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Trivalent ion conducting solid electrolytes
Solid State Ionics 136–137 (2000) 319–324 www.elsevier.com / locate / ssi
Trivalent ion conducting solid electrolytes N. Imanaka, Y. Kobayashi, S. Tamura, G. Adachi* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan
Abstract A trivalent ion conduction in solids was realized with Sc 2 (WO 4 ) 3 -type structure by the consideration of the stability and size of mobile trivalent ions and a structure to reduce the electrostatic interaction between the framework and the mobile ionic species as much as possible. Among the tungstates and the molybdates with the Sc 2 (WO 4 ) 3 -type structure, Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 were found to hold the most suitable lattice size for trivalent ion migration in the individual series. By the dc electrolysis and EPMA measurements, the mobile species was clearly identified to be trivalent ions in the Sc 2 (WO 4 ) 3 type structure. 2000 Elsevier Science B.V. All rights reserved. Keywords: Rare earths; Trivalent ion; Ion conduction; Aluminum; Solid electrolyte; Molybdates; Tungstates
1. Introduction In general, ionic conduction in solid electrolytes is directly dependent on the valency and ionic radius of the mobile ion species. In contrast, trivalent cations have been considered to be an extremely poor migrant species in solids due to their high electrostatic interaction of the highly charged cation species with the constitute anions of the surrounding framework such as oxide anion (O 22). The mobile ion species should be selected from consideration of the trivalent ion properties such as stability and the ionic size, that is, relatively small in ionic radius. In addition, the framework of solid electrolytes should have such characteristics as to possess a larger tunnel size for ion migration so to reduce the electrostatic interaction between the mo*Corresponding author. Tel.: 1 81-6-6879-7353; fax: 1 81-66879-7354. E-mail address: [email protected] (G. Adachi).
bile ions and anions comprising the framework as much as possible. The suitable structure for the trivalent ion migration selected here is the Sc 2 (WO 4 ) 3 -type structure [1–5] with the mobile ion species of aluminum and rare earths. In the structure, the hexavalent tungsten ion, W 61 , bonds strongly to the constituent oxide anions and the interaction between mobile trivalent ions and the oxide ions was greatly reduced as a result of it. Recently, we have directly and quantitatively demonstrated a trivalent ion conduction in the Sc 2 (WO 4 ) 3 -type structure [6– 11] for the trivalent ion species of Al 31 , Sc 31 , Y 31 , and Er 31 . Molybdenum is one of those cations to hold a hexavalent state and molybdates with aluminum and rare earths also have the same Sc 2 (WO 4 ) 3 -type 61 structure. Since the ionic radius of Mo (0.055 nm) 61 [12] is smaller than that of W (0.056 nm) [12], the oxide anions around Mo 61 bond more strongly than those around W 61 . Therefore the electrostatic interaction between the mobile trivalent ions and the
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oxide anions in the framework is reduced and the trivalent ion conducting characteristics are expected to be improved. In this paper, the trivalent ion dependencies on both the trivalent ion conducting properties and the structure with the Sc 2 (WO 4 ) 3 -type structure are clarified.
2. Experimental Molybdates and tungstates, M 2 (M9O 4 ) 3 (M 5 Al, In, Sc, Er, Tm, Yb, and Lu, M9 5 Mo, W), were synthesized by a conventional solid state reaction. In the case of the molybdate preparation, a stoichiometric amount of R 2 O 3 (purity: 99.9%) and MoO 3 (purity: 99.9%) was mixed and calcined on a Pt boat at 700–10008C for 5–17 h in air. The calcined powder was ground and heated again at 800–11008C for 12 h in air. The resulting powder was pelletized (10 mm in diameter) and sintered at 800–11008C for 12 h in air. For the molybdates of heavy rare earths, Er-Lu, the sample powder was dried in vacuum at 1508C before sintering because of their highly hygroscopic nature. The Al 2 (MoO 4 ) 3 pellet preparation is the same as described above except for using Al(OH) 3 as one of starting materials. For the preparation of rare earth tungstates, a stoichiometric amount of R 2 O 3 (purity: 99.9%) and WO 3 (purity: 99.9%) was mixed and calcined on a Pt boat at 10008C for 12 h in air and then reheated at 12008C for 12 h in air. The resulting powder was made into pellets (10 mm in diameter) and sintered at 1300–14008C for 12 h in air. The tungstate sample powder of Y and heavy rare earths, Er-Lu, was also preliminary dried in a similar manner mentioned above. The Al 2 (WO 4 ) 3 preparation method is described previously [8]. Sample characterization and the details of the measurements are reported in our previous papers [8–11].
corner-linked ScO 6 octahedra and M9O 4 (M9 5 Mo, W) tetrahedra. Each ScO 6 octahedron is connected to six M9O 4 tetrahedra and each M9 tetrahedron is bonded to four ScO 6 octahedra. By the X-ray powder diffraction measurements of the compounds for Y and Er-Lu, all tungstates and molybdates were found to be a single Sc 2 (WO 4 ) 3 -type phase with an orthorhombic symmetry (Pbcn) and the peaks were shifted to lower angles with increasing the trivalent ionic size. Fig. 1 presents the dependencies of the unit cell volume for the molybdate and the tungstate system on the trivalent ionic radius. The cell volume increase monotonously with increasing the ionic size. Since the expansion ratio of each lattice parameter toward the trivalent ionic radius was found to be similar and an isotropic expansion was observed. The electrical conductivity at 6008C and the activation energy (Ea) variation for the tungstate and the molybdate series is shown in Fig. 2 as a function of the trivalent ionic radius. The conducting properties depend on a kind of trivalent ion in the tungstates and the molybdates, and the most appropriate trivalent ion size was found to exist in each series. From the figure, Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3
3. Results and discussion Both tungstates and molybdates possess the Sc 2 (WO 4 ) 3 -type structure with an orthorhombic symmetry (space group Pbcn). The structure is built up with a three-dimensional skeleton framework of
Fig. 1. Trivalent ionic radius dependencies on the unit cell volume for the molybdate (d) and the tungstate (s) system.
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Fig. 3. The oxygen partial pressure dependencies of the electrical conductivity for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 . Fig. 2. The trivalent ionic radius dependence of the electrical conductivity at 6008C and the activation energy for M 2 (M9O 4 ) 3 (M9 5 W, Mo).
were found to show the highest conductivity, 6.5 3 10 25 S-cm 21 and 1.8 3 10 24 S-cm 21 at 6008C and the lowest activation energy, 44.1 kJ ? mol 21 and 44.4 kJ ? mol 21 among the tungstate and the molybdate series with the Sc 2 (WO 4 ) 3 -type structure, respectively. For the smallest ion of Al 31 in the series, the conductivity was the lowest and Ea was the highest in both the tungstate and the molybdate series. The extraordinary poor Al 31 conduction in the Sc 2 (WO 4 ) 3 -type structure is mainly attributed to the high electrostatic interaction caused by the low polarizability of Al 31 with surrounding anions in the structure. The oxygen partial pressure dependencies of the electrical conductivity for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 , which show the highest electrical conductivity in the tungstate and molybdate series, respectively, were measured and presented in Fig. 3. Since Mo 61 ions are easier to be reduced in comparison with W 61 , the conductivity abruptly increased at the oxygen partial pressure lower than 10 212 Pa, indicating an appearance of n-type (electron) conduction in the case of Sc 2 (MoO 4 ) 3 . The conductivities for both Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 show no dependence in such a wide pressure region from 10 217 Pa to 10 5 Pa, and from 10 212 Pa to 10 5
Pa, respectively. From these results, both Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 were clarified to be mainly ionic conductors without showing any p- or n-type conduction in such a wide oxygen pressure range. Fig. 4 presents the time dependencies of the dc to ac conductivity ratio (sdc /sac ) in both oxygen (PO 2 : 10 5 Pa) and helium (PO 2 : 4 Pa) for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 . The dc conductivities decreased abruptly in comparison to the ac conductivities and approached steady current after 4 min and the sdc / sac ratio is lower than 0.01. Since the electrical
Fig. 4. The time dependence of the dc to ac conductivity (sdc /sac ) for Sc 2 (MoO 4 ) 3 (s,d) and Sc 2 (WO 4 ) 3 (^,m) in oxygen (open) or helium (closed) atmosphere at 7008C.
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conductivity of Sc 2 (MoO 4 ) 3 is higher than that of Sc 2 (WO 4 ) 3 , Sc 2 (MoO 4 ) 3 shows the greater polarizing behavior in comparison to Sc 2 (WO 4 ) 3 . To ensure the possibility of the O 22 conduction in Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 , the time dependencies of the sdc /sac ratio was compared in both oxygen and helium (Fig. 4). A clear polarization phenomenon was similarly observed in both atmospheres for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 . If O 22 ions are a predominant mobile ionic species in the tungstates and molybdates, a clear polarization must be observed in the helium atmosphere, while the dc conductivity should be the same as that obtained by the ac method in the oxygen atmosphere and any polarizing phenomenon should not be observed as described in reference [8]. The similar polarizing behavior in both atmospheres clearly denies the possibility of oxide ion conduction in the series. The above mentioned large polarization behavior indicates that the predominant migrating species is neither electron nor hole but only ion and that the ionic transference number is estimated to be higher than 0.99 for both series. For the purpose of determining the mobile ionic species in the tungstates and molybdates, a dc electrolysis was performed by sandwiching the electrolyte between two platinum electrodes. Here, as the candidate, Sc 2 (MoO 4 ) 3 which shows the highest conductivity in the Sc 2 (WO 4 ) 3 type structure was selected. The SEM photographs of the cathodic bulk surface of Sc 2 (MoO 4 ) 3 after the electrolysis is shown in Fig. 5a. Many plate-shape deposits were recognized on the surface. From EPMA measurements, scandium was only found to exist in the plate-shape deposits (Fig. 5b). The cross-sectional EPMA line analysis for Sc 2 (MoO 4 ) 3 after the electrolysis is presented in Fig. 6. From the profile of Sc, a large Sc segregation at the cathodic surface of the electrolyte was clearly observed, while that of Mo was almost flat in the region except for the cathodic surface area where Mo decreased due to the Sc deposition. The behaviors observed above strongly indicate that trivalent Sc 31 ion macroscopically migrates from anode to cathode direction and scandium segregates on the cathodic surface as a metal state and then is immediately oxidized to scandium oxide since the electrolysis was conducted in air atmosphere. A similar phenomenon
Fig. 5. SEM photograph of the (a) cathodic surface of Sc 2 (MoO 4 ) 3 after the electrolysis and (b) EPMA point analysis of the plate deposits in Fig. 5a.
is also observed for the Sc 31 ion conducting Sc 2 (WO 4 ) 3 solid electrolyte, showing the highest ion conductivity in the tungstate series.
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Fig. 7 presents the temperature dependencies of the trivalent ion conductivity for Al 2 (WO 4 ) 3 and Sc 2 (WO 4 ) 3 , which are the lowest and the highest conductors, respectively, in the tungstate series. The data for Sc 2 (MoO 4 ) 3 are also depicted in the same figure. The activation energies for Al 2 (WO 4 ) 3 , Sc 2 (WO 4 ) 3 , and Sc 2 (MoO 4 ) 3 are 70.5, 50.6 and 44.4 kJ mol 21 , respectively (some curvature is observed in low temperature. Therefore, the activation energy was calculated from the values obtained in the relationship of log (s T ) 2 1 /T between 400 and 6008C.). From the results obtained above, Sc 2 (MoO 4 ) 3 was found to show higher trivalent ion conductivity in the whole temperature region with the lowest activation energy among three electrolytes. Since Mo 61 bonds to O 22 in the polyhedron stronger than W 61 , trivalent ions are in such a circumstance to ionically migrate easier in Sc 2 (MoO 4 ) 3 in comparison to the tungstate series. Fig. 6. Cross-sectional EPMA line analysis of the Sc 2 (MoO 4 ) 3 pellet after the electrolysis.
4. Conclusion The Sc 2 (WO 4 ) 3 type structure which holds a quasi layered structure to reduce the electrostatic interaction between the mobile trivalent ions and anions in the framework as much as possible, was selected. Among the tungstate and the molybdate series with the Sc 2 (WO 4 ) 3 -type structure, Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 were found to show the highest trivalent ion conducting properties, indicating that they hold the most suitable lattice size for trivalent ion migration. The mobile species was clearly identified to be the trivalent ions in the Sc 2 (WO 4 ) 3 -type structure by the dc electrolysis and the EPMA measurements.
Acknowledgements
Fig. 7. Temperature dependencies of trivalent ion conductivity for Sc 2 (MoO 4 ) 3 and Sc 2 (WO 4 ) 3 [10] with the Sc 2 (WO 4 ) 3 -type structure. The data obtained for Al 2 (WO 4 ) 3 are also presented.
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
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Solid Electrolytes (JSPS-RFTF96P00102)’ Program from the Japan Society for the Promotion of Science.
References [1] S.C. Abrahams, J.L. Bernstein, J. Chem. Phys. 45 (1966) 2745. [2] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 26 (1965) 1805. [3] H.J. Borchardt, J. Chem. Phys. 39 (1963) 504. [4] K. Nassau, J.W. Shiever, E.T. Keve, J. Solid State Chem. 3 (1971) 411. [5] E.Y. Rode, G.V. Lysanova, V.G. Kuznetsov, L.Z. Gokhman, Russ. J. Inorg. Chem. 13 (1968) 678.
[6] N. Imanaka, Y. Kobayashi, G. Adachi, Chem. Lett. (1995) 433. [7] N. Imanaka, G. Adachi, J. Alloys Comp. 250 (1997) 492. [8] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Chem. Mater. 9 (1997) 1649. [9] S. Tamura, T. Egawa, Y. Okazaki, Y. Kobayashi, N. Imanaka, G. Adachi, Chem. Mater. 10 (1998) 1958. [10] N. Imanaka, Y. Kobayashi, K. Fujiwara, T. Asano, Y. Okazaki, G. Adachi, Chem. Mater. 10 (1998) 2006. [11] N. Imanaka, M. Hiraiwa, S. Tamura, G. Adachi, H. Dabkowska, A. Dabkowski, J.E. Greedan, Chem. Mater. 10 (1998) 2542. [12] R.D. Shannon, Acta Cryst. A32 (1976) 751.
Trivalent ion conducting solid electrolytes N. Imanaka, Y. Kobayashi, S. Tamura, G. Adachi* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan
Abstract A trivalent ion conduction in solids was realized with Sc 2 (WO 4 ) 3 -type structure by the consideration of the stability and size of mobile trivalent ions and a structure to reduce the electrostatic interaction between the framework and the mobile ionic species as much as possible. Among the tungstates and the molybdates with the Sc 2 (WO 4 ) 3 -type structure, Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 were found to hold the most suitable lattice size for trivalent ion migration in the individual series. By the dc electrolysis and EPMA measurements, the mobile species was clearly identified to be trivalent ions in the Sc 2 (WO 4 ) 3 type structure. 2000 Elsevier Science B.V. All rights reserved. Keywords: Rare earths; Trivalent ion; Ion conduction; Aluminum; Solid electrolyte; Molybdates; Tungstates
1. Introduction In general, ionic conduction in solid electrolytes is directly dependent on the valency and ionic radius of the mobile ion species. In contrast, trivalent cations have been considered to be an extremely poor migrant species in solids due to their high electrostatic interaction of the highly charged cation species with the constitute anions of the surrounding framework such as oxide anion (O 22). The mobile ion species should be selected from consideration of the trivalent ion properties such as stability and the ionic size, that is, relatively small in ionic radius. In addition, the framework of solid electrolytes should have such characteristics as to possess a larger tunnel size for ion migration so to reduce the electrostatic interaction between the mo*Corresponding author. Tel.: 1 81-6-6879-7353; fax: 1 81-66879-7354. E-mail address: [email protected] (G. Adachi).
bile ions and anions comprising the framework as much as possible. The suitable structure for the trivalent ion migration selected here is the Sc 2 (WO 4 ) 3 -type structure [1–5] with the mobile ion species of aluminum and rare earths. In the structure, the hexavalent tungsten ion, W 61 , bonds strongly to the constituent oxide anions and the interaction between mobile trivalent ions and the oxide ions was greatly reduced as a result of it. Recently, we have directly and quantitatively demonstrated a trivalent ion conduction in the Sc 2 (WO 4 ) 3 -type structure [6– 11] for the trivalent ion species of Al 31 , Sc 31 , Y 31 , and Er 31 . Molybdenum is one of those cations to hold a hexavalent state and molybdates with aluminum and rare earths also have the same Sc 2 (WO 4 ) 3 -type 61 structure. Since the ionic radius of Mo (0.055 nm) 61 [12] is smaller than that of W (0.056 nm) [12], the oxide anions around Mo 61 bond more strongly than those around W 61 . Therefore the electrostatic interaction between the mobile trivalent ions and the
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oxide anions in the framework is reduced and the trivalent ion conducting characteristics are expected to be improved. In this paper, the trivalent ion dependencies on both the trivalent ion conducting properties and the structure with the Sc 2 (WO 4 ) 3 -type structure are clarified.
2. Experimental Molybdates and tungstates, M 2 (M9O 4 ) 3 (M 5 Al, In, Sc, Er, Tm, Yb, and Lu, M9 5 Mo, W), were synthesized by a conventional solid state reaction. In the case of the molybdate preparation, a stoichiometric amount of R 2 O 3 (purity: 99.9%) and MoO 3 (purity: 99.9%) was mixed and calcined on a Pt boat at 700–10008C for 5–17 h in air. The calcined powder was ground and heated again at 800–11008C for 12 h in air. The resulting powder was pelletized (10 mm in diameter) and sintered at 800–11008C for 12 h in air. For the molybdates of heavy rare earths, Er-Lu, the sample powder was dried in vacuum at 1508C before sintering because of their highly hygroscopic nature. The Al 2 (MoO 4 ) 3 pellet preparation is the same as described above except for using Al(OH) 3 as one of starting materials. For the preparation of rare earth tungstates, a stoichiometric amount of R 2 O 3 (purity: 99.9%) and WO 3 (purity: 99.9%) was mixed and calcined on a Pt boat at 10008C for 12 h in air and then reheated at 12008C for 12 h in air. The resulting powder was made into pellets (10 mm in diameter) and sintered at 1300–14008C for 12 h in air. The tungstate sample powder of Y and heavy rare earths, Er-Lu, was also preliminary dried in a similar manner mentioned above. The Al 2 (WO 4 ) 3 preparation method is described previously [8]. Sample characterization and the details of the measurements are reported in our previous papers [8–11].
corner-linked ScO 6 octahedra and M9O 4 (M9 5 Mo, W) tetrahedra. Each ScO 6 octahedron is connected to six M9O 4 tetrahedra and each M9 tetrahedron is bonded to four ScO 6 octahedra. By the X-ray powder diffraction measurements of the compounds for Y and Er-Lu, all tungstates and molybdates were found to be a single Sc 2 (WO 4 ) 3 -type phase with an orthorhombic symmetry (Pbcn) and the peaks were shifted to lower angles with increasing the trivalent ionic size. Fig. 1 presents the dependencies of the unit cell volume for the molybdate and the tungstate system on the trivalent ionic radius. The cell volume increase monotonously with increasing the ionic size. Since the expansion ratio of each lattice parameter toward the trivalent ionic radius was found to be similar and an isotropic expansion was observed. The electrical conductivity at 6008C and the activation energy (Ea) variation for the tungstate and the molybdate series is shown in Fig. 2 as a function of the trivalent ionic radius. The conducting properties depend on a kind of trivalent ion in the tungstates and the molybdates, and the most appropriate trivalent ion size was found to exist in each series. From the figure, Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3
3. Results and discussion Both tungstates and molybdates possess the Sc 2 (WO 4 ) 3 -type structure with an orthorhombic symmetry (space group Pbcn). The structure is built up with a three-dimensional skeleton framework of
Fig. 1. Trivalent ionic radius dependencies on the unit cell volume for the molybdate (d) and the tungstate (s) system.
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Fig. 3. The oxygen partial pressure dependencies of the electrical conductivity for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 . Fig. 2. The trivalent ionic radius dependence of the electrical conductivity at 6008C and the activation energy for M 2 (M9O 4 ) 3 (M9 5 W, Mo).
were found to show the highest conductivity, 6.5 3 10 25 S-cm 21 and 1.8 3 10 24 S-cm 21 at 6008C and the lowest activation energy, 44.1 kJ ? mol 21 and 44.4 kJ ? mol 21 among the tungstate and the molybdate series with the Sc 2 (WO 4 ) 3 -type structure, respectively. For the smallest ion of Al 31 in the series, the conductivity was the lowest and Ea was the highest in both the tungstate and the molybdate series. The extraordinary poor Al 31 conduction in the Sc 2 (WO 4 ) 3 -type structure is mainly attributed to the high electrostatic interaction caused by the low polarizability of Al 31 with surrounding anions in the structure. The oxygen partial pressure dependencies of the electrical conductivity for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 , which show the highest electrical conductivity in the tungstate and molybdate series, respectively, were measured and presented in Fig. 3. Since Mo 61 ions are easier to be reduced in comparison with W 61 , the conductivity abruptly increased at the oxygen partial pressure lower than 10 212 Pa, indicating an appearance of n-type (electron) conduction in the case of Sc 2 (MoO 4 ) 3 . The conductivities for both Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 show no dependence in such a wide pressure region from 10 217 Pa to 10 5 Pa, and from 10 212 Pa to 10 5
Pa, respectively. From these results, both Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 were clarified to be mainly ionic conductors without showing any p- or n-type conduction in such a wide oxygen pressure range. Fig. 4 presents the time dependencies of the dc to ac conductivity ratio (sdc /sac ) in both oxygen (PO 2 : 10 5 Pa) and helium (PO 2 : 4 Pa) for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 . The dc conductivities decreased abruptly in comparison to the ac conductivities and approached steady current after 4 min and the sdc / sac ratio is lower than 0.01. Since the electrical
Fig. 4. The time dependence of the dc to ac conductivity (sdc /sac ) for Sc 2 (MoO 4 ) 3 (s,d) and Sc 2 (WO 4 ) 3 (^,m) in oxygen (open) or helium (closed) atmosphere at 7008C.
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conductivity of Sc 2 (MoO 4 ) 3 is higher than that of Sc 2 (WO 4 ) 3 , Sc 2 (MoO 4 ) 3 shows the greater polarizing behavior in comparison to Sc 2 (WO 4 ) 3 . To ensure the possibility of the O 22 conduction in Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 , the time dependencies of the sdc /sac ratio was compared in both oxygen and helium (Fig. 4). A clear polarization phenomenon was similarly observed in both atmospheres for Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 . If O 22 ions are a predominant mobile ionic species in the tungstates and molybdates, a clear polarization must be observed in the helium atmosphere, while the dc conductivity should be the same as that obtained by the ac method in the oxygen atmosphere and any polarizing phenomenon should not be observed as described in reference [8]. The similar polarizing behavior in both atmospheres clearly denies the possibility of oxide ion conduction in the series. The above mentioned large polarization behavior indicates that the predominant migrating species is neither electron nor hole but only ion and that the ionic transference number is estimated to be higher than 0.99 for both series. For the purpose of determining the mobile ionic species in the tungstates and molybdates, a dc electrolysis was performed by sandwiching the electrolyte between two platinum electrodes. Here, as the candidate, Sc 2 (MoO 4 ) 3 which shows the highest conductivity in the Sc 2 (WO 4 ) 3 type structure was selected. The SEM photographs of the cathodic bulk surface of Sc 2 (MoO 4 ) 3 after the electrolysis is shown in Fig. 5a. Many plate-shape deposits were recognized on the surface. From EPMA measurements, scandium was only found to exist in the plate-shape deposits (Fig. 5b). The cross-sectional EPMA line analysis for Sc 2 (MoO 4 ) 3 after the electrolysis is presented in Fig. 6. From the profile of Sc, a large Sc segregation at the cathodic surface of the electrolyte was clearly observed, while that of Mo was almost flat in the region except for the cathodic surface area where Mo decreased due to the Sc deposition. The behaviors observed above strongly indicate that trivalent Sc 31 ion macroscopically migrates from anode to cathode direction and scandium segregates on the cathodic surface as a metal state and then is immediately oxidized to scandium oxide since the electrolysis was conducted in air atmosphere. A similar phenomenon
Fig. 5. SEM photograph of the (a) cathodic surface of Sc 2 (MoO 4 ) 3 after the electrolysis and (b) EPMA point analysis of the plate deposits in Fig. 5a.
is also observed for the Sc 31 ion conducting Sc 2 (WO 4 ) 3 solid electrolyte, showing the highest ion conductivity in the tungstate series.
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Fig. 7 presents the temperature dependencies of the trivalent ion conductivity for Al 2 (WO 4 ) 3 and Sc 2 (WO 4 ) 3 , which are the lowest and the highest conductors, respectively, in the tungstate series. The data for Sc 2 (MoO 4 ) 3 are also depicted in the same figure. The activation energies for Al 2 (WO 4 ) 3 , Sc 2 (WO 4 ) 3 , and Sc 2 (MoO 4 ) 3 are 70.5, 50.6 and 44.4 kJ mol 21 , respectively (some curvature is observed in low temperature. Therefore, the activation energy was calculated from the values obtained in the relationship of log (s T ) 2 1 /T between 400 and 6008C.). From the results obtained above, Sc 2 (MoO 4 ) 3 was found to show higher trivalent ion conductivity in the whole temperature region with the lowest activation energy among three electrolytes. Since Mo 61 bonds to O 22 in the polyhedron stronger than W 61 , trivalent ions are in such a circumstance to ionically migrate easier in Sc 2 (MoO 4 ) 3 in comparison to the tungstate series. Fig. 6. Cross-sectional EPMA line analysis of the Sc 2 (MoO 4 ) 3 pellet after the electrolysis.
4. Conclusion The Sc 2 (WO 4 ) 3 type structure which holds a quasi layered structure to reduce the electrostatic interaction between the mobile trivalent ions and anions in the framework as much as possible, was selected. Among the tungstate and the molybdate series with the Sc 2 (WO 4 ) 3 -type structure, Sc 2 (WO 4 ) 3 and Sc 2 (MoO 4 ) 3 were found to show the highest trivalent ion conducting properties, indicating that they hold the most suitable lattice size for trivalent ion migration. The mobile species was clearly identified to be the trivalent ions in the Sc 2 (WO 4 ) 3 -type structure by the dc electrolysis and the EPMA measurements.
Acknowledgements
Fig. 7. Temperature dependencies of trivalent ion conductivity for Sc 2 (MoO 4 ) 3 and Sc 2 (WO 4 ) 3 [10] with the Sc 2 (WO 4 ) 3 -type structure. The data obtained for Al 2 (WO 4 ) 3 are also presented.
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
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Solid Electrolytes (JSPS-RFTF96P00102)’ Program from the Japan Society for the Promotion of Science.
References [1] S.C. Abrahams, J.L. Bernstein, J. Chem. Phys. 45 (1966) 2745. [2] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 26 (1965) 1805. [3] H.J. Borchardt, J. Chem. Phys. 39 (1963) 504. [4] K. Nassau, J.W. Shiever, E.T. Keve, J. Solid State Chem. 3 (1971) 411. [5] E.Y. Rode, G.V. Lysanova, V.G. Kuznetsov, L.Z. Gokhman, Russ. J. Inorg. Chem. 13 (1968) 678.
[6] N. Imanaka, Y. Kobayashi, G. Adachi, Chem. Lett. (1995) 433. [7] N. Imanaka, G. Adachi, J. Alloys Comp. 250 (1997) 492. [8] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Chem. Mater. 9 (1997) 1649. [9] S. Tamura, T. Egawa, Y. Okazaki, Y. Kobayashi, N. Imanaka, G. Adachi, Chem. Mater. 10 (1998) 1958. [10] N. Imanaka, Y. Kobayashi, K. Fujiwara, T. Asano, Y. Okazaki, G. Adachi, Chem. Mater. 10 (1998) 2006. [11] N. Imanaka, M. Hiraiwa, S. Tamura, G. Adachi, H. Dabkowska, A. Dabkowski, J.E. Greedan, Chem. Mater. 10 (1998) 2542. [12] R.D. Shannon, Acta Cryst. A32 (1976) 751.