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Trivalent Sc3+ ion conduction in the Sc2(WO4)3–Sc2(MoO4)3 solid solution
Solid State Ionics 136–137 (2000) 437–440 www.elsevier.com / locate / ssi
Trivalent Sc 31 ion conduction in the Sc 2 (WO 4 ) 3 –Sc 2 (MoO 4 ) 3 solid solution Y. Okazaki, T. Ueda, S. Tamura, N. Imanaka, G. Adachi* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan
Abstract The solid solutions of the (1 2 x)Sc 2 (WO 4 ) 3 2 xSc 2 (MoO 4 ) 3 system were synthesized and their ion conducting characteristics were investigated. The electrical conductivity increased and the activation energy decreased linearly with increasing the Mo content. Sc 2 (MoO 4 ) 3 (x 5 1) showed the highest conductivity of 2.6 times as high as that of Sc 2 (WO 4 ) 3 (x 5 0). By DC electrolysis, the conducting ion species in this system were identified to be trivalent Sc 31 ion. The enhancement of the Sc 31 ion conduction in Sc 2 (MoO 4 ) 3 is due to the reduction of the electrostatic interaction between mobile Sc 31 and O 22 by introducing a smaller Mo 61 ion into the tetrahedral unit of (MO 4 )22 (M 5 W, Mo). Sc 2 (MoO 4 ) 3 was found to possess the most suitable lattice size for the Sc 31 ion migration, exhibiting the highest Sc 31 ion conductivity among the (1 2 x)Sc 2 (WO 4 ) 3 2 xSc 2 (MoO 4 ) 3 system. 2000 Elsevier Science B.V. All rights reserved. Keywords: Trivalent ion conductor; Solid electrolytes; Solid solution; Scandium; Tungstate; Molybdate
1. Introduction The trivalent ion has been regarded as an ionically very poor migrating species in solids compared with the mono and divalent ions such as Li 1 and O 22 , whose conductors have been already utilized for commercial applications. To realize a trivalent ion conduction in solids, the Sc 2 (WO 4 ) 3 type structure which effectively acts to reduce the electrostatic interaction between the trivalent cation and the surrounding framework of anions such as O 22 , was selected and the trivalent ion conduction in R 2 (WO 4 ) 3 (R 5 Sc, Y, Er, Al) has been directly demonstrated [1–7]. The highest trivalent ion conduction and the lowest activation energy were *Corresponding author. Tel. / fax: 181-6-6879-7354. E-mail address: [email protected] (G. Adachi).
attained for Sc 2 (WO 4 ) 3 [3,4] among rare earth tungstate with the Sc 2 (WO 4 ) 3 -type structure. The Sc 31 ion conduction in the structure was successfully enhanced by partially substituting Sc site for larger Ln 31 (Ln 5 Gd [8], Lu [9]) ion to form the Sc 2 (WO 4 ) 3 –Ln 2 (WO 4 ) 3 solid solutions. In this study, the relationship between the lattice size and the ion conduction was investigated in order to clarify the suitable lattice size for Sc 31 ion migration in the Sc 2 (WO 4 ) 3 -type structure by replac61 61 ing another cation of W in R 2 (WO 4 ) 3 for Mo 61 ion. Due to the introduction of the smaller Mo ion (0.055 nm) [10] compared to W 61 (0.056 nm) [10], the strength of M–O (M 5 W, Mo) bonding in the tetrahedral unit of (MO 4 )22 is expected to increase, and as a result, the electrostatic interaction between Sc 31 and O 22 would be expected to be reduced and
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00445-8
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to have a good effect on the Sc 31 ion conduction in the Sc 2 (WO 4 ) 3 -type structure.
2. Experimental The solid solutions of (1 2 x)Sc 2 (WO 4 ) 3 2 xSc 2 (MoO 4 ) 3 were prepared by heating the appropriate mixture of Sc 2 O 3 , WO 3 and MoO 3 at 7008C for 5 h and then at 11008C for 12 h in air. The powder obtained was pelletized after mixing with the 3% poly(vinyl alcohol) aqueous solution, and the pellets were sintered at 11008C for 12 h in air. The identification of the solid solutions was carried out by X-ray powder diffraction (M18XHF, Mac Science). The lattice parameters of the solid solutions were calculated by the Rietveld method (RIETAN94) [11]. The electrical conductivity of the sample pellets was measured by AC complex impedance method (Precision LCR meter 4192A, Hewlett Packard) at temperatures between 200 and 6008C in various oxygen pressures (10 5 –10 218 Pa). The DC conductivity was calculated by the current passed (0.1 mA), the steady voltage generated by passing the current, the cross section of the surface and the thickness of the pellet. In order to identify the conducting ion species in the solid solutions, the DC electrolysis was carried out by applying a voltage of 3 V for 250 h at 9008C in air. Scanning electron microscope (SEM, S-800, Hitachi) measurement and electron probe microscope analysis (EPMA-1500, Shimadzu) were performed for the cathodic surface of the sample pellets after electrolysis.
Fig. 1. The lattice parameter a (s), b (m), and c (♦) changes with the W site substitution for Mo 61 in (12x)Sc 2 (WO 4 ) 3 – xSc 2 (MoO 4 ) 3 .
of Mo 61 , indicating the formation of the solid solutions. The compositional dependencies of the electrical conductivity at 6008C and the activation energy in the temperature range of 400–6008C are presented in Fig. 2. In the (12x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 system, the conductivity increased and the activation energy decreased linearly with increasing the Mo content. Among the samples prepared, Sc 2 (MoO 4 ) 3 (x51) exhibits the highest electrical conductivity (s6008C 51.84310 24 S cm 21 ), which is 2.6 times as high as that of Sc 2 (WO 4 ) 3 , and the lowest activation
3. Results and discussion From the X-ray powder diffraction analysis, the (1 2 x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 system is found to possess the Sc 2 (WO 4 ) 3 type structure (orthorhombic) in the whole compositional range examined. Fig. 1 shows the lattice parameters, a, b, and c, calculated by the Rietveld method for the solid solutions. The lattice parameters were linearly decreased in all three axes by the substitution of W site for the smaller ion
Fig. 2. The compositional dependencies of the electrical conductivity at 6008C (d) and activation energy (m) for (12 x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 .
Y. Okazaki et al. / Solid State Ionics 136 – 137 (2000) 437 – 440
energy (44.4 kJ mol 21 ), which is about 10 kJ mol 21 lower than that of Sc 2 (WO 4 ) 3 . The oxygen partial pressure dependencies of the electrical conductivity were studied to verify the ionic conduction as described in Ref. [1]. The electrical conductivities of 0.6Sc 2 (WO 4 ) 3 – 0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 are constant in the oxygen partial pressure (PO 2 ) range of 10 216 –10 5 Pa and 10 214 –10 5 Pa, respectively. It is suggested that from these results the conducting species in the solid solutions are predominantly ionic in the PO 2 range mentioned above. The polarization behavior in the atmosphere of O 2 (PO 2 : 10 5 Pa) and He (PO 2 : 4 Pa) was further examined to check the possibility of O 22 conduction as demonstrated in Ref. [1]. The same polarization was observed in O 2 and He for both 0.6Sc 2 (WO 4 ) 3 – 0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 . Therefore, the O 22 ion is excluded from the mobile ion species in 0.6Sc 2 (WO 4 ) 3 –0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 . In order to directly identify the mobile ion species in the (12x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 system, DC electrolysis was carried out by applying 3 V for the samples of 0.6Sc 2 (WO 4 ) 3 –0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 , respectively. In both cases, a considerable amount of the deposits was recognized on the cathodic sample pellet surface after the electrolysis. The SEM photograph and the result of the EPMA spot analysis of the deposits on the cathodic surface of Sc 2 (MoO 4 ) 3 are shown in Figs. 3 and 4, respectively. From the EPMA measurement, the element identified was only Sc, indicating that the deposits are scandium oxide. Sc 31 ion migrates from anode to cathode direction and deposits on the cathodic surface as a metal state. Since the electrolysis was carried out in air atmosphere, the Sc metal deposited was immediately oxidized. In the case of 0.6Sc 2 (WO 4 ) 3 –0.4Sc 2 (MoO 4 ) 3 , the elements identified were only Sc and W, and the Sc / W ratio of the deposits was nine times as high as that of the sample before electrolysis. This result means that the deposits are Sc 6 WO 12 , produced by the chemical reaction on the cathodic surface between Sc 2 O 3 and Sc 2 (WO 4 ) 3 in a molar ratio of 8:1 as reported in Refs. [3,4]. It is suggested that from the results mentioned above the predominant conducting ion species in the Sc 2 (WO 4 ) 3 –Sc 2 (MoO 4 ) 3 solid solu-
439
Fig. 3. The SEM photograph of the deposits on the cathodic surface of Sc 2 (MoO 4 ) 3 after electrolysis.
Fig. 4. The EPMA spot analysis result of the deposits on the cathodic surface of Sc 2 (MoO 4 ) 3 after electrolysis.
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tions and Sc 2 (MoO 4 ) 3 is clearly verified to be only Sc 31 .
4. Conclusion The Sc 31 ion conduction in the Sc 2 (WO 4 ) 3 – Sc 2 (MoO 4 ) 3 system is enhanced with increasing the Mo content and Sc 2 (MoO 4 ) 3 shows the highest Sc 31 ion conductivity in the system. By replacing W site for the smaller ion of Mo 61 , the electrostatic interaction between Sc 31 and O 22 seems to be reduced due to the increase of the M–O (M5W, Mo) bonding in the tetrahedral unit of (MO 4 )22 and the enhancement of the trivalent Sc 31 ion conductivity was realized.
Acknowledgements We thank Dr. K. Yamada for EPMA measurements and for helpful discussions. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 09215223 on priority areas (No. 260), Nos. 06241106, 06241107, and 093065 from The Ministry of Education, Science, Sports and Culture.
This work was also supported by the ‘Research for the Future, Preparation and Application of Newly Designed Solid Electrolytes (JSPS-RFTF96P00102)’ Program from the Japan Society for the Promotion of Science.
References [1] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Chem. Mater. 9 (1997) 1649. [2] N. Imanaka, Y. Kobayashi, G. Adachi, Chem. Lett. (1995) 433. [3] N. Imanaka, G. Adachi, J. Alloys Comp. 250 (1997) 492. [4] N. Imanaka, Y. Kobayashi, K. Fujisawa, T. Asano, Y. Okazaki, G. Adachi, Chem. Mater. 10 (1998) 2006. [5] N. Imanaka, G. Adachi, Molten Salts 41 (1998) 177. [6] N. Imanaka, Y. Kobayashi, S. Tamura, G. Adachi, Electrochem. Solid-State Lett. 1 (1998) 271. [7] Y. Kobayashi, S. Tamura, N. Imanaka, G. Adachi, Solid State Ionics 113–115 (1998) 545. [8] Y. Kobayashi, T. Egawa, Y. Okazaki, S. Tamura, N. Imanaka, G. Adachi, Solid State Ionics 111 (1998) 59. [9] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Solid State Ionics 118 (1999) 325. [10] R.D. Shannon, Acta Cryst. A32 (1976) 751. [11] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford University Press, Oxford, 1993, Chapter 13.
Trivalent Sc 31 ion conduction in the Sc 2 (WO 4 ) 3 –Sc 2 (MoO 4 ) 3 solid solution Y. Okazaki, T. Ueda, S. Tamura, N. Imanaka, G. Adachi* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan
Abstract The solid solutions of the (1 2 x)Sc 2 (WO 4 ) 3 2 xSc 2 (MoO 4 ) 3 system were synthesized and their ion conducting characteristics were investigated. The electrical conductivity increased and the activation energy decreased linearly with increasing the Mo content. Sc 2 (MoO 4 ) 3 (x 5 1) showed the highest conductivity of 2.6 times as high as that of Sc 2 (WO 4 ) 3 (x 5 0). By DC electrolysis, the conducting ion species in this system were identified to be trivalent Sc 31 ion. The enhancement of the Sc 31 ion conduction in Sc 2 (MoO 4 ) 3 is due to the reduction of the electrostatic interaction between mobile Sc 31 and O 22 by introducing a smaller Mo 61 ion into the tetrahedral unit of (MO 4 )22 (M 5 W, Mo). Sc 2 (MoO 4 ) 3 was found to possess the most suitable lattice size for the Sc 31 ion migration, exhibiting the highest Sc 31 ion conductivity among the (1 2 x)Sc 2 (WO 4 ) 3 2 xSc 2 (MoO 4 ) 3 system. 2000 Elsevier Science B.V. All rights reserved. Keywords: Trivalent ion conductor; Solid electrolytes; Solid solution; Scandium; Tungstate; Molybdate
1. Introduction The trivalent ion has been regarded as an ionically very poor migrating species in solids compared with the mono and divalent ions such as Li 1 and O 22 , whose conductors have been already utilized for commercial applications. To realize a trivalent ion conduction in solids, the Sc 2 (WO 4 ) 3 type structure which effectively acts to reduce the electrostatic interaction between the trivalent cation and the surrounding framework of anions such as O 22 , was selected and the trivalent ion conduction in R 2 (WO 4 ) 3 (R 5 Sc, Y, Er, Al) has been directly demonstrated [1–7]. The highest trivalent ion conduction and the lowest activation energy were *Corresponding author. Tel. / fax: 181-6-6879-7354. E-mail address: [email protected] (G. Adachi).
attained for Sc 2 (WO 4 ) 3 [3,4] among rare earth tungstate with the Sc 2 (WO 4 ) 3 -type structure. The Sc 31 ion conduction in the structure was successfully enhanced by partially substituting Sc site for larger Ln 31 (Ln 5 Gd [8], Lu [9]) ion to form the Sc 2 (WO 4 ) 3 –Ln 2 (WO 4 ) 3 solid solutions. In this study, the relationship between the lattice size and the ion conduction was investigated in order to clarify the suitable lattice size for Sc 31 ion migration in the Sc 2 (WO 4 ) 3 -type structure by replac61 61 ing another cation of W in R 2 (WO 4 ) 3 for Mo 61 ion. Due to the introduction of the smaller Mo ion (0.055 nm) [10] compared to W 61 (0.056 nm) [10], the strength of M–O (M 5 W, Mo) bonding in the tetrahedral unit of (MO 4 )22 is expected to increase, and as a result, the electrostatic interaction between Sc 31 and O 22 would be expected to be reduced and
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00445-8
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Y. Okazaki et al. / Solid State Ionics 136 – 137 (2000) 437 – 440
to have a good effect on the Sc 31 ion conduction in the Sc 2 (WO 4 ) 3 -type structure.
2. Experimental The solid solutions of (1 2 x)Sc 2 (WO 4 ) 3 2 xSc 2 (MoO 4 ) 3 were prepared by heating the appropriate mixture of Sc 2 O 3 , WO 3 and MoO 3 at 7008C for 5 h and then at 11008C for 12 h in air. The powder obtained was pelletized after mixing with the 3% poly(vinyl alcohol) aqueous solution, and the pellets were sintered at 11008C for 12 h in air. The identification of the solid solutions was carried out by X-ray powder diffraction (M18XHF, Mac Science). The lattice parameters of the solid solutions were calculated by the Rietveld method (RIETAN94) [11]. The electrical conductivity of the sample pellets was measured by AC complex impedance method (Precision LCR meter 4192A, Hewlett Packard) at temperatures between 200 and 6008C in various oxygen pressures (10 5 –10 218 Pa). The DC conductivity was calculated by the current passed (0.1 mA), the steady voltage generated by passing the current, the cross section of the surface and the thickness of the pellet. In order to identify the conducting ion species in the solid solutions, the DC electrolysis was carried out by applying a voltage of 3 V for 250 h at 9008C in air. Scanning electron microscope (SEM, S-800, Hitachi) measurement and electron probe microscope analysis (EPMA-1500, Shimadzu) were performed for the cathodic surface of the sample pellets after electrolysis.
Fig. 1. The lattice parameter a (s), b (m), and c (♦) changes with the W site substitution for Mo 61 in (12x)Sc 2 (WO 4 ) 3 – xSc 2 (MoO 4 ) 3 .
of Mo 61 , indicating the formation of the solid solutions. The compositional dependencies of the electrical conductivity at 6008C and the activation energy in the temperature range of 400–6008C are presented in Fig. 2. In the (12x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 system, the conductivity increased and the activation energy decreased linearly with increasing the Mo content. Among the samples prepared, Sc 2 (MoO 4 ) 3 (x51) exhibits the highest electrical conductivity (s6008C 51.84310 24 S cm 21 ), which is 2.6 times as high as that of Sc 2 (WO 4 ) 3 , and the lowest activation
3. Results and discussion From the X-ray powder diffraction analysis, the (1 2 x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 system is found to possess the Sc 2 (WO 4 ) 3 type structure (orthorhombic) in the whole compositional range examined. Fig. 1 shows the lattice parameters, a, b, and c, calculated by the Rietveld method for the solid solutions. The lattice parameters were linearly decreased in all three axes by the substitution of W site for the smaller ion
Fig. 2. The compositional dependencies of the electrical conductivity at 6008C (d) and activation energy (m) for (12 x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 .
Y. Okazaki et al. / Solid State Ionics 136 – 137 (2000) 437 – 440
energy (44.4 kJ mol 21 ), which is about 10 kJ mol 21 lower than that of Sc 2 (WO 4 ) 3 . The oxygen partial pressure dependencies of the electrical conductivity were studied to verify the ionic conduction as described in Ref. [1]. The electrical conductivities of 0.6Sc 2 (WO 4 ) 3 – 0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 are constant in the oxygen partial pressure (PO 2 ) range of 10 216 –10 5 Pa and 10 214 –10 5 Pa, respectively. It is suggested that from these results the conducting species in the solid solutions are predominantly ionic in the PO 2 range mentioned above. The polarization behavior in the atmosphere of O 2 (PO 2 : 10 5 Pa) and He (PO 2 : 4 Pa) was further examined to check the possibility of O 22 conduction as demonstrated in Ref. [1]. The same polarization was observed in O 2 and He for both 0.6Sc 2 (WO 4 ) 3 – 0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 . Therefore, the O 22 ion is excluded from the mobile ion species in 0.6Sc 2 (WO 4 ) 3 –0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 . In order to directly identify the mobile ion species in the (12x)Sc 2 (WO 4 ) 3 –xSc 2 (MoO 4 ) 3 system, DC electrolysis was carried out by applying 3 V for the samples of 0.6Sc 2 (WO 4 ) 3 –0.4Sc 2 (MoO 4 ) 3 and Sc 2 (MoO 4 ) 3 , respectively. In both cases, a considerable amount of the deposits was recognized on the cathodic sample pellet surface after the electrolysis. The SEM photograph and the result of the EPMA spot analysis of the deposits on the cathodic surface of Sc 2 (MoO 4 ) 3 are shown in Figs. 3 and 4, respectively. From the EPMA measurement, the element identified was only Sc, indicating that the deposits are scandium oxide. Sc 31 ion migrates from anode to cathode direction and deposits on the cathodic surface as a metal state. Since the electrolysis was carried out in air atmosphere, the Sc metal deposited was immediately oxidized. In the case of 0.6Sc 2 (WO 4 ) 3 –0.4Sc 2 (MoO 4 ) 3 , the elements identified were only Sc and W, and the Sc / W ratio of the deposits was nine times as high as that of the sample before electrolysis. This result means that the deposits are Sc 6 WO 12 , produced by the chemical reaction on the cathodic surface between Sc 2 O 3 and Sc 2 (WO 4 ) 3 in a molar ratio of 8:1 as reported in Refs. [3,4]. It is suggested that from the results mentioned above the predominant conducting ion species in the Sc 2 (WO 4 ) 3 –Sc 2 (MoO 4 ) 3 solid solu-
439
Fig. 3. The SEM photograph of the deposits on the cathodic surface of Sc 2 (MoO 4 ) 3 after electrolysis.
Fig. 4. The EPMA spot analysis result of the deposits on the cathodic surface of Sc 2 (MoO 4 ) 3 after electrolysis.
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tions and Sc 2 (MoO 4 ) 3 is clearly verified to be only Sc 31 .
4. Conclusion The Sc 31 ion conduction in the Sc 2 (WO 4 ) 3 – Sc 2 (MoO 4 ) 3 system is enhanced with increasing the Mo content and Sc 2 (MoO 4 ) 3 shows the highest Sc 31 ion conductivity in the system. By replacing W site for the smaller ion of Mo 61 , the electrostatic interaction between Sc 31 and O 22 seems to be reduced due to the increase of the M–O (M5W, Mo) bonding in the tetrahedral unit of (MO 4 )22 and the enhancement of the trivalent Sc 31 ion conductivity was realized.
Acknowledgements We thank Dr. K. Yamada for EPMA measurements and for helpful discussions. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 09215223 on priority areas (No. 260), Nos. 06241106, 06241107, and 093065 from The Ministry of Education, Science, Sports and Culture.
This work was also supported by the ‘Research for the Future, Preparation and Application of Newly Designed Solid Electrolytes (JSPS-RFTF96P00102)’ Program from the Japan Society for the Promotion of Science.
References [1] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Chem. Mater. 9 (1997) 1649. [2] N. Imanaka, Y. Kobayashi, G. Adachi, Chem. Lett. (1995) 433. [3] N. Imanaka, G. Adachi, J. Alloys Comp. 250 (1997) 492. [4] N. Imanaka, Y. Kobayashi, K. Fujisawa, T. Asano, Y. Okazaki, G. Adachi, Chem. Mater. 10 (1998) 2006. [5] N. Imanaka, G. Adachi, Molten Salts 41 (1998) 177. [6] N. Imanaka, Y. Kobayashi, S. Tamura, G. Adachi, Electrochem. Solid-State Lett. 1 (1998) 271. [7] Y. Kobayashi, S. Tamura, N. Imanaka, G. Adachi, Solid State Ionics 113–115 (1998) 545. [8] Y. Kobayashi, T. Egawa, Y. Okazaki, S. Tamura, N. Imanaka, G. Adachi, Solid State Ionics 111 (1998) 59. [9] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Solid State Ionics 118 (1999) 325. [10] R.D. Shannon, Acta Cryst. A32 (1976) 751. [11] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford University Press, Oxford, 1993, Chapter 13.