Vibrational Dynamics and Oxygen Diffusion in a Nanoporous Oxide

Sep 15, 2007 - Transparent Electro-Active Materials Project, ERATO-SORST, Japan Science and Technology Agency, in Frontier Collaborative Research Cent...
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J. Phys. Chem. C 2007, 111, 14855-14861

14855

Vibrational Dynamics and Oxygen Diffusion in a Nanoporous Oxide Ion Conductor 12CaO‚7Al2O3 Studied by 18O Labeling and Micro-Raman Spectroscopy Koichi Kajihara,*,†,‡ Satoru Matsuishi,§ Katsuro Hayashi,§ Masahiro Hirano,†,§ and Hideo Hosono†,§ Transparent Electro-ActiVe Materials Project, ERATO-SORST, Japan Science and Technology Agency, in Frontier CollaboratiVe Research Center, Mail Box S2-13, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Department of Applied Chemistry, Graduate School of Urban EnVironmental Sciences, Tokyo Metropolitan UniVersity, 1-1 Minami-Osawa, Hachioji 192-0397, Japan, and Frontier CollaboratiVe Research Center and Materials & Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: June 1, 2007; In Final Form: July 26, 2007

Nanoporous oxide 12CaO‚7Al2O3 (C12A7) is a fast oxide-ion conductor that may encage various types of anionic oxygen species such as O2-, O2-, O22-, and O- in its crystallographic cages. The C12A7 single crystals thermally annealed in 18O2 were examined by micro-Raman spectroscopy. The symmetric properties and the origin of Raman bands were analyzed by the isotope shift and the depolarization on the bases of the crystal symmetry of C12A7, and the Raman band due to the stretching mode of extraframework O22- was identified. Spatially resolved measurements of the isotope shift of the Raman bands in a partially 18O-substituted sample indicated that the local fraction of 18O is equal between the framework and extraframework sites, demonstrating that the substitution of O2- ions in the C12A7 framework is as fast as the formation of extraframework oxygen species by thermal oxygen loading. It strongly suggests that the extraframework oxygen species diffuse by replacing oxygens in the framework sites, rather than simply permeating through intercage openings.

1. Introduction Development of fast oxide ion conductors1-5 is a key issue for the commercialization of fuel cells, oxygen sensors, electricfield oxygen emitters, and heterogeneous oxidation catalysts. The compound 12CaO‚7Al2O3 (C12A7) exhibits oxide ion conduction with a conductivity ∼1/10 that of yttria-stabilized zirconia (YSZ),6 and the fast ion conduction is most likely because of the unique oxygen coordination structure in C12A7. Table 1 lists the coordinates and site occupancies of atoms in stoichiometric C12A7 (space group I 4h3d).7 The unit cell composition is expressed by 2[12CaO‚7Al2O3] ) [Ca24Al28O64]4+‚ 2O2-. The former part represents a positively charged framework containing 12 ellipsoidal interstitial cages with an inner diameter of ∼4 Å. The cage center is located at the 12a site (Table 1), whose site symmetry is 4h (S4). The cage structure is schematically shown in Figure 1, indicating that the O2- belonging to the framework [“framework O2- ion”, O(1) and O(2)] is fourcoordinated by Ca2+ and Al3+ ions. The O(1) is termed a “bridging oxygen” because it bridges Al(1) and Al(2), and O(2) is referred to as a “nonbriding oxygen” because it is bonded to only one Al3+ ion, Al(2). The remaining two O2- ions [O(3)], termed “extraframework O2-” or “free O2-”, are randomly distributed over the cages, occupying 1/6 of them. The extraframework O2- ion is not bonded to Al3+ ions and is loosely bound between two Ca2+ ions on the cage wall with defining the S4 symmetry axis of the cage. Each cage is connected to * Corresponding author. E-mail: [email protected]. † Japan Science and Technology Agency. ‡ Tokyo Metropolitan University. § Tokyo Institute of Technology.

eight neighboring cages via Ca-O(1)-Al(2)-O(1)-Al(1)O(2) six-membered openings. Thus, it has been thought that the permeation of the extraframework O2- through the openings is responsible for the fast oxide ion conduction in C12A7. It is also known that thermal annealing in dry oxygen atmosphere gives C12A7 an oxygen-excess because of the formation of extraframework O2- and O- ions, with a total concentration of up to 2.3 × 1021 cm-3.8-11 The oxygen uptake process has been tentatively assigned to a charge-transfer reaction among oxygen species in the cages (eq 1).

O2(gas) + O2- (cage) f O- (cage) + O2- (cage)

(1)

Here, it is implicitly assumed that O2 in the atmosphere penetrates the cage wall and is incorporated into the cages without dissociation. Reaction eq 1 is seemingly supported by an electron spin resonance observation of a small amount of C12A7 powder treated in a 17O-enriched oxygen atmosphere, in which the isotopic compositions of the atmosphere and encaged O2- were nearly the same.11 However, eq 1 does not explain two experimental observations implying cleavage and reproduction of diatomic oxygen species: (1) the larger concentration of O2- than O-,9 and (2) the formation of a noticeable amount of O22-.12,13 Moreover, a recent theoretical study using both molecular dynamics simulations and ab initio calculations indicates that the easiest diffusion path of the extraframework oxygen species involves in the transient formation of O-O bonds in the framework, as shown in Figure 2; therefore, we expect significant exchange between the framework and extraframework oxygen species.14 However, the microscopic oxygen diffusion mechanism in C12A7 has not been experimentally studied to date.

10.1021/jp074248n CCC: $37.00 © 2007 American Chemical Society Published on Web 09/15/2007

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TABLE 1: Structure Parameters of Stoichiometric C12A7 with Space Group I 4h3d a,b coordinates

a

atom

site

x

y

z

occupancy

type

Ca Al(1) Al(2) O(1) O(2) O(3) cage center

24d 16c 12b 48e 16c 24d 12a

0 0.0187 -0.125 0.151 -0.064 0.337 0.375

0.25 0.0187 0 -0.037 -0.064 0 0

0.1397 0.0187 0.25 0.057 -0.064 0.25 0.25

1 1 1 1 1 0.083

framework framework framework framework framework extraframework

Reference 7. b The extraframework O2- ion is denoted by O(3). The other ions form the framework of C12A7.

We examined the thermal uptake of oxygen species in singlecrystalline C12A7 samples using 18O isotope labeling and microRaman spectroscopy. Polarized micro-Raman spectra provide detailed information on the vibrational symmetry and origin of the Raman bands as well as the spatial distribution of 18O. Thus, this technique makes it possible to simultaneously investigate the incorporation of 18O into the framework oxygen sites and into the extraframework O2- ions, through the isotope shifts of Raman bands associated with these structures. The results clearly indicate that transient incorporation of oxygen species in the framework plays a vital role both in the oxygen uptake from atmosphere and in the diffusion of extraframework oxygen species in C12A7.

TABLE 2: Factor Group Analysis for the Framework Part of C12A7 with Space Group I 4h3d a,b site

site symmetry

A1

A2

E

T1

T2

atoms

48e 24d 16c 12b

1 2.. .3. 4h..

3 1 1

3 1 1 1

6 2 2 1

9 5 3 2

9 5 3 3

O(1) Ca Al(1), O(2) Al(2)

total

vibrational translational rotational

6

7

13

21

22 1

1

a

References 18 and 19. b The translational and rotational modes (T2 and T1, respectively) are included. Note that E and T modes are doubly and triply degenerated, respectively. Normal vibrations of symmetry A1, E, and T2 are Raman active.

2. Experimental Procedure Two types of samples were prepared from single crystalline C12A7 grown by the Czochralski method:16 sample A, ∼2 × 2 × 0.5 mm3 with (001) top and (110) side faces; sample B, ∼2 × 2 × 2 mm3 with an unknown crystallographic orientation. Sample A was sealed in a silica tube (∼110 mm long, ∼12 mm inner diameter, and 1.2 mm thick) with ∼1 atm of 18O2 gas (∼99% 18O fraction) at room temperature and was subsequently thermally annealed at 1000 °C for 96 h, which is expected to be sufficiently long to prepare a fully 18O-substituted sample.6,9 To restore the partial pressure and isotopic fraction, the 18O2 gas inside the tube was refilled every 24 h. The 16O -treated sample A was also prepared by the same procedure. 2 Sample B was subjected to partial 18O substitution; it was sealed in a silica tube with ∼1 atm of 18O2 at room temperature and was treated for 48 h at a much lower temperature (700 °C) without refilling the O2 gas inside the tube. Oxygen-unloaded references of samples A and B were prepared by treating them in an evacuated silica tube using thermal annealing programs identical to the respective O2 treatments. The isotopic composition of the O2 gas in the silica tube was examined by conventional Raman spectroscopy, where continuous-wave (cw) light from a frequency-doubled Nd:YVO4 laser (λ ) 532 nm, ∼2 W) was focused at the center of the tube, and the scattered light was collected in a right-angle geometry and was analyzed by a monochromator (HR-640, Jobin Yvon) with a cooled photomultiplier (R4632, Hamamatsu). The spectral resolution was ∼3 cm-1. After the thermal annealing, the samples were characterized by a micro-Raman spectrometer (Nanofinder, Tokyo Instruments), where frequency-doubled cw Nd:YVO4 laser light (λ ) 532 nm, ∼200 mW) was focused onto the sample surface through an objective lens and backscattered light was collected by the same lens. The spectral resolution was ∼2 cm-1. Polarized micro-Raman spectra were measured on the (001) face of sample A. The [001] axis of the sample A was aligned parallel to the incident laser light, which is on the Z-axis of the laboratory frame. The excitation laser light was originally

polarized, and the polarization direction was set parallel to the X- or Y-axes of the laboratory frame by rotating a half-wave plate placed in front of the laser head. The scattered light was corrected through a polarizer. To minimize errors due to the uneven spectral sensitivity of the monochromator against the polarization direction, the orientation of the polarizer was fixed along the X-axis. Thus, the scattering configuration was denoted by Z(XX)Z h or Z(YX)Z h . In addition, the sample was allowed to be rotated about the Z-axis, and the angle between the [100]axis of the sample and the X-axis of the laboratory frame was defined by θ. The numerical aperture (NA) of the objective lens (50×) was 0.45, and it was sufficiently small to neglect the depolarization correction associated with wide-aperture lenses.17 Nonpolarized micro-Raman spectra were taken without the polarizer for the scattered light. Using a 100× objective lens (NA 0.95), only a small area, limited by the focused laser spot (∼10 µm) with a depth of ∼10 µm, was detected. This configuration was used to measure the spatially resolved Raman spectra. 3. Results 3.1. Theory of Raman Scattering from the Framework Part of C12A7 Crystal Structure. The number of normal vibrations in crystals is mechanically calculated from the space group and the number of the crystallographic sites occupied by atoms.18,19 This method is often referred to as factor group analysis, and Table 2 lists the result of the analysis performed using the primitive cell (1/2 of the unit cell) of the C12A7 framework. Among the normal vibrations listed in Table 2, the A1, E, and T2 modes are Raman active. It should be noted that Table 2 lists just the numbers of normal vibrations originating from each crystallographic site in the C12A7 framework; actual vibration is not localized on a single site but is linearly combined with vibrations of the same symmetry, originating from the other sites. Important consequences derived from Table 2 are (i) the number of the totally symmetric modes (A1), which generally produce much more intense Raman bands than the other

Oxygen Diffusion in a Nanoporous Oxide Ion Conductor TABLE 3: Nonzero Elements of Raman Scattering Tensor for a Cubic Crystal of Point Group 4h3m.a,b symmetry

tensor

nonzero elements Rxx ) Ryy ) Rzz ) a

A1 E

1 2

Rxx ) -31/2b, Ryy ) 31/2b Rxx ) Ryy ) b, Rzz ) -2b

T2

1 2 3

Ryz ) Rzy ) c Rxz ) Rzx ) c Rxy ) Ryx ) c

a Reference 17. b The E and T modes are degenerate and have two 2 and three tensors, respectively.

modes,20 is six, and (ii) Al(2), which occupies the 12b site, does not participate in the A1 modes. Raman scattering tensors for a cubic crystal of space group I 4h3d follow the description for point group 4h3m ) Td.17 Using the tensor elements listed in Table 3, the relative intensity of Raman bands associated with A1, E, and T2 modes was derived as a function of θ, as listed in Table 4. We defined the depolarization ratio as F ) IZ(YX)Zh /IZ(XX)Zh , where IZ(XX)Zh and IZ(YX)Zh denote the Raman intensity in the respective polarization configurations. Table 4 indicates that F is zero irrespective of θ for Raman bands with A1 symmetry. In contrast, F depends on θ for Raman bands with E and T2 symmetry; the F maxima appears at each 90° rotation of θ, whereas the phase angles for the maxima are different by 45° between the E and T2 bands. Thus, the A1, E, and T2 Raman bands can be distinguished by examining F at θ ) 0 and 45°, as listed in Table 4. 3.2. Polarized Raman Measurements of 18O-Free and Fully 18O-Substituted Samples. Figure 3 shows nonpolarized micro-Raman spectra of C12A7 samples A thermally annealed for 96 h at 1000 °C in 16O2 or 18O2 gas or in a vacuum (reference). The Raman bands observed below 1000 cm-1 were mainly attributed to normal vibrations of the framework of C12A7.21 The positions of the Raman bands were nearly the same between the 16O2-treated and reference samples. In contrast, Raman bands of the 18O2-treated sample were shifted to a lower energy, indicating the incorporation of 18O into the framework. The Raman bands emerging at 1131, 1099, and 1066 cm-1 in the O2-treated samples were assigned to an isotopeshifted series of the O-O stretching mode of extraframework 16O -, 16O18O-, and 18O -, respectively.22 The degree of the 2 2 16O-18O substitution evaluated from the intensity ratio of the Raman bands of extraframework O2- in the 18O2-treated sample was ∼97%, and the reasoning for why this value represents the isotopic composition of C12A7 samples is shown in Section 3.3. The peak position and polarization properties of the Raman bands were investigated in detail by polarized micro-Raman spectroscopy. Figure 4, panels a and b, show Raman spectra measured at four scattering configurations listed in Table 4 for the 16O2- and 18O2-treated samples A, which were also used to obtain Figure 3. From the recorded spectra, 14 distinct Raman bands were labeled and analyzed. The observed peak positions (ν16 and ν18), relative peak shifts due to the 16O-18O substitution (ν18/ν16), and symmetry evaluated in Table 4 were summarized in Table 5. The possible assignment of these Raman bands are described in detail in the Discussion section. 3.3. Spatially Resolved Raman Measurements of a Partially 18O-Substituted Sample. The fraction of 18O near the surface of a partially 18O-substituted C12A7 sample may be sensitive to the variation of the isotopic composition of sealed O2 gas during thermal annealing. Thus, the 18O fraction of the sealed O2 gas was measured before and after the partial 18O

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14857 substitution of sample B for 48 h at 700 °C. Figure 5 shows Raman spectra of the sealed O2 gas. A Raman band associated with the O-O stretching mode (Q band) of 18O2 was observed at ∼1467 cm-1 along with the rotational-vibrational sidebands of the O (lower energy side, ∆J ) -2) and S (higher energy side, ∆J ) 2) branches. Before, the thermal annealing Raman bands due to 16O18O and 16O2 were below detection limit (relative fraction j2%). After the thermal annealing, the intensity of the Q band of 18O2 decreased slightly, and a new Raman band, assigned to the Q band of 16O18O, emerged at ∼1513 cm-1. However, the Q band due to 16O2 (∼1556 cm-1 23) was not detected. From the ratio of the Q band intensities, the fraction of 18O in the atmosphere was evaluated to be ∼95% after the thermal annealing. Such small decrease in the 18O fraction of O2 gas may provide no significant influence on that of the C12A7 sample, and thus we neglect the change in the isotopic composition of the sealed O2 gas during the partial 18O substitution. Figure 6 shows nonpolarized micro-Raman spectra of the partially 18O-substituted sample B, taken at three different positions (corner, edge center, and face center). The spectrum of a vacuum-annealed sample B (reference) is also shown for comparison. Both the total intensity of the Raman bands of extraframework O2- and the 18O fraction increased in the order of the face, edge, and corner positions. Thus, it is most likely that the oxygen uptake from atmosphere becomes fast with an increase in the surface-to-volume ratio. The incorporation of 18O to the C12A7 framework was monitored using band G (a peak located at 522 cm-1 for 18Ofree samples), which is the strongest among the Raman bands originating from the framework21 and is sufficiently isolated from other bands, as shown in Figure 4. Figure 6 shows that the 18O2 treatment shifted band G to the lower-energy side, and the degree of the shift was varied with the sample position, similar to that observed for extraframework O2-. To examine how 18O is distributed among the framework and extraframework oxygen sites, the normalized position of band G was plotted against the 18O fraction of extraframework O2measured at the same sample position, as shown in Figure 7. The position of band G was defined by the weight center because the peak shift may be because of the formation of new bands at lower energy side. The peak position was first normalized to the value for the reference sample (left axis of Figure 7), then renormalized to the maximum peak shift determined from the fully 18O-substituted sample A described in Section 3.2 (right axis of Figure 7). The normalized band position decreased almost linearly with the 18O fraction in extraframework O2- measured at the same position, and the slope was ∼1. Hence, 18O is homogeneously distributed among the framework and extraframework oxygen sites, and thus the isotopic composition of extraframework O2- represents that of C12A7 samples. 4. Discussion 4.1. Assignment of Raman Bands by Isotope Shift and Depolarization Ratio. The relative peak shift due to 16O-18O isotope exchange provides useful information on the nature of normal vibrations. Normal vibrations localized to oxygen atoms exhibit the largest shift determined by the ratio of the square root of the atomic masses of 16O and 18O, ν18/ν16 ) (m16O/m18O)1/2 ) 0.943. Table 5 shows that bands F, I, J, and O undergo such largest isotope shift. Band O is assigned to the stretching mode of extraframework O2-.22 The ν18/ν16 value is equal to the theoretical limit,

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TABLE 4: Dependence of Relative Raman Scattering Intensity on the Symmetry of the Normal Vibration and the Polarization Configuration for a Cubic Crystal of Point Group 4h3m, Calculated Using Tensor Elements Listed in Table 3a Z(XX)Z h A1 E T2

Z(YX)Z h

F

Z(XX)Z h

Z(YX)Z h

θ ) 0°

θ ) 45°

θ ) 0°

θ ) 45°

θ ) 0°

θ ) 45°

b2(1 + 3 cos2 2θ) c2 sin2 2θ

0 3b2 sin2 2θ c2 cos2 2θ

1 1 0

1 0.25 1

0 0 1

0 0.75 0

0 0 ∞

0 3 0

symmetry a2

a The [001]-axis of the crystal is parallel to the Z-axis of the laboratory frame. The incident laser light is polarized parallel to the X- or Y-axes, and a polarizer for the scattered light is along the X-axis, making the polarization configuration Z(XX)Z h or Z(YX)Z h . θ denotes the angle between the X-axis and the [100]-axis of the crystal. Relative scattering intensities and depolarization ratio [F ) IZ(YX)Zh /IZ(XX)Zh ] calculated at θ ) 0° and 45° are also shown.

Figure 1. Schematic illustrations of cages in C12A7; side and top views of a cage whose principal (S4 symmetry) axis is directed along the [001] axis of C12A7 (a), and two adjacent cages expressed by coordination polyhedra (b) and ball-stick (c) models, viewed from the same direction. Red, pale-red, and yellow polyhedra in panels (a) and (b) denote Al(1)O4, Al(2)O4, and CaO6 units in the C12A7 framework, respectively. Ca in panel (a) is represented by yellow spheres. O(1) (“bridging oxygen”) and O(2) (“nonbridging oxygen”) are shown by small pale-blue and blue spheres, respectively. In panel (b), the directions of [100], [010], and [001] axes are shown on the bottom-left. Ca is coordinated by four O(1) and two O(2). Al(1) is bonded to three O(1) and one O(2), and Al(2) is bonded to four O(1). O(1) is bonded to one Al(1), one Al(2), and two Ca, and O(2) is coordinated by one Al(1) and three Ca. Thus, O(1) bridges Al(1) and Al(2), and there are no Al(1)-O-Al(1) or Al(2)-O-Al(2) bonds. Each cage is connected to eight other cages via Ca-O(1)-Al(2)-O(1)Al(1)-O(2) six-membered openings, which can be seen as triangularshaped windows in panel (b). The blue sphere in the left-hand side cage in panel (c) denotes an extraframework O2- ion, O(3), which is not bonded to Al but is loosely bonded to Ca on the S4 axis of the cage. For more details on the crystal and cage structures refer articles (e.g., refs 11, 14, and 15).

verifying that the O-O stretching mode is sufficiently isolated from the framework modes of C12A7. The stretching mode of such homonuclear diatomic species is totally symmetric, but the Raman scattering tensor is not spherically symmetric (Rzz > Rxx ) Ryy), leading to a nonzero F value. The observed F value for extraframework O2- (band O) was Fθ)0° = 0.07 and Fθ)45° = 0.24. The variation of F with θ indicates that the orientation of extraframework O2- is not perfectly random such as that in liquid and gas phases, and it probably reflects the anisotropic thermal motion, which has been confirmed by an electron paramagnetic resonance study.11 In 18O-free samples, band J is located at ∼772 cm-1 and is hard to distinguish from band K. However, the intensity of the

770 cm-1 band is increased considerably by thermal annealing in oxygen atmosphere, and this observation indicates the contribution of oxygen-related species to this band.12,13 The oxygen-related band, which corresponds to band J in this study, has been assigned to extraframework O22-,12,13 on the bases of the fact that the stretching mode of O22- is commonly located at this frequency range.20 This 18O exchange experiment clearly shows the presence of band J, which appears as a result of the large isotope shift as compared with that of band K (Table 5). In addition, the value of F of band K is nearly zero, whereas that of band J is roughly estimated to be Fθ)0° = Fθ)45° = 0.2. This makes it possible to selectively detect band J, even in 18O-free samples under the Z(YX)Z h configuration [Figure 4a]. The close similarity of the average F values between bands J and O suggests that the chemical species responsible for band J is diatomic as well. Thus, this study provides distinct evidence that extraframework O22- is responsible for band J, and the stretching mode is well isolated from the normal vibrations in the framework. The assignments of bands F and I are less certain. The halfwidth of band F is significantly larger than that of the bands because of extraframework molecular species (bands J and O), and the intensity is almost proportional to that of other bands originating from the framework (e.g., band G). The most probable origin of band F is normal vibration localized to O2ions in the framework. Band F might have similar origin to that of the adjacent band E because these bands have the same symmetry, and the ν18/ν16 value of band E is relatively close to that of band F. In contrast, band I disappears after thermal annealing in O2 at 700 °C (Figure 6). Thus, this band is unlikely to be due to the framework and might be related to some extraframework oxygen species, whose concentration is easily changed by thermal annealing. Raman bands arising from normal vibrations in the C12A7 framework are usually associated with the movement of Al3+ and Ca2+ ions, and the isotope shift due to 16O-18O exchange is smaller than the theoretical limit defined by the atomic masses of 16O and 18O. Because most of the Raman bands belong to this group, it is necessary to consider both the isotope shift and the polarization properties to reliably assign these bands. Among the normal vibrations, A1 modes are the most important because such totally symmetric modes usually produce strong Raman bands. Bands B, G, K, L, and M exhibit F = 0 at θ ) 0° and 45°, which is characteristic to A1 modes (Table 4). Bands D and N may also have the A1 character because the observed variation of F with θ can originate from overlaps with E or T2 modes. However, the number of A1 modes is theoretically predicted to be six for the C12A7 framework with I 4h3d symmetry (Table 2). This discrepancy may be because of the presence of extraframework oxygen species, which randomly occupy the cages and locally reduce the symmetry by distorting the framework.

Oxygen Diffusion in a Nanoporous Oxide Ion Conductor

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14859

Figure 2. Schematic illustration of diffusion of extraframework O2- and O2- ions (blue spheres) in C12A7, predicted by a theoretical study.14 Atoms and bonds near the cage opening are emphasized. The diffusion process involves the splitting of the O-O bond of O2- ions and the oxygen exchange with framework O2- ions (pale-blue spheres).

Figure 3. Nonpolarized surface micro-Raman spectra of sample A treated for 96 h at 1000 °C in a vacuum (reference) or in 16O2 or 18O2 gas.

We suggest that the six theoretically predicted A1 bands include the five strongest bands (bands B, D, G, K, and M) among the seven bands. Table 2 indicates that these bands with A1 symmetry originate from the movement of Al(1), O(1), O(2), and Ca. The Al(1) is coordinated by one O(2) and three O(1). Both Al(1) and O(2) occupy 16c sites, and in A1 modes their displacements are confined along the Al(1)-O(2) bond, which is parallel to the C3 symmetry axes of C12A7 along the direction.18,19 The movement of O(1) (48e site), which bridges Al(1) and Al(2) with an Al(1)-O(1)-Al(2) angle of ∼140°, is complicated, because the displacement vectors originate from linear combination of three vectors along the [100], [010], and [001] axes. However, it is most likely that one displacement vector is almost parallel to the line connecting Al(1) and Al(2) (stretching-like mode); the other two vectors are on the plane equidistant from Al(1) and Al(2), one being nearly on the plane defined by Al(1), Al(2), and O(1) (bending-like mode), and the other being orthogonal to this plane (rocking mode). The displacement vector of the Ca (24d site) responsible for the A1 mode is parallel to the S4 symmetry axis and is symmetric about the cage center (“breathing mode”). The Al(2) (12b site), which is coordinated by four O(1), does not move in A1 modes because the displacements of O(1) are always symmetric about Al(2). The two strongest Raman bands (G and K) exhibit similar ν18/ν16 values of ∼0.96, which are close to that of the stretching mode of a free Al-O oscillator (0.965). Furthermore, Ca is expected to be less mobile at this frequency range.24 Thus, bands G and K are both attributed to vibrations involving O and Al motions. The ν18/ν16 value of band B is larger than that expected for free Al-O and Ca-O (0.959) oscillators, indicating that the movement of Al(1) and/or Ca is important for band B. It is likely that the contribution of Ca is dominant because the

displacement of Ca becomes distinct below ∼300 cm-1.24 Thus, band B is probably ascribed to the Ca breathing mode. The remaining two A1 bands (D and M) show relatively small ν18/ ν16 values (∼0.95), indicating that the movement of O(1) and O(2) is dominant. Such vibrations would be formed by a combination of the stretching-like mode of O(1) and the Al(1)-O(2) stretching mode, leading to the expansion of Al(1)O(1)3O(2) tetrahedra synchronized with the contraction of neighboring Al(2)O(1)4 tetrahedra, because this minimizes the displacement of Al(1), located at the center of the Al(1)O(1)3O(2) tetrahedron. The ν18/ν16 value of band C is the largest among the observed Raman bands, indicating a small contribution of O motion. This band is located below 300 cm-1 and is probably due to the movement of Ca. The T2 symmetry of this band suggests that the displacement vector of Ca is either perpendicular to the S4 symmetry axis or is along the S4 axis but asymmetric about the cage center.18,19 Bands H, K, and N show ν18/ν16 values around ∼0.96 and are located close to bands G and K. Thus, these bands are ascribed to the movement of O2- and Al3+ ions in the framework. The depolarization of band A is not simple, and the symmetry remains unclear. This makes the assignment of band A difficult. 4.2. Analysis of Oxygen Diffusion by Isotope Shift of Raman Bands. Figure 7 shows that the normalized position of band G, which is due to a normal vibration of the C12A7 framework involving O and Al motions, depends linearly on the 18O fraction of extraframework O2- with a slope of ∼1. This observation demonstrates that 18O distributes homogeneously over the framework and extraframework sites. The direct incorporation of O2 molecules from the atmosphere into the cages should generate only 18O2- in the cage (eq 1). However, the 18O fraction of extraframework O2- formed at the face center is extremely low (∼5%, Figure 6), despite of the high 18O purity of ambient O2 gas during the thermal annealing (J95%, Figure 5). The concentration of oxygen atoms that form O2- at the face center was ∼1 × 1020 cm-3,25 and it is less than 1% of oxygen atoms in the framework (∼4 × 1022 cm-3). Thus, it is evident that oxygen atoms that form extraframework O2- originate from the framework almost exclusively. We therefore conclude that the exchange between the framework and extraframework oxygen species is an important step for the oxygen dissolution and diffusion in C12A7. The simple permeation of extraframework oxygen species through intercage openings is not the main oxygen diffusion mechanism. This observation confirms the result of a recent theoretical study performed using an embedded cluster approach.14

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Figure 4. Polarized surface micro-Raman spectra of sample A treated for 96 h at 1000 °C in 16O2 (a) or 18O2 (b) gas, taken under the Z(XX)Z h or Z(YX)Z h configuration. The detail of the configuration is described in the text and Table 4 caption.

TABLE 5: Peak Position (ν), Relative 16O-18O Isotope Shift of the Peak Position (ν18/ν16), Symmetry, and Main Origin of the Raman Bands Observed in C12A7 peak position ν (cm-1) label

16O

18O

ν18/ν16

symmetry

A B C D E F G H I

183 239 268 312 333 388 522 589 670

178 232 265 296 317 366 501 566 635

0.973 0.971 0.989 0.949 0.952 0.943 0.960 0.961 0.948

? A1(+E?) T2 A1 + T2 E E A1 T2

J K L M N O

771 772 841 886 918 1131

727 740 807 843 885 1067

0.943 0.959 0.960 0.951 0.964 0.943

A1 A1? A1 T2(+A1?)

main origin framework Ca framework Ca framework O framework O framework O framework O, Al framework O, Al extraframework O species? extraframework O22 framework O, Al framework O, Al framework O framework O, Al extraframework O2

Figure 6. Surface micro-Raman spectra of sample B partially 18Osubstituted or vacuum-annealed (reference) for 48 h at 700 °C. The spectra of the 18O-substituted sample were taken at the corner, edge center, and face center of the sample.

Figure 6 shows that the concentration of extraframework O2at the corner position of the sample is ∼2-3 times larger than that at the face center, indicating that there exists an activation barrier for the dissolution of oxygen in C12A7. Such an activation barrier is typical of solid oxide electrolytes, where O2 molecules are converted to O2- ions at the oxide surfaces.26,27 However, it contrasts with the oxygen dissolution in amorphous silica (a-SiO2), another oxide with a lot of interstitial voids where O2 dissolves in without dissociation, leading to a rapid saturation of the O2 concentration near the surface.28-30 Thus, Figure 7. Relation between the 18O fraction in extraframework O2and the normalized position of Raman band G (522 cm-1 for 18O-free samples) measured for sample B. The bar in the figure shows the statistic error of the experiment. The normalized band positions (weight centers) expected for an isolated Al-O oscillator and a symmetric stretching mode of an AlO4 tetrahedron are also shown for comparison.

the slow oxygen dissolution into C12A7 is most likely due to the kinetic barrier for the splitting of O2 and the subsequent incorporation into the framework, again exhibiting an important role of the framework oxygens for the formation and diffusion of extraframework oxygen species in C12A7. 5. Conclusions Figure 5. Raman spectra of O2 gas sealed in a silica tube measured before and after thermal annealing (partial 18O substitution) with sample B for 48 h at 700 °C.

The vibrational dynamics and mechanism of thermal oxygen diffusion in a nanoporous oxide 12CaO‚7Al2O3 (C12A7), which

Oxygen Diffusion in a Nanoporous Oxide Ion Conductor accommodates free negatively charged extraframework oxygen species in its interstitial cages, were examined. Polarized microRaman spectroscopy and factor-group analysis of singlecrystalline C12A7 samples allowed to examine the symmetry of normal vibrations responsible for Raman bands, and to qualitatively predict the orientations of atomic displacement vectors. Measurements of isotope shift of Raman bands due to thermal 16O-18O exchange revealed that 18O is easily incorporated in the C12A7 framework. The 18O fraction in the framework is almost equal to that of extraframework O2- ions evaluated at the same sample position. Furthermore, the presence of extraframework O22- ions, which may be formed by reactions involving monatomic extraframework oxygen species, was evidenced by selectively detecting the stretching mode. These observations confirm that the extraframework oxygen species easily exchanges with O2- ions in the framework during their formation through oxygen uptake and subsequent diffusion in C12A7. Such a mechanism provides insight into oxygen diffusion in oxygen clathrate compounds. Acknowledgment. We thank Dr. K. Kawamura of Japan Science and Technology Agency for the assistance of microRaman measurements. This work was supported by the Grantin-Aid for Creative Scientific Research (Grant No. 16GS0205) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. References and Notes (1) Kendall, K. R.; Navas, C.; Thomas, J. K.; zur Loye, H. C. Chem. Mater. 1996, 8, 642-649. (2) Boivin, J. C.; Mairesse, G. Chem. Mater. 1998, 10, 2870-2888. (3) Sammes, N. M.; Tompsett, G. A.; Na¨fe, H.; Aldingera, F. J. Euro. Ceram. Soc. 1999, 19, 1801-1826. (4) Slater, P. R.; Sansom, J. E. H.; Tolchard, J. R. Chem. Rec. 2004, 4, 373-384. (5) Stoukides, M. Ind. Eng. Chem. Res. 1988, 27, 1745-1750. (6) Lacerda, M.; Irvine, J. T. S.; Glasser, F. P.; West, A. W. Nature 1988, 332, 525-526.

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