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ISSN: 2052-5206

In situ observation of the tetragonal–cubic phase transition in the CeZrO4 solid solution – a high-temperature neutron diffraction study

aDepartment of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta-cho 4259-J2-61, Midori-ku, Yokohama 226-8502, Japan, and bDaiichi Kigenso Kagaku Kogyo Co. Ltd, Hirabayashi-Minami 1-6-38, Suminoe-ku, Osaka 559-0025, Japan
*Correspondence e-mail: yashima@materia.titech.ac.jp

(Received 20 November 2006; accepted 15 February 2007)

The crystal structure of the compositionally homogeneous ceria–zirconia solid solution CeZrO4 is refined by Rietveld analysis of neutron diffraction data measured in situ over the temperature range 296–1831 K. The CeZrO4 exhibits a tetragonal structure with the space group P42/nmc at temperatures from 296 to 1542 K (Z = 1), and a cubic fluorite-type form with the space group [Fm\overline 3 m] at 1831 K (Z = 2). The isotropic atomic displacement parameters of Ce and Zr atoms B(Ce,Zr) and O atoms B(O) are found to increase with temperature, with B(O) being larger than B(Ce,Zr), suggesting the higher diffusivity of oxygen ions. The ratio of the c axial length to the a length of the pseudo-fluorite lattice (c/aF axial ratio) for the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching unity between 1542 and 1831 K. The displacement of O atoms along the c axis in the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching 0.0 Å between 1542 and 1831 K. These results indicate that the cubic-to-tetragonal phase transition between 1542 and 1831 K is accompanied by oxygen displacement along the c axis and the increase of the c/aF axial ratio from unity.

1. Introduction

Materials containing ceria and zirconia have become the focus of intense study in recent years as a result of several applications in the field of high-temperature materials for technologies such as fuel cells, oxygen gas sensors (Inaba & Tagawa, 1996[Inaba, H. & Tagawa, H. (1996). Solid State Ion. 83, 1-16.]) and structural ceramics (Tsukuma & Shimada, 1985[Tsukuma, K. & Shimada, M. (1985). J. Mater. Sci. 20, 1178-1184.]), and as components of automotive exhaust catalysts for the removal of noxious compounds (Yao & Yao, 1984[Yao, H. C. & Yao, T. F. Yu (1984). J. Catal. 86, 254-265.]; Ozawa et al., 1993[Ozawa, M., Kimura, M. & Isogai, A. (1993). J. Alloys Compd. 193, 73-75.]; Murota et al., 1993[Murota, T., Hasegawa, T., Aozasa, S., Matsui, H. & Motoyama, M. (1993). J. Alloys Compd. 193, 298-299.]; Trovarelli, 1996[Trovarelli, A. (1996). Catal. Rev. Sci. Eng. 38, 439-520.]). The material properties of these compounds are strongly dependent on the crystal structure and phase, yet the form of these compounds at high temperature, at which most function most efficiently, remains poorly understood. In the case of the Ce1 − xZrxO2 solid solutions, there is strong interest in the compositionally homogeneous metastable forms of the tetragonal phase, since these materials are used extensively as promoters in three-way catalysts for automotive exhausts. The compositional dependence of the phase and crystal structure of compositionally homogeneous metastable Ce1 − xZrxO2 solid solutions have been investigated by many researchers (Meriani, 1985[Meriani, S. (1985). J. Mater. Sci. Eng. 71, 369-370.]; Yashima et al., 1993a[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993a). J. Am. Ceram. Soc. 76, 1745-1750.],b[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993b). J. Am. Ceram. Soc. 76, 2865-2868.]; Yashima, Arashi, Kakihana & Yoshimura, 1994[Yashima, M., Arashi, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1067-1071.]; Yashima, Takashina, Kakihana & Yoshimura, 1994[Yashima, M., Takashina, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1869-1874.]; Yashima, Ohtake, Kakihana, Arashi & Yoshimura, 1996[Yashima, M., Ohtake, K., Kakihana, M., Arashi, H. & Yoshimura, M. (1996). J. Phys. Chem. Solids, 57, 17-24.]; Yashima et al., 1998[Yashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. & Mori, T. (1998). Appl. Phys. Lett. 72, 182-184.]; Omata et al., 1999[Omata, T., Kishimoto, H., Otsuka-Yao-Matsuo, S., Ohtori, N. & Umesaki, N. (1999). J. Solid State Chem. 147, 573-583.]; Kaspar & Fornasiero, 2003[Kaspar, J., Fornasiero, P., Balducci, G., DiMonte, R., Hickey, N. & Sergo, V. (2003). Inorg. Chim. Acta, 349, 217-226.]; Enzo et al., 2000[Enzo, S., Delogu, F., Frattini, R., Primavera, A. & Trovarelli, A. (2000). J. Mater. Res. 15, 1538-1545.]; Lamas et al., 2005[Lamas, D. G., Fuentes, R. O., Fabregas, I. O., Fernandez de Rapp, M. E., Lascalea, G. E., Casanova, J. R., Walsoe de Reca, N. E. & Craievich, A. F. (2005). J. Appl. Cryst. 38, 867-873.]; Zhang et al., 2006[Zhang, F., Chen, C.-H., Hanson, J. C., Robinson, R. D., Herman, I. P. & Chan, S.-W. (2006). J. Am. Ceram. Soc. 89, 1028-1036.]). As the compositionally homogeneous CeZrO4 solid solution also exhibits high catalytic performance in the Ce1 − xZrxO2 (Trovarelli et al., 1997[Trovarelli, A., Zamar, F., Llorca, J., Leitenburg, C., Dolcetti, G. & Kiss, J. T. (1997). J. Catal. 169, 490-502.]; Suda et al., 2001[Suda, A., Sobukawa, H., Suzuki, T., Kandori, T., Ukyo, Y. & Sugiura, M. (2001). J. Ceram. Soc. Jpn, 109, 177-180.]), it is therefore important to investigate the crystal structure and phase transition in the CeZrO4 solid solution at high temperatures.

The crystal structure of the compositionally homogeneous Ce1 − xZrxO2 solid solutions has been studied by Yashima et al. (1993a[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993a). J. Am. Ceram. Soc. 76, 1745-1750.],b[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993b). J. Am. Ceram. Soc. 76, 2865-2868.]), Yashima, Arashi, Kakihana & Yoshimura (1994[Yashima, M., Arashi, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1067-1071.]), Yashima, Takashina, Kakihana & Yoshimura (1994[Yashima, M., Takashina, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1869-1874.]) and Yashima et al. (1998[Yashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. & Mori, T. (1998). Appl. Phys. Lett. 72, 182-184.]), who have reported three forms of the tetragonal phase (t, t′, t′′), all belonging to the space group P42/nmc. The stable form of the tetragonal phase is called the t form and is restricted to the solubility limit in the equilibrium phase diagram (Yashima et al., 1993a[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993a). J. Am. Ceram. Soc. 76, 1745-1750.],b[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993b). J. Am. Ceram. Soc. 76, 2865-2868.]; Yashima, Arashi, Kakihana & Yoshimura, 1994[Yashima, M., Arashi, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1067-1071.]; Yashima, Takashina, Kakihana & Yoshimura, 1994[Yashima, M., Takashina, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1869-1874.]). The t′ form is metastable and unstable compared with the coexistence of the t form and cubic (c) phase. The t′ form has an axial ratio larger than unity, that is, ct/(21/2at) > 1, where ct and at are the unit-cell parameters c and a of the t′ form (Fig. 1[link]). The t′′ form is another metastable tetragonal phase with an axial ratio of unity [ct/(21/2at)  =  1]. The t′′ form is designated as a tetragonal symmetry owing to the displacement of O atoms along the c axis from the regular position in the cubic phase (Fig. 1[link]).

[Figure 1]
Figure 1
Crystal structure of metastable tetragonal (t′) CeZrO4 solid solution and relationship between tetragonal cell and pseudo-cubic fluorite (F) cell. Filled and shaded circles denote cations and anions, respectively. Arrows denote the displacement of O atoms along the c axis. Thick dashed lines denote the primitive cell after Teufer (1962[Teufer, G. (1962). Acta Cryst. 15, 1187.]), and thin solid lines delineate two pseudo-fluorite cells.

Yashima et al. (1993b[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993b). J. Am. Ceram. Soc. 76, 2865-2868.]) investigated the temperature dependence of the ct/(21/2at) ratio in the CeZrO4 solid solution by ex situ X-ray diffraction measurements of quenched samples at room temperature. In the present work the temperature dependence of the unit-cell parameters and axial ratio of the CeZrO4 solid solution are investigated by in situ neutron diffraction measurements at high temperatures. Yashima and co-workers (Yashima, Arashi, Kakihana & Yoshimura, 1994[Yashima, M., Arashi, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1067-1071.]; Yashima, Takashina, Kakihana & Yoshimura, 1994[Yashima, M., Takashina, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1869-1874.]; Yashima et al., 1998[Yashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. & Mori, T. (1998). Appl. Phys. Lett. 72, 182-184.]) suggested that the cubic-to-tetragonal phase transition is induced by the displacement of O atoms (Fig. 1[link]) on the basis of the high-temperature Raman spectra of Ce0.8Zr0.2O2 and ex situ neutron and synchrotron powder diffraction data for Ce1 − xZrxO2 (x  =  0.1, 0.2, 0.3, 0.35, 0.4 and 0.5) obtained at room temperature. As no high-temperature in situ studies of the crystal structure or cubic–tetragonal phase transition have been conducted for the CeZrO4 solid solution, the temperature dependence of the oxygen displacement and atomic displacement parameters remains an unresolved issue. Through the investigation of the temperature dependence of the unit-cell parameters, the axial ratio ct/(21/2at), positional and atomic displacement parameters, and oxygen displacement of the CeZrO4 solid solution, it is demonstrated in this study for the first time that the ct′ transition between 1542 and 1831 K is accompanied by oxygen displacement along the c axis and an increase in the c/aF axial ratio from unity.

2. Experiments and data analysis

Fig. 2[link] shows the synthesis procedure of CeZrO4 powders and pellets. Ce(NO3)3 (99.9% purity) and ZrO(NO3)2 (99.9% purity) aqueous solutions were supplied from Daiichi Kigenso Kagaku Kogyo Co. Ltd. These solutions were mixed at an atomic ratio of Zr:Ce  =  1:1 and added to a 5 mass % ammonia aqueous solution, resulting in the formation of hydroxides containing Ce and Zr species. The resultant precipitates were washed after filtration and then calcined at 1073 K for 3 h. The CeZrO4 powders thus obtained were pressed uniaxially at 17 MPa, and then pressed isostatically into pellets at 98 MPa. The pellets were sintered at 1973 K for 5 h in air and then annealed at 1073 K for 24 h in air to afford a cylindrical product of 19 mm in diameter and 76 mm in height. The CeZrO4 − δ solid solution thus obtained was pale yellow, indicating the stoichiometry to be δ  =  0, where δ denotes the proportion of oxygen vacancies. Chemical analysis by inductively coupled plasma optical-emission spectroscopy (ICP-OES) indicated a small amount of hafnium, corresponding to an average chemical composition of Ce0.4943 (3)(Zr0.993 (2)Hf0.007 (2))0.5057 (3)O2, where the values in parentheses denote the error bar in the last digit.

[Figure 2]
Figure 2
Synthesis procedure for CeZrO4 pellets.

Neutron powder diffraction measurements were performed in air using a 150 detector HERMES system (Ohoyama et al., 1998[Ohoyama, K., Kanouchi, T., Nemoto, K., Ohashi, M., Kajitani, T. & Yamaguchi, Y. (1998). Jpn. J. Appl. Phys. 37, 3319-3326.]) installed at the JRR-3M reactor of the Japan Atomic Energy Agency, Tokai, Japan. Neutrons with wavelength of 1.81430 (7) Å were obtained from the (311) reflection of a Ge monochromator. Diffraction data were collected in the 2θ range 5–155° at intervals of 0.1° over the temperature range 296–1831 K. A furnace equipped with MoSi2 heaters (Yashima, 2002[Yashima, M. (2002). J. Am. Ceram. Soc. 85, 2925-2930.]) was used for the high-temperature neutron diffraction measurements. Sample temperatures were maintained constant during data acquisition.

The crystal structures of the CeZrO4 material were refined by the Rietveld method using the computer program RIETAN-2000 (Izumi & Ikeda, 2000[Izumi, F. & Ikeda, T. (2000). Mater. Sci. Forum, 321-324, 198-203.]). The peak shape was assumed to be a modified split-type pseudo-Voigt function and a cut-off value of 7.00 (Toraya, 1990[Toraya, H. (1990). J. Appl. Cryst. 23, 485-491.]). The background was approximated by a 12-parameter polynomial in 2θn (n  =  0–11). The n parameters were refined simultaneously with the unit-cell, zero-point scale, profile-shape and crystal structural parameters.

3. Results

Figs. 3[link] and 4[link] show the neutron diffraction profiles of the CeZrO4 solid solution measured at 296, 1542 and 1831 K. All reflections are indexed by a tetragonal cell (P42/nmc) between 296 and 1542 K. The peak splitting between the 004t and 220t reflections was clearly observed between 296 and 1542 K (Figs. 3[link]a and b). Here hklt denotes the hkl reflection of the t′ form. The 102t reflection was clearly detected in this temperature range (Fig. 4[link]), allowing the CeZrO4 solid solution to be identified as the single phase of the t′ form with an axial ratio ct/(21/2at) larger than unity [ct/(21/2at) > 1] at temperatures between 296 and 1542 K. All reflections in the neutron diffraction profile measured at 1831 K are indexed by a cubic fluorite-type cell ([Fm\bar 3m]; Fig. 3[link]c). The 400c reflection exhibits a single feature without splitting between the 004t and 220t reflections. No 102t reflection was detected at 1831 K (Fig. 4[link]c). Thus, the CeZrO4 solid solution at 1831 K is identified as having a cubic structure. No impurity phases were detected in the neutron diffraction profiles between 296 and 1831 K. Each peak did not exhibit anisotropic peak broadening. These results indicate that the CeO2 composition in the sample is homogeneous in the whole temperature range. The t′ form is metastable and unstable compared with the stable t + c two-phase coexistence. The stable t and c phases did not form, because the heating rate (ca 10 K min−1) was too high and the measurement time (ca 90 min) was too short for the phase separation to occur.

[Figure 3]
Figure 3
Rietveld fitting patterns for neutron diffraction data of CeZrO4 solid solution measured at (a) 296, (b) 1542 and (c) 1831 K. Crosses and lines denote observed and calculated intensities, respectively. Short vertical lines indicate the positions of possible Bragg reflections of (a), (b) tetragonal and (c) cubic CrZrO4 phases. Lines below the profiles denote the difference between observed and calculated intensities.
[Figure 4]
Figure 4
Neutron diffraction profiles around the 102t reflection of CeZrO4 solid solution measured at (a) 296, (b) 1542 and (c) 1831 K.

Rietveld analysis of the CeZrO4 solid solution was carried out for the tetragonal structure with the P42/nmc space group (Z = 1) at 296–1542 K, where the cation (Ce4+ and Zr4+) and anion (O2−) were placed at the special positions of 2(a) 0,0,0 and 4(d) 0,1/2,z, respectively. Data at 1831 K were analyzed assuming the cubic fluorite-type structure with the space group [Fm\bar 3m] (Z = 2), where the cation (Ce4+ and Zr4+) and anion (O2−) were placed at 4(a) 0,0,0 and 8(c) 1/4,1/4,1/4, respectively. In a preliminary analysis, the occupancy factor of the O atom g(O) was refined. The refined g(O) was unity within the error bar, thus we fixed the g(O) to be unity in the final refinement. The calculated profile is in good agreement with the observed data (Fig. 3[link]). Table 1[link]1 lists the crystal parameters and reliability factors in the Rietveld analyses of the neutron diffraction data for the CeZrO4 solid solution between 296 and 1831 K. The refined axial ratio ct/(21/2at) at room temperature was 1.0088 (1), consistent with that reported previously (Yashima et al., 1998[Yashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. & Mori, T. (1998). Appl. Phys. Lett. 72, 182-184.]). The refined fractional coordinate z of oxygen atoms z(O) was 0.2189 (2) at room temperature, which is also consistent with the results of Yashima et al. (1998[Yashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. & Mori, T. (1998). Appl. Phys. Lett. 72, 182-184.]) and Lamas et al. (2005[Lamas, D. G., Fuentes, R. O., Fabregas, I. O., Fernandez de Rapp, M. E., Lascalea, G. E., Casanova, J. R., Walsoe de Reca, N. E. & Craievich, A. F. (2005). J. Appl. Cryst. 38, 867-873.]) within ±3 standard deviations (σ) of z(O).

Table 1
Crystal parameters and reliability factors of Rietveld analysis of CeZrO4 solid solution measured at different temperatures

Mr = 295.40, θmin = 25°, θmax = 150°.

Temperature (K)   296 507 772 1034 1291 1542 1831
Space group   P42/nmc P42/nmc P42/nmc P42/nmc P42/nmc P42/nmc [Fm\bar 3m]
Unit-cell parameters a (Å) 3.7191 (2) 3.7286 (2) 3.7386 (3) 3.7503 (3) 3.7640 (3) 3.7781 (4) 5.3848 (6)
c (Å) 5.3057 (4) 5.3215 (4) 5.3381 (4) 5.3564 (5) 5.3745 (5) 5.3833 (7)  
Ce, Zr B2) 0.62 (4) 0.90 (4) 1.15 (4) 1.58 (5) 2.07 (5) 2.61 (7) 2.49 (6)
O g (O) 1.0 1.0 1.0 1.0 1.0 1.0 1.0
  B2) 1.18 (3) 1.59 (4) 2.01 (4) 2.63 (5) 3.49 (6) 4.64 (7) 6.05 (8)
  z (O) 0.2190 (2) 0.2184 (2) 0.2179 (2) 0.2170 (2) 0.2170 (2) 0.2199 (3) 0.25
Reliability factors in the Rietveld analysis Rwp§ 0.0879 0.0787 0.0834 0.0749 0.0685 0.0720 0.0821
Rp§ 0.0628 0.0573 0.0618 0.0563 0.0514 0.0551 0.0605
GoF§ 4.91 4.40 4.70 4.74 4.88 5.69 6.40
RI§ 0.0483 0.0512 0.0587 0.0553 0.0562 0.0582 0.0797
RF§ 0.0325 0.0350 0.0394 0.0353 0.0343 0.0378 0.0441
V3)   73.39 (1) 73.98 (1) 74.61 (1) 75.34 (1) 76.14 (1) 76.84 (2) 156.13 (3)
Z   1 1 1 1 1 1 2
No. of parameters   32 32 32 32 32 32 27
B: isotropic atomic displacement parameter.
z(O): fractional coordinate z of O atoms.
§Standard Rietveld agreement index (Young et al., 1982[Young, R. A., Prince, E. & Sparks, R. A. (1982). J. Appl. Cryst. 15, 357-359.]).

The unit-cell parameters a and c of the CeZrO4 solid solution increased with temperature, coinciding between 1542 and 1831 K because of the t′–c transformation (Fig. 5[link]). The axial ratio ct/(21/2at) of the metastable t′-CeZrO4 increased slightly from 296 to 1034 K, and decreased from 1291 to 1542 K (Fig. 6[link]). The axial ratio became unity between 1542 and 1831 K, corresponding to the t′–c phase transition. The fractional coordinate z(O) of CeZrO4 decreased from 296 to 1034 K, and increased from 1291 K, reaching 1/4 between 1542 and 1831 K (Fig. 7[link]). The oxygen displacement d(O) from the regular 8(c) position of the cubic fluorite-type structure can be estimated by the equation d(O)  =  c[0.25 − z(O)]. The d(O) value of CeZrO4 increased slightly from 296 to 1034 K, and then decreased to 0.0 Å between 1542 and 1831 K (Fig. 8[link]), corresponding to the t′–c phase transition. The isotropic atomic displacement parameters of Ce and Zr atoms B(Ce,Zr) and O atoms B(O) increased with temperature (Fig. 9[link]). B(O) was larger than B(Ce,Zr), suggesting the higher diffusivity of oxygen ions. This is consistent with the relation of B(Ce,Zr) < B(O) in Ce1 − xZrxO2 at room temperature (Lamas et al., 2005[Lamas, D. G., Fuentes, R. O., Fabregas, I. O., Fernandez de Rapp, M. E., Lascalea, G. E., Casanova, J. R., Walsoe de Reca, N. E. & Craievich, A. F. (2005). J. Appl. Cryst. 38, 867-873.]) and the diffusivity experiments in the literature (Yashima et al., 1993a[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993a). J. Am. Ceram. Soc. 76, 1745-1750.]; Yashima, Kakihara & Yoshimura, 1996[Yashima, M., Kakihana, M. & Yoshimura, M. (1996). Solid State Ion. 86-88, 1131-1149.]).

[Figure 5]
Figure 5
Temperature dependence of the unit-cell parameters of CeZrO4 solid solution. Open circles and squares denote the unit-cell parameters 21/2at and ct of the metastable tetragonal t′ form, respectively. The filled circle denotes the unit-cell parameter ac of the cubic phase. Error bars of standard uncertainties in the Rietveld analysis are smaller than the datum symbols. The solid line is the least-squares fit (quadratic polynomial) and the dashed line is a guide to the eye.
[Figure 6]
Figure 6
Relationship between temperature and axial ratio ct/aF for CeZrO4 solid solution. Open and filled circles denote the ct/aF ratios of the t′ and c phases, respectively. The solid line is a least-squares fit (polynomial) and the dashed line is a guide to the eye.
[Figure 7]
Figure 7
Temperature dependence of the fractional coordinate z of O atoms in CeZrO4 solid solution. Open circles denote t′-CeZrO4 and the solid circle denotes c-CeZrO4. Error bars denote the standard uncertainties in the Rietveld analysis. The solid line is a least-squares fit (polynomial) and the dashed line is a guide to the eye.
[Figure 8]
Figure 8
Temperature dependence of the oxygen displacement of CeZrO4 solid solution. Open circles denote t′-CeZrO4 and the solid circle denotes c-CeZrO4. Error bars were estimated from the standard uncertainties of c and z obtained in the Rietveld analysis. The solid line is a least-squares fit (polynomial) and the dashed line is a guide to the eye.
[Figure 9]
Figure 9
Temperature dependence of isotropic atomic displacement parameters of Ce and Zr atoms [B(Ce, Zr)] and O atoms [B(O)] in CeZrO4 solid solution. Open circles denote B(Ce,Zr) and triangles denote B(O) for t′-CeZrO4. Filled symbols denote c-CeZrO4. The solid line is a least-squares fit (quadratic polynomial) and the dashed line is a guide to the eye.

4. Discussion

The present work based on in situ experiments has demonstrated that the ctphase transition of the CeZrO4 solid solution occurs between 1542 and 1831 K (Figs. 3–8[link][link][link][link][link][link]). The ct′ transition temperature between 1542 and 1831 K is consistent with the previous ex situ study (Yashima et al., 1993b[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993b). J. Am. Ceram. Soc. 76, 2865-2868.]). The ct/(21/2at) value is 1.0 for the cubic phase and increases with decreasing temperature below the ctphase transition temperature. This indicates that the ctphase transition is accompanied by the increase of c/aF ratio from unity. The present in situ experiments also revealed the temperature dependence of the atomic coordinate z(O) and oxygen displacement d(O) for the first time (Figs. 7[link] and 8[link]). The d(O) value is 0.0 Å for the cubic phase and increases with decreasing temperature below the ct′ phase-transition temperature. This indicates that the ctphase transition is accompanied by oxygen displacement. Yashima et al. suggested oxygen-induced structural change as a mechanism for the ct′ transformation in ZrO2–CeO2 systems (Yashima et al., 1993a[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993a). J. Am. Ceram. Soc. 76, 1745-1750.],b[Yashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993b). J. Am. Ceram. Soc. 76, 2865-2868.]; Yashima, Arashi, Kakihana & Yoshimura, 1994[Yashima, M., Arashi, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1067-1071.]; Yashima, Takashina, Kakihana & Yoshimura, 1994[Yashima, M., Takashina, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1869-1874.]; Yashima et al., 1998[Yashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. & Mori, T. (1998). Appl. Phys. Lett. 72, 182-184.]) and ZrO2RO1.5 (R  =  Y, Nd, Sm, Er, Yb) systems (Yashima, Ohtake, Kakihana, Arashi & Yoshimura, 1996[Yashima, M., Ohtake, K., Kakihana, M., Arashi, H. & Yoshimura, M. (1996). J. Phys. Chem. Solids, 57, 17-24.]) on the basis of data acquired at room temperature. Yashima, Arashi, Kakihana & Yoshimura (1994[Yashima, M., Arashi, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1067-1071.]) also suggested such oxygen-induced behavior in the ZrO2–CeO2 system through in situ Raman studies. The present work has provided direct evidence of the temperature dependence of oxygen displacement d(O) as an oxygen-induced structural change responsible for the ct′ transition.

5. Conclusions

The crystal structure of the compositionally homogenous CeZrO4 solid solution was investigated by in situ neutron powder diffraction analysis and Rietveld refinement over a temperature range of 296 to 1831 K. The CeZrO4 solid solution was found to transform from the tetragonal t′ phase to the cubic phase between 1542 and 1831 K, accompanied by an increase in the isotropic atomic displacement parameters B(Ce,Zr) and B(O) with increasing temperature. B(O) was found to be larger than B(Ce,Zr), suggesting a higher diffusivity of oxygen ions. The axial ratio c/aF of the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching unity between 1542 and 1831 K. The displacement of O atoms along the c axis in the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching 0.00 between 1542 and 1831 K. These results indicate that the tetragonal-to-cubic phase transition is accompanied by oxygen displacement along the c axis and the increase of the c/aF axial ratio from unity.

Supporting information


Computing details top

For all compounds, cell refinement: RIETAN-2000 (Izumi and Ikeda, 2000); program(s) used to solve structure: RIETAN-2000; program(s) used to refine structure: RIETAN-2000; software used to prepare material for publication: VICS (Izumi and Dilanian, 2002).

Figures top
[Figure 1]
[Figure 2]
[Figure 3]
[Figure 4]
[Figure 5]
[Figure 6]
[Figure 7]
[Figure 8]
[Figure 9]
(296K) cerium zirconiuim oxide top
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Dx = 6.684 Mg m3
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Tetragonal, P42/nmcT = 296 K
a = 3.7191 (2) Åyellow
c = 5.3057 (4) Åcylinder, 76 × 19 mm
V = 73.39 (1) Å3Specimen preparation: Prepared at 1973 K and 100 kPa
Z = 2
Data collection top
150 detector system HERMES
diffractometer
Data collection mode: transmission
Radiation source: neutronScan method: step
None monochromator2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Specimen mounting: Polycrystalline pellets without container
Refinement top
Rp = ?Profile function: pseudo-Voigt
Rwp = 0.08832 parameters
Rexp = 0.02
χ2 = NOT FOUNDBackground function: 12-parameter polynomial
1500 data pointsPreferred orientation correction: no
Excluded region(s): 5-25, 150-155
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022V = 73.39 (1) Å3
Mr = 147.70Z = 2
Tetragonal, P42/nmcNeutron radiation, λ = 1.81430 (7) Å
a = 3.7191 (2) ÅT = 296 K
c = 5.3057 (4) Åcylinder, 76 × 19 mm
Data collection top
150 detector system HERMES
diffractometer
Scan method: step
Specimen mounting: Polycrystalline pellets without container2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Data collection mode: transmission
Refinement top
Rp = ?χ2 = NOT FOUND
Rwp = 0.0881500 data points
Rexp = 0.0232 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ce0000.0079 (5)*0.4943
Zr0000.0079 (5)*0.5022
Hf0000.0079 (5)*0.0035
O00.500000.2190 (2)0.0150 (4)*
Geometric parameters (Å, º) top
Ce—O2.1928 (6)Zr—O2.1928 (6)
Ce—O—Zr115.99 (9)
(507K) cerium zirconiuim oxide top
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Dx = 6.630 Mg m3
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Tetragonal, P42/nmcT = 507 K
a = 3.7286 (2) Åyellow
c = 5.3215 (4) Åcylinder, 76 × 19 mm
V = 73.98 (1) Å3Specimen preparation: Prepared at 1973 K and 100 kPa
Z = 2
Data collection top
150 detector system HERMES
diffractometer
Data collection mode: transmission
Radiation source: neutronScan method: step
None monochromator2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Specimen mounting: Polycrystalline pellets without container
Refinement top
Rp = ?Profile function: pseudo-Voigt
Rwp = 0.07932 parameters
Rexp = 0.02
χ2 = NOT FOUNDBackground function: 12-parameter polynomial
1500 data pointsPreferred orientation correction: no
Excluded region(s): 5-25, 150-155
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022V = 73.98 (1) Å3
Mr = 147.70Z = 2
Tetragonal, P42/nmcNeutron radiation, λ = 1.81430 (7) Å
a = 3.7286 (2) ÅT = 507 K
c = 5.3215 (4) Åcylinder, 76 × 19 mm
Data collection top
150 detector system HERMES
diffractometer
Scan method: step
Specimen mounting: Polycrystalline pellets without container2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Data collection mode: transmission
Refinement top
Rp = ?χ2 = NOT FOUND
Rwp = 0.0791500 data points
Rexp = 0.0232 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ce0000.0114 (5)*0.4943
Zr0000.0114 (5)*0.5022
Hf0000.0114 (5)*0.0035
O00.500000.2184 (2)0.0201 (5)*
Geometric parameters (Å, º) top
Ce—O2.1970 (6)Zr—O2.1970 (6)
Ce—O—Zr116.11 (9)
(772K) cerium zirconiuim oxide top
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Dx = 6.574 Mg m3
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Tetragonal, P42/nmcT = 772 K
a = 3.7386 (3) Åyellow
c = 5.3381 (4) Åcylinder, 76 × 19 mm
V = 74.61 (1) Å3Specimen preparation: Prepared at 1973 K and 100 kPa
Z = 2
Data collection top
150 detector system HERMES
diffractometer
Data collection mode: transmission
Radiation source: neutronScan method: step
None monochromator2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Specimen mounting: Polycrystalline pellets without container
Refinement top
Rp = ?Profile function: pseudo-Voigt
Rwp = 0.08332 parameters
Rexp = 0.02
χ2 = NOT FOUNDBackground function: 12-parameter polynomial
1500 data pointsPreferred orientation correction: no
Excluded region(s): 5-25, 150-155
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022V = 74.61 (1) Å3
Mr = 147.70Z = 2
Tetragonal, P42/nmcNeutron radiation, λ = 1.81430 (7) Å
a = 3.7386 (3) ÅT = 772 K
c = 5.3381 (4) Åcylinder, 76 × 19 mm
Data collection top
150 detector system HERMES
diffractometer
Scan method: step
Specimen mounting: Polycrystalline pellets without container2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Data collection mode: transmission
Refinement top
Rp = ?χ2 = NOT FOUND
Rwp = 0.0831500 data points
Rexp = 0.0232 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ce0000.0146 (6)*0.4943
Zr0000.0146 (6)*0.5022
Hf0000.0146 (6)*0.0035
O00.500000.2179 (2)0.0254 (6)*
Geometric parameters (Å, º) top
Ce—O2.2018 (7)Zr—O2.2018 (7)
Ce—O—Zr116.21 (9)
(1034K) cerium zirconiuim oxide top
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Dx = 6.511 Mg m3
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Tetragonal, P42/nmcT = 1034 K
a = 3.7503 (3) Åyellow
c = 5.3564 (5) Åcylinder, 76 × 19 mm
V = 75.34 (1) Å3Specimen preparation: Prepared at 1973 K and 100 kPa
Z = 2
Data collection top
150 detector system HERMES
diffractometer
Data collection mode: transmission
Radiation source: neutronScan method: step
None monochromator2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Specimen mounting: Polycrystalline pellets without container
Refinement top
Rp = ?Profile function: pseudo-Voigt
Rwp = 0.07532 parameters
Rexp = 0.02
χ2 = NOT FOUNDBackground function: 12-parameter polynomial
1500 data pointsPreferred orientation correction: no
Excluded region(s): 5-25, 150-155
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022V = 75.34 (1) Å3
Mr = 147.70Z = 2
Tetragonal, P42/nmcNeutron radiation, λ = 1.81430 (7) Å
a = 3.7503 (3) ÅT = 1034 K
c = 5.3564 (5) Åcylinder, 76 × 19 mm
Data collection top
150 detector system HERMES
diffractometer
Scan method: step
Specimen mounting: Polycrystalline pellets without container2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Data collection mode: transmission
Refinement top
Rp = ?χ2 = NOT FOUND
Rwp = 0.0751500 data points
Rexp = 0.0232 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ce0000.0200 (6)*0.4943
Zr0000.0200 (6)*0.5022
Hf0000.0200 (6)*0.0035
O00.500000.2170 (2)0.0333 (6)*
Geometric parameters (Å, º) top
Ce—O2.2063 (8)Zr—O2.2063 (8)
Ce—O—Zr116.40 (10)
(1291K) cerium zirconiuim oxide top
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Dx = 6.442 Mg m3
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Tetragonal, P42/nmcT = 1291 K
a = 3.7640 (3) Åyellow
c = 5.3745 (5) Åcylinder, 76 × 19 mm
V = 76.14 (1) Å3Specimen preparation: Prepared at 1973 K and 100 kPa
Z = 2
Data collection top
150 detector system HERMES
diffractometer
Data collection mode: transmission
Radiation source: neutronScan method: step
None monochromator2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Specimen mounting: Polycrystalline pellets without container
Refinement top
Rp = ?Profile function: pseudo-Voigt
Rwp = 0.06932 parameters
Rexp = 0.02
χ2 = NOT FOUNDBackground function: 12-parameter polynomial
1500 data pointsPreferred orientation correction: no
Excluded region(s): 5-25, 150-155
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022V = 76.14 (1) Å3
Mr = 147.70Z = 2
Tetragonal, P42/nmcNeutron radiation, λ = 1.81430 (7) Å
a = 3.7640 (3) ÅT = 1291 K
c = 5.3745 (5) Åcylinder, 76 × 19 mm
Data collection top
150 detector system HERMES
diffractometer
Scan method: step
Specimen mounting: Polycrystalline pellets without container2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Data collection mode: transmission
Refinement top
Rp = ?χ2 = NOT FOUND
Rwp = 0.0691500 data points
Rexp = 0.0232 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ce0000.0262 (7)*0.4943
Zr0000.0262 (7)*0.5022
Hf0000.0262 (7)*0.0035
O00.500000.2170 (2)0.0442 (7)*
Geometric parameters (Å, º) top
Ce—O2.2142 (8)Zr—O2.2142 (8)
Ce—O—Zr116.41 (10)
(1542K) cerium zirconiuim oxide top
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Dx = 6.383 Mg m3
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Tetragonal, P42/nmcT = 1542 K
a = 3.7781 (4) Åyellow
c = 5.3833 (7) Åcylinder, 76 × 19 mm
V = 76.84 (2) Å3Specimen preparation: Prepared at 1973 K and 100 kPa
Z = 2
Data collection top
150 detector system HERMES
diffractometer
Data collection mode: transmission
Radiation source: neutronScan method: step
None monochromator2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Specimen mounting: Polycrystalline pellets without container
Refinement top
Rp = ?Profile function: pseudo-Voigt
Rwp = 0.07232 parameters
Rexp = 0.02
χ2 = NOT FOUNDBackground function: 12-parameter polynomial
1500 data pointsPreferred orientation correction: no
Excluded region(s): 5-25, 150-155
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022V = 76.84 (2) Å3
Mr = 147.70Z = 2
Tetragonal, P42/nmcNeutron radiation, λ = 1.81430 (7) Å
a = 3.7781 (4) ÅT = 1542 K
c = 5.3833 (7) Åcylinder, 76 × 19 mm
Data collection top
150 detector system HERMES
diffractometer
Scan method: step
Specimen mounting: Polycrystalline pellets without container2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Data collection mode: transmission
Refinement top
Rp = ?χ2 = NOT FOUND
Rwp = 0.0721500 data points
Rexp = 0.0232 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ce0000.0330 (9)*0.4943
Zr0000.0330 (9)*0.5022
Hf0000.0330 (9)*0.0035
O00.500000.2199 (3)0.0587 (10)*
Geometric parameters (Å, º) top
Ce—O2.2296 (11)Zr—O2.2296 (11)
Ce—O—Zr115.83 (12)
(1831K) cerium zirconiuim oxide top
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Dx = 6.283 Mg m3
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Cubic, Fm3mT = 1831 K
a = 5.3848 (7) Åyellow
V = 156.13 (3) Å3cylinder, 76 × 19 mm
Z = 4Specimen preparation: Prepared at 1973 K and 100 kPa
Data collection top
150 detector system HERMES
diffractometer
Data collection mode: transmission
Radiation source: neutronScan method: step
None monochromator2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Specimen mounting: Polycrystalline pellets without container
Refinement top
Rp = ?Profile function: pseudo-Voigt
Rwp = 0.08227 parameters
Rexp = 0.02
χ2 = NOT FOUNDBackground function: 12-parameter polynomial
1500 data pointsPreferred orientation correction: no
Excluded region(s): 5-25, 150-155
Crystal data top
Ce0.4943Hf0.0035O2Zr0.5022Z = 4
Mr = 147.70Neutron radiation, λ = 1.81430 (7) Å
Cubic, Fm3mT = 1831 K
a = 5.3848 (7) Åcylinder, 76 × 19 mm
V = 156.13 (3) Å3
Data collection top
150 detector system HERMES
diffractometer
Scan method: step
Specimen mounting: Polycrystalline pellets without container2θmin = 5°, 2θmax = 155°, 2θstep = 0.1°
Data collection mode: transmission
Refinement top
Rp = ?χ2 = NOT FOUND
Rwp = 0.0821500 data points
Rexp = 0.0227 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ce0000.0315 (7)*0.4943
Zr0000.0315 (7)*0.5022
Hf0000.0315 (7)*0.0035
O0.250.250.250.0766 (11)*
Geometric parameters (Å, º) top
Ce—O2.3317 (2)Zr—O2.3317 (2)
Ce—O—Zr109.4712 (1)

Experimental details

(296K)(507K)(772K)(1034K)
Crystal data
Chemical formulaCe0.4943Hf0.0035O2Zr0.5022Ce0.4943Hf0.0035O2Zr0.5022Ce0.4943Hf0.0035O2Zr0.5022Ce0.4943Hf0.0035O2Zr0.5022
Mr147.70147.70147.70147.70
Crystal system, space groupTetragonal, P42/nmcTetragonal, P42/nmcTetragonal, P42/nmcTetragonal, P42/nmc
Temperature (K)2965077721034
a, b, c (Å)3.7191 (2), 3.7191 (2), 5.3057 (4)3.7286 (2), 3.7286 (2), 5.3215 (4)3.7386 (3), 3.7386 (3), 5.3381 (4)3.7503 (3), 3.7503 (3), 5.3564 (5)
α, β, γ (°)90, 90, 9090, 90, 9090, 90, 9090, 90, 90
V3)73.39 (1)73.98 (1)74.61 (1)75.34 (1)
Z2222
Radiation typeNeutron, λ = 1.81430 (7) ÅNeutron, λ = 1.81430 (7) ÅNeutron, λ = 1.81430 (7) ÅNeutron, λ = 1.81430 (7) Å
µ (mm1)????
Specimen shape, size (mm)Cylinder, 76 × 19Cylinder, 76 × 19Cylinder, 76 × 19Cylinder, 76 × 19
Data collection
Diffractometer150 detector system HERMES
diffractometer
150 detector system HERMES
diffractometer
150 detector system HERMES
diffractometer
150 detector system HERMES
diffractometer
Specimen mountingPolycrystalline pellets without containerPolycrystalline pellets without containerPolycrystalline pellets without containerPolycrystalline pellets without container
Data collection modeTransmissionTransmissionTransmissionTransmission
Scan methodStepStepStepStep
2θ values (°)2θmin = 5 2θmax = 155 2θstep = 0.12θmin = 5 2θmax = 155 2θstep = 0.12θmin = 5 2θmax = 155 2θstep = 0.12θmin = 5 2θmax = 155 2θstep = 0.1
Refinement
R factors and goodness of fitRp = ?, Rwp = 0.088, Rexp = 0.02, χ2 = NOT FOUNDRp = ?, Rwp = 0.079, Rexp = 0.02, χ2 = NOT FOUNDRp = ?, Rwp = 0.083, Rexp = 0.02, χ2 = NOT FOUNDRp = ?, Rwp = 0.075, Rexp = 0.02, χ2 = NOT FOUND
No. of data points1500150015001500
No. of parameters32323232
No. of restraints????


(1291K)(1542K)(1831K)
Crystal data
Chemical formulaCe0.4943Hf0.0035O2Zr0.5022Ce0.4943Hf0.0035O2Zr0.5022Ce0.4943Hf0.0035O2Zr0.5022
Mr147.70147.70147.70
Crystal system, space groupTetragonal, P42/nmcTetragonal, P42/nmcCubic, Fm3m
Temperature (K)129115421831
a, b, c (Å)3.7640 (3), 3.7640 (3), 5.3745 (5)3.7781 (4), 3.7781 (4), 5.3833 (7)5.3848 (7), 5.3848 (7), 5.3848 (7)
α, β, γ (°)90, 90, 9090, 90, 9090, 90, 90
V3)76.14 (1)76.84 (2)156.13 (3)
Z224
Radiation typeNeutron, λ = 1.81430 (7) ÅNeutron, λ = 1.81430 (7) ÅNeutron, λ = 1.81430 (7) Å
µ (mm1)???
Specimen shape, size (mm)Cylinder, 76 × 19Cylinder, 76 × 19Cylinder, 76 × 19
Data collection
Diffractometer150 detector system HERMES
diffractometer
150 detector system HERMES
diffractometer
150 detector system HERMES
diffractometer
Specimen mountingPolycrystalline pellets without containerPolycrystalline pellets without containerPolycrystalline pellets without container
Data collection modeTransmissionTransmissionTransmission
Scan methodStepStepStep
2θ values (°)2θmin = 5 2θmax = 155 2θstep = 0.12θmin = 5 2θmax = 155 2θstep = 0.12θmin = 5 2θmax = 155 2θstep = 0.1
Refinement
R factors and goodness of fitRp = ?, Rwp = 0.069, Rexp = 0.02, χ2 = NOT FOUNDRp = ?, Rwp = 0.072, Rexp = 0.02, χ2 = NOT FOUNDRp = ?, Rwp = 0.082, Rexp = 0.02, χ2 = NOT FOUND
No. of data points150015001500
No. of parameters323227
No. of restraints???

Computer programs: RIETAN-2000 (Izumi and Ikeda, 2000), RIETAN-2000, VICS (Izumi and Dilanian, 2002).

 

Footnotes

1Supplementary data for this paper are available from the IUCr electronic archives (Reference: OG5020 ). Services for accessing these data are described at the back of the journal.

Acknowledgements

This research was supported in part by the Ministry of Education, Culture, Sport, Science and Technology of Japan through a Grant-in-Aid for Scientific Research (B). The authors would like to thank to Dr K. Ohoyama and Mr K. Nemoto (Tohoku University) for facilitating neutron diffraction measurements, and Dr R. Ali (National Institute for Materials Science), Mr Y. Ando, Mr T. Tsuji, Mr Q. Xu, Mr T. Ueda and Mr Y. Kawaike (Tokyo Institute of Technology) for experimental assistance in the neutron powder diffraction measurements.

References

First citationEnzo, S., Delogu, F., Frattini, R., Primavera, A. & Trovarelli, A. (2000). J. Mater. Res. 15, 1538–1545.  Web of Science CrossRef CAS Google Scholar
First citationInaba, H. & Tagawa, H. (1996). Solid State Ion. 83, 1–16.  CrossRef CAS Web of Science Google Scholar
First citationIzumi, F. & Ikeda, T. (2000). Mater. Sci. Forum, 321–324, 198–203.  CrossRef CAS Google Scholar
First citationKaspar, J., Fornasiero, P., Balducci, G., DiMonte, R., Hickey, N. & Sergo, V. (2003). Inorg. Chim. Acta, 349, 217–226.  Web of Science CrossRef CAS Google Scholar
First citationLamas, D. G., Fuentes, R. O., Fabregas, I. O., Fernandez de Rapp, M. E., Lascalea, G. E., Casanova, J. R., Walsoe de Reca, N. E. & Craievich, A. F. (2005). J. Appl. Cryst. 38, 867–873.  Web of Science CrossRef CAS IUCr Journals Google Scholar
First citationMeriani, S. (1985). J. Mater. Sci. Eng. 71, 369–370.  CrossRef Web of Science Google Scholar
First citationMurota, T., Hasegawa, T., Aozasa, S., Matsui, H. & Motoyama, M. (1993). J. Alloys Compd. 193, 298–299.  CrossRef CAS Web of Science Google Scholar
First citationOhoyama, K., Kanouchi, T., Nemoto, K., Ohashi, M., Kajitani, T. & Yamaguchi, Y. (1998). Jpn. J. Appl. Phys. 37, 3319–3326.  Web of Science CrossRef CAS Google Scholar
First citationOmata, T., Kishimoto, H., Otsuka-Yao-Matsuo, S., Ohtori, N. & Umesaki, N. (1999). J. Solid State Chem. 147, 573–583.  Web of Science CrossRef CAS Google Scholar
First citationOzawa, M., Kimura, M. & Isogai, A. (1993). J. Alloys Compd. 193, 73–75.  CrossRef CAS Web of Science Google Scholar
First citationSuda, A., Sobukawa, H., Suzuki, T., Kandori, T., Ukyo, Y. & Sugiura, M. (2001). J. Ceram. Soc. Jpn, 109, 177–180.  CrossRef CAS Google Scholar
First citationTeufer, G. (1962). Acta Cryst. 15, 1187.  CrossRef IUCr Journals Web of Science Google Scholar
First citationToraya, H. (1990). J. Appl. Cryst. 23, 485–491.  CrossRef CAS Web of Science IUCr Journals Google Scholar
First citationTrovarelli, A. (1996). Catal. Rev. Sci. Eng. 38, 439–520.  CrossRef CAS Google Scholar
First citationTrovarelli, A., Zamar, F., Llorca, J., Leitenburg, C., Dolcetti, G. & Kiss, J. T. (1997). J. Catal. 169, 490–502.  CrossRef CAS Web of Science Google Scholar
First citationTsukuma, K. & Shimada, M. (1985). J. Mater. Sci. 20, 1178–1184.  CrossRef CAS Web of Science Google Scholar
First citationYao, H. C. & Yao, T. F. Yu (1984). J. Catal. 86, 254–265.  CrossRef CAS Web of Science Google Scholar
First citationYashima, M. (2002). J. Am. Ceram. Soc. 85, 2925–2930.  CrossRef CAS Google Scholar
First citationYashima, M., Arashi, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1067–1071.  CrossRef CAS Web of Science Google Scholar
First citationYashima, M., Kakihana, M. & Yoshimura, M. (1996). Solid State Ion. 86–88, 1131–1149.  CrossRef CAS Google Scholar
First citationYashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993a). J. Am. Ceram. Soc. 76, 1745–1750.  CrossRef CAS Web of Science Google Scholar
First citationYashima, M., Morimoto, K., Ishizawa, N. & Yoshimura, M. (1993b). J. Am. Ceram. Soc. 76, 2865–2868.  CrossRef CAS Web of Science Google Scholar
First citationYashima, M., Ohtake, K., Kakihana, M., Arashi, H. & Yoshimura, M. (1996). J. Phys. Chem. Solids, 57, 17–24.  CrossRef Web of Science Google Scholar
First citationYashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. & Mori, T. (1998). Appl. Phys. Lett. 72, 182–184.  Web of Science CrossRef CAS Google Scholar
First citationYashima, M., Takashina, H., Kakihana, M. & Yoshimura, M. (1994). J. Am. Ceram. Soc. 77, 1869–1874.  CrossRef CAS Web of Science Google Scholar
First citationYoung, R. A., Prince, E. & Sparks, R. A. (1982). J. Appl. Cryst. 15, 357–359.  CrossRef Web of Science IUCr Journals Google Scholar
First citationZhang, F., Chen, C.-H., Hanson, J. C., Robinson, R. D., Herman, I. P. & Chan, S.-W. (2006). J. Am. Ceram. Soc. 89, 1028–1036.  Web of Science CrossRef CAS Google Scholar

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