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Rare earth ion conduction in tungstate and phosphate solids
Journal of Alloys and Compounds 344 (2002) 137–140
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Rare earth ion conduction in tungstate and phosphate solids N. Imanaka*, G.-Y. Adachi Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan
Abstract The quasi two-dimensional Sc 2 (WO 4 ) 3 type structure and the three-dimensional NASICON type R 1 / 3 Zr 2 (PO 4 ) 3 (R5Al 31 , rare earths) series were prepared and their trivalent ion conducting characteristics were compared. Among the Sc 2 (WO 4 ) 3 type and the NASICON type series, the solid electrolytes with Sc 31 as a migrating species show the highest ion conductivity. Especially, the NASICON type phosphate series holds such advantages of having a considerable high relative density and also more than seven times as high hardness compared with the Sc 2 (WO 4 ) 3 type series, promising applications for various types of devices are greatly expected. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earths; Trivalent; Ion conduction; Solid electrolyte; Tungstate; Molybdate; Phosphate
1. Introduction Ion migration in solids has been well known for the ion species such as mono-, divalent cations and anions. In contrast, for trivalent ions, it has been commonly accepted that the ion species whose valency is higher than trivalent state are extremely poor conductors in solids. Some papers have reported that Ln 31 -b0-alumina [1–7], b-LaNbO 3 [8] and LaA 11 O 18 [9] are trivalent ion conductors. However, neither direct nor quantitative demonstration has been carried out. In 1995, a direct demonstration of trivalent Sc 31 cation conduction in solids has been successfully realized by choosing the constituent structure such as a quasi two-dimensional one [10]. The higher the valency of the conducting cation is, the stronger the interaction between the cation and surrounding anions becomes. For the purpose of reducing the interaction between the mobile trivalent ion and the anion species surrounding the trivalent ion as much as possible, the pathway for the trivalent ion needs to possess enough space to migrate at least, twodimensionally, preferentially, to be located three dimensionally. In addition, to release the migrating cation to conduct smoother, higher electrostatic interaction between the lattice forming cations and anions is needed compared
*Corresponding author. Tel.: 181-6-6879-7353; fax: 181-6-68797354. E-mail address: [email protected] (N. Imanaka).
with the interaction between the mobile trivalent cations and the anions. Furthermore, the inclusion of lower valent cations than the trivalent state should be eliminated because mono- or divalent cations can easily conduct in solids for the objective trivalent cations. The needs described above indicate that the solid should contain the elements which can stably hold the tetravalent state or higher. Recently, we have succeeded in developing the R 2 (WO 4 ) 3 (R5Al, In, Sc, Y, Er–Lu) solid electrolytes with the orthorhombic Sc 2 (WO 4 ) 3 type structure, showing a pure trivalent ion conduction in solids [11–16]. The Sc 2 (WO 4 ) 3 type series possesses a quasi-layered structure and holds a large pathway where trivalent ion can conduct smoothly. In the structure, the hexavalent tungsten ion (W 61 ) bonds strongly to surrounding four oxide anions to constitute a WO 22 tetrahedron unit and a considerable 4 reduction of the electrostatic interaction between trivalent ions and O 22 anions was successfully realized. In a similar manner, we have also developed a new type of trivalent rare earth ion conducting solid electrolyte based on phosphate [17]. In this paper, the quasi two-dimensional Sc 2 (WO 4 ) 3 type and the three-dimensional NASICON type structures which contain hexavalent W 61 or tetravalent Zr 41 , and pentavalent P 51 , were prepared as the candidates for the promising trivalent ion conductors and the characteristics of two series of solid electrolytes were compared in detail with various kinds of rare earths as a migrating trivalent ion species.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00352-3
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N. Imanaka, G. Adachi / Journal of Alloys and Compounds 344 (2002) 137–140
2. Experimental The tungstates, M 2 (WO 4 ) 3 (M5Al, Sc, Y, Er, Tm, Yb, and Lu) were prepared by a conventional solid-state reaction method. A stoichiometric amount of Al(OH) 3 (purity: 99.99%), M 2 O 3 (99.9%), and WO 3 (99.9%) was mixed in a mortar and heated at 1000 8C for 12 h. The calcined powder was ground and sintered at 1000–1300 8C for 12 h. In the case of the samples of Y and heavy rare earths, Er–Lu, the mixed powder was dried in a vacuum at 150 8C before sintering because these compounds are considerably hygroscopic. R 1 / 3 Zr 2 (PO 4 ) 3 phosphates (R5 Sc, Eu, Gd, Er, Tm, Yb, and Lu) were synthesized by a sol–gel method from high-purity R 2 O 3 (99.9%), ZrO(NO 3 ) 2 ?2H 2 O (99.95%) and (NH 4 ) 2 HPO 4 (.99.99%) as the starting materials. R 2 O 3 and ZrO(NO 3 ) 2 ?2H 2 O were separately dissolved in HNO 3 solution (3 M), then mixed together. The (NH 4 ) 2 HPO 4 solution (3%) was dropped into the mixed HNO 3 solution afterwards. After precipitations were obtained, the solution was heated at 75 8C for 24 h, then water in the solution was vaporized at 130 8C. The dried precipitant was heated at 300 8C for 6 h and the resulting powder was pelletized and sintered at 850 8C for 24 h. The Sc 1 / 3 Zr 2 (PO 4 ) 3 samples were also prepared by a ball milling method with the same starting materials. The powder was mixed in an agent pot at a rotation speed of 300 rpm for 12 h. After identifying the resulting powder to be an amorphous state by X-ray powder diffraction method (M18XHF, Mac Science), the powder obtained was made into pellet (10 mm in diameter) and sintered at 850 8C for 12 h in an air atmosphere. The X-ray diffraction (XRD) data were collected by a step-scanning method in the 2u range from 10 to 708 with a step width of 0.048 and a scan time of 4 s. The relative density of the pellet was calculated by dividing sintered pellet density by the powder one by the Archimedes method. The Vickers hardness of sintered pellets of Sc 2 (WO 4 ) 3 and Sc 1 / 3 Zr 2 (PO 4 ) 3 was measured by using the Micro Hardness Tester (HMV-2, Shimadzu). Electrical conductivity was measured by an a.c. and a d.c. method, using the sintered sample pellet with two platinum electrodes in the temperature range between 200 and 600 8C. The a.c. conductivity measurement was carried out by an a.c. complex impedance method in the frequency range from 20 Hz to 1 MHz by using the Precision LCR meter (8284A, Hewlett-Packard).
3. Results and discussion Fig. 1 shows the trivalent ion conductivity dependencies on the A ratio, which is defined as the conducting trivalent cation size for the crystal lattice volume of the tungstate (d) series. Among various types of trivalent ions investigated, the solid electrolyte with Sc 31 ion (ionic radius: 0.0885 nm [18]) shows the highest trivalent ion con-
Fig. 1. The trivalent ion conductivity dependencies on the A ratio, which is defined as the conducting trivalent cation size for the crystal lattice volume of the tungstate (d) series.
ductivity. Especially, for the ion species of Al 31 which has a considerably smaller ionic radius (0.0675 nm) [18], the ion conductivity decreased considerably owing to the smaller A ratio due to the larger contraction of the volume of the migrating trivalent ion from Sc 31 to Al 31 compared with that of the crystal lattice from Sc 2 (WO 4 ) 3 to Al 2 (WO 4 ) 3 (a direct trivalent ion conduction in the tungstate series was clearly demonstrated by d.c. electrolysis and polarization measurements [11,12]). The tungstates hold a quasi-layered structure and contain the hexavalent cation of W 61 and W 61 can be easily reduced. In order to obtain trivalent cation conductors which possess such characteristics not to be reduced, the phosphate based materials were selected as the promising candidate. The A ratio dependencies on the ionic conductivity of the phosphate series [R 1 / 3 Zr 2 (PO 4 ) 3 ] prepared by a sol– gel method, are presented in Fig. 2. With the decrease of the rare earth ion radius by a lanthanide contraction, the ion conductivity monotonously increased and the highest ion conductivity was observed for the migrating ion species of Sc 31 . In contrast, the conductivity of the NASICON structure whose crystal lattice was intentionally expanded by doping the larger In 31 ion (ionic radius: 0.094 nm [18]) on the Sc site to form the 0.8Sc 1 / 3 Zr 2 (PO 4 ) 3 –0.2In 1 / 3 Zr 2 (PO 4 ) 3 solid solution, decreased compared with that of Sc 1 / 3 Zr 2 (PO 4 ) 3 (see the figure inserted), indicating that a further A ratio reduction decreased the ion conductivity (the trivalent rare earth cation conduction in the phosphate solids was also directly
N. Imanaka, G. Adachi / Journal of Alloys and Compounds 344 (2002) 137–140
139
Fig. 2. The A ratio dependencies on the ionic conductivity of the phosphate series [R 1 / 3 Zr 2 (PO 4 ) 3 ]. The ion conductivity of the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte prepared by a ball mill method is also plotted (s).
demonstrated both by polarization and d.c. electrolysis measurements [17]). It can be easily recognized that Sc 31 ion conducting Sc 1 / 3 Zr 2 (PO 4 ) 3 shows the highest ion conductivity among the R 1 / 3 Zr 2 (PO 4 ) 3 series investigated, demonstrating that the most suitable NASICON type R 1 / 3 Zr 2 (PO 4 ) 3 crystal lattice is realized for the migrating cation of Sc 31 . In addition, by preparing the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte by a ball mill method, the ion conductivity enhanced approximately three times as high as that prepared by a sol–gel method, as indicated in Fig. 2(s). The temperature dependencies of the ionic conductivity for Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by both sol–gel and ball mill methods are presented with the data for Sc 2 (WO 4 ) 3 and 25 Al 2 (WO 4 ) 3 in Fig. 3. The conductivity (2.91310 S 21 cm ) of Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by a ball mill method is found to be comparable to the series of the Sc 2 (WO 4 ) 3 type structure at 600 8C. The activation energy for ion conduction of Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by sol–gel and ball mill methods, which is derived form the relationship between log (s T ) and 1 /T are 67.9 and 72.7 kJ mol 21 , respectively, which is a little higher than those of Sc 2 (WO 4 ) 3 (56.4 kJ mol 21 ) and Al 2 (WO 4 ) 3 (65.8 kJ mol 21 ), but indicating that similar suitable ion pathway was realized in both NASICON and Sc 2 (WO 4 ) 3 type structure. Table 1 tabulates the trivalent ion conductivity at 600 8C, Vickers hardness, and relative density for
Fig. 3. The temperature dependencies of the ionic conductivity for Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by both sol–gel and ball mill methods with the data for Sc 2 (WO 4 ) 3 and Al 2 (WO 4 ) 3 .
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Table 1 The trivalent Sc 31 ion conductivity, Vickers hardness and relative density for Sc 2 (WO 4 ) 3 and Sc 1 / 3 Zr 2 (PO 4 ) 3 Sc 31 ion conductors
Conductivity (s / S cm 21 )
Vickers hardness (Hv)
Relative density (%)
Sc 2 (WO 4 ) 3 Sc 1 / 3 Zr 2 (PO 4 ) 3 (sol–gel) Sc 1 / 3 Zr 2 (PO 4 ) 3 (ball mill)
3.74310 25 1.07310 25 2.91310 25
40 293 305
89 99.9 99.9
Sc 1 / 3 Zr 2 (PO 4 ) 3 (both sol–gel and ball mill methods) and Sc 2 (WO 4 ) 3 which shows the highest trivalent ion conductivity in the Sc 2 (WO 4 ) 3 type series. The relative densities of both Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolytes was considerably high in comparison to Sc 2 (WO 4 ) 3 . In addition, the Vickers hardness of the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte is more than seven times as high as that of Sc 2 (WO 4 ) 3 . The results described above clearly indicate the fact that the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte based on phosphate which is highly stable in various atmospheres was successfully obtained with having a high relative density and a considerably high mechanical hardness which contribute greatly to applying as an element for various types of devices.
4. Conclusions Among the Sc 2 (WO 4 ) 3 type and the NASICON type R 1 / 3 Zr 2 (PO 4 ) 3 (R5Al 31 , rare earths) series, the trivalent ion conducting solid electrolytes with Sc 31 as a conducting species were found to show the highest ion conductivity. Especially, Sc 1 / 3 Zr 2 (PO 4 ) 3 possesses such advanced merits to have not only a considerable high relative density but also considerably higher mechanical strength in comparison to the Sc 2 (WO 4 ) 3 type structure series, promising applications such as a component of functional materials are highly expected.
Acknowledgements The present work was partially supported by a Grant-inAid 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] B. Dunn, G.C. Farrington, Solid State Ionics 9 / 10 (1983) 223. [2] W. Carrillo-Cabrera, J.O. Thomas, G.C. Farrington, Solid State Ionics 9 / 10 (1983) 245. [3] B. Ghosal, E.A. Mangle, M.R. Tipp, B. Dunn, G.C. Farrington, Solid State Ionics 9 / 10 (1983) 273. [4] G.C. Farrington, B. Dunn, J.O. Thomas, Appl. Phys. A A32 (1983) 159. ¨ [5] T. Dedecke, J. Kohler, F. Tietz, W. Urland, Eur. J. Solid State Inorg. Chem. 33 (1996) 185. ¨ [6] J. Kohler, W. Urland, Solid State Ionics 86–88 (1996) 93. ¨ [7] J. Kohler, W. Urland, Angew. Chem. 109 (1997) 105. [8] A.M. George, A.N. Virkar, J. Phys. Chem. Solids 49 (1998) 743. [9] T.E. Warner, D.J. Fray, A. Davies, Solid State Ionics 92 (1996) 99. [10] N. Imanaka, Y. Kobayashi, G. Adachi, Chem. Lett. (1995) 433. [11] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Chem. Mater. 9 (1997) 1649. [12] N. Imanaka, Y. Kobayashi, K. Fujiwara, T. Asano, Y. Okazaki, G. Adachi, Chem. Mater. 10 (1998) 2006. [13] S. Tamura, T. Egawa, Y. Okazaki, Y. Kobayashi, N. Imanaka, G. Adachi, Chem. Mater. 10 (1998) 1958. [14] N. Imanaka, M. Hiraiwa, S. Tamura, G. Adachi, H. Dabkowska, A. Dabkowski, J.E. Greedan, Chem. Mater. 10 (1998) 2542. ¨ [15] J. Kohler, N. Imanaka, G. Adachi, J. Mater. Chem. 9 (1999) 1357. [16] N. Imanaka, Y. Kobayashi, S. Tamura, G. Adachi, Solid State Ionics 136 / 137 (2000) 319. [17] S. Tamura, N. Imanaka, G. Adachi, Adv. Mater. 11 (1999) 1521. [18] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.
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Rare earth ion conduction in tungstate and phosphate solids N. Imanaka*, G.-Y. Adachi Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan
Abstract The quasi two-dimensional Sc 2 (WO 4 ) 3 type structure and the three-dimensional NASICON type R 1 / 3 Zr 2 (PO 4 ) 3 (R5Al 31 , rare earths) series were prepared and their trivalent ion conducting characteristics were compared. Among the Sc 2 (WO 4 ) 3 type and the NASICON type series, the solid electrolytes with Sc 31 as a migrating species show the highest ion conductivity. Especially, the NASICON type phosphate series holds such advantages of having a considerable high relative density and also more than seven times as high hardness compared with the Sc 2 (WO 4 ) 3 type series, promising applications for various types of devices are greatly expected. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earths; Trivalent; Ion conduction; Solid electrolyte; Tungstate; Molybdate; Phosphate
1. Introduction Ion migration in solids has been well known for the ion species such as mono-, divalent cations and anions. In contrast, for trivalent ions, it has been commonly accepted that the ion species whose valency is higher than trivalent state are extremely poor conductors in solids. Some papers have reported that Ln 31 -b0-alumina [1–7], b-LaNbO 3 [8] and LaA 11 O 18 [9] are trivalent ion conductors. However, neither direct nor quantitative demonstration has been carried out. In 1995, a direct demonstration of trivalent Sc 31 cation conduction in solids has been successfully realized by choosing the constituent structure such as a quasi two-dimensional one [10]. The higher the valency of the conducting cation is, the stronger the interaction between the cation and surrounding anions becomes. For the purpose of reducing the interaction between the mobile trivalent ion and the anion species surrounding the trivalent ion as much as possible, the pathway for the trivalent ion needs to possess enough space to migrate at least, twodimensionally, preferentially, to be located three dimensionally. In addition, to release the migrating cation to conduct smoother, higher electrostatic interaction between the lattice forming cations and anions is needed compared
*Corresponding author. Tel.: 181-6-6879-7353; fax: 181-6-68797354. E-mail address: [email protected] (N. Imanaka).
with the interaction between the mobile trivalent cations and the anions. Furthermore, the inclusion of lower valent cations than the trivalent state should be eliminated because mono- or divalent cations can easily conduct in solids for the objective trivalent cations. The needs described above indicate that the solid should contain the elements which can stably hold the tetravalent state or higher. Recently, we have succeeded in developing the R 2 (WO 4 ) 3 (R5Al, In, Sc, Y, Er–Lu) solid electrolytes with the orthorhombic Sc 2 (WO 4 ) 3 type structure, showing a pure trivalent ion conduction in solids [11–16]. The Sc 2 (WO 4 ) 3 type series possesses a quasi-layered structure and holds a large pathway where trivalent ion can conduct smoothly. In the structure, the hexavalent tungsten ion (W 61 ) bonds strongly to surrounding four oxide anions to constitute a WO 22 tetrahedron unit and a considerable 4 reduction of the electrostatic interaction between trivalent ions and O 22 anions was successfully realized. In a similar manner, we have also developed a new type of trivalent rare earth ion conducting solid electrolyte based on phosphate [17]. In this paper, the quasi two-dimensional Sc 2 (WO 4 ) 3 type and the three-dimensional NASICON type structures which contain hexavalent W 61 or tetravalent Zr 41 , and pentavalent P 51 , were prepared as the candidates for the promising trivalent ion conductors and the characteristics of two series of solid electrolytes were compared in detail with various kinds of rare earths as a migrating trivalent ion species.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00352-3
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N. Imanaka, G. Adachi / Journal of Alloys and Compounds 344 (2002) 137–140
2. Experimental The tungstates, M 2 (WO 4 ) 3 (M5Al, Sc, Y, Er, Tm, Yb, and Lu) were prepared by a conventional solid-state reaction method. A stoichiometric amount of Al(OH) 3 (purity: 99.99%), M 2 O 3 (99.9%), and WO 3 (99.9%) was mixed in a mortar and heated at 1000 8C for 12 h. The calcined powder was ground and sintered at 1000–1300 8C for 12 h. In the case of the samples of Y and heavy rare earths, Er–Lu, the mixed powder was dried in a vacuum at 150 8C before sintering because these compounds are considerably hygroscopic. R 1 / 3 Zr 2 (PO 4 ) 3 phosphates (R5 Sc, Eu, Gd, Er, Tm, Yb, and Lu) were synthesized by a sol–gel method from high-purity R 2 O 3 (99.9%), ZrO(NO 3 ) 2 ?2H 2 O (99.95%) and (NH 4 ) 2 HPO 4 (.99.99%) as the starting materials. R 2 O 3 and ZrO(NO 3 ) 2 ?2H 2 O were separately dissolved in HNO 3 solution (3 M), then mixed together. The (NH 4 ) 2 HPO 4 solution (3%) was dropped into the mixed HNO 3 solution afterwards. After precipitations were obtained, the solution was heated at 75 8C for 24 h, then water in the solution was vaporized at 130 8C. The dried precipitant was heated at 300 8C for 6 h and the resulting powder was pelletized and sintered at 850 8C for 24 h. The Sc 1 / 3 Zr 2 (PO 4 ) 3 samples were also prepared by a ball milling method with the same starting materials. The powder was mixed in an agent pot at a rotation speed of 300 rpm for 12 h. After identifying the resulting powder to be an amorphous state by X-ray powder diffraction method (M18XHF, Mac Science), the powder obtained was made into pellet (10 mm in diameter) and sintered at 850 8C for 12 h in an air atmosphere. The X-ray diffraction (XRD) data were collected by a step-scanning method in the 2u range from 10 to 708 with a step width of 0.048 and a scan time of 4 s. The relative density of the pellet was calculated by dividing sintered pellet density by the powder one by the Archimedes method. The Vickers hardness of sintered pellets of Sc 2 (WO 4 ) 3 and Sc 1 / 3 Zr 2 (PO 4 ) 3 was measured by using the Micro Hardness Tester (HMV-2, Shimadzu). Electrical conductivity was measured by an a.c. and a d.c. method, using the sintered sample pellet with two platinum electrodes in the temperature range between 200 and 600 8C. The a.c. conductivity measurement was carried out by an a.c. complex impedance method in the frequency range from 20 Hz to 1 MHz by using the Precision LCR meter (8284A, Hewlett-Packard).
3. Results and discussion Fig. 1 shows the trivalent ion conductivity dependencies on the A ratio, which is defined as the conducting trivalent cation size for the crystal lattice volume of the tungstate (d) series. Among various types of trivalent ions investigated, the solid electrolyte with Sc 31 ion (ionic radius: 0.0885 nm [18]) shows the highest trivalent ion con-
Fig. 1. The trivalent ion conductivity dependencies on the A ratio, which is defined as the conducting trivalent cation size for the crystal lattice volume of the tungstate (d) series.
ductivity. Especially, for the ion species of Al 31 which has a considerably smaller ionic radius (0.0675 nm) [18], the ion conductivity decreased considerably owing to the smaller A ratio due to the larger contraction of the volume of the migrating trivalent ion from Sc 31 to Al 31 compared with that of the crystal lattice from Sc 2 (WO 4 ) 3 to Al 2 (WO 4 ) 3 (a direct trivalent ion conduction in the tungstate series was clearly demonstrated by d.c. electrolysis and polarization measurements [11,12]). The tungstates hold a quasi-layered structure and contain the hexavalent cation of W 61 and W 61 can be easily reduced. In order to obtain trivalent cation conductors which possess such characteristics not to be reduced, the phosphate based materials were selected as the promising candidate. The A ratio dependencies on the ionic conductivity of the phosphate series [R 1 / 3 Zr 2 (PO 4 ) 3 ] prepared by a sol– gel method, are presented in Fig. 2. With the decrease of the rare earth ion radius by a lanthanide contraction, the ion conductivity monotonously increased and the highest ion conductivity was observed for the migrating ion species of Sc 31 . In contrast, the conductivity of the NASICON structure whose crystal lattice was intentionally expanded by doping the larger In 31 ion (ionic radius: 0.094 nm [18]) on the Sc site to form the 0.8Sc 1 / 3 Zr 2 (PO 4 ) 3 –0.2In 1 / 3 Zr 2 (PO 4 ) 3 solid solution, decreased compared with that of Sc 1 / 3 Zr 2 (PO 4 ) 3 (see the figure inserted), indicating that a further A ratio reduction decreased the ion conductivity (the trivalent rare earth cation conduction in the phosphate solids was also directly
N. Imanaka, G. Adachi / Journal of Alloys and Compounds 344 (2002) 137–140
139
Fig. 2. The A ratio dependencies on the ionic conductivity of the phosphate series [R 1 / 3 Zr 2 (PO 4 ) 3 ]. The ion conductivity of the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte prepared by a ball mill method is also plotted (s).
demonstrated both by polarization and d.c. electrolysis measurements [17]). It can be easily recognized that Sc 31 ion conducting Sc 1 / 3 Zr 2 (PO 4 ) 3 shows the highest ion conductivity among the R 1 / 3 Zr 2 (PO 4 ) 3 series investigated, demonstrating that the most suitable NASICON type R 1 / 3 Zr 2 (PO 4 ) 3 crystal lattice is realized for the migrating cation of Sc 31 . In addition, by preparing the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte by a ball mill method, the ion conductivity enhanced approximately three times as high as that prepared by a sol–gel method, as indicated in Fig. 2(s). The temperature dependencies of the ionic conductivity for Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by both sol–gel and ball mill methods are presented with the data for Sc 2 (WO 4 ) 3 and 25 Al 2 (WO 4 ) 3 in Fig. 3. The conductivity (2.91310 S 21 cm ) of Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by a ball mill method is found to be comparable to the series of the Sc 2 (WO 4 ) 3 type structure at 600 8C. The activation energy for ion conduction of Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by sol–gel and ball mill methods, which is derived form the relationship between log (s T ) and 1 /T are 67.9 and 72.7 kJ mol 21 , respectively, which is a little higher than those of Sc 2 (WO 4 ) 3 (56.4 kJ mol 21 ) and Al 2 (WO 4 ) 3 (65.8 kJ mol 21 ), but indicating that similar suitable ion pathway was realized in both NASICON and Sc 2 (WO 4 ) 3 type structure. Table 1 tabulates the trivalent ion conductivity at 600 8C, Vickers hardness, and relative density for
Fig. 3. The temperature dependencies of the ionic conductivity for Sc 1 / 3 Zr 2 (PO 4 ) 3 prepared by both sol–gel and ball mill methods with the data for Sc 2 (WO 4 ) 3 and Al 2 (WO 4 ) 3 .
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Table 1 The trivalent Sc 31 ion conductivity, Vickers hardness and relative density for Sc 2 (WO 4 ) 3 and Sc 1 / 3 Zr 2 (PO 4 ) 3 Sc 31 ion conductors
Conductivity (s / S cm 21 )
Vickers hardness (Hv)
Relative density (%)
Sc 2 (WO 4 ) 3 Sc 1 / 3 Zr 2 (PO 4 ) 3 (sol–gel) Sc 1 / 3 Zr 2 (PO 4 ) 3 (ball mill)
3.74310 25 1.07310 25 2.91310 25
40 293 305
89 99.9 99.9
Sc 1 / 3 Zr 2 (PO 4 ) 3 (both sol–gel and ball mill methods) and Sc 2 (WO 4 ) 3 which shows the highest trivalent ion conductivity in the Sc 2 (WO 4 ) 3 type series. The relative densities of both Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolytes was considerably high in comparison to Sc 2 (WO 4 ) 3 . In addition, the Vickers hardness of the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte is more than seven times as high as that of Sc 2 (WO 4 ) 3 . The results described above clearly indicate the fact that the Sc 1 / 3 Zr 2 (PO 4 ) 3 solid electrolyte based on phosphate which is highly stable in various atmospheres was successfully obtained with having a high relative density and a considerably high mechanical hardness which contribute greatly to applying as an element for various types of devices.
4. Conclusions Among the Sc 2 (WO 4 ) 3 type and the NASICON type R 1 / 3 Zr 2 (PO 4 ) 3 (R5Al 31 , rare earths) series, the trivalent ion conducting solid electrolytes with Sc 31 as a conducting species were found to show the highest ion conductivity. Especially, Sc 1 / 3 Zr 2 (PO 4 ) 3 possesses such advanced merits to have not only a considerable high relative density but also considerably higher mechanical strength in comparison to the Sc 2 (WO 4 ) 3 type structure series, promising applications such as a component of functional materials are highly expected.
Acknowledgements The present work was partially supported by a Grant-inAid 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] B. Dunn, G.C. Farrington, Solid State Ionics 9 / 10 (1983) 223. [2] W. Carrillo-Cabrera, J.O. Thomas, G.C. Farrington, Solid State Ionics 9 / 10 (1983) 245. [3] B. Ghosal, E.A. Mangle, M.R. Tipp, B. Dunn, G.C. Farrington, Solid State Ionics 9 / 10 (1983) 273. [4] G.C. Farrington, B. Dunn, J.O. Thomas, Appl. Phys. A A32 (1983) 159. ¨ [5] T. Dedecke, J. Kohler, F. Tietz, W. Urland, Eur. J. Solid State Inorg. Chem. 33 (1996) 185. ¨ [6] J. Kohler, W. Urland, Solid State Ionics 86–88 (1996) 93. ¨ [7] J. Kohler, W. Urland, Angew. Chem. 109 (1997) 105. [8] A.M. George, A.N. Virkar, J. Phys. Chem. Solids 49 (1998) 743. [9] T.E. Warner, D.J. Fray, A. Davies, Solid State Ionics 92 (1996) 99. [10] N. Imanaka, Y. Kobayashi, G. Adachi, Chem. Lett. (1995) 433. [11] Y. Kobayashi, T. Egawa, S. Tamura, N. Imanaka, G. Adachi, Chem. Mater. 9 (1997) 1649. [12] N. Imanaka, Y. Kobayashi, K. Fujiwara, T. Asano, Y. Okazaki, G. Adachi, Chem. Mater. 10 (1998) 2006. [13] S. Tamura, T. Egawa, Y. Okazaki, Y. Kobayashi, N. Imanaka, G. Adachi, Chem. Mater. 10 (1998) 1958. [14] N. Imanaka, M. Hiraiwa, S. Tamura, G. Adachi, H. Dabkowska, A. Dabkowski, J.E. Greedan, Chem. Mater. 10 (1998) 2542. ¨ [15] J. Kohler, N. Imanaka, G. Adachi, J. Mater. Chem. 9 (1999) 1357. [16] N. Imanaka, Y. Kobayashi, S. Tamura, G. Adachi, Solid State Ionics 136 / 137 (2000) 319. [17] S. Tamura, N. Imanaka, G. Adachi, Adv. Mater. 11 (1999) 1521. [18] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.