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Ag L3-Edge X-ray Absorption Near-Edge Structure of 4d10 (Ag+) Compounds: Origin of the Edge Peak and Its Chemical Relevance Takeshi Miyamoto,†,‡ Hironobu Niimi,‡ Yoshinori Kitajima,§ Toshio Naito,| and Kiyotaka Asakura*,†,‡ Department of Quantum Science and Engineering, Graduate School of Engineering, Hokkaido UniVersity, 21-10, Sapporo 001-0021, Japan, Catalysis Research Center (CRC), Hokkaido UniVersity 21-10, Sapporo 001-0021, Japan, Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba 305-0801, Japan, and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity 10-8, Sapporo 060-0810, Japan ReceiVed: August 8, 2009; ReVised Manuscript ReceiVed: January 11, 2010
A peak appearing at the L2,3 X-ray absorption edge often provides the number of empty d states of the X-ray absorbing atoms. Ag+ compounds have a d10 state (no d empty states) but show a small peak at the edge. In this research, we systematically studied the edge peak of Ag+ compounds to understand its origin on the basis of the molecular orbital picture and to obtain a relation of the edge peak intensity to chemical and physical quantities. The edge peak can be formally assigned to the transition from 2p to 5s enhanced by the s-d hybridization. The peak intensity has a negative correlation with a coordination charge but has a positive correlation with the strength of the covalent bond, which is in the reverse order to the other dn (n < 10) elements. 1. Introduction The transition metals give a strong edge peak referred to as a white line in their L2,3 absorption edge, which is assigned to the 2p f md (m ) 3, 4, 5) dipole transition. Since the d electrons in transition metals are important to determine the physical properties and chemical reactivities, the L2,3-edge white lines have been extensively investigated since the late 1970s, when high-resolution X-ray spectra became available with the advent of synchrotron radiation. The analysis of the L2,3-edge white line intensity of 4d and 5d transition metals is now wellestablished.1-4 The peak intensity reflects the number of d vacancies on 4d and 5d.5 The edge peak intensity increases with the oxidation number of the central atoms. A more oxidized cation gives a stronger edge peak.6,7 Recently, the strong white line has been used as a high-speed monitor for the dynamic change of catalyst particle structures under catalytic conditions.8-11 In addition to the research on 5d transition elements, white line analysis has been performed on 4d transition metals such as Mo, Ru, and Pd.12 Although the origin of the L2,3 edge can be explained on the basis of the d vacancy, the detailed features of the L2,3-edge white line are modified by several factors. For example, atomic X-ray absorption fine structure, Fano resonances, and multiple scattering from the adsorbates affect the Pt L3-edge white line shape.13-15 Ag+ plays an important role in catalysts for the epoxidation reaction, antibacterial materials, electronic devices, and photography film. Ag+ has a d10 configuration, and no white line peak is expected due to the fully occupied d state. Recently, we measured the Ag L3-edge X-ray absorption near-edge structure (XANES) of Ag(DMe-DCNQI)2 to characterize the * Corresponding author. Phone and fax: 81-11-706-9113. E-mail:
[email protected]. † Graduate School of Engineering, Hokkaido University. ‡ CRC, Hokkaido University. § High Energy Accelerator Research Organization. | Graduate School of Science, Hokkaido University.
Ag valence state.16 Ag(DMe-DCNQI)2 is a photosensitive organic conductor and a promising material for organic microdevices.17-21 We found that Ag(DMe-DCNQI)2 has a small but distinctive peak in the middle of the X-ray absorption edge. Behrens et al. assigned the peak in the middle of the edge appearing at Ag2O to a 2p f 4d transition in which a d hole is created by the s-d hybridization through the covalent bond of the ligand.22-24 We have two questions in this context. The assignment of the peak to the 2p f 4d transition suggests the presence of a d hole or the d9 state in the Ag of Ag(DMe-DCNQI)2; however, the presence of the d9 state contradicts other experimental results about Ag(DMe-DCNQI)2, such as electric conductivities and magnetic properties.25 In addition, if the d hole is created due to the covalent bond, the edge height should increase with the covalency, not with the ionicity. The more electronegative ligand gives a smaller edge peak. It is the reverse order expected from the general rule for the L3 white line peak intensity. As far as we know, there have been no systematic studies in the L3-edge XANES of Ag+ compounds. We carried out measurements of L3-edge XANES for Ag+ (d10) compounds to better understand the origin of the edge peak and to obtain empirical relations between the peak intensity and other physical parameters, such as electronegativity. We interpret the intensity of the absorption edge peak in the framework of a molecular orbital picture instead of band theory throughout this paper. 2. Experimental Section The X-ray absorption experiments were performed using a beamline 11B at the Photon Factory of the High Energy Accelerator Research Organization (KEK-PF). Ag L3-edge XANES spectra were recorded in a sample current mode. The Photon Factory was operated at 2.5 GeV and 400 mA. The X-ray was monochromatized using a Ge(111) double crystal. All measurements were carried out in a UHV chamber with a base pressure of 1 × 10-7 Pa. The samples were fixed on conducting
10.1021/jp907669s 2010 American Chemical Society Published on Web 03/08/2010
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TABLE 1: Structural Parameters of Ag Compoundsa formal valence
nearest neighbor atoms, coordination numbers
Ag metal Ag2O Ag2CO3 Ag2SO3
0 1 1 1
AgNO3 Ag2SO4 AgClO4 AgNO2
1 1 1 1
K(Ag(CN)2) AgCN
1 1
AgSCN
1
AgF AgCl Ag2S
1 1 1
Ag × 12 O×2 O×3 Ag(1)-O × 4 Ag(2)-O × 4 Ag(2)-S × 1 O×7 O×6 O×8 O×6 N×1 C×2 C×1 N×1 S×1 N×1 F×6 Cl × 6 Ag(1)-S × 3 Ag(2)-S × 4
Ag(CH3COO) AgNCO
1
compound
N×2 O×2
site symmetry around Ag atom Oh D3d C1 C1 C1 C1 C2 D2d,x C2V C1 C3V C1 Oh Oh C1 C1 no data C2V
bond distance/0.1 nm 2.89 × 12 2.04 × 2 2.23, 2.24, 2.44 2.23, 2.30, 2.47, 2.50 2.40, 2.45 × 2, 2.73 2.465 2.48 × 2, 2.50, 2.56, 2.58, 2.75, 2.77 2.41 × 2, 2.43 × 2, 2.69 × 2 2.53 × 4, 2.73 × 4 2.44 × 2, 2.72 × 4 2.30 2.13 × 2 2.15 × 1 1.85 × 1 2.43 2.22 2.46 × 6 2.77 × 6 2.42, 2.45, 2.92 2.49, 2.57, 2.60, 2.98 2.163 × 2 2.163 × 2
ref
symbol
35 36 37
A B C D
38 39 40 41
E F G H
42 43
I J
44
K
45 46 47
L M N
48
O P
a
In the case that more than two different kinds of chemical elements coordinate with a Ag atom, they are listed separately. More than two different Ag atoms are included in the unit cell; they are listed by notation as Ag(1) and Ag(2) etc. (for example, see the Ag2SO3 row). The number of the first coordination atoms is written like “O × 1”, which means one oxygen atom coordinates to a Ag atom.
carbon tape. The spectra were recorded at ambient temperature under UHV conditions. The spectra did not change, even after prolonged X-ray irradiation. The E0 (edge energy) was calibrated by Ag metal. The samples are detailed in Table 1. Ag metal foil (99.95%), AgClO4 (99.9%), AgSCN (99%), and KAg(CN)2 (99.9%) were purchased from Strem Chemicals, Inc. Ag2O (99.0%), Ag2SO4 (99.5%), AgNO3 (99.8%), AgCN (99%), and Ag(CH3COO) (99.9%) were purchased from Wako Pure Chemical Industries, Ltd. AgNCO (silver cyanate, 99%), Ag2CO3 (99%), AgNO2 (99%), Ag2S (99.9%), and Ag2SO3 (99.9%) were purchased from Sigma-Aldrich Co. The samples were quickly mounted on the sample holder to prevent photo reduction. The background absorption originating from other elements in the system and from Ag edges other than L3 was removed by a linear extrapolation from the pre-edge region. The edge height was normalized by dividing the whole spectra by the value of a smooth postedge background absorption at 3455 eV. The smooth postedge background absorption curve was obtained using a cubic smoothing method in the postedge region. An absorption edge was then approximated with the arctangent function µ ) arctan{(E - E0)/ JW}, as shown in Figure 1. The edge peak was fitted with a Lorentz function, µ ) A/(1 + B(E - Ep)2). Peak intensity was estimated by the area of the peak. 3. Results and Discussion 3.1. X-ray Edge Spectra of Ag+ Compounds. Structure data of a series of Ag+ compounds are listed in Table 1 with their coordination numbers and bond distances. The L3-edge XANES spectra of all the Ag+ compounds are depicted in Figures 2-5. All the spectra exhibit an edge peak around 3352 eV, except for the Ag foil spectra. Figure 2 shows the spectra of Ag metal and Ag halide (AgCl and AgF). Ag metal has a d10s1 configuration and an fcc (face centered cubic) structure, whereas AgCl and AgF have a mainly d10 configuration with a NaCl structure.
Figure 1. Ag L3-absorption edge spectrum of AgCN as an example for the fitting analysis. The lower graph shows the fitting result of a normalized spectrum. The thick, solid line is a normalized spectrum, and the broken line is the best-fit curve. The upper graph shows the residual spectrum (solid line) and fitted curve (broken).
AgCl shows a small but distinctive shoulder around 3352 eV and an edge peak around 3358 eV. Very small shoulder structures are found in AgF at the corresponding positions, even though F has the largest electronegativity. This means that the peak intensity does not have a positive correlation with the electronegativity. Figure 3 shows the spectra of various Ag+ compounds where oxygen atoms are the first coordination atoms, except for AgNO2, which has one extra nitrogen atom in addition to six oxygen atoms in the first coordination shell. Note that the peak intensities are scattered from compound to compound even if the compounds have the same types of atoms in the first coordination shell. Figure 4 shows the spectra for Ag+ compounds containing cyano groups. AgCN and KAg(CN)2 have especially high edge peak intensities relative to the edge height. Figure 5 shows the spectra for other compounds, such as Ag2S and Ag(CH3COO).
Ag L3-EDGE Ag L3-Edge XANES of 4d10
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Figure 2. Ag L3-X-ray absorption edge spectra of (a) silver metal foil, (b) AgF, and (c) AgCl.
Figure 3. Ag L3-X-ray absorption edge spectra of various Ag+ compounds: (a) AgClO4, (b) Ag2SO4, (c) AgNO3, (d) Ag2SO3, (e) Ag2CO3, (f) Ag2O, and (g) AgNO2.
Figure 4. Ag L3-X-ray absorption edge spectra of various Ag+ compounds: (a) AgNCO, (b) AgSCN, (c) AgCN, and (d) KAg(CN)2.
3.2. Origin of the Edge Peak. As mentioned above, the peak intensity has no positive relation to the coordination charge, as was found in the comparison of AgF and AgCl. It means the edge peak does not obey the general rule proposed for the white lines in other 4d and 5d transition metals.6 Figure 6 shows the relation between the edge peak intensity and the coordination charge. Coordination charge is defined as
(
η)Z1-
∑ N1 exp(- 41 (χM - χL)2))
(1)
where χM and χL are the electronegativity of metals and ligands, respectively. N is the cooridation number. We found a negative
Figure 5. Ag L3-X-ray absorption edge spectra of Ag+ compounds: (a) Ag2S and (b) Ag(CH3COO).
Figure 6. Plot of the edge peak intensity against the coordination charge. (B) Ag2O, (C) Ag2CO3, (D) Ag2SO3, (E) AgNO3, (G) AgClO4, (H) AgNO2, (I) KAg(CN)2, (J) AgCN, (K) AgSCN, (L)AgF, (M) AgCl, and (N) Ag2S.
correlation, as shown in Figure 6. It means the edge peak in the Ag L3-edge is, rather, related to the covalency of Ag-L (L represents a ligand coordinating to Ag) bond and not to its ionicity. We tried to interpret this situation in a molecular orbital framework. If the filled 4dz2 and empty 5s orbitals, φ4dz2 and φ5s, have the same symmetry and are hybridized, the 4d vacancy is created, as suggested by Behrens.22,23 In a spherical system such as an atom, hybridization of 4d and 5s orbitals does not occur due to the symmetry demands. When Ag+ is present in the molecules and the rotational symmetry around an axis perpendicular to the principal axis (z axis here) is e2-fold, the 4dz2 and 5s orbitals can belong to the same irreducible representations, such as A(Cn), A1(CnV, Dn, D2d), A1g(C2nh, S2), and A′(C(2n-1)h, S4, S6). Actually, many Ag+ compounds have a linear structure or other symmetries lower than tetrahedral (T) or octahedral (O), as shown in Table 1. For a linear AgL2 molecule, the ligand group orbitals are represented by symmetric and antisymmetric molecular orbitals φLg and φLu as follows:
1 (φL1 + φL2) √2 1 ) (φL1 - φL2) √2
φLg ) φLu
(2)
where φL1 and φL2 are the ligands’ coordination orbitals. Since φLg has an A1g symmetry, it can interact with φ5s and φ4dz2, which have an A1g symmetry. The molecular orbitals ψi (i ) 1, 2, 3), which are composed of Ag orbitals φ5s and φ4dz2 and ligand orbital φLg, can be written as follows:
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ψ1 ) (c11φ5s + c12φ4dz2 + c13φLg) ψ2 ) (c21φ5s + c22φ4dz2 + c23φLg) ψ3 ) (c31φ5s + c32φ4dz2 + c33φLg)
Miyamoto et al.
(3)
Ψ1, Ψ2, and Ψ3 are dominated by φ5s, φ4dz2, and φLg (|c11| ≈ |c22| ≈ |c33| ≈ 1), respectively, and Ψ2 and Ψ3 are occupied. Thus, the initial state can be written as
Φi ) |φ4d4 φ4d4 ψ2ψ2ψ3ψ3|
(4)
where φ4d’s express the d electron wave functions other than φ4dz2, and core electrons are omitted. The relaxation process by the electron transfer from the ligand to the central atom 5s orbital is small because the 4d states to which the 2p absorption occurs at the peak edge energy are fully occupied and the core hole is well screened. Thus we can adopt a frozen orbital approximation. The final state is written as
Φf ) 2p|φ4d4 φ4d4 ψ2ψ2ψ3ψ3ψ1|
(5)
where 2p_ is a 2p hole. The edge peak intensity is written as
Figure 7. Plot of the edge peak intensity against the 4d contribution to 5s (∑m|cm(2) q2m|2) in some Ag oxygen compounds. (B) Ag2O, (C) Ag2CO3, (D) Ag2SO3, (E) AgNO3, (F)Ag2SO4, and (G) AgClO4.
TABLE 2: Oxidative Dimerization Values of Ag Compounds Ag compounds
oxidative dimerization values/V
AgNO3 Ag2SO4 Ag(CH3COO) AgCl AgNO2 AgSCN Ag2S AgCN
0.29 0.59 0.95 1.24 1.73 1.83 3.08 2.79
I ∝ |〈Φi |z|Φf〉|2 ≈ |〈2p|z|ψ1〉|2 ) |c12|2|〈φ2pz |z|φ4dz2〉|2
(6) where only 2pz f 4dz2 is taken into account in the Ψ1, since the 2p f 5s transition probability is generally much smaller than that of the 2p f 4d transition and negligible.26-31 Since hybridization occurs due to the perturbation of the ligand field V, c12 can be expressed as
c12 )
∫ φ*5s Vφ4d
z2
dr
ε4dz2 - ε5s
)
V5s,4dz2 ε4dz2 - ε5s
(7)
As shown in the Supporting Information, the value of V5s,4dz2 is roughly proportional to the inverse cube of the interatomic distance between the absorption atom and the ligand, (1/ rML)3, and hence,
I∝
(
V5s,4dz2 ε4dz2 - ε5s
)
2
∝
( ) 1
rML
6
(8)
where rML is the interatomic distance between the absorption atom and the ligand. The perturbation from the ligand field yields the hybridization. If the Ag+ ion core were hard, the ligand field could not induce the s-d hybridization, but Ag is classified to the soft acid, where the Ag+ ion core is easily reformed. Consequently, the d orbital is hybridized with the s state and contributes more to the ψ1 antibonding to make a d hole. At the same time, the s-d hybridized orbital and ligand orbital makes a strong covalent bond. The edge peak intensity is related to the strength in the covalent bond, the Ag-L bond strength, or both. To demonstrate the above conclusions, we plotted the edge peak intensity of the compounds with oxygen atoms in the first coordination shell against the ligand field effects expressed by ∑m|cm(2) q2m|2 (see the Supporting Information). The m is the
Figure 8. Plot of edge peak intensity against oxidative dimerization potential: (E) AgNO3, (F) Ag2SO4, (H) AgNO2, (J) AgCN, (K) AgSCN, (M) AgCl, (N) Ag2S, and (O) Ag(CH3COO).
magnetic quantum number and c(2) m and q2m express the contribution of different 4d obribals and the geometry of the ligands. As shown in Figure 7, there is a linear correlation between the two (correlation coefficient, 0.996). Therefore, the edge intensity is related to the degree of contribution of the d state to ψ1. Such a direct comparison is possible when the coordinating atom is the same. To compare the edge peak intensities with the different coordinating atoms, we need the other index. The above discussion has suggested that the edge peak intensity is related to the polarization of the ion core, which can be associated with the idea of the “soft and hard acid and base” theory proposed by Pearson.32 Ag is a typical soft acid. A soft acid makes a strong bond with a soft base. It is wellknown that soft bases and acids have a large polarizability in the electron cloud. Thus, the edge peak intensity may depend on the softness of the base. An empirically available index for the base softness is the oxidative dimerization potential,33 which is shown in Table 2. A smaller value means a harder base. Figure 8 shows a plot of the edge peak intensity against the oxidative dimerization values. There appears to be a positive correlation with oxidative dimerization (R ) 0.74). Although the edge peak intensity (s-d hybridization) seemed to have a good correlation with the softness of the ligand, the number of ligands with their softness known is not insufficient.
Ag L3-EDGE Ag L3-Edge XANES of 4d10
J. Phys. Chem. A, Vol. 114, No. 12, 2010 4097 4. Conclusions The distinctive edge peak appearing at the Ag L3 X-ray absorption edge is formally assigned to 2p to 5s. The 2p-to-5s transition in itself is weak, but the intensity is borrowed from 4d through the 5s-4d hybridization. The edge peak intensity is directly related to the covalency between Ag and the ligand, which enhances the 5s-4d hybridization. This assignment prevents us from a misunderstanding of the valence state of Ag based on the L3 edge peak.
Figure 9. Plot of edge peak intensity against enthalpy change Hex in the exchange reaction, Hex in eq 9: (B)Ag2O, (C) Ag2CO3, (D) Ag2SO3, (E) AgNO3, (F) Ag2SO4, (G) AgClO4, (H) AgNO2, (J) AgCN, (K) AgSCN, (L) AgF, (M) AgCl, (N) Ag2S, (O) Ag(CH3COO), and (P) AgNCO.
As shown in the Supporting Information, the enthalpy change of the following reaction 9 expresses the degree of covalency of the bond:
AgX + NaF f AgF + NaX-Hex
Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
(9) References and Notes
Figure 9 shows the relation between the exchange enthalpy and the peak intensity. The exchange energy of KAg(CN)2(I) is missing and was omitted. The figure shows a good positive correlation, except for AgCN (J), AgCl (M), and Ag(CH3COO) (O). AgCN is off the line because Ag d orbitals in AgCN are interacted with empty CN π* bonds, which directly create the d vacancy. Ag(CH3COO) can have a d-π* interaction, although its crystal structure is unknown. AgCl is another exception in which the s-d hybridization occurs through a different mechanism, which will be discussed elsewhere.34 We can get a linear fit on the other data with a correlation coefficient, R ) 0.90, indicating that the edge peak intensity in many Ag+ compounds is related to the covalency of the bond. To clear up the misunderstanding in the edge peak intensity of the d10 element, we propose the edge peak of the d10 element should be assigned to the 2pf 5s transition enhanced by s-d hybridization. We expand the initial and final states, 4 and 5 in the atomic orbitals using eq 3 and then compare the dominant terms of the initial and final states after X-ray absorption (see the Supporting Information):
Φi (main):|φ4d φ4d φLφL| Φf (main):2p|φ4d φ4d φLφLφ5s|
Acknowledgment. One of the authors (T. Miyamoto) is supported by the Research Fellowships of JSPS. This work is also financially supported by a Grand-in-Aid for JSPS and JST Research Promotion Project EXPEEM. The XANES experiments were carried out under the approval of the PF Advisory Committee (PAC No. 2004G062).
(10)
The transition can formally be regarded as 2p f 5s. As mentioned above, the 2p f 5s transition is much weaker than the 2p f 5d transition. The peak intensity is enhanced through the hybridization with 4d. This phenomenon is analogous to the one where the strong pre-edge peak in the K-edge XANES of 3d or 4d elements in distorted compounds is assigned to 1s f 3d or 4d through the hybridization of the p component.1 The edge peak intensity of Ag+ compounds is an indicator of the degree of 5s-4d hybridization or covalence. It does not necessarily indicate the presence of Ag2+ (d9s0), but means that the hybridization of the d9s1 configuration occurs to the main d10s0 configuration, consequently forming a 4d hole. Finally, we should point out that the edge peak of the other d10 transition metals, such as Au+ and Hg2+, may be assigned similarly and should be interpreted carefully, although further investigation for their compounds is necessary.
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