Crystal Structure, Disorder, and Diffusion Path of Oxygen Ion

Conductors Y1-xTaxO1.5+x (x ) 0.215 and 0.30). Masatomo Yashima* and Takayuki Tsuji. Department of Materials Science and Engineering, Interdisciplinar...
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Chem. Mater. 2007, 19, 3539-3544

3539

Crystal Structure, Disorder, and Diffusion Path of Oxygen Ion Conductors Y1-xTaxO1.5+x (x ) 0.215 and 0.30) Masatomo Yashima* and Takayuki Tsuji Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama-shi, Kanagawa, 226-8502, Japan ReceiVed April 2, 2007. ReVised Manuscript ReceiVed April 25, 2007

We report the results of a neutron powder diffraction study of the yttrium tantalates Y0.785Ta0.215O1.715 and Y0.7Ta0.3O1.8. In the orthorhombic fluorite-related Y0.7Ta0.3O1.8 compound, the Y and Ta atoms were occupationally ordered, and the oxygen ions were localized at around the stable positions. On the contrary, in the defect fluorite-type cubic Y0.785Ta0.215O1.715 material, the Y and Ta atoms were occupationally disordered, and the oxygen ions exhibited a large spatial distribution and positional disorder. At a higher temperature of 808 K, the spatial distribution of the oxygen ions was larger, and the diffusion paths of the oxygen ions were observed along the 〈100〉 directions. The high diffusion coefficient and conductivity of oxygen ions in cubic Y0.8Ta0.2O1.7 are attributable to the occupational and positional disorder.

Introduction Fluorite-type structured compounds such as zirconia-, ceria-, and bismuth oxide-based materials have a high oxygen ion conductivity.1 These materials continue to attract researchers because of a wide variety of applications in solidoxide fuel cells, gas sensors, pumps, and catalysts. Rare earth tantalates R1-xTaxO1.5+x (R: rare earth, Y, and Sc elements) have different crystal structures depending on the R species, chemical composition x, temperature, and preparation method, varying from a cubic defect fluorite-type structure to an orthorhombic fluorite-related structure.2-10 The lattice diffusion coefficient of oxygen ions and its activation energy in fluorite-type Y0.8Ta0.2O1.7 are close to those in stabilized zirconia.11 Oxygen ion conductivity in the Y1-xTaxO1.5+x materials (0 < x < 0.25) is highest around the Y0.8Ta0.2O1.7 composition.12,13 However, the structural origin of the high diffusion coefficient and conductivity of oxygen ions for Y0.8Ta0.2O1.7 has not been understood satisfactorily. * Corresponding author. E-mail: [email protected].

(1) Boivin, J. C.; Mairesse, G. Chem. Mater. 1998, 10, 2870-2888. (2) Rooksby, H. P.; White, E. A. D. J. Am. Ceram. Soc. 1964, 47, 9496. (3) Allpress, J. G.; Rossell, H. J. J. Solid State Chem. 1979, 27, 105114. (4) Rossell, H. J. J. Solid State Chem. 1979, 27, 115-122. (5) Rossell, H. J. J. Solid State Chem. 1979, 27, 287-292. (6) Shirotinkin, V. P.; Evdokimov, A. A.; Trunov, V. K. Russ. J. Inorg. Chem. 1982, 27, 931-933. (7) Yokogawa, Y.; Yoshimura, M. J. Am. Ceram. Soc. 1991, 74, 20772081. (8) Tanaka, T.; Ishizawa, N.; Yoshimura, M.; Marumo, F.; Oyanagi, H. J. Solid State Chem. 1995, 114, 79-87. (9) Yokogawa, Y.; Yoshimura, M. J. Am. Ceram. Soc. 1997, 80, 19651974. (10) Yokota, O.; Yashima, M.; Yamamoto, N.; Yoshimura, M. J. Am. Ceram. Soc. 1997, 80, 2429-2432. (11) Ikuma, Y.; Tsubaki, Y.; Nakao, Y.; Yokogawa, Y.; Yoshimura, M. Solid State Ionics 1990, 40-41, 258-261. (12) Kim, S.; Yashima, M.; Kakihana, M.; Yoshimura, M. J. Alloys Compds. 1993, 192, 72-74. (13) Petric, A.; Huang, P. J. Mater. Chem. 1995, 5, 607-610.

The crystal structure of yttrium tantalates Y1-xTaxO1.5+x has been studied by some researchers.4,5,8,10 Rossel determined the crystal structure of the orthorhombic fluoriterelated compounds Y0.75Ta0.25O1.75 (space group C2221)4 and Y0.71Ta0.29O1.79 (space group Cmmm).5 However, the atomic displacement parameters and the spatial distribution of mobile oxygen ions were not reported. Tanaka et al.8 investigated the crystal structure of fluorite-type Y0.75Ta0.25O1.75 using single-crystal X-ray diffraction, EXAFS, split-atom models, and difference Fourier method and concluded that a part of the oxygen atoms is displaced along 〈100〉 by 0.54(4) Å. Here, we report the results of a neutron powder diffraction study of the Y0.785Ta0.215O1.715 and Y0.7Ta0.3O1.8 compounds. The spatial distribution and positional disorder of oxygen atoms were investigated by a combined technique of Rietveld method, maximum-entropy method (MEM),14-17 and MEMbased pattern fitting (MPF).15-17 The MEM and MPF techniques are least biased with respect to unobserved structure factors and can produce a reliable nuclear density distribution in detail from neutron powder diffraction data. In the MEM analysis, any kind of complicated nuclear density distribution is allowed as long as it satisfies the symmetry requirements. The present work demonstrates for the first time that the oxygen ions in Y0.785Ta0.215O1.715 diffuse along the 〈100〉 directions. Experimental Procedures Sample Preparation and Characterization. The Y0.785Ta0.215O1.715 and Y0.7Ta0.3O1.8 materials were prepared by solid-state reactions. (14) Takata, M.; Umeda, B.; Nishibori, E.; Sakata, M.; Saito, Y.; Ohno, M.; Shinohara, H. Nature 1995, 377, 46-49. (15) Izumi, F.; Dilanian, R. A. Recent Res. DeV. Phys. 2002, 3, 699-726. (16) Yashima, M.; Ishimura, D. Chem. Phys. Lett. 2003, 378, 395-399. (17) Yashima, M.; Itoh, M.; Inaguma, Y.; Morii, Y. J. Am. Chem. Soc. 2005, 127, 3491-3495.

10.1021/cm070910g CCC: $37.00 © 2007 American Chemical Society Published on Web 06/12/2007

3540 Chem. Mater., Vol. 19, No. 14, 2007 We did not choose the Y0.8Ta0.2O1.7 composition because Y0.8Ta0.2O1.7 contained small amounts of impurities due to the solubility limit. We used the Y0.785Ta0.215O1.715 composition, which is a single cubic fluorite-structured phase. High-purity yttrium oxide (Y2O3, 99.99% purity) and tantalum oxide (Ta2O3, 99.99% purity) powders were mixed in an agate mortar for about 3 h as ethanol slurries and as dried powders. The mixture was uniaxially pressed into pellets at 200 MPa. Pellets with the Y0.7Ta0.3O1.8 composition were sintered in air at 1500 °C for 6 h. Sintered pellets of Y0.7Ta0.3O1.8 were crushed and ground to obtain powdered samples for neutrondiffraction measurements. Pellets with the Y0.785Ta0.215O1.715 composition were heated in air at 1700 °C for 3 h. Chemical analysis indicated that the composition of cubic Y0.785Ta0.215O1.715 was Y0.7848(7)Ta0.2152(7)O1.715, where the number in parentheses is the error bar of the last digit. Neutron Powder Diffraction Experiments. The crystal structure and structural disorder of the Y0.785Ta0.215O1.715 and Y0.7Ta0.3O1.8 compositions were studied by the neutron-diffraction technique because there is no interference from the electron density distribution, and the scattering power for the O atom is relatively large in contrast to X-ray diffractometry. The powdered sample of Y0.7Ta0.3O1.8 was contained in a vanadium can of 10 mm inner diameter and 40 mm height for the data collection from neutron diffraction. Neutron-diffraction data for both samples were collected on a multidetector fixed wavelength powder diffractometer (HERMES) installed at the JRR-3M research reactor of the Japan Atomic Energy Agency (JAEA), by the Institute for Materials Research, Tohoku University.18 A neutron beam with a 1.82 Å wavelength was obtained by the (331) plane of a Ge monochromator. Wavelengths of the neutron beams were 1.8207 and 1.8243 Å for Y0.7Ta0.3O1.8 and Y0.785Ta0.215O1.715, respectively. The relatively long wavelength λ makes it impossible to collect the data for the lattice spacing d value smaller than λ/2. Since the total pattern is able to be collected in a short time, the HERMES diffractometer is a powerful tool to study the MEM nuclear density map. The profile data were measured by scanning at intervals of 0.10° in the 2θ range from 5.0 to 150.0° using 150 detectors of the HERMES diffractometer. Neutron-diffraction data of Y0.7Ta0.3O1.8 were collected at 299 K, while those of Y0.785Ta0.215O1.715 were measured at 299 and 808(6) K. For the measurement at 808 K, a furnace with molybdenum disilicide heaters was used.19 Data Analyses of the Neutron Powder Diffraction Data. The neutron-diffraction data were iteratively analyzed by a combination of the Rietveld analysis, MEM,14-17 MPF.15-17 The crystal structures of Y0.785Ta0.215O1.715 and Y0.7Ta0.3O1.8 were refined by Rietveld analysis of the neutron-diffraction data. The calculations were performed by a Rietveld analysis computer program RIETAN200020 with the following neutron scattering lengths: Y, 7.75; Ta, 6.91; and O, 5.803 fm. The peak shape was assumed to be a splittype pseudo-Voigt function. The cutoff value was 7.00 × (fwhm). The background was approximated by a 12-parameter polynomial 2θn, where n has values between 0 and 11. The 12 parameters were simultaneously refined with the unit cell, zero-point, scale, profile shape, and crystal structural parameters. The isotropic atomic displacement parameters were used for all the atoms. The diffraction pattern of Y0.785Ta0.215O1.715 exhibited a background with a complicated profile shape due to diffuse scattering. Thus, we subtracted the background using PowderX21 before the Rietveld analysis. The MEM calculations for the Y0.785Ta0.215O1.715 and Y0.7Ta0.3O1.8 (18) Ohoyama, K.; Kanouchi, T.; Nemoto, K.; Ohashi, M.; Kajitani, T.; Yamaguchi, Y. Jpn. J. Appl. Phys., Part 1 1998, 37, 3319-3326. (19) Yashima, M. J. Am. Ceram. Soc. 2002, 85, 2925-2930. (20) Izumi, F.; Ikeda, T. Mater. Sci. Forum 2000, 321-324, 198-203. (21) Dong, C. J. Appl. Crystallogr. 1999, 32, 838-838.

Yashima and Tsuji Table 1. Refined Crystallographic Parameters and Reliability Factors of Y0.7Ta0.3O1.8 (299 K)a atom Y Ta O

site

g

x

y

z

U (Å2)

Y1 2c Y2 4e Ta1 2d Ta2 4e O1 8q O2 4g O3 4g

1 0.9 1 0.1 1 1 0.6

0 1/4 0 1/4 0.1239(5) 0.1368(6) 0.0756(10)

1/2 1/4 0 1/4 0.2060(5) 1/2 0

1/2 0 1/2 0 1/2 0 0

0.0116(12) ) U(Y1) 0.0109(5) ) U(Ta1) 0.0257(10) 0.0181(11) 0.026(3)

a Orthorhombic space group Cmmm (No. 65); number of formula unit Y0.7Ta0.3O1.8 in a unit cell: Z ) 8. Unit cell parameters: a ) 10.4734(7) Å, b ) 7.3868(3) Å, c ) 3.7132(2) Å, R ) β ) γ ) 90°; unit cell volume: 287.27(3) Å3; g: occupancy; x, y, z: fractional coordinates; and U: atomic displacement parameters. Reliability factors in the Rietveld analysis: Rwp ) 8.82%, Rp ) 6.34%, Re ) 1.88%, Rwp/Re ) 4.68, RI ) 5.14%, RF ) 4.10%. Reliability factors in the first MPF analysis: RI ) 3.71% and RF ) 3.12%.

samples were performed using the computer program PRIMA,15 with 64 × 64 × 64 and 128 × 128 × 128 pixels, respectively. To reduce the bias imposed by the simple structural model, an iterative procedure named by the REMEDY cycle15 was employed after the MEM analysis.

Results and Discussion Structural Refinement of Y0.7Ta0.3O1.8. All the peaks of the neutron-diffraction profile of Y0.7Ta0.3O1.8 were indexed by an orthorhombic lattice. Rietveld analyses of Y0.7Ta0.3O1.8 were performed with the fluorite-related Cmmm structure.5 In a preliminary analysis, the occupancy factors were refined, and the refined occupancy factor of Y atoms at the 2c site g(Y1) was 1.02(24). Thus, the value of g(Y1) was fixed to unity in the subsequent analyses. In other analyses, the refined occupancy factor of Y atoms at the 2d site had a negative value; therefore, it was fixed to zero in the subsequent analyses. Refined occupancy factors g(O1) and g(O2) were unity within the error bars; thus, they were fixed to unity in the subsequent analyses. Consequently, all the occupancy factors in the final refinement were fixed to the values in Table 1. The calculated profile agreed well with the observed one (Figure 1). Crystallographic parameters and reliability factors are shown in Table 1. The atomic coordinates of the present Cmmm Y0.7Ta0.3O1.8 agree with those of Cmmm Y0.71Ta0.29O1.79 reported in the literature.5 The atomic displacement parameters of Y and Ta atoms U(Y) ) 0.0116(12) Å2 and U(Ta) ) 0.0109(5) Å2 were smaller than those of oxygen atoms at the O1, O2, and O3 sites U(O1) ) 0.0257(10), U(O2) ) 0.0181(11), and U(O3) ) 0.026(3) Å2, suggesting a larger mobility of oxygen ions than that of cations. Figure 2a shows the crystal structure of Cmmm Y0.7Ta0.3O1.8 depicted with the TaO6.6, YO8, and (Y0.9Ta0.1)O7.2 polyhedra, using refined crystallographic parameters. The relationship of the lattice vectors between Cmmm and pseudo-fluorite-type structures can be described as aortho ) 2cF, bortho ) aF + bF, cortho ) (aF - bF)/2 where aortho and cF are lattice vectors of the Cmmm and pseudo-fluorite cells, respectively. Most of the Ta atoms are localized at the 2d site (Wyckoff notation in the Cmmm space group, corresponding fractional coordinate: 0,0,1/2). On the

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Figure 1. Rietveld pattern fitting result for neutron-diffraction data of Y0.7Ta0.3O1.8 measured at 299 K. Red crosses and green line denote observed and calculated intensities, respectively. The short green vertical lines denote the possible Bragg peak positions for Cmmm Y0.7Ta0.3O1.8. The blue profile denotes the difference between observed and calculated intensities.

Figure 2. (a) Refined crystal structure of orthorhombic Cmmm Y0.7Ta0.3O1.8 depicted using the TaO6.6, YO8, and (Y0.9Ta0.1)O7.2 polyhedra (299 K). (b) Crystal structural model for the cubic defect fluorite-type Y0.785Ta0.215O1.715 phase (space group: Fm3hm) depicted with (Y,Ta)O6.86 cubes. Average distribution of 6.86 oxygen atoms at the 8c site of Fm3hm, (1/4,1/4,1/4). Solid line denotes the unit cell of the crystal structure.

contrary, the Y atoms exist at the 2c (0,1/2,1/2) and 4e (1/ 4,1/4,0) sites (Table 1), indicating the occupational order of

Y and Ta atoms (Figure 2a). The oxygen vacancy exists at the nearest neighbor site of Ta atom, making the TaO6.6 and (Y0.9Ta0.1)O7.2 polyhedra. On the contrary, the O1 and O2 sites, which are the nearest neighbor sites of the Y1 site, are fully occupied by oxygen atoms, making the YO8 polyhedron. These results indicate the occupational order of oxygen atoms and their vacancies among the O1, O2, and O3 sites. It is interesting that the oxygen vacancy favors the nearest neighbor sites of Ta5+ cations because, from the viewpoint of the electrostatic energy, the oxygen vacancy could favor the Y3+ cation. Bond valence sum (BVS) calculations22 were performed to confirm the validity of the crystal structure and occupational order of Y0.7Ta0.3O1.8. The BVS value for the 2c site was estimated to be 2.6, which is consistent with the valence of the Y3+ cation. Thus, the occupation of the 2c site by the Y atom is valid. The BVS value for the 2d site, 6.3, could be consistent with occupation by the Ta5+ cation. The BVS value for the 4e site, 3.4, is nearly equaled to the averaged valence value 3.2 for Y0.9Ta0.1. These results indicate that the refined crystal structure and occupational order of Y0.7Ta0.3O1.8 are valid. Structural Refinement of Y0.785Ta0.215O1.715. The neutrondiffraction profile of the Y0.785Ta0.215O1.715 material exhibited a single fluorite-type structured phase at 299 and 808 K. The data were analyzed assuming a simple defect cubic fluoritetype structure (Figure 2b). We did not utilize the split-atom model with specific atomic displacements because only a small number of diffraction peaks (nine) was observed. In the present simple defect cubic fluorite-type structure, the oxygen atoms exist at the 8c site in Fm3hm symmetry (Table 2). The Y and Ta atoms were put at the special position 4a of Fm3hm (0,0,0), where the occupancy factors of Y and Ta (22) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192-197.

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Table 2. Crystallographic Parameters and Reliability Factors of Y0.785Ta0.215O1.715 at 299 and 808 Ka temperature 299 K unit cell parameter unit cell volume Y

site occupancy g(Y) fractional coordinates atomic displacement parameter U(Y) Ta site occupancy g(Ta) fractional coordinates atomic displacement parameter U(Ta) O site occupancy g(O) fractional coordinates atomic displacement parameter U(O) reliability factors in the Rietveld analysis reliability factors in the nth MPF analysesd

c

808(6) K

a ) 5.2415(3) Å 144.00(2) Å3

a ) 5.2610(4) Å 145.61(2) Å3

4a 0.7848b 0, 0, 0 0.0253(5) Å2 4a 0.2152b 0, 0, 0 ) U(Y) 8c 0.8576c 1/4,1/4,1/4 0.0650(7) Å2 Rwp ) 7.64%, Rp ) 5.52%, Re ) 3.60%, Rwp/ Re ) 2.12, RI ) 2.54%, RF ) 2.54% Rwp ) 7.51%, Rp ) 5.37%, Re ) 3.60%, Rwp/Re ) 2.08, RI ) 2.14%, RF ) 1.98%

4a 0.7848b 0, 0, 0 0.0335(6) Å2 4a 0.2152b 0, 0, 0 ) U(Y) 8c 0.8576c 1/4,1/4,1/4 0.0787(8) Å2 Rwp ) 7.09%, Rp ) 5.10 %, Re ) 2.83%, Rwp/ Re ) 2.51, RI ) 2.72%, RF ) 3.70% Rwp ) 6.72%, Rp ) 4.77%, Re ) 2.83%, Rwp/Re ) 2.38, RI ) 1.61%, RF ) 2.02%

a Cubic space group Fm3 hm (No. 225); number of formula unit Y0.785Ta0.215O1.715 in a unit cell: Z ) 4. b Values from the chemical analysis results. Calculated value with electrical neutrality. d n ) 1 and 2 for data at 299 and 808 K, respectively.

Figure 3. Rietveld pattern fitting result for neutron-diffraction data of cubic fluorite-type Y0.785Ta0.215O1.715 measured (a) at 299 K and (b) at 808 K. Red crosses and green line denote observed and calculated intensities, respectively. The short green vertical lines denote the possible Bragg peak positions for Fm3hm Y0.785Ta0.215O1.715. The blue profile denotes the difference between observed and calculated intensities.

were 0.7848 and 0.2152, respectively, indicating the occupational disorder of Y and Ta atoms. The calculated diffraction profiles of Y0.785Ta0.215O1.715 agreed well with the observed ones (Figure 3). The resultant crystallographic parameters and reliability factors are shown in Table 2. In the cubic defect fluorite-type structure, the cation Y,Ta is surrounded by 6.86 oxygen ions forming a (Y,Ta)-

Figure 4. Scattering amplitude distribution of orthorhombic Cmmm Y0.7Ta0.3O1.8 at 299 K with an equicontour surface at 0.2 fm Å-3 obtained by the combination technique of Rietveld refinement, MEM, and MPF analyses.

O6.86 polyhedron. The bond length between the cation and the oxygen atom was calculated to be 2.27 Å. The BVS for the (Y,Ta)O6.86 polyhedron was estimated to be 3.3, which is consistent with the averaged valence of cations, 3.4. Here, we used the averaged bond valence parameter 1.994. The unit cell parameter a at 808 K (5.2610(4) Å) is larger than that at 299 K (5.2415(3) Å) due to the thermal expansion. The thermal expansion coefficient between 299 and 808 K is estimated to be 7.31 × 10-6 (K-1). Atomic displacement parameters of cations and anions at 808 K are

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Chem. Mater., Vol. 19, No. 14, 2007 3543

Figure 5. Scattering amplitude distribution of cubic fluorite-type Y0.785Ta0.215O1.715 at 299 K with an equicontour surface at 0.2 fm Å-3 obtained by the combination technique of Rietveld refinement, MEM, and MPF analyses.

Figure 7. Scattering amplitude distribution (a) on the (002) plane of orthorhombic Cmmm fluorite-related Y0.7Ta0.3O1.8 and on the (110) planes of cubic fluorite-type Y0.785Ta0.215O1.715 (b) at 299 K and (c) at 808 K. Contour lines: from 0.2 to 2.0 by a step of 0.2 fm Å-3. The line with arrows indicates the diffusion path along the 〈100〉 directions.

Figure 6. Scattering amplitude distribution of Y0.785Ta0.215O1.715 at 808 K with an equicontour surface at 0.2 fm Å-3 obtained by the combination technique of Rietveld refinement, MEM, and MPF analyses. Lines with arrows indicate the diffusion paths along the 〈100〉 directions.

larger than those at 299 K (Table 2). The atomic displacement parameter of anions is higher than that of cations, suggesting the higher diffusion coefficient of anions. At 299

K, the atomic displacement parameters of cubic fluorite-type Y0.785Ta0.215O1.715 (U(Y,Ta) ) 0.0253(5) and U(O) ) 0.0650(7) Å2) are larger than those of orthorhombic Y0.7Ta0.3O1.8 (U(Y) ) 0.0116(12), U(Ta) ) 0.0109(5) Å2, U(O1) ) 0.0257(10), U(O2) ) 0.0181(11), and U(O3) ) 0.026(3) Å2), indicating the larger positional disorder in cubic Y0.785Ta0.215O1.715. MEM Nuclear Density of Y0.7Ta0.3O1.8 and Y0.785Ta0.215O1.715. MEM calculations for the Y0.7Ta0.3O1.8 and Y0.785Ta0.215O1.715 compositions were performed using the observed structure factors obtained in the Rietveld analyses of neutron-diffraction data. Numbers of structure factors derived in the analyses of the Y0.7Ta0.3O1.8 and Y0.785Ta0.215O1.715 compounds were 109 and 11, respectively. The reliability factors based on the integrated intensity RI and structure factor RF of Cmmm Y0.7Ta0.3O1.8 were improved from 5.14% (Rietveld analysis) to 3.71% (first MPF analysis) and from 4.10% (Rietveld analysis) to 3.12% (first MPF analysis), respectively. The RI and RF factors for the cubic Y0.785Ta0.215O1.715 data taken at 299 K were improved from 2.54% (Rietveld analysis) to 2.14% (first MPF analysis) and from 2.54% (Rietveld analysis) to 1.98% (first MPF analysis), respectively. The RI and RF factors for the Y0.785Ta0.215O1.715 data taken at 808 K were considerably

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improved from 2.72% (Rietveld analysis) to 1.61% (second MPF analysis) and 3.70% (Rietveld analysis) to 2.02% (second MPF analysis), respectively. Figure 4 shows the equicontour surface and nuclear density distribution on the (002) plane of the Cmmm Y0.7Ta0.3O1.8 material. Figures 5 and 6 show the equicontour surface and nuclear density distribution on the corresponding (110) plane of fluorite-type Y0.785Ta0.215O1.715 material at 299 and 808 K, respectively. Figure 7 compares the nuclear density distributions on the (002) plane of the Cmmm Y0.7Ta0.3O1.8 material at 299 K (panel a) and on the corresponding (110) plane of the fluorite-type Y0.785Ta0.215O1.715 material at 299 K (panel b) and at 808 K (panel c). The oxygen ions are localized near the stable positions in Cmmm Y0.7Ta0.3O1.8, indicating the positional order. On the contrary, the oxygen ions have a large spatial distribution and positional disorder in the cubic fluorite-structured Y0.785Ta0.215O1.715 (Figures 5-7). The spatial distribution of Y0.785Ta0.215O1.715 at 808 K is larger than that at 299 K. These differences in the nuclear density distribution (Figures 4-7) correspond to those in the atomic displacement parameters (Tables 1 and 2). The lattice diffusion coefficient of oxygen ions and its activation energy in fluorite-type Y0.8Ta0.2O1.7 are close to those in stabilized zirconia.11 Oxygen ion conductivity in the Y1-xTaxO1.5+x materials (0 < x < 0.25) is highest around the Y0.8Ta0.2O1.7 composition.12,13 The high diffusivity and ionic conductivity of oxygen ions in cubic Y0.8Ta0.2O1.7 are attributable to occupational and positional disorder (Figures 2b and 5-7). The large distribution and positional disorder of oxygen ions in cubic Y0.785Ta0.215O1.715 lie in the 〈100〉 directions (Figures 5-7). The large distribution along the 〈100〉 directions is consistent with the split atom site at x,0,0 of oxygen atoms reported in Y0.75Ta0.25O1.75 through singlecrystal X-ray diffraction analysis.8 The spatial distribution of oxygen ions along the 〈100〉 directions in Figures 5-7 is continuous, suggesting that the split atom site is an approximation to describe the spatial distribution. It should be noted that the diffusion pathways along the 〈100〉 directions are clearly observed in the MEM nuclear density map of Y0.785Ta0.215O1.715 at 808 K (Figures 6 and 7). Thus, the

Yashima and Tsuji

oxygen ions would also diffuse along the 〈100〉 directions at high temperatures. A similar diffusion path along the 〈100〉 direction has been reported in other fluorite-structured materials such as δ-Bi1.4Yb0.6O3,23 CeO2,24,25 Ce0.93Y0.7O1.97,26 R-CuI,27 Zr0.85Ca0.85O1.85,28 and Zr1-xYxO2-x/2.29 Thus, the diffusion pathway along the 〈100〉 directions is a common feature of fluorite-structured ionic conductors. The diffusion pathway is strongly dependent on the crystal structure. For example, in the perovskitetype ABO3-δ oxide, the oxygen ions diffuse along the 〈100〉 direction near the stable position and then rotate around the B cation, keeping the B-O distance constant to some degree.30,31 The common feature of the disorder and diffusion path depending on the crystal structure helps the design and development of ionic conductors. Acknowledgment. We thank Prof. K. Ohoyama and K. Nemoto for the use of the HERMES diffractometer. We also thank T. Wakita, Dr. Y. Matsushita, Dr. R. Ali, Y. Kawaike, T. Komatsu, and Y. Phat for experimental assistance. Gratitude is extended to Daiichi-Kigenso-Kagaku Kogyo Co. for the chemical analysis. We thank Daiichi-Kigenso-Kagaku Kogyo Co. and Mitsui Co. for Y2O3 and Ta2O5 powders, respectively. This research was partially supported by the Ministry of Education, Science, Sports and Culture of Japan, Grant-in-Aid. Figures 2, 4, 5, 6, and 7 were drawn using the computer program VESTA developed by K. Momma and Dr. F. Izumi.32 CM070910G (23) Yashima, M.; Ishimura, D. Appl. Phys. Lett. 2005, 87, 221909221912. (24) Yashima, M.; Kobayashi, S. Appl. Phys. Lett. 2004, 84, 526-528. (25) Yashima, M.; Kobayashi, S.; Yasuda, T. Solid State Ionics 2006, 177, 211-215. (26) Yashima, M.; Kobayashi, S.; Yasuda, T. Faraday Discuss. 2007, 134, 369-376. (27) Yashima, M.; Xu, Q.; Yoshiasa, A.; Wada, S. J. Mater. Chem. 2006, 16, 4393-4396. (28) Lorenz, G.; Frey, F.; Schulz, H.; Boysen, H. Solid State Ionics 1988, 28-30, 497-502. (29) Shimojo, F.; Okazaki, H. J. Phys. Soc. Jpn. 1992, 61, 41064118. (30) Yashima, M.; Nomura, K.; Kageyama, H.; Miyazaki, Y.; Chitose, N.; Adachi, K. Chem. Phys. Lett. 2003, 380, 391-396. (31) Islam, M. S. J. Mater. Chem. 2000, 10, 1027-1038. (32) Momma, K.; Izumi, F. Int. Union Crystallogr., Comm. Crystallogr. Comput. News. 2006, 7, 106-119.