Correlation between Structure of Titanium Additives and

Materials Transactions, Vol. 52, No. 4 (2011) pp. 635 to 640 Special Issue on Advanced Materials for Hydrogen Energy Applications #2011 The Japan Institute of Metals

Correlation between Structure of Titanium Additives and Dehydrogenation Reaction of Magnesium Borohydride Studied by Continuous Observation of X-Ray Absorption Spectroscopy Daiju Matsumura1 , Takahiro Ohyama1; *1 , Yuka Okajima1; *2 , Yasuo Nishihata1 , Hai-Wen Li2 and Shin-ichi Orimo2 1 2

Quantum Beam Research Directorate, Japan Atomic Energy Agency, Hyogo 679-5148, Japan Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Local structure of Ti-based additives in Mg(BH4 )2 was studied by X-ray absorption fine structure spectroscopy with conventional and dispersive mode in order to understand the correlation between the structure of the additives and dehydrogenation property of Mg(BH4 )2 . Simultaneous measurement of the dehydrogenation curve and the X-ray absorption spectroscopy by the dispersive optics revealed that a part of TiCl3 additive is converted to Tix Mg1
3x=2 (BH4 )2 (x ¼ 00:67) just after ball milling mixture and then promptly resolved to TiB2 at 100–150 C with the first dehydrogenation peak. TiO2 additive is slowly converted to lower oxidation state in wide temperature range around the second dehydrogenation peak at 350 C where the main dehydrogenation reaction of Mg(BH4 )2 occurs. [doi:10.2320/matertrans.MA201018] (Received October 25, 2010; Accepted December 7, 2010; Published January 26, 2011) Keywords: magnesium borohydride, X-ray absorption fine structure, dehydrogenation reaction, hydrogen, hydride

1.

Introduction

Development of advanced hydrogen storage materials with high hydrogen volumetric and gravimetric densities, moderate operation temperatures, and high dehydrogenation and rehydrogenation rates is highly required for onboard storage materials. Complex hydride is one of the candidates for the hydrogen storage materials because it contains many hydrogen atoms in one molecule.1–3) Magnesium borohydride Mg(BH4 )2 , has a high hydrogen density of 14.9 mass% and 125 kgH2 /m3 , has been regarded as one of the potential hydrogen storage materials.4–7) However, its high dehydrogenation temperature (>300 C) highly restricted its potential applications. Recent study reported that Ti compounds largely lowered the dehydrogenation temperature of Mg(BH4 )2 : TiCl3 and TiO2 addition decreases the initial dehydrogenation temperature of Mg(BH4 )2 by 170 and 50 C, respectively.4) These results motivate us to clarify the effect of the addition of TiCl3 and TiO2 on the dehydrogenation reaction of Mg(BH4 )2 . X-ray absorption fine structure (XAFS) technique is one of the powerful tools for the element selective determination of local structure. Observation of static XAFS spectra of the additives below and above the dehydrogenation temperature can clarify the contribution of the additives to the dehydrogenation reaction. However, because the dehydrogenation reaction itself is the time-dependent effect, the continuous observation of the XAFS spectra is desired for the exact determination of the correlation between the local structure of additive and the dehydrogenation reaction. Simultaneous observation of the local structure and the dehydrogenation reaction is suitable for understanding the actual active phase of additives contributing to the dehydrogenation reaction. *1Graduate

Student, Kwansei Gakuin University. Present address: Fuji Electric Holdings Co., Ltd., Matsumoto 390-0921, Japan *2Present address: SPring-8 Service Co., Ltd., Hyogo 679-5148, Japan

In this study, the local structure of Ti-based additives in Mg(BH4 )2 was studied from the viewpoint of dynamics. Dispersive optics for XAFS observation is an attractive tool because it does not contain the mechanical motion module during observation of the X-ray absorption spectroscopy, which enables us to get the stable and fast XAFS spectra.8,9) Simultaneous observation of XAFS spectra by the dispersive optics and dehydrogenation curves by quadruple mass spectroscopy (QMS) was operated. The effectiveness of a simultaneous measurement of a chemical reaction by several detection systems has been established in the study of the catalysis.10) Completely same time axes for the XAFS spectra and the dehydrogenation curves make the experiment reliable and clarify the correlation between the local structure of the Ti additives and the dehydrogenation properties of the storage materials of Mg(BH4 )2 . 2.

Experiments

Mg(BH4 )2 was synthesized from MgCl2 (95%, Aldrich) and NaBH4 (99.99%, Aldrich) in dehydrated diethyl ether (Cica-Reagent, Kanto Chemical), as described in the previous report.4) TiCl3 (95%, Toho Titanium Co., Ltd.) and TiO2 (mixture of anatase and rutile) were used for an additive without purification. Samples of Mg(BH4 )2 þ xTiCl3 with x ¼ 0:02 and 0.10 mol, and Mg(BH4 )2 þ 0:02 mol TiO2 were prepared by planetary ball milling (Fritsch P-7) with 20 steel balls (7 mm in diameter) in a hardened steel vial (30 cm3 in volume) under 0.1 MPa Ar for 5 h. The handling of the samples was always performed in a glove box filled with purified Ar/He gas (dew point below
90 C; oxygen concentration lower than 1 ppm) to avoid (hydro-)oxidation. The XAFS measurements were performed at the bending magnet beamline BL14B1 of SPring-8. This beamline has the apparatus of both the conventional and dispersive XAFS observation systems.11,12) The synchrotron beam current was kept to 100 mA by top-up mode at 8 GeV storage ring.

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A Si(111) double-crystal monochromator was used for the conventional XAFS measurements of the Ti K-edge (4965 eV). Two Rh-coated mirrors were used for the vertical focusing and the elimination of the higher-order harmonics. In the dispersive optics, a curved crystal of Si(111) was used for making the dispersive X rays in the Bragg configuration. The polychromator was bent horizontally by leading movements at the both fixed sides of the crystal. The crystal was cooled by soaking in an In-Ga pool. The focus point of the dispersed X rays corresponding to the sample position was set to 1000 mm from the polychromater. A phosphor screen (10 mm thick Gd2 O2 S(Tb)) was exposed to the dispersed X rays from the sample and emitted lights were collected using a charge coupled device (CCD) camera (640  480 channels, 12 bits). The intensities in the vertical direction (about 200 channels) were summed to enable a onedimensional spectroscopy. The horizontal focus size of the X rays was measured to be 0.05 mm in full width at half maximum (FWHM) and the vertical size is equal to the sample pellet height for accumulating the intensity of transmitted X rays. Two Rh-coated mirrors were used in the same way as the conventional setup. In the dispersive measurement, only the X-ray absorption near-edge structure (XANES) spectra were obtained because of high intensity of the higher-order harmonics at the energy of the Ti K-edge in the SPring-8 ring and the low linearity of the detector, especially phosphor screen. All spectra were recorded in the transmission mode. The samples (about 5 mg) were mixed with boron nitride matrix (20 mg) by a mortar in about 10 minutes under He atmosphere. Homogeneously dispersed samples were pressed to make a pellet (’ ¼ 7 mm). The pellets were attached to the sample cell filled with He gas. During XAFS measurements, He gas was always flowed in the sample cell to avoid exposing to air. XAFS spectra were taken with an acquisition time of about 60 minutes and 10 seconds for conventional and dispersive mode, respectively. During continuous measurement of thermal desorption analysis, samples were heated from room temperature to 500 C with a constant rate of 10 K/min. Desorbed hydrogen during XAFS monitoring was observed by QMS. 3.

Results

3.1 TiCl3 additive to Mg(BH4 )2 3.1.1 Conventional XAFS analyses Figure 1(a) shows the Ti K-edge XANES spectra of 2 mol% TiCl3 additive in Mg(BH4 )2 . This figure indicates that 2 mol% TiCl3 additive has been already converted to other compound just after the ball milling mixture even at the room temperature.13,14) By heating the sample, the drastic change is observed between 25 C and 250 C. Then, the shape of the spectra keeps same until 500 C. The similar tendency is observed in the case of 10 mol% TiCl3 addition (Fig. 1(b)), though the change of the spectra is not so drastic as the case of the 2 mol% TiCl3 addition. Fourier transform intensities of k3 -weighted extended X-ray absorption fine structure (EXAFS) spectra are summarized in Fig. 2 as well as the fitted curves.15,16) Estimated EXAFS parameters are shown in Table 1. The fitting

(a) 1

0

Normalized absorption

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(b) 1

0

(c) 1

0 4960

4980

5000

Photon energy, E/eV Fig. 1 Ti K-edge XANES spectra for Mg(BH4 )2 + (a) 2 mol% TiCl3 , (b) 10 mol% TiCl3 and (c) 2 mol% TiO2 additives. Observation temperatures are 25 C (thick solid line), 250 C (dashed line) and 500 C (thin solid line). Start samples of TiCl3 (circle) and TiO2 (triangle) are included as references.

parameters were coordination number (CN), interatomic distance (R), edge energy shift and mean square relative displacement (C2 ). For the standard spectra, simulated results from FEFF 8.4 code were used.17) As for the 2 mol% and 10 mol% TiCl3 additives, a similar trend is observed in Figs. 2(a) and 2(b), which means that structural transformation occurs between 25 and 250 C. This is consistent with the change of the XANES spectra seen in Figs. 1(a) and 1(b). For the TiCl3 addition samples, judging from the value of interatomic distance, the nearest-neighbor peak is a result of boron coordination. Determined interatomic distance of the nearest-neighbor Ti-B bonding indicates that the samples have shorter distances of Ti-B ˚ ) than those at higher temperbonding at 25 C (2.11–2.12 A ˚ ature (2.35–2.41 A). The interatomic distance of Ti-B ˚ for TiB2 and 2.201 A ˚ for Ti(BH4 )3 in bonding is 2.375 A 18) literature, this suggests that borohydride sample can show a shorter interatomic distance than boride sample. It is reasonable to consider that the nearest-neighbor peak of Fourier transform intensities of TiCl3 additives at 25 C is assigned to titanium-related borohydride, which is probably expressed as Tix Mg1
3x=2 (BH4 )2 (x ¼ 00:67).7)

Correlation between Structure of Titanium Additives and Dehydrogenation Reaction of Magnesium Borohydride 2.4

2.4

(a)

(b)

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Fourier transform intensity (a. u.)

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(c)

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25 °C

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500 °C

0 0

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500 °C

4

0.8

2

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0 0

2

4

Interatomic distance, R/Å

Table 1 EXAFS parameters determined from the fitting seen in Fig. 2. ˚
1 . Fitting was Range of the Fourier transform is typically 3–10 A ˚ (2 mol% performed in the R-space with a typical range of 0.5–2.5 A ˚ (10 mol% TiCl3 ) and 0.5–2.0 A ˚ (2 mol% TiO2 ). The TiCl3 ), 0.5–2.8 A intrinsic loss factor was estimated to be 0.7 from a Ti foil at room temperature. Additive 2 mol% TiCl3

10 mol% TiCl3

T ( C)

R ˚) (A

C2 ˚ 2) (0.01 A

R-fac. (%) 2.0

25

B

1.9(3)

2.12(1)

0.4(2)

B

8.9(30)

2.41(4)

1.4(5)

2.6

500

B

10.1(15)

2.39(1)

1.6(2)

1.0

25

B Cl

2.9(7) 0.5(1)

2.11(2) 2.51(2)

0.9(3) 1.3

1.5

Ti

1.0(1)

2.99(1)

0.9

B

7.2(7)

2.37(1)

1.2(1)

Ti

2.1(2)

3.02(1)

1.2(1)

500



CN

250

250

2 mol% TiO2

shell

Normalized absorption

Fig. 2 Fourier transform intensities of k3 -weighted EXAFS functions for Mg(BH4 )2 + (a) 2 mol% TiCl3 , (b) 10 mol% TiCl3 and (c) 2 mol% TiO2 additives. Dotted lines indicate the fitted ones.

1

0 4950

1.0

B

8.1(9)

2.35(1)

1.4(2)

Ti

2.6(3)

3.03(1)

1.4(2)

1.5

25

O

6.0(4)

1.96(1)

0.5(1)

0.4

250 500

O O(B)

5.3(3) 7.3(15)

1.96(1) 2.04(2)

0.5(1) 1.9(4)

0.4 1.5

Fixed values obtained from the reference samples. Same parameter with B shell.



Elongated Ti-B interatomic distance observed at higher temperature is considered to originate in the formation of TiB2 , which is estimated by the similarity of the bond length. Boride formation by heating was also pointed out in the cobalt chloride addition system,19) but in that case, they reported that the cobalt chloride was converted to metal cobalt phase by ball milling procedure. For the TiCl3 10 mol% addition sample, some other peaks are observed in the Fourier transform intensities. The analysis under the supposition that residuals of left TiCl3 and reduced Ti metal are contained reproduces the experimental spectra well. Ti metal component still exists in the higher temperature.

4980

5010

5040

Photon energy, E/eV Fig. 3 Ti K-edge XANES spectra for Mg(BH4 )2 + 2 mol% TiCl3 obtained from the dispersive optics at 25 C (solid circle) and 500 C (open circle). Exposure time of each spectroscopy is 10 seconds.

3.1.2 Dispersive XAFS + QMS analyses In order to study the correlation between the local structure of Ti additives and the dehydrogenation mechanism, the simultaneous and continuous observation of the local structure of Ti atoms by dispersive XAFS and the dehydrogenation curve by QMS was operated. At first, as the typical information, the Ti K-edge XANES spectra obtained by the dispersive XAFS system were depicted in Fig. 3. Because of the high intensity of higher-order harmonics at the photon energy of Ti K-edge, the XANES spectra show many irregular jumps and dips. Nevertheless, these jumps and dips originate from the position dependence of the light intensity, these irregular jumps and dips do not change during monitoring XAFS spectra. The uneven curves in XANES spectra may hinder the precise observation of the small peak of XANES curve, but in the case of observation for the

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(a) −2

−4

(b)

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H2 intensity (a. u.)

Absorption edge energy shift, E/eV

2

0 0

(c) −2

−4

(d)

0

100

200

300

400

500

Temperature, T/°C Fig. 4 (a)–(c) Results of the simultaneous measurements of XAFS and QMS during thermal heating. (Left axis) Shift of the absorption energy of Ti K-edge determined by the dispersive optics. (Right axis) Intensity of desorbed hydrogen determined by QMS. Studied samples are Mg(BH4 )2 + (a) 2 mol% TiCl3 , (b) 10 mol% TiCl3 and (c) 2 mol% TiO2 additive. (d) Intensity of desorbed hydrogen for Mg(BH4 )2 without using catalyst.7) The data were obtained by different system from that for the simultaneous measurements of (a)–(c).

relative shift of spectra, we can receive the advantage of the dispersive XAFS optics that serves stable and fast monitoring because of the unmoving optical mechanism. By fitting the XANES spectra with the simple step function, we obtain the relative shift of the absorption edge position for the Ti additives. Although the XANES spectra contain many fine peaks at the edge energy position, such easy approach is still useful for studying the dynamic information by the continuous observation. The edge energy shift of the Ti additives from the continuous measurement of the Ti K-edge XANES spectra by the dispersive XAFS optics is summarized in Fig. 4 as well as the dehydrogenation curves obtained by the simultaneous measurement of QMS. In the case of the 2 mol% TiCl3 addition sample shown in Fig. 4(a), the edge energy shift shows the abrupt change at 100–150 C, which indicates the structural change of the Ti compound occurs around 100–

150 C. After that, no critical change is observed until 500 C. The very small peak at 100–150 C and the large peak at 350 C are distinguished in the dehydrogenation curve. The main peak originates from the dehydrogenation reaction of Mg(BH4 )2 .7) Recent study has indicated that Mg(BH4 )2 itself shows the three dehydrogenation peaks between 300 and 380 C, as shown in Fig. 4(d).7) In this study, lower resolution of the QMS system produces the single peak at 350 C. By comparing the dehydrogenation curve of the 10 mol% TiCl3 addition sample shown in Fig. 4(b) with the 2 mol% TiCl3 addition sample, it is indicated that the peak at 100– 150 C is much enhanced in the 10 mol% TiCl3 addition sample. This is consistent with the recent result which has reported the growth of the peak at 100–150 C as the increase of amount of TiCl3 additive.7) As for the absorption edge energy, the abrupt change of the edge energy position seems to be related to the peak of the dehydrogenation curve at 100– 150 C. On the other hand, the absorption energy shift does not show the change at 350 C where the main peak of the hydrogen desorption curve appears. The 10 mol% TiCl3 addition sample shows opposite direction of the edge energy shift compared with the 2 mol% TiCl3 addition sample. As seen in the Fig. 1, the shape of XANES spectra of the 10 mol% TiCl3 addition sample at 25 C do not resemble much that of the 2 mol% addition sample. The intersection energy of XANES spectra between 25 C and higher temperature locates at the middle of edge jump in the 10 mol% TiCl3 addition sample, which indicates that a simple analysis of edge energy position can be shifted to either direction. However, because the structure change of Ti additive occurs at the same temperature, we consider that the correlation between local structural change of Ti and dehydrogenation property is similar in both samples. The 10 mol% TiCl3 addition sample contains Tix Mg1
3x=2 (BH4 )2 , TiCl3 , and metallic Ti, which can influence the shift style of absorption edge due to the change of ratio of components. As a result, we have concluded that the local structural change of the Ti compound is directly related to the first dehydrogenation peak at 100–150 C, and is not much influenced by the main dehydrogenation peat at 350 C. Both of the dehydrogenation peak and the change of the Ti K-edge absorption energy position at 100–150 C originate from the dehydrogenation reaction of Tix Mg1
3x=2 (BH4 )2 . At higher temperature, Tix Mg1
3x=2 (BH4 )2 is converted to TiB2 which is more stable compound and keeps its structure until 500 C. 3.2 TiO2 additive to Mg(BH4 )2 3.2.1 Conventional XAFS analyses The local structure of Ti atoms for 2 mol% TiO2 additive in Mg(BH4 )2 shows different character with the case of the TiCl3 additive. Figure 1(c) indicates that the local structure of Ti atoms just after the ball milling mixture is the same with the start compound of TiO2 . The structure of TiO2 is kept till 250 C by heating. However, the spectra show difference at 500 C. This indicates that TiO2 is converted between 250 and 500 C. By comparing the case of the TiCl3 addition, TiO2 is more stable compound than TiCl3 during the ball milling mixture with Mg(BH4 )2 . Moreover, TiO2 shows more stability for thermal activation than the case of TiCl3 addition because the TiO2 kept its structure even at 250 C.

Correlation between Structure of Titanium Additives and Dehydrogenation Reaction of Magnesium Borohydride

The interatomic distance of Ti-O bonding for the sample is ˚ , as shown in Fig. 2(c) and Table 1. determined to be 1.96 A This agrees well with that for anatase and rutile TiO2 (1.94– ˚ ), and is clearly distinguished with the longer distance 1.98 A of Ti-B bonding including boride and borohydride. Large value of the mean square relative displacement and the increase of the interatomic distance at 500 C indicate that some Ti compounds are mixed. Combined with the results of XANES spectra, it is considered that some reduced state of Ti3þ or Ti2þ , such as Ti2 O3 and TiB2 ,14) are formed by the reaction of TiO2 and the released boron and hydrogen from Mg(BH4 )2 . 3.2.2 Dispersive XAFS + QMS analyses Only single peak at 350 C is observed in the dehydrogenation curve in Fig. 4(c) for the 2 mol% TiO2 addition sample. While the edge energy position changes smoothly from 250 to 500 C. It is noted that the character of the shift of the absorption edge position is different in the samples of TiCl3 and TiO2 addition. The former shows the abrupt change in the narrow temperature region, but the latter shows the gentle shift of the edge energy position in the wide temperature range. In the case of the TiCl3 addition sample, the temperature region where the local structural change occurs agrees with that where the dehydrogenation peak exists. The direct contribution of the Ti compound concerning the dehydrogenation mechanism is concluded. While in the case of the TiO2 addition sample, the slope of the edge energy position spreads in wider temperature range than the peak width of the dehydrogenation curve at 350 C. This implies that the structural change of TiO2 does not directly form the dehydrogenation peak, but catalytic effect for dehydrogenation reaction by reducing TiO2 is expected. 4.

Discussion

Preedge peak of XANES spectra around 4970 eV observed in 2 mol% TiCl3 additive of Fig. 1(a) at 25 C can be related to specific structure. This peak is well discussed as the evidence of the specific titanium local structure. Among titanium(IV) oxides, Ti-O4 tetrahedral structure is known to show a sharp single peak at the preedge region.20,21) Ti-O6 octahedral structure, which is seen in the anatase and rutile TiO2 used in this study as additive, shows small three peaks at the preedge region. In the past study, a similar single sharp peak in the preedge region was observed in the titanium isopropoxide additive.14) It was reported that amorphous TiO2 was created by ball milling procedure. Because titanium(IV) isopropoxide has Ti-O4 tetrahedral structure, if the titanium isopropoxide remained after the ball milling procedure, the sharp single preedge peak is considered to be observed. In our study, because the start compound is titanium(III) chloride, reaction to form amorphous TiO2 is less active than the case of titanium(IV) isopropoxide addition. Different reaction mechanism between titanium(IV) chloride and titanium(III) chloride was also reported about the titanium chloride addition in complex hydride system.22) This also supports that amorphous TiO2 is not formed in the condition used in our study. Dehydrogenation of Mg(BH4 )2 is expressed as the below chemical formulae:7)

Mg(BH4 )2 ! 1/6MgB12 H12 þ 5/6MgH2 þ 13/6H2 $ MgH2 þ 2B þ 3H2 $ Mg þ 2B þ 4H2 :

639

ð1Þ ð2Þ ð3Þ

Because the initial dehydrogenation temperature decreases by the TiO2 addition,4) the TiO2 additive can contribute to the reaction formula (1). In Fig. 4(c), the TiO2 additive starts to be reduced before the dehydrogenation reaction occurs. The reduced state of TiO2 , which may be Ti2 O3 , is considered to provide a new chemical reaction path. Such different chemical state created at the interface of TiO2 and Mg(BH4 )2 can show the catalysis for the dehydrogenation reaction formula (1).23) The emission of B atoms is explained in formulae (2) and (3), which may relate to the creation of lower oxidation state of Ti via the reaction with B atoms and the wide temperature range of the edge position change for the TiO2 additive. Similar mechanism is not considered in the main hydrogen desorption peak for the TiCl3 addition sample because TiCl3 has been already converted to TiB2 at this temperature. 5.

Conclusions

We have studied the local structure of Ti additives in hydrogen storage material Mg(BH4 )2 by Ti K-edge XAFS spectra. TiCl3 additive is converted to Tix Mg1
3x=2 (BH4 )2 by ball milling mixture, while TiO2 additive keeps its structure. Simultaneous and continuous measurement of the local structure of Ti and the dehydrogenation curve during thermal heating clearly demonstrates the correlation between the local structure of additives and the dehydrogenation property. The first peak of the dehydrogenation curve at 150 C is concluded to originate from the decomposition reaction of Tix Mg1
3x=2 (BH4 )2 for the TiCl3 addition sample. Although TiO2 additive does not show the first peak of the dehydrogenation curve because TiO2 is stable by ball milling mixture, catalytic effect for the dehydrogenation of Mg(BH4 )2 is discussed from the viewpoint of the reduction of TiO2 additive. Acknowledgements We are grateful to Mr. K. Kato for constructing the dispersive XAFS system and stimulating discussion. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under ‘‘Advanced Fundamental Research Project on Hydrogen Storage Materials’’. REFERENCES 1) S. Orimo, Y. Nakamori, J. R. Eliseo, A. Zu¨ttel and C. M. Jensen: Chem. Rev. 107 (2007) 4111–4132. 2) H.-W. Li, S. Orimo, Y. Nakamori, K. Miwa, N. Ohba, S. Towata and A. Zu¨ttel: J. Alloy. Compd. 446–447 (2007) 315–318. 3) T. Sato, K. Miwa, Y. Nakamori, K. Ohoyama, H.-W. Li, T. Noritake, M. Aoki, S. Towata and S. Orimo: Phys. Rev. B 77 (2008) 104114. 4) H.-W. Li, K. Kikuchi, Y. Nakamori, K. Miwa, S. Towata and S. Orimo: Scr. Mater. 57 (2007) 679–682. ˇ erny´, Y. Filinchuk, H. Hagemann and K. Yvon: Angew. Chem. Int. 5) R. C Ed. 46 (2007) 5765–5767.

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6) J.-H. Her, P. W. Stephens, Y. Gao, G. L. Solveichik, J. Riijssenbeek, M. Andrus and J.-C. Zhao: Acta Crystallogr. Sec. B 63 (2007) 561–568. 7) H.-W. Li, D. Matsumura, Y. Nishihata, Y. Yan, K. Miwa, S. Towata and S. Orimo: in preparation. 8) T. Matsushita and R. P. Phizackerley: Jpn. J. Appl. Phys. 20 (1981) 2223–2228. 9) U. Kaminaga, T. Matsushita and K. Kohra: Jpn. J. Appl. Phys. 20 (1981) L355–L358. 10) M. A. Newton, C. Belver-Colderia, A. Martı´nez-Arias and M. Ferna´ndez-Garcı´a: Nature Mater. 6 (2007) 528–532. 11) D. Matsumura, Y. Okajima, Y. Nishihata, J. Mizuki, M. Taniguchi, M. Uenishi and H. Tanaka: J. Phys.: Conf. Ser. 190 (2009) 012154. 12) D. Matsumura, Y. Nishihata, J. Mizuki, M. Taniguchi, M. Uenishi and H. Tanaka: J. Appl. Phys. 107 (2010) 124319. 13) S. Isobe, T. Ichikawa, Y. Kojima and H. Fujii: J. Alloy. Compd. 446– 447 (2007) 360–362. 14) E. Deprez, M. A. Mun˜oz-Ma´rquez, M. A. Rolda´n, C. Prestipino, F. J. Palomares, C. B. Minella, U. Bo¨senberg, M. Dornheim, R. Bormann and A. Ferna´ndez: J. Phys. Chem. C 114 (2010) 3309–3317.

15) T. Yokoyama, H. Hamamatsu and T. Ohta: EXAFSH, 2nd ed., (The University of Tokyo, 1998). 16) M. Newville: J. Synchrotron Rad. 8 (2001) 322–324. 17) A. L. Ankudinov, A. I. Nesvizhskii and J. J. Rehr: Phys. Rev. B 67 (2003) 115120. 18) C. J. Dain, A. J. Downs, M. J. Goode, D. G. Evans, K. T. Nicholls, D. W. H. Rankin and H. E. Robertson: J. Chem. Soc. Dalton Trans. (1991) 967–977. 19) W. S. Tang, G. Wu, T. Liu, A. T. S. Wee, C. K. Yong, Z. Xiong, A. T. S. Hor and P. Chen: Dalton Trans. (2008) 2395–2399. 20) T. Tanaka, K. Teramura, T. Yamamoto, S. Takenaka, S. Yoshida and T. Funabiki: J. Photochem. Photobio. A 148 (2002) 277–281. 21) F. Farges, G. E. Brown Jr. and J. J. Rehr: Phys. Rev. B 56 (1997) 1809– 1819. 22) A. Le´on, J. Rothe and M. Fichtner: J. Phys. Chem. C 111 (2007) 16664–16669. 23) S. Isobe, A. Ono, H. Yao, Y. Wang, N. Hashimoto and S. Ohnuki: Appl. Phys. Lett. 96 (2010) 223109.