実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report
課題番号 Project No. 2010A0031

提出日 Date of ReportMar. 04, 2011
装置責任者 Name of responsible person
Yasuhiro Miyake

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

The doping effect on the formation of HμH bond in MgH2

D1

実験責任者名 Name of principal investigator

実施日 Date of Experiment

Jun Sugiyama

October 20, 2011 ‒ October 24, 2011

所属 Affiliation
Toyota Central Research and Development Laboratories, Inc.
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.

Two powder samples of Magnesium hydride were synthesized at TCRDL. One is as hydrogenatedpowder sample, the other is the powder milled for 24 hours.
Each powder was packed in an Au-sealed titanium cell with Kapton window.

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

Magnesium is considered to be one of the most promising materials for reversible hydrogenstorage, because of its high storage capacity. However, since a magnesium hydride (MgH2) isthermodynamically very stable, its dehydrogenation temperature (Td) for an as prepared powder
sample is reported to be ~ 600 K. But, Td for a milled sample decreases by about 50 K [1]. In orderto understand the bonding nature of Mg and H, MEM/Rietveld analyses were performed usingsynchrotron radiation x-ray [2]. However, the dynamics for the hydrogenation and/ordehydrogenation is still not well understood. We have, therefore, measured wTF- and ZF-spectra
for the two MgH2 powder samples; namely, a hydrogenated powder and a milled hydrogenated-powder, in the T range between 50 and 500 K in order to study the mechanism for lowering Td bymilling. The obtained spectra were fitted using a combination of H-µ-H signal, a dynamic
Gaussian Kubo-Toyabe signal, three oscillatory signals, and an offset background signal; A0P(t) =AHµHPHµH(t)exp(-λHµHt) + AKTGKT(t) + ΣAiexp(-λit)cos(ωit+φi) + ABG. Here, A0 is the initial (t=0)
asymmetry, AHµH, Ai, AKT and ABG are the asymmetry for each signal, λHµH and λi is the exponential
relaxation rate of the oscillatory signals, GKT(t) is the GKT function.

12. 実 験 方 法 及 び 結 果 （ つ づ き ） Experimental
method and results (continued)

Figure 1 shows the T variation of theZF-spectra for the two samples in the T range
between 50 and 500 K. The ZF spectrum forthe non-milled sample shows a minimum
around 5 µs, indicating the presence of a small
random nuclear field (Hintn). However, there isno composition of a H-µ-H signal. Since thewhole signal from the sample is well fitted bya dynamic Kubo-Toyabe function, the originof the muon-spin depolarization is assigned asa nuclear magnetic field caused by H.
On the other hand, the ZF-spectra for themilled sample shows a damped oscillation
below 250K indicating the formation of theH-µ-H bond. Above 300K, the H-µ-H signal
disappears and the whole spectrum shows adynamic Kubo-Toyabe behavior.
Figure 2 shows the T dependence of theasymmetry of each signal for the milled MgH2sample. Below 250K, AHµH decreases withincreasing T, while Ai (i=1-3) remains. This
result suggests that the µ+ forms H-µ+-H bondand the extra combination to other H.
Furthermore, the bonds between µ+ and H are
supposed to be formed on the surface of theMgH2 particle, because the surface area of themilled sample is larger than the non-milledsample. The absence of the oscillatory signal
above 300 K indicates that the bond betweenµ+ and H is unstable and the µ+ feels only theFig. 1 T variation of the ZF-spectra for MgH2(a) as hydrogenated and (b) milled for 24 hours.

nuclear magnetic moment of H.
[1] R. A. Varin, et al., Nanotechnology, 17, 3856
(2006).
[2] T. Noritake, et al., Appl. Phys. Lett., 81, 2008
(2002).

Fig. 2 T dependences of the asymmetry for
the ZF-µ+SR spectra for the milledsample.
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