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Novel valence state of cerium in Ce2Zr2O7.5 elucidated by electron energy-loss spectroscopy under electron channeling conditions
Solid State Communications 135 (2005) 664–667 www.elsevier.com/locate/ssc
Novel valence state of cerium in Ce2Zr2O7.5 elucidated by electron energy-loss spectroscopy under electron channeling conditions Shigeo Araia, Shunsuke Mutob,*, Tsuyoshi Sasakic, Kazuyoshi Tatsumib, Yoshio Ukyoc, Kotaro Kurodad, Hiroyasu Sakad a
1MV Electron Microscopy Laboratory, Eco Topia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan c Toyota Central R&D Laboratories, Inc., 41-1, Nagakute-cho, Aichi-gun 480-1192, Japan d Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 21 April 2005; received in revised form 15 June 2005; accepted 16 June 2005 by H. Akai Available online 11 July 2005
Abstract Rare earth compounds are known to often exhibit peculiar valence states. In the present study, we show, by electron energy loss spectroscopy (EELS) measurements under electron channeling conditions, the existence of a novel valence state for the cerium atoms in Ce2Zr2O7.5, where the valence state is split into two states, with the ions exhibiting a trivalent character (Ce3C) when facing oxygen planes with oxygen vacancies, and a tetravalent character (Ce4C) when facing oxygen planes with no oxygen vacancies. This result should prompt reconsideration of the conventional concept of the valence fluctuation in rare earth compounds. q 2005 Elsevier Ltd. All rights reserved. PACS: 71.28.Cd; 79.20.Uv; 61.18.Kj Keywords: D. Electron energy loss spectroscopy; D. Valence fluctuation
Valence fluctuation phenomena in heavy Fermion systems are known as a manifestation of the many-body effect of valence electrons in rare earth metal ions. This effect is considered as a mixture of 4fn and 4fnK1 (n: number of valence electrons) ions, the energies of which are nearly degenerate near the Fermi level [1]. The valence fluctuation is caused by either (i) time fluctuation of the 4f charge between the two configurations on the time scale tvf at any given site or (ii) spatial charge ordering due to a periodic arrangement of the rare earth atoms with different valence states [2,3]. Here we show, by electron energy loss spectroscopy (EELS) measurements under channeling conditions, a new third case, where the valence state of
* Corresponding author. Tel.: C81 52 789 5200; fax: C81 52 789 5137. E-mail address: [email protected] (S. Muto).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.06.023
the cerium atoms in Ce2Zr2O7.5, a recently discovered metastable phase [4], splits into two states, with the ions exhibiting a trivalent character (Ce3C) when facing towards oxygen planes with oxygen vacancies, and a tetravalent character (Ce4C) when facing towards oxygen planes with no oxygen vacancies. Ceria-zirconia solid solutions, Ce2Zr2O7Cx (0%x%1), are now used to promote proper functioning of noble metal catalysts that purify automotive emissions by controlling the partial oxygen pressure around the reaction field [5–11]. Ce2Zr2O7 has a pyrochlore structure with cubic symmetry. Its building blocks are cerium coordinated by eight oxygen atoms and zirconium by six oxygen atoms, and these two cations form a NaCl-type ordered structure. Thus, Ce2Zr2O7 intrinsically possesses ordered oxygen vacancies at the nearest neighbor sites of zirconium [8], and hence, can store oxygen to fill oxygen vacancies up to the composition Ce2Zr2O8, maintaining cubic symmetry.
S. Arai et al. / Solid State Communications 135 (2005) 664–667
In previous research, we found that the starting material, Ce2Zr2O7, absorbed oxygen from the ambient atmosphere even at room temperature to form the metastable intermediate phase Ce2Zr2O7.5, where the absorbed oxygen selectively occupies the oxygen vacancy sites in every other {400}Oxygen plane [4,12,13], as shown in Fig. 1. In addition, EELS of this material revealed that the apparent valence of the cerium ion was Ce3C, Ce3.5C, and Ce4C for the Ce2Zr2O7, Ce2Zr2O7.5 and Ce2Zr2O8 phases, respectively [13]. The relative intensity ratio of the white-line in rare earth elements is known to correspond well to the occupancy of the 4f orbitals [14]. Indeed, the intensity ratio of the Ce–M4,5 edge (3d/4f* transition) was 1.25, 1.1 and 0.95 for Ce2Zr2O7 (Ce3C), Ce2Zr2O7.5 (Ce3.5C) and Ce2Zr2O8 (Ce4C), respectively. On the other hand, the valence state of the zirconium ion remained Zr4C for all three cases although the oxygen atoms filling the structural vacancies are located at the nearest neighbor sites of the zirconium [13]. It thus follows that the valence change in the cerium ions compensates for the local charge neutrality associated with the oxygen absorption/release. The valence of the cerium ion in the Ce2Zr2O7.5 intermediate phase can be interpreted as the electronic configuration of the cerium ions fluctuating between the Ce3C and Ce4C configurations with the characteristic time constant tvf [1]. EELS probes the sample on a shorter time scale than the valence
Fig. 1. Crystal structure of Ce2Zr2O7.5 to observe the stacking of the [h00] plane. The A and A 0 planes correspond to the oxygen planes. The A 0 planes contain oxygen vacancies (VO), which are indicated by the open (white) circles. The actual displacements of the atom positions from the rational lattice points are not shown.
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fluctuation time tvf [1] and can thus detect both the 4f1 and 4f0 configurations of cerium. Previously, we showed that the Ce–M4,5 edge profile consisted of the two components from the Ce2Zr2O7 (Ce3C) and Ce2Zr2O8 (Ce4C) phases [13]. The excellent reversible oxygen absorption/emission properties of this material are presumably ascribable to an exquisite combination of the physical properties of zirconia and ceria, since both ZrO2 and CeO2 exhibit oxygen exchange; ZrO2 accompanies the oxygen mobility enhancement associated with the distortion of the ZrO6 polyhedra, while CeO2Kx introduces oxygen vacancies and the changeable valence state of cerium bears the local charge compensation. The intermediate valence state could originate from the anisotropic configuration of the oxygen vacancies about the cerium site [12,13]. Hence, additional information on this peculiar valence state may be obtained by examining the Ce–M4,5 white-line under electron channeling conditions with the high energy electron probe localized on either the oxygen-abundant or -deficient planes. Electron channeling EELS can be applied after the ALCHEMI (atom location by channeling enhanced microanalysis) method, which was first applied to site-selective X-ray microanalysis [15]. The Ce2Zr 2O7 sample was prepared by the coprecipitation method from aqueous solutions containing the corresponding nitrates [16]. A powder precursor of the solid solution with composition Ce/ZrZ50/50 (mole ratio) was reduced in a graphite furnace under a static argon atmosphere at 1673 K for 8 h. The reduced powder was cooled to room temperature, removed from the furnace and exposed to air. This treatment is known to transform the powder precursor into Ce2Zr2O7 [4,17]. In order to obtain b–Ce2Zr2O7.5 powder, the Ce2Zr2O7 powder was exposed to air at room temperature for over 1 year [4,12]. The sample powder was dispersed on a carbon microgrid film, and observed using a 300-kV electron microscope (Hitachi H-9000NAR) equipped with a GATAN imaging filter. A grain of an appropriate size and thickness was selected. The Ce2Zr2O7.5 phase was confirmed by emerging the {002} superlattice reflections and the white-line intensity ratio of the Ce–M4,5 absorption edge [13] because the Ce2Zr2O7.5 phase can revert to the Ce2Zr2O7 phase due to the intense electron illumination. The incident electron beam was focused on the thin edge of the sample, approximately 50–100 nm thick, judging from the thickness contours. A systematic orientation was chosen to observe the stacking sequence ABA 0 BABA 0 B. along the [h00] direction. The A and A 0 planes contain all the oxygen atoms, while the oxygen vacancies entirely exist on the A 0 planes, with all the Ce and Zr atoms on the B planes (Fig. 1). Hence, by recording the EEL spectra under the 200 reflection excitation condition with positive and negative excitation errors, the thickness-averaged electron intensity can be maximized on the atomic plane pairs AB and A 0 B pairs, respectively [18]. The entrance aperture of the EELS
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detector should be placed off axis, because this increases the localization of the inelastic excitation, thus strengthening the channeling effect [18]. Fig. 2(a)–(c) show the calculated electron intensities propagating along the [h00] planes for a negative, zero and a positive excitation error, respectively, based on the two-beam dynamical electron scattering theory [19], in which the atomic positions in the Ce2Zr2O7.5 unit cell were taken from the X-ray diffraction results [20] and the absorption effect was phenomenologically taken into account. The electron mainly propagates along the channels between the A and B planes for the negative excitation error, and between the A 0 and B planes for the positive excitation error. For sZ0 (exact Bragg condition), the electron intensity maxima are located exactly on the cation planes. Fig. 3(a) shows the experimental Ce–M4,5 edge spectra from the fixed area with the 200 reflection excited for a negative, zero and positive excitation error, and the corresponding relative positions of the transmitted beam, Kikuchi lines and entrance aperture of the detector are shown in Fig. 3(b). The approximate valence state can be easily distinguished by the relative height of the M4 and M5 peaks; the M5 peak is higher than M4 for Ce3C, but slightly lower for Ce4C. The relative height of the M4 and M5 edges does change with the diffraction condition, as shown in Fig. 3, suggesting that the apparent valence of the Ce atom fluctuates spatially; the valence of Ce seems to be nearly C4 when the primary electron excites the atomic planes along the fully occupied oxygen planes and their neighboring rows (s!0), but mostly C3 when exciting the atomic planes along the oxygen plane containing oxygen vacancies and their neighboring rows (sO0). At exactly the Bragg condition, where sZ0, Ce exhibits a mixed valence of w3.5, as judged from the relative peak height. The relative white-line peak area ratio, IM4/IM5, was estimated to be 0.99, 1.08 and 1.19 for negative, zero and positive excitation errors, respectively. The atomic structure of Ce2Zr2O7.5 is established by the Rietveld analysis of a precise X-ray diffraction experiment which has single equivalent [20]: the space group is F43m, sites (16e) for cerium and zirconium, and oxygen occupies 4a, 4b, 4c and two 24f sites, as well as a small portion of the 4d site. Cation positions are slightly shifted from their
Fig. 2. Calculated diffracted intensity distributions of electrons with the 200 reflection excited in Ce2Zr2O7.5 (E0Z300 kV, x200/x000Z 0.1, xhkl: extinction distance for the hkl reflection). (a) Negative excitation error (sx200Z0.5). (b) Exact Bragg condition (sZ0). (c) Positive excitation error (sx200ZK0.5). The solid and broken lines represent the A and A 0 oxygen planes in Fig. 1, respectively.
Fig. 3. Ce–M4,5 edge EEL spectra and the corresponding diffraction conditions obtained from the fixed area. (a) Change in the Ce–M4,5 white-line peak area ratio for the excitation error indicated. (b) Corresponding diffraction conditions for the spectra in (a). The solid lines indicate the positions of the set of 200 Kikuchi lines, and the open circles indicate the size and position of the EELS entrance aperture for recording.
rational positions in the h111i directions, and the oxygen 24f sites are significantly displaced in one of the h100i directions. The most stable structure calculated by the ab initio pseudo-potential method coincides almost exactly with experimental results. Thus, the present atomic structure is thought to be highly reliable. Moreover, electron diffraction patterns and high-resolution electron micrographs were also consistent with the X-ray results [4,12], and modulated structure associated with possible local charge ordering or lattice distortions was not observed. Thus the present results cannot be interpreted as two different alternating Ce ion sites because the periodic charge ordering along one of the cubic axes is not compatible with the cubic symmetry. In order for the present double-valence state to be compatible with the experimentally determined crystal symmetry, the valence state of each cerium ion must be split into two along one of the h111i directions, as schematized in Fig. 4, where the electron-deficient h111i direction (trivalent side) in each cerium site always points obliquely to the oxygen-deficient {400} plane, while the direction (tetravalent side) points to electron-abundant h1 1 1i the fully occupied oxygen plane, which is totally consistent with the present experimental results. From the point of view of crystallographic symmetry, each cerium ion has three nearest oxygen vacancies in the h311i directions, separated by 0.41 nm, forming an equilateral triangle in the
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Scientific Research from Japan Society for the Promotion of Science.
References
Fig. 4. (a) Local atom configuration of a Ce ion and oxygen vacancies. (b) The valence states of the Ce ions in the unit cell. Vo and Onew stand for oxygen vacancies and oxygen atoms filled during the transformation from Ce2Zr2O7 to Ce2Zr2O7.5. The bonds connecting the Ce ions and their nearest neighbor oxygen atoms are shown. It is seen that the Ce4C and Ce3C ‘faces’ look towards the neighboring oxygen-abundant and oxygen-deficient planes, respectively. Note that (a) and (b) are viewed from different directions to allow a better view of the local configurations.
{111} crystallographic plane. The cerium ions are shifted slightly towards, rather than away from, the vacancy direction) [20], presumably triangle along its normal (h1 1 1i as a result of the increased Coulomb attraction between the Ce tetravalent side and the nearest oxygen ion. In conclusion, this is the first experimental evidence of a novel valence state for cerium ions, as if they were in a double-faced (Ce3C and Ce4C) state in which the two valencies face in opposite directions. There have been a number of recent reports on mixed-valence states in cerium compounds [21–24] in which the analysis and discussion were restricted to conventional concepts. We examined the charge distributions around the cerium ions with the first principles APWClo (augmented plane-waveClocal orbital) band method [25] and observed only slight anisotropy in the Ce-4f occupied states, compatible with the present results. Hence, the present extraordinary anisotropic state should be understood in the framework of the many-electron system, rather than the one-electron approximation, because the latter does not allow the double-valence state as a ground state. The present results urge us to reconsider the mixedvalence state of cerium ions in other ordered cerium compounds, which may help elucidate the mechanism of the excellent and stable oxygen storage properties of this series of compounds.
Acknowledgements This work is partly supported by a Grant-in-Aid for
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Novel valence state of cerium in Ce2Zr2O7.5 elucidated by electron energy-loss spectroscopy under electron channeling conditions Shigeo Araia, Shunsuke Mutob,*, Tsuyoshi Sasakic, Kazuyoshi Tatsumib, Yoshio Ukyoc, Kotaro Kurodad, Hiroyasu Sakad a
1MV Electron Microscopy Laboratory, Eco Topia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan c Toyota Central R&D Laboratories, Inc., 41-1, Nagakute-cho, Aichi-gun 480-1192, Japan d Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 21 April 2005; received in revised form 15 June 2005; accepted 16 June 2005 by H. Akai Available online 11 July 2005
Abstract Rare earth compounds are known to often exhibit peculiar valence states. In the present study, we show, by electron energy loss spectroscopy (EELS) measurements under electron channeling conditions, the existence of a novel valence state for the cerium atoms in Ce2Zr2O7.5, where the valence state is split into two states, with the ions exhibiting a trivalent character (Ce3C) when facing oxygen planes with oxygen vacancies, and a tetravalent character (Ce4C) when facing oxygen planes with no oxygen vacancies. This result should prompt reconsideration of the conventional concept of the valence fluctuation in rare earth compounds. q 2005 Elsevier Ltd. All rights reserved. PACS: 71.28.Cd; 79.20.Uv; 61.18.Kj Keywords: D. Electron energy loss spectroscopy; D. Valence fluctuation
Valence fluctuation phenomena in heavy Fermion systems are known as a manifestation of the many-body effect of valence electrons in rare earth metal ions. This effect is considered as a mixture of 4fn and 4fnK1 (n: number of valence electrons) ions, the energies of which are nearly degenerate near the Fermi level [1]. The valence fluctuation is caused by either (i) time fluctuation of the 4f charge between the two configurations on the time scale tvf at any given site or (ii) spatial charge ordering due to a periodic arrangement of the rare earth atoms with different valence states [2,3]. Here we show, by electron energy loss spectroscopy (EELS) measurements under channeling conditions, a new third case, where the valence state of
* Corresponding author. Tel.: C81 52 789 5200; fax: C81 52 789 5137. E-mail address: [email protected] (S. Muto).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.06.023
the cerium atoms in Ce2Zr2O7.5, a recently discovered metastable phase [4], splits into two states, with the ions exhibiting a trivalent character (Ce3C) when facing towards oxygen planes with oxygen vacancies, and a tetravalent character (Ce4C) when facing towards oxygen planes with no oxygen vacancies. Ceria-zirconia solid solutions, Ce2Zr2O7Cx (0%x%1), are now used to promote proper functioning of noble metal catalysts that purify automotive emissions by controlling the partial oxygen pressure around the reaction field [5–11]. Ce2Zr2O7 has a pyrochlore structure with cubic symmetry. Its building blocks are cerium coordinated by eight oxygen atoms and zirconium by six oxygen atoms, and these two cations form a NaCl-type ordered structure. Thus, Ce2Zr2O7 intrinsically possesses ordered oxygen vacancies at the nearest neighbor sites of zirconium [8], and hence, can store oxygen to fill oxygen vacancies up to the composition Ce2Zr2O8, maintaining cubic symmetry.
S. Arai et al. / Solid State Communications 135 (2005) 664–667
In previous research, we found that the starting material, Ce2Zr2O7, absorbed oxygen from the ambient atmosphere even at room temperature to form the metastable intermediate phase Ce2Zr2O7.5, where the absorbed oxygen selectively occupies the oxygen vacancy sites in every other {400}Oxygen plane [4,12,13], as shown in Fig. 1. In addition, EELS of this material revealed that the apparent valence of the cerium ion was Ce3C, Ce3.5C, and Ce4C for the Ce2Zr2O7, Ce2Zr2O7.5 and Ce2Zr2O8 phases, respectively [13]. The relative intensity ratio of the white-line in rare earth elements is known to correspond well to the occupancy of the 4f orbitals [14]. Indeed, the intensity ratio of the Ce–M4,5 edge (3d/4f* transition) was 1.25, 1.1 and 0.95 for Ce2Zr2O7 (Ce3C), Ce2Zr2O7.5 (Ce3.5C) and Ce2Zr2O8 (Ce4C), respectively. On the other hand, the valence state of the zirconium ion remained Zr4C for all three cases although the oxygen atoms filling the structural vacancies are located at the nearest neighbor sites of the zirconium [13]. It thus follows that the valence change in the cerium ions compensates for the local charge neutrality associated with the oxygen absorption/release. The valence of the cerium ion in the Ce2Zr2O7.5 intermediate phase can be interpreted as the electronic configuration of the cerium ions fluctuating between the Ce3C and Ce4C configurations with the characteristic time constant tvf [1]. EELS probes the sample on a shorter time scale than the valence
Fig. 1. Crystal structure of Ce2Zr2O7.5 to observe the stacking of the [h00] plane. The A and A 0 planes correspond to the oxygen planes. The A 0 planes contain oxygen vacancies (VO), which are indicated by the open (white) circles. The actual displacements of the atom positions from the rational lattice points are not shown.
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fluctuation time tvf [1] and can thus detect both the 4f1 and 4f0 configurations of cerium. Previously, we showed that the Ce–M4,5 edge profile consisted of the two components from the Ce2Zr2O7 (Ce3C) and Ce2Zr2O8 (Ce4C) phases [13]. The excellent reversible oxygen absorption/emission properties of this material are presumably ascribable to an exquisite combination of the physical properties of zirconia and ceria, since both ZrO2 and CeO2 exhibit oxygen exchange; ZrO2 accompanies the oxygen mobility enhancement associated with the distortion of the ZrO6 polyhedra, while CeO2Kx introduces oxygen vacancies and the changeable valence state of cerium bears the local charge compensation. The intermediate valence state could originate from the anisotropic configuration of the oxygen vacancies about the cerium site [12,13]. Hence, additional information on this peculiar valence state may be obtained by examining the Ce–M4,5 white-line under electron channeling conditions with the high energy electron probe localized on either the oxygen-abundant or -deficient planes. Electron channeling EELS can be applied after the ALCHEMI (atom location by channeling enhanced microanalysis) method, which was first applied to site-selective X-ray microanalysis [15]. The Ce2Zr 2O7 sample was prepared by the coprecipitation method from aqueous solutions containing the corresponding nitrates [16]. A powder precursor of the solid solution with composition Ce/ZrZ50/50 (mole ratio) was reduced in a graphite furnace under a static argon atmosphere at 1673 K for 8 h. The reduced powder was cooled to room temperature, removed from the furnace and exposed to air. This treatment is known to transform the powder precursor into Ce2Zr2O7 [4,17]. In order to obtain b–Ce2Zr2O7.5 powder, the Ce2Zr2O7 powder was exposed to air at room temperature for over 1 year [4,12]. The sample powder was dispersed on a carbon microgrid film, and observed using a 300-kV electron microscope (Hitachi H-9000NAR) equipped with a GATAN imaging filter. A grain of an appropriate size and thickness was selected. The Ce2Zr2O7.5 phase was confirmed by emerging the {002} superlattice reflections and the white-line intensity ratio of the Ce–M4,5 absorption edge [13] because the Ce2Zr2O7.5 phase can revert to the Ce2Zr2O7 phase due to the intense electron illumination. The incident electron beam was focused on the thin edge of the sample, approximately 50–100 nm thick, judging from the thickness contours. A systematic orientation was chosen to observe the stacking sequence ABA 0 BABA 0 B. along the [h00] direction. The A and A 0 planes contain all the oxygen atoms, while the oxygen vacancies entirely exist on the A 0 planes, with all the Ce and Zr atoms on the B planes (Fig. 1). Hence, by recording the EEL spectra under the 200 reflection excitation condition with positive and negative excitation errors, the thickness-averaged electron intensity can be maximized on the atomic plane pairs AB and A 0 B pairs, respectively [18]. The entrance aperture of the EELS
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detector should be placed off axis, because this increases the localization of the inelastic excitation, thus strengthening the channeling effect [18]. Fig. 2(a)–(c) show the calculated electron intensities propagating along the [h00] planes for a negative, zero and a positive excitation error, respectively, based on the two-beam dynamical electron scattering theory [19], in which the atomic positions in the Ce2Zr2O7.5 unit cell were taken from the X-ray diffraction results [20] and the absorption effect was phenomenologically taken into account. The electron mainly propagates along the channels between the A and B planes for the negative excitation error, and between the A 0 and B planes for the positive excitation error. For sZ0 (exact Bragg condition), the electron intensity maxima are located exactly on the cation planes. Fig. 3(a) shows the experimental Ce–M4,5 edge spectra from the fixed area with the 200 reflection excited for a negative, zero and positive excitation error, and the corresponding relative positions of the transmitted beam, Kikuchi lines and entrance aperture of the detector are shown in Fig. 3(b). The approximate valence state can be easily distinguished by the relative height of the M4 and M5 peaks; the M5 peak is higher than M4 for Ce3C, but slightly lower for Ce4C. The relative height of the M4 and M5 edges does change with the diffraction condition, as shown in Fig. 3, suggesting that the apparent valence of the Ce atom fluctuates spatially; the valence of Ce seems to be nearly C4 when the primary electron excites the atomic planes along the fully occupied oxygen planes and their neighboring rows (s!0), but mostly C3 when exciting the atomic planes along the oxygen plane containing oxygen vacancies and their neighboring rows (sO0). At exactly the Bragg condition, where sZ0, Ce exhibits a mixed valence of w3.5, as judged from the relative peak height. The relative white-line peak area ratio, IM4/IM5, was estimated to be 0.99, 1.08 and 1.19 for negative, zero and positive excitation errors, respectively. The atomic structure of Ce2Zr2O7.5 is established by the Rietveld analysis of a precise X-ray diffraction experiment which has single equivalent [20]: the space group is F43m, sites (16e) for cerium and zirconium, and oxygen occupies 4a, 4b, 4c and two 24f sites, as well as a small portion of the 4d site. Cation positions are slightly shifted from their
Fig. 2. Calculated diffracted intensity distributions of electrons with the 200 reflection excited in Ce2Zr2O7.5 (E0Z300 kV, x200/x000Z 0.1, xhkl: extinction distance for the hkl reflection). (a) Negative excitation error (sx200Z0.5). (b) Exact Bragg condition (sZ0). (c) Positive excitation error (sx200ZK0.5). The solid and broken lines represent the A and A 0 oxygen planes in Fig. 1, respectively.
Fig. 3. Ce–M4,5 edge EEL spectra and the corresponding diffraction conditions obtained from the fixed area. (a) Change in the Ce–M4,5 white-line peak area ratio for the excitation error indicated. (b) Corresponding diffraction conditions for the spectra in (a). The solid lines indicate the positions of the set of 200 Kikuchi lines, and the open circles indicate the size and position of the EELS entrance aperture for recording.
rational positions in the h111i directions, and the oxygen 24f sites are significantly displaced in one of the h100i directions. The most stable structure calculated by the ab initio pseudo-potential method coincides almost exactly with experimental results. Thus, the present atomic structure is thought to be highly reliable. Moreover, electron diffraction patterns and high-resolution electron micrographs were also consistent with the X-ray results [4,12], and modulated structure associated with possible local charge ordering or lattice distortions was not observed. Thus the present results cannot be interpreted as two different alternating Ce ion sites because the periodic charge ordering along one of the cubic axes is not compatible with the cubic symmetry. In order for the present double-valence state to be compatible with the experimentally determined crystal symmetry, the valence state of each cerium ion must be split into two along one of the h111i directions, as schematized in Fig. 4, where the electron-deficient h111i direction (trivalent side) in each cerium site always points obliquely to the oxygen-deficient {400} plane, while the direction (tetravalent side) points to electron-abundant h1 1 1i the fully occupied oxygen plane, which is totally consistent with the present experimental results. From the point of view of crystallographic symmetry, each cerium ion has three nearest oxygen vacancies in the h311i directions, separated by 0.41 nm, forming an equilateral triangle in the
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Scientific Research from Japan Society for the Promotion of Science.
References
Fig. 4. (a) Local atom configuration of a Ce ion and oxygen vacancies. (b) The valence states of the Ce ions in the unit cell. Vo and Onew stand for oxygen vacancies and oxygen atoms filled during the transformation from Ce2Zr2O7 to Ce2Zr2O7.5. The bonds connecting the Ce ions and their nearest neighbor oxygen atoms are shown. It is seen that the Ce4C and Ce3C ‘faces’ look towards the neighboring oxygen-abundant and oxygen-deficient planes, respectively. Note that (a) and (b) are viewed from different directions to allow a better view of the local configurations.
{111} crystallographic plane. The cerium ions are shifted slightly towards, rather than away from, the vacancy direction) [20], presumably triangle along its normal (h1 1 1i as a result of the increased Coulomb attraction between the Ce tetravalent side and the nearest oxygen ion. In conclusion, this is the first experimental evidence of a novel valence state for cerium ions, as if they were in a double-faced (Ce3C and Ce4C) state in which the two valencies face in opposite directions. There have been a number of recent reports on mixed-valence states in cerium compounds [21–24] in which the analysis and discussion were restricted to conventional concepts. We examined the charge distributions around the cerium ions with the first principles APWClo (augmented plane-waveClocal orbital) band method [25] and observed only slight anisotropy in the Ce-4f occupied states, compatible with the present results. Hence, the present extraordinary anisotropic state should be understood in the framework of the many-electron system, rather than the one-electron approximation, because the latter does not allow the double-valence state as a ground state. The present results urge us to reconsider the mixedvalence state of cerium ions in other ordered cerium compounds, which may help elucidate the mechanism of the excellent and stable oxygen storage properties of this series of compounds.
Acknowledgements This work is partly supported by a Grant-in-Aid for
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