miller [8]. In Refs. [6,8], which are essentially the same except subtle difference in eutectic and peritectic temperatures, the apatite phase is identified as Nd8(SiO4)6 that has a defective apatite structure and melts congruently. In a preliminary experiment, however, we obtained a poor-quality crystal having a trace of cellular growth by using a feed rod with a composition of
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Nd8(SiO4)6. High-quality crystals were grown only when a feed composition of Nd9.33(SiO4)6O2 or Nd9.20(SiO4)6O1.8 was used [9,10]. On the other hand, sub-solidus phase relations are investigated in Ref. [7], which indicates that there is an appreciably wide solid solution region of the oxyapatite phase with a composition of Nd9.33x(SiO4)6O21.5x (x ¼ 021:33). The general formula of the apatite is expressed as A10(BO4)6X2, in which cations A occupy 4f and 6h sites and anions X occupy 2a site. If an apatite phase having a composition of Nd8(SiO4)6 exists, it should contain a number of cation and anion vacancies: that is, two-tenth of the 4f and 6h sites are vacant and the occupancy of the 2a site is zero. Such a structure is expected to be very unstable and existence of the Nd8(SiO4)6 apatite phase is accordingly suspicious. Knowledge of correct phase diagrams is inevitable to grow high-quality crystals. In this study, the phase relations around the oxyapatite phase in the Nd2O3–SiO2 system have been reinvestigated using a slow-cooling floating zone (SCFZ) method [11] and conventional solid-state reactions. A new phase diagram for this system is proposed in the present report compared with previous ones.
appropriate length was formed, both upper and lower shafts were slowly pulled up and down, respectively, and the lamp power was slowly decreased so that solid–liquid interfaces moved at less than 1 mm/h. The solidified specimens were cut parallel to the pulling direction and polished. After observation of the solidified specimens with an optical microscope, chemical compositions in the specimens were analyzed with a wavelength dispersive-type electron probe microanalyzer (EPMA). Crystalline phases in the specimen were determined with X-ray powder diffraction (XRD). 2.2. Solid-state reaction In order to clarify the compositional region within which a single phase of oxyapatite can be obtained, a conventional solid-state reaction was carried out. Powders of Nd2O3 and SiO2 were mixed with desired compositions between Nd9.33(SiO4)6O2 and Nd8(SiO4)6. The mixed powders were molded into pellets under a uniaxial pressure of 100 MPa. The molded specimens were then fired at 16501C for 20 h. The fired specimens were ground and crystalline phases were identified with XRD. The cycle of grinding, firing and XRD measurement was repeated five times at least to complete the solid-state reaction in each specimen.
2. Experimental procedure 2.1. SCFZ method
3. Results and discussion
The principle and basic technique of the SCFZ method is detailed in Ref. [11]. Commercial Nd2O3 (99.9%) and SiO2 (99.9%) powders were used as raw materials. This study does not cover the whole compositional range but deals with the phase relations around the oxyapatite phase in the Nd2O3–SiO2 system, that is, compositions of Nd2O3:SiO2=7:9, 2:3, 1:2 and 5:6 were examined. The powders were mixed with desired compositions and molded into rods by a rubber press technique under 100 MPa, followed by sintering at 16501C for 20 h. They were fixed to the upper and lower shafts of an image furnace with double ellipsoidal mirrors (NEC Machinery: SC-N35HDL), in which 1.5 kW halogen lamps were used as a heating source. After a molten zone with an
3.1. Results on SCFZ 3.1.1. Starting composition Nd2O3:SiO2=2:3 According to Refs. [6,8], Nd8(SiO4)6 has been believed to melt congruently, and to have an apatite structure. However, a zonal structure is clearly seen in the cut and polished specimen solidified with a starting composition of Nd2O3: SiO2=2:3 as shown in Fig. 1. The first region corresponds to the original sintered portion. The second and third region are found to be single phases with the compositions of Nd:Si=9.33:6, corresponding to the oxyapatite phase, and Nd:Si=2:2, corresponding to Nd2Si2O7 phase, respectively. These results indicate that the 2:3 phase (Nd8(SiO4)6) does not melt congruently and
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Fig. 2. Solidified specimen with a starting composition Nd2O3: SiO2=7:9. Original sintered rod (a) and oxyapatite phase (b).
does not exist as a pure single phase in the Nd2O3– SiO2 system. Furthermore, the solidified boules are not cone shape, which do not reflect the shape of the meniscus. This is because the solidification occurred in the mode of solution growth due to a large difference in the compositions between liquid and solid phases.
original sintered rod. This result indicates that the oxyapatite phase melts congruently at a composition of Nd2O3:SiO2=7:9 in the Nd2O3–SiO2 system. Higuchi et al. reported that the congruent composition of the oxyapatite phase of the neodymium silicate was at Nd:Si=9.20:6 [12]. In principle, the congruent composition can be determined by using SCFZ. In this system, however, a little evaporation of SiO2 occurred during the SCFZ run, and the exact congruent composition could consequently not be determined.
3.1.2. Starting composition Nd2O3:SiO2=7:9 This composition corresponds to the formula Nd9.33(SiO4)6O2, for which single crystals with good quality were obtained [3]. In contrast with Nd2O3:SiO2=2:3, the SCFZ run for this composition gave a pair of boules having a simple coneshape portion recrystallized at each end as shown in Fig. 2. By means of EPMA and XRD, the solidified phase was identified to be a single phase of oxyapatite with a composition of approximately Nd:Si=9.33:6, which is the same as that of the
3.1.3. Starting composition Nd2O3:SiO2=1:2 Fig. 3 shows a solidified specimen which has a starting composition of Nd2O3:SiO2=1:2, indicating three regions with a zonal structure. The first region corresponds to the original sintered rod. According to the EPMA and XRD analyses, the second region consists of a single phase of Nd2Si2O7. In the third region, a typical eutectic lamellar structure is observed. The results of the analyses reveal that the lamellar structure consists of two phases, Nd2Si2O7 and SiO2. The eutectic
Fig. 1. Vertical cross section of the solidified specimen with a starting composition Nd2O3:SiO2=2:3. Original sintered rod (a), oxyapatite phase (b) and Nd2Si2O7 (c).
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Fig. 3. Solidified specimen with a starting composition Nd2O3: SiO2=1:2. Original sintered rod (a), Nd2Si2O7 (b) and eutectic lamellar structure of the Nd2Si2O7 and SiO2 (c).
compositional ratio of Nd2O3:SiO2 could not be determined, because the composition of whole region in the third portion consisting of the clear lamellar structure could not be measured by EPMA. According to Refs. [6,8], the eutectic compositional ratio Nd2O3:SiO2 was about 28:72 (mole ratio). The results on the starting composition of Nd2O3:SiO2=2:3 indicate that Nd2Si2O7 melts incongruently as suggested by Toropov et al. [6] and Miller et al. [8], since the single phase of Nd2Si2O7 precipitates next to oxyapatite. However, the single phase of oxyapatite did not precipitate as an initial phase when the starting composition of Nd2O3:SiO2=1:2 was used. Furthermore, the Nd2Si2O7 single crystals were easily grown using feed rods with the same composition. From these facts, the peritectic composition appears to be close to Nd2Si2O7. 3.1.4. Starting composition Nd2O3:SiO2=5:6 With sintered rods which had a composition of Nd2O3:SiO2=5:6, the melt infiltrated into the upper and lower sintered rods, and consequently
Fig. 4. Solidified specimen with a starting composition Nd2O3: SiO2=5:6. Original single crystal rod (a), oxyapatite phase (b) and eutectic mixture of the Nd2SiO5 and Nd2O3 (c).
slow cooling of the molten zone could not be carried out. In a preliminary experiment, it was revealed that the single phase of Nd9.33(SiO4)6O2 precipitated as the initial phase on an SCFZ run with the starting composition Nd2O3:SiO2=5:6. Thus, we used Nd9.33(SiO4)6O2 single crystals, instead of the sintered rods, as melt holders. These crystals were fixed to upper and lower shafts and a small sintered compact with the composition of Nd2O3:SiO2=5:6 was put onto the lower crystal. The sintered compact was then melted to form a stable molten zone with an appropriate length, and SCFZ run was performed without infiltration. Fig. 4 shows a vertical cross section of the solidified specimen with a starting composition of Nd2O3:SiO2=5:6. The first region (a) in Fig. 4 corresponds to the original single crystal rod. The second region (b) is the single phase of oxyapatite. The third region (c) is the eutectic mixture, which consists of the Nd2SiO5 and Nd2O3, and eutectic composition was found to be Nd2O3:SiO2=65:35
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(mole ratio). The composition of Nd2O3:SiO2=5:6 is between Nd9.33(SiO4)6O2 and Nd2SiO5 (Nd2O3: SiO2=1:1) which has been thought to melt congruently. Since eutectic composition was on an Nd-rich composition of about 65:35 (mole ratio), and most portion of the solidified specimen comprises the oxyapatite phase, the phase of Nd2O3:SiO2=1:1 is found to melt incongruently. Furthermore, the peritectic temperature of the Nd2SiO5 appears to be close to the eutectic temperature of the Nd2SiO5 and Nd2O3, since the Nd2SiO5 single phase does not precipitate between the oxyapatite phase and the eutectic mixture of the Nd2O3 and Nd2SiO5. Miller et al. reported the existence of the anomalously low eutectic temperature between Nd2SiO5 and Nd8(SiO4)6 phases. The liquid they found between those phases will correspond to the liquid phase which coexist with Nd9.33(SiO4)6O2 down to the peritectic temperature of Nd2SiO5.
the oxyapatite phase lies from Nd9.33(SiO4)6O2 to Nd8(SiO4)6 in Ref. [7], the specimens with compositions of Nd:Si=9.20:6 and 8:6 consist of the oxyapatite phase and Nd2Si2O7 phase, and the Nd2Si2O7 content increases with increasing Si content. These results indicate that the solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C. The oxyapatite single crystal without macroscopic defects was successfully grown from a feed rod with a composition of Nd9.20(SiO4)6O1.8, and it showed a higher conductivity than that of the Nd9.33(SiO4)6O2 single crystals [9,10]. The increase in the conductivity is attributable to the existence of oxygen vacancy. Therefore, the solid solution region of neodymium and oxygen-deficient-type oxyapatite may exist at a higher temperature.
3.2. Solid solution region
The results of SCFZ runs and solid-state reactions reveal the following facts:
Fig. 5 shows XRD patterns for the specimens with compositions of Nd:Si=9.33:6, 9.20:6 and 8:6, after the solid-state reaction at 16501C. Only the specimen with a composition of Nd:Si=9.33:6 is identified to comprise a single phase of oxyapatite. Although the solid solution region of
Fig. 5. X-ray diffraction patterns of x ¼ 0; 0.13, 1.33 in Nd9.33x(SiO4)6O21.5x. Oxyapatite phase (J) and Nd2Si2O7 (m).
3.3. Reconstruction of the phase diagram of the Nd2O3–SiO2 system
(1) The solid phases encountered were Nd2SiO5, Nd9.33(SiO4)6O2 and Nd2Si2O7. No other compounds were observed, except for the end members of Nd2O3 and SiO2. (2) Only the oxyapatite phase melted congruently at a composition of Nd2O3:SiO2=7:9, corresponding to Nd9.33(SiO4)6O2. Both Nd2SiO5 and Nd2Si2O7 melted incongruently. (3) The solidification occurs in the following sequence: Nd9.33(SiO4)6O2, Nd2Si2O7 and the mixture of Nd2Si2O7 and SiO2, towards Sirich from Nd9.33(SiO4)6O2. Towards Nd-rich from Nd9.33(SiO4)6O2, the solidification sequence is Nd9.33(SiO4)6O2, Nd2SiO5, and the mixture of Nd2SiO5 and Nd2O3. (4) The peritectic composition at which Nd9.33(SiO4)6O2, Nd2Si2O7 and liquid coexist may be close to Nd2Si2O7. (5) The peritectic temperature of Nd2SiO5 appeared to be close to the eutectic temperature between Nd2SiO5 and Nd2O3. (6) The solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C.
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Fig. 6. Modified phase diagram of the Nd2O3–SiO2 system.
On the basis of these results, a modified phase diagram around the oxyapatite phase in the Nd2O3–SiO2 system is obtained as shown in Fig. 6. One of the disadvantages of the SCFZ method is that every temperature, such as melting point, cannot be measured, and thus we referred to Refs. [6,8] for the temperature scale in Fig. 6. Although the exact temperature scale is important from the viewpoint of physical chemistry, it is not so important in crystal growth, especially in the floating zone method. In crystal growth, the most important knowledge is whether a desired phase melts congruently or what kind of liquid coexists with a desired phase. From this viewpoint, the phase diagram obtained on this study is satisfactory for the crystal growth of oxyapatite.
4. Conclusion The present phase diagram of the Nd2O3–SiO2 system determined by using the SCFZ method and
solid-state reactions shows some different features from Refs. [6–8]. The first point is the congruent composition of the oxyapatite phase. The oxyapatite phase melts congruently at a composition of Nd2O3:SiO2=7:9, corresponding to Nd9.33(SiO4)6O2. In the second point, the 1:1 phase (Nd2SiO5), which has been thought to melt congruently, melts incongruently. The third point is that the solid solution region of the oxyapatite phase is extremely narrow or does not exist below 16501C. With the aid of the phase diagram of the Nd2O3–SiO2 system modified in this study, high-quality single crystal of oxyapatite-type neodymium silicate, which is a new type of oxide ionic conductor, is expected to be successfully grown.
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