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X-ray Absorption Spectroscopic Study on Valence State and Local Atomic Structure of Transition Metal Oxides Doped in MgH2 Nobuko Hanada,†,‡ Takayuki Ichikawa,*,† Shigehito Isobe,† Tessui Nakagawa,§ Kazuhiko Tokoyoda,| Tetsuo Honma,⊥ Hironobu Fujii,† and Yoshitsugu Kojima† Institute for AdVanced Materials Research, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan; Department of Quantum Matter, ADSM, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan; R & D Center, Taiheiyo Cement Corporation, 2-4-2 Osaku, Sakura, 285-8655, Japan; and Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan ReceiVed: February 28, 2009; ReVised Manuscript ReceiVed: June 14, 2009
A valence state and a local structure of transition metals (Nb, V, and Ti) in MgH2 doped with metal oxides (Nb2O5, V2O5, and TiO2nano) by ball milling were examined by X-ray absorption spectroscopy (XAS). The main edge regions of the Nb, V, and Ti K-edges in the X-ray absorption near edge structure (XANES) profiles are located between 0 and +5 in the oxidation states. Since these spectra coincide with those of NbO, VO, and Ti2O3, respectively, the additives are reduced by MgH2 to the metal oxides, which have lower oxidation states than those of the starting materials. Furthermore, in order to examine the local structures around the transition metal atoms, the extended X-ray absorption fine structure (EXAFS) spectra were analyzed. In the Fourier transformation curves of the EXAFS spectra, all samples doped with the metal oxides show two peaks corresponding to metal-oxygen and metal-metal bonds, being the same as the references of NbO, VO, and Ti2O3. The local structure formed after ball milling or dehydrogenation is close to that of each of the reduced metal oxides (NbO, VO, and Ti2O3) but in a more disarrangement state. 1. Introduction Magnesium (Mg) is one of the attractive hydrogen storage materials because of its high hydrogen capacity (7.6 mass %). However, the reaction kinetics of hydrogen absorption and desorption is too low for practical use and needs temperatures higher than 300 °C. In order to improve the kinetics, the hydrogen storage properties of the ball milled Mg or MgH2 composites with a small amount of transition metals1-5 and transition metal oxides6-14 have been investigated. The results indicated that transition metal oxides exhibit superior catalytic effect on hydrogen sorption kinetics of MgH2 to the transition metals. However, the role of metal oxide as a catalyst has not been clarified yet. Oelerich et al. have reported that the oxidation of the transition metals played an important role for catalysis to improve the kinetics of absorption reaction, because after exposing MgH2 with ultrapure V metal to air, the product exhibited much better hydrogen absorption properties than the nonexposed one.7 In addition, Bobet et al. have studied the hydrogen absorption/desorption kinetics of Mg with Cr2O3 ball milled.12-14 It was suggested that the nanosized catalyst Cr2O3 was reduced by MgH2 during ball milling to form Cr atom clusters, although those metallic phases have not been detected by X-ray diffraction (XRD) or any other experiments yet.12 Furthermore, de Castro et al. have shown the hydrogen desorption kinetics of MgH2 with nanoparticle ball milled FeF3,15,16 which has better kinetics than that of pure MgH2 itself. * To whom correspondence should be addressed. E-mail: tichi@ hiroshima-u.ac.jp. Fax: +81-82-424-5744. † Institute for Advanced Materials Research, Hiroshima University. ‡ Current address: Department of Engineering and Applied Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo, 102-8554, Japan. § Department of Quantum Matter, ADSM, Hiroshima University. | Taiheiyo Cement Corporation. ⊥ Japan Synchrotron Radiation Research Institute.
In that case, both Fe and MgF2 phases were observed in the sample by synchrotron X-ray diffraction measurement. This indicates that FeF3 was reduced by MgH2 to form Fe and MgF2 during ball milling. So far, we have reported that 20 h ball milling treatment is necessary to obtain the optimum catalytic effect of a metal oxide such as Nb2O5 on hydrogen sorption properties of MgH2. However, such a long milling treatment gave a much worse catalytic effect on MgH2 with a transition metal nanometer size particle such as Ninano.17,18 Furthermore, it has been found that the MgH2 with 1 mol % Nb2O5 ball milled for 20 h after dehydrogenation at 200 °C could absorb a hydrogen of more than 4.5 mass % even at room temperature under hydrogen gas at 1 MPa within 15 s.19 We presumed that Nb2O5 distributing in the Mg was reduced to form metal Nb and MgO during ball milling or heat treatment. However, only the MgO was observed in the ball milled sample by XRD measurement, while any traces of the Nb metal or the Nb related phases could not be observed.17-19 X-ray absorption spectroscopy (XAS) has been applied to investigate the valence state and the local structure of the catalysts in this work. The method is suited to detect the local structure of nano crystalline or amorphous phase. Recently, Le´on et al. have obtained such information around Ti in NaAlH4 doped with TiCl3 by using this method.20 In this paper, we studied the valence state and the local structure of metal oxides (Nb2O5, V2O5, and TiO2nano) ball milled with MgH2 to evaluate the precise structures of the catalysts. 2. Experimental Procedures Magnesium hydride (MgH2) powder (90% purity, several micrometer meters in size) and transition metal oxides Nb2O5 with mesopores of 3.2 nm in diameter (99.5% purity) and V2O5
10.1021/jp901859f CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
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Figure 1. TG-TDMS profiles of pure MgH2 after ball milling and MgH2 with 1 mol % Nb2O5, V2O5, and TiO2nano after ball milling. The solid line represents the sample after milling, and the dashed line represents the sample after rehydrogenation.
Figure 3. (a) Nb K-edge XANES spectra of MgH2 with 1 mol % Nb2O5 after ball milling, dehydrogenation, and subsequent rehydrogenation. The XANES spectra of Nb, NbO, and Nb2O5 are included for comparison. (b) V K-edge XANES spectra of MgH2 with 1 mol % V2O5 after ball milling, dehydrogenation, and subsequent rehydrogenation. The XANES spectra of V, VO, V2O3, and V2O5 are included for comparison. (c) Ti K-edge XANES spectra of MgH2 with 1 mol % TiO2nano after ball milling, dehydrogenation, and subsequent rehydrogenation. The XANES spectra of Ti, TiH2, Ti2O3, and TiO2 are included for comparison.
Figure 2. X-ray diffraction profiles of pure MgH2 after ball milling heated to 470 °C and MgH2 with 1 mol % Nb2O5, V2O5, and TiO2nano after ball milling heated to 450 °C in a He flow (i.e., after TG-TDMS measurement). The dashed line and the black circles correspond to the peaks of Mg and MgO phase, respectively.
(99.99% purity) were purchased from Sigma-Aldrich. TiO2nano (82.8% purity) with the particles less than a hundred nanometers was provided from Millennium Chemicals. As references of the XAS measurements, Nb (99.96% purity), NbO, V (99.7% purity), and Ti (99.9% purity) powders were from Rare Metallic, TiH2 (98% purity), V2O3 (99.99% purity), and Ti2O3 (99.9% purity) powders were from Sigma-Aldrich, and VO (99.5% purity) was purchased from Soekawa Chemical. The samples before and after ball milling were always handled in an argon glovebox equipped with a recycling purification system (Miwa MFG, MP-P60W), in which oxygen and water levels are below
1 ppm. Totally, 300 mg of each mixture of MgH2 and 1 mol % Nb2O5, V2O5, or TiO2nano was put into a Cr-steel pot (30 cm3 in inner volume) together with 20 steel balls (7 mm in diameter). Then, the pot was degassed below 1 × 10-4 Pa for 12 h, and high-purity hydrogen gas (6 N) at 1.0 MPa was introduced into it. After that, the mixture was ball milled for 20 h at 400 rpm by a planetary ball mill apparatus (Fritsch P7). To synthesize the samples dehydrogenated and rehydrogenated, the ball milled samples are dehydrogenated at 200 °C in a high vacuum for 8 h and then hydrogenated at 200 °C under 3 MPa of H2 for 8 h. The hydrogen desorption (HD) properties of the products were examined by a thermal desorption mass spectrometer (TDMS) (Anelva, M-QA200TS) combined with a thermogravimetric (TG) analyzer (Rigaku, TG8120). This equipment was specially designed and installed in another Ar glovebox, so that the measurements of TDMS and TG could be achieved simultaneously without exposing the samples to air. The measurements were conducted under a high-purity He gas (6 N) flow with a flow rate of 300 mL/min and a heating rate of 5 °C/min (maximum temperature range was up to 450 °C).
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XAS measurements were carried out at the BL19B2 beamline of the SPring-8 synchrotron radiation (SR) facility in Japan. The SPring-8 storage ring was operated with an electron beam of 8 GeV and a maximum electron current of 100 mA. This beamline is a hard X-ray bending magnet one. The Nb K-edge and V K-edge XAS spectra were measured in transmission mode. The Ti K-edge spectrum of the samples and the references were measured in fluorescence mode and transmission mode, respectively. X-ray from SR was monochromated by using an X-ray monochromator with a pair of Si (111) crystals in fixedexit mode. All the preparations of the sample were performed in the glovebox. The samples were formed to be in the shape of a 1 cm diameter pellet under a pressure of 60 MPa, and boron nitride (BN) powder (99% purity, High purity chemicals) was used as a diluent. To avoid the oxidation of the sample during the sample transfer and the XAS measurement, the samples were protected by a transparent polyimide film with 8 µm thickness. All XAS data were analyzed by using the REX2000 program package (Rigaku). For extended X-ray absorption fine structure (EXAFS) spectra, background subtraction was performed by fitting the Victoreen function to the pre-edge region and the McMaster coefficient to the postedge region. The absorption threshold was decided from the first inflection point in the second derivative of the near-edge region. EXAFS oscillation function, χ(k), was obtained by removing the atomic background using a spline smoothing method. The radical structure function, FT(χ(k)*k3), was obtained by Fourier transforming of the k3 weighted χ(k) function multiplied by a Hanning window function. The curve-fitting was performed by least-squares fits on first and second neighbor peak of FT curves. The fitting parameters were neighboring atomic distances Ri, EXAFS Debye-Waller factors σi, and coordination numbers Ni for the coordination shell i. The electron mean free path λ was fixed to 6 in all the fittings. Backscattering functions and phase shift functions for each shell were obtained from FEFF8.2. The EXAFS spectra and FT curves of NbH2 and VH2 were simulated by using FEFF8.2. The XANES data for all the samples were subtracted by the Victoreen function in the background region before the K-edge and were normalized by the edge jump.
Hanada et al.
Figure 4. EXAFS spectra of Nb, NbO, and MgH2 with 1 mol % Nb2O5 after ball milling, dehydrogenation, and rehydrogenation.
3. Results and Discussion Figure 1 shows TDMS and TG curves of pure MgH2 ball milled for 20 h and MgH2 with 1 mol % Nb2O5, V2O5, or TiO2nano ball milled for 20 h. It can be seen that all the samples with the additives desorb a hydrogen of more than 6 mass % at lower than 250 °C. The peak temperatures of the samples with the additives decrease by about 150 °C compared to that of pure MgH2. Therefore, a small amount of transition metal oxides reveal a significant catalytic effect on the hydrogen desorption properties. Furthermore, the rehydrogenated sample shows almost the same TDMS and TG results as the ball milled samples (Figure 1). But, the decreases in hydrogen amount of less than 1 mass % are observed for all samples. The samples with Nb2O5 and V2O5 rehydrogenated desorb hydrogen at lower temperature ranges than those of the ball milled samples, indicating that the chemical states of those additives might slightly change during the heating process at 200 °C for the dehydrogenation and subsequent rehydrogenation treatment. However, the sample with TiO2nano after the rehydrogenation desorbs hydrogen in the same temperature range as that after ball milling. The XRD profiles of MgH2 with Nb2O5, V2O5, and TiO2nano after dehydrogenation by heating to 450 °C are shown together with the result of pure ball milled MgH2 after heating to 470
Figure 5. Fourier transformation curves of (a) MgH2 with 1 mol % Nb2O5 after ball milling, dehydrogenation, and rehydrogenation and (b) the sample after dehydrogenation, Nb, NbH2, and NbO.
°C in Figure 2. The MgO phase is clearly observed in the samples with additives compared to that of pure MgH2. From this fact, we can deduce that the metal oxides in MgH2 interact with Mg or MgH2 during the ball milling or the heating process and are reduced to generate the MgO phase, even though the reduced phase of the transition metals could not be detected in the XRD profiles. In order to clarify whether the transition metal oxides are really reduced or not, the XANES measurements have been
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TABLE 1: EXAFS Fitting Parameters of NbO and MgH2 with 1 mol % Nb2O5 Ball Milled after Dehydrogantion, VO and MgH2 with 1 mol % V2O5 after Ball Milling, and Ti2O3 and MgH2 with 1 mol % TiO2nano after Ball Millinga sample NbO (Pm3m, a ) 4.210 Å)
fit range (R) (Å)
22
NbO MgH2 + 1 mol % Nb2O5dehydrogenation
1.166-1.994 2.178-3.007 1.258-3.191
VO (Fm3m, a ) 4.073 Å)22 MgH2 + 1 mol % V2O5 milling for 20 h Ti2O3 (R3cj, a ) 5.158 Å, c ) 13.611 Å, R ) 90°, γ ) 120°)22
MgH2 + 1 mol % TiO2milling for 20 h a
1.381-2.270 2.270-2.884
1.074-3.007
shell
N
r (Å)
O Nb O Nb O Nb O V O V O O Ti Ti O Ti
4.0 8.0 4.0 7.1 2.7 7.0 6.0 12.0 3.4 1.3 3.0 3.0 1.0 3.0 2.5 1.0
2.11 2.98 2.11 2.98 2.21 2.99 2.04 2.88 2.19 2.87 2.03 2.07 2.58 2.99 2.00 2.86
∆E (eV)
σ (Å)
R-factor
0.112 0.951 19.975 -7.287
0.059 0.062 0.109 0.131
0.495 0.264 0.096
6.964 12.795
0.066 0.072
2.330 0.172
1.274 18.962
0.058 0.056
1.047
Literature values of NbO, VO, and Ti2O3 are shown as the references.
performed. Figure 3a,b,c show the XANES spectra of, respectively, Nb, V, and Ti K-edges for the samples after ball milling, dehydrogenation, and subsequent rehydrogenation. For comparison, the XANES spectra of the reference samples, Nb, Nb2O5, V, V2O3, V2O5, Ti, TiH2, Ti2O3, and TiO2nano, are shown in Figure 3. The Nb, V, and Ti K-edge XANES patterns of MgH2 ball milled with the corresponding metal oxides (Nb2O5, V2O5, and TiO2nano) are located between 0 and +5 in the oxidation states. Especially, in the sample with V2O5, the preedge around at 5470 eV observed in the reference of V2O5 completely disappears. Furthermore, these spectra are consistent with those of NbO, VO, and Ti2O3, in which the valence states of the transition metal are +2 or +3. Therefore, the metal oxides in the samples are reduced during ball milling by MgH2 to metal oxides, which have a lower oxidation state than that of starting materials. In details, the Nb K-edge spectrum of the sample after dehydrogenation is located at a slightly lower energy position than that of the ball milled sample, and then, the sample after rehydrogenation shifts back to the same position of the ball milled one as shown in Figure 3a. Thus, ball milling mainly affects the decrease of the Nb valence state from +5 to +2, while the dehydrogenation and rehydrogenation treatment affect only slight decrease and increase of the Nb valence state. Friedrichs et al. have reported a similar result of XANES spectra for MgH2 with 10 mass % nanocrystalline Nb2O5 ball milled for 50 h.21 Nb2O5 is reduced during the ball milling process to NbO2 with an oxidation state of +4. Further reduction occurs during the first desorption process to an oxidation state of +2.5.21 On the other hand, in the V and Ti K-edges of the metal oxides, the shift of oxidation state is not clearly observed during dehydrogenation and hydrogenation treatment. Figure 4 displays the Nb K-edge EXAFS spectra (χ(k)*k3) of the samples after ball milling, dehydrogenation, and rehydrogenation with the references of Nb and NbO. The EXFAS oscillations of all the samples are extremely small compared to those of Nb and NbO. This indicates that the samples ball milled and heated have strong disarrangements of the local structure. Figure 5a shows the Fourier transformation (FT) curves of EXAFS spectra for the samples. The sample after ball milling has a broad peak in the R range of around 1-3 Å. After dehydrogenation, two peaks clearly appear in the same R range, indicating that the ordering local structure is formed by the heating treatment to 200 °C. After subsequent rehydrogenation, the intensity of both peaks decreases. Therefore, the disarrange-
ment of the local structure might occur again during the hydrogenation. Figure 5b shows FT curves of Nb, NbO, and NbH2 simulated by FEFF (space group Fm3m, a ) 4.566 Å)22 to compare with that of the sample after dehydrogenation. Nb and NbH2 have a first peak corresponding to an Nb-Nb bond at 2.0-3.5 Å of the R range, respectively. On the other hand, in NbO, which has the same +2 oxidation state as the hydride phase of NbH2, the first peak corresponding to a Nb-O bond and the second peak corresponding to a Nb-Nb bond are observed at 1.0-3.0 Å of the R range. Although the peak intensity of the sample is smaller than those of references, two peaks corresponding to Nb-O and Nb-Nb bonds are clearly observed, indicating that the local structure of the additive in MgH2 is close to that of NbO. To decide the local structure around Nb atoms, the curve fittings of EXAFS data for the sample after dehydrogenation and NbO are performed as shown in Table 1. According to the literature structure of NbO with a space group of Pm3m and a lattice constant of 4.210 Å,22 Nb should be coordinated by four oxygen atoms with the coordination distance of 2.11 Å and then coordinated by eight niobium atoms with 2.98 Å as a second neighbor. The fitting result of NbO shows the same coordination number and distance for both O and Nb shells as those of literature NbO. However, in the sample, the fitting result is not exactly the same as NbO. For the first neighbor of O shell, the distance of the O shell is 0.1 Å longer than that of NbO. Furthermore the coordination number and Debye-Waller factor (σ) are 1.3 smaller and 0.05 larger those of NbO, respectively. It has been pointed out in the literature23 that generally the decrease in the coordination number and the increase in the Debye-Waller factor in the powder compared to the bulk materials are caused by decreasing the particle size to nanometer scale. From the TEM observation of the ball milled sample with Nb2O5, the additive particles were homogeneously distributed in the MgH2 particles in nanometer scale as it is difficult to be distinguishable in the observation level.24 Therefore, the additive in the sample would have the short-range ordering structure of NbO as nanometer scale particles. Figures 6 and 7 show the V and Ti K-edge EXAFS spectra of the samples after ball milling, dehydrogenation, and subsequent rehydrogenation with the references of V, VO, Ti, TiH2, and Ti2O3. In the V K-edge, the spectra after dehydrogenation and subsequent hydrogenation do not change from that after
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Figure 6. EXAFS spectra of V, VO, and MgH2 with 1 mol % V2O5 after ball milling, dehydrogenation, and rehydrogenation.
Figure 8. Fourier transformation curves of (a) MgH2 with 1 mol % V2O5 after ball milling, dehydrogenation, and rehydrogenation and (b) the sample after ball milling, V, VH2, and VO.
Figure 7. EXAFS spectra of Ti, TiH2, Ti2O3, and MgH2 with 1 mol % TiO2nano after ball milling.
ball milling (Figure 6). Of course all samples show the identical patterns in FT curves as shown in Figure 8a, which exhibit two peaks at 1-3 Å of the R range. This indicates that the local structure of the sample with V2O5 is kept during hydrogen sorption with heat treatment. In the Ti K-edge EXAFS spectra, the samples after ball milling show similar oscillation patterns to Ti2O3 (Figure 7). Figures 8b and 9 show the FT curves of V, VO, and VH2 simulated by FEFF (space group Fm3m, a ) 4.073 Å)22 and Ti, Ti2O3, and TiH2 simulated by FEFF (space group R3cj, a ) 5.158 Å, c ) 13.611 Å, R ) 90°, γ ) 120°),22 respectively, to compare with the samples after ball milling. Both VO and Ti2O3 have two peaks corresponding to metaloxygen and metal-metal bonds at 1-3 Å, being the same as both of the samples after ball milling. To decide the local structure around V and Ti atoms, the curve fittings of EXAFS data for the samples after ball milling and the VO and Ti2O3 are performed as shown in Table 1. In the sample with V2O5, the coordination distance and the number of the first neighbor are 0.15 larger and 2.6 smaller than literature values of VO, respectively. Furthermore, in the second neighbor, although the coordination distance of vanadium atoms is the same as that of VO, the coordination number is much smaller than VO. In the sample with TiO2, the coordination numbers of both first and second neighbors are much smaller than those of Ti2O3. From
Figure 9. Fourier transformation curves of MgH2 with 1 mol % TiO2nano after ball milling, Ti, TiH2, and Ti2O3.
these results, the local structures of the additive are close to those of bulk VO and Ti2O3, respectively, but in a more disordered local state, which might be formed by ball milling as the nanometer scale particles in MgH2 phase. 4. Conclusions A small amount of transition metal oxides of Nb2O5, V2O5, and TiO2nano brought a significant catalytic effect on the hydrogen desorption properties of MgH2. In the XRD profiles for all the products after dehydrogenation, the growth of the MgO phase in addition to the Mg phase was observed. However, any traces of the additives could not be detected. The Nb, V, and Ti K-edge XANES spectra of the MgH2 with the corresponding metal oxides after ball milling coincide with those of NbO, VO, and Ti2O3. Hence, it is confirmed that the additives are reduced by MgH2 during ball milling to the metal oxides, which have lower oxidation states of +2 or +3 than the starting materials. On the other hand, the dehydrogenation and rehy-
XAS Study of Transition Metal Oxides Doped in MgH2 drogenation treatment scarcely affect oxidation state of the additives. In the Fourier transformation curves of the EXAFS spectra for all samples, two peaks corresponding to metal-oxygen and metal-metal bonds are observed. The local structure formed after ball milling or dehydrogenation is close to that of each low oxidation state metal oxides but in a more disarrangement state, which might be formed by ball milling as the nanometer scale particles in MgH2 phase. Therefore, the improvement of hydrogen sorption of MgH2 occurs by the catalytic effect of the metal oxides, which has low oxidation state and the disarranged local structure. Hydrogen would take the special pathways on or in the catalytic metal oxides to decrease the activation energy of the sorption process. Acknowledgment. The synchrotron radiation experiments were performed at the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2004B0898-RI-np-TU). The authors gratefully acknowledge Prof. N. Umesaki (JASRI) for useful discussions. References and Notes (1) Liang, G.; Huot, J.; Boily, S.; VanNeste, A.; Schulz, R. J. Alloys Compd. 1999, 291, 295. (2) Liang, G.; Huot, J.; Boily, S.; VanNeste, A.; Schulz, R. J. Alloys Compd. 1999, 292, 247. (3) Zaluska, A.; Zaluski, L.; Stro¨m-Olsen, J. O. J.Appl. Phys. A 2001, 72, 157. (4) Bobet, J.-L.; Chevalier, B.; Song, M. Y.; Darriet, B.; Etourneau, J. J. Alloys Compd. 2003, 336, 292. (5) Hanada, N.; Ichikawa, T.; Fuji, H. J. Phys. Chem. B 2005, 109, 7188. (6) Oelerich, W.; Klassen, T.; Bormann, R. J. Alloys Compd. 2001, 315, 237.
J. Phys. Chem. C, Vol. 113, No. 30, 2009 13455 (7) Oelerich, W.; Klassen, T.; Bormann, R. J. Alloys Compd. 2001, 322, L5. (8) Oelerich, W.; Klassen, T.; Bormann, R. AdV. Eng. Mater. 2001, 3, 487. (9) Dehouche, Z.; Klassen, T.; Oelerich, W.; Goyette, J.; Bose, T. K.; Schulz, R. J. Alloys Compd. 2002, 347, 319. (10) Barkhordarian, G.; Klassen, T.; Bormann, R. Scripta Mater. 2003, 49, 213. (11) Barkhordarian, G.; Klassen, T.; Bormann, R. J. Alloys Compd. 2004, 364, 242. (12) Bobet, J.-L.; Desmoulins-Krawiec, S.; Grigorova, E.; Cansell, F.; Chevalier, B. J. Alloys Compd. 2003, 351, 217. (13) Bobet, J.-L.; Castro, F. J.; Chevalier, B. J. Alloys Compd. 2004, 376, 205. (14) Bobet, J.-L.; Castro, F. J.; Chevalier, B. Scripta Mater. 2005, 52, 33. (15) de Castro, J. F. R.; Yavari, A. R.; LeMoulec, A.; Ishikawa, T. T.; Botta F, W. J. J. Alloys Compd. 2005, 389, 270. (16) Yavari, A. R.; LeMoulec, A.; de Castro, F. R.; Deledda, S.; Friedrichs, O.; Botta, W. J.; Vaugham, G.; Klassen, T.; Fernandez, A.; Lvick, A. Scripta Mater. 2005, 52, 719. (17) Ichikawa, T.; Hanada, N.; Isobe, S.; Leng, H. Y.; Fujii, H. Mater. Trans. JIM 2005, 46, 1. (18) Hanada, N.; Ichikawa, T.; Fujii, H. J. Alloys Compd. 2005, 404406, 716. (19) Hanada, N.; Ichikawa, T.; Hino, S.; Fujii, H. J. Alloys Compd. 2006, 420, 46. (20) Le´on, A.; Kircher, O.; Rothe, J.; Fichtner, M. J. Phys. Chem. B 2004, 108, 16372. (21) Friedrichs, O.; Martı´nez, D.; Guilera, G.; Lo´pez, J. C. A.; Ferna´ndez, A. J. Phys. Chem. C 2007, 111, 10700. (22) Frenkel, A. I.; Hills, C. W.; Nuzzo, R. G. J. Phys. Chem. B 2001, 105, 12689. (23) Villars, P.; Calvert, D. Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, desk edition; American Society for Metals: Metal Park, OH, 1997. (24) Hanada, N.; Hirotoshi, E.; Ichikawa, T.; Akiba, E.; Fujii, H. J. Alloys Compd. 2008, 450, 395.
JP901859F
