Mechanochemical transformations in NaNH2-MgH2 mixtures

Article abstract of DOI:10.1016/j.jallcom.2011.10.041

Mechanochemical transformations occurring during ball milling of sodium amide (NaNH₂) with magnesium hydride (MgH₂) taken in 2:3 and 2:1 molar ratios have been investigated using X-ray powder diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) techniques. For the 2NaNH₂-3MgH₂ system the mechanochemical reaction proceeds via the formation of MgNH as an intermediate, whereas magnesium nitride (Mg ₃N₂), sodium hydride (NaH) and hydrogen (～5 wt%) form as the final products. The overall solid state reaction for this system is 2NaNH₂ + 3MgH₂ → Mg₃N₂ + 2NaH + 4H₂. However, the mechanochemical transformation of the 2NaNH ₂-MgH₂ system proceeds through the reaction: 2NaNH ₂ + MgH₂ → Mg(NH₂)₂ + 2NaH, without any hydrogen release. Comparison of the mechanochemical transformations with the previously studied thermochemical transformations reveals that the two approaches lead to the same final products via different reaction pathways.

Full text of DOI:10.1016/j.jallcom.2011.10.041

Journal of Alloys and Compounds 513 (2012) 324–327
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Mechanochemical transformations in NaNH2-MgH2 mixtures
Niraj K. Singha, Takeshi Kobayashia, Oleksandr Dolotkoa, Jerzy W. Wiencha, Marek Pruskia,b,∗
,
Vitalij K. Pecharskya,c
a Ames Laboratory, Iowa State University, Ames, IA 500011-3020, USA
b Department of Chemistry, Iowa State University, Ames, IA 500011-3111, USA
c Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011-2300, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2011
Received in revised form 8 October 2011
Accepted 10 October 2011
Available online 24 October 2011
Mechanochemical transformations occurring during ball milling of sodium amide (NaNH2) with mag-
nesium hydride (MgH2) taken in 2:3 and 2:1 molar ratios have been investigated using X-ray
powder diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) techniques. For the
2NaNH2–3MgH2 system the mechanochemical reaction proceeds via the formation of MgNH as an inter-
mediate, whereas magnesium nitride (Mg3N2), sodium hydride (NaH) and hydrogen (∼5 wt%) form as the
final products. The overall solid state reaction for this system is 2NaNH2 + 3MgH2 → Mg3N2 + 2NaH + 4H2.
However, the mechanochemical transformation of the 2NaNH2–MgH2 system proceeds through
the reaction: 2NaNH2 + MgH2 → Mg(NH2)2 + 2NaH, without any hydrogen release. Comparison of the
mechanochemical transformations with the previously studied thermochemical transformations reveals
that the two approaches lead to the same final products via different reaction pathways.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Hydrogen storage
Mechanochemical transformation
Solid-state NMR
X-ray diffraction
1. Introduction
affects the dehydrogenation properties, and 4.5 wt% of hydrogen
can be reversibly stored in the 2LiNH2–MgH2 mixture. The ther-
Continuously increasing demand for energy stimulates the
exploration of alternative energy sources, and in this context var-
ious solid-state hydrogen storage materials have been examined
[1,2]. Among these, complex hydrides formed with lightweight
alkali metals, such as lithium and sodium, have attracted consid-
erable attention mainly due to their high gravimetric hydrogen
content [1,3,4]. Recently, Chen et al. [5] reported that hydrogena-
tion of lithium nitride (Li3N) and subsequent dehydrogenation can
be performed via the following reversible two-step reaction:
mal dehydrogenation of this system leads to the formation of
Li2Mg(NH)2 through the first step of reaction (R2) [7,8]. How-
ever, the hydrogenation of Li2Mg(NH)2 leads to the formation of
Mg(NH2)2 and LiH through the second step of (R2), and it is this step
of the reaction which is responsible for the reversibility of hydrogen
in the 2LiNH2–MgH2 system [8–11]
2LiNH2 + MgH2 → Li2Mg(NH)2 + 2H2↑ ↔ Mg(NH2)2 + 2LiH (R2)
The ball milling of the same system yields directly Mg(NH2)2
and 2LiH [11–13], which demonstrates that reactions induced
by mechanical energy may proceed through nonequilibrium con-
figurations with different final products. A similar discrepancy
was recently reported in 2LiNH2–CaH2 system, which under ther-
mochemical conditions decomposes to Li2Ca(NH)2 and can be
hydrogenated to Ca(NH2)2 and LiH, in analogy to (R2) [18]. How-
ever, neither Li2Ca(NH)2 nor Ca(NH2)2 is formed during mechanical
milling of 2LiNH2–CaH2 mixture [19].
Two recent reports focused on thermal decomposition of the
mixture of MgH2 and sodium amide, NaNH2 [20,21]. In the
2MNH2–3MgH2 system (M = Li or Na) studied by Dolotko et al. [20],
the dehydrogenation proceeds according to
Li3N + 2H2 ↔ Li2NH + LiH + H2 ↔ LiNH2 + 2LiH
(R1)
Theoretically, as much as 10.4 wt% of hydrogen can be stored
using this system. However, the strongly endothermic nature of the
above reaction requires high operation temperature, which hinders
practical application of this LiNH2–LiH system.
Subsequently, Nakamori and Orimo [6] reported that replace-
ment of Li by an element with higher electronegativity, such as
Mg, improves the dehydrogenation properties of LiNH2. Later stud-
ies involved mechanochemistry and thermochemistry of various
mixtures of LiNH2 and MgH2 [7–17]. Luo [7] and Xiong et al. [8]
independently demonstrated that the replacement of LiH by MgH2
2MNH2 + 3MgH2 → Mg3N2 + 2MH + 4H2↑
(R3)
Although the final products are the same, different inter-
mediates arise for M = Li (Li2Mg(NH)2) and M = Na (Mg(NH2)2
and NaMgH3). The attempt of hydrogenation of the decomposed
∗
Corresponding author at: Ames Laboratory, Iowa State University, Ames, IA
500011-3020, USA. Tel.: +1 515 294 2017.
E-mail address: mpruski@iastate.edu (M. Pruski).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.10.041

N.K. Singh et al. / Journal of Alloys and Compounds 513 (2012) 324–327
325
products for M = Li was unsuccessful [20]. However, in the case of
M = Na, partial rehydrogenation has been observed:
Mg3N2 + 2NaH + 2H2 → 2MgNH + 2NaH + MgH2
(R4)
Most recently, hydrogen desorption from LiNH2–MgH2 mixture
(1:1) has been reported [21], which yields 3.3 wt% between 70 and
335 ◦C. The temperature programmed dehydrogenation curve was
different than that reported by Dolotko et al. [20], and two new,
unidentified magnesium-containing phases corresponding to an
imide and nitride were involved in the decomposition pathway.
Herein, we describe the solid-state transformations occurring
during mechanical milling of two mixtures of NaNH2 and MgH2. In
view of the aforementioned differences between thermochemical
and mechanochemical transformations in other hydride systems,
we re-examined the 2NaNH2–3MgH2 mixture, which indeed pro-
duces a new intermediate (MgNH). We have also investigated the
previously unexplored 2NaNH2–MgH2 system. Our earlier studies
have demonstrated that in order to properly characterize the crys-
talline and amorphous phases of reacting hydrides, such studies
can be best performed by combining the X-ray powder diffrac-
tion (XRD) and solid-state nuclear magnetic resonance (SSNMR)
spectroscopy [22,23].
Fig. 1. The XRD patterns of the 2NaNH2–3MgH2 mixture, reaction products after
ball milling and reference compounds. Only the locations (but not intensities) of
the Braggs peaks of the starting compounds and two reaction products are shown
as the vertical markers at the bottom of the plot. Braggs peaks of the intermediate
(MgNH) are observed at 2ꢀ = 34.7◦, 40.8◦, and 59.3◦ in samples after 1, 3, and 6 h of
processing.
mechanochemical transformations. During the milling of 2:1 mix-
ture no pressure was developed in the vial, which indicates that the
mechanochemical changes do not involve formation of any gaseous
products.
2. Experimental details
2.1. Materials and mechanochemical processing
The starting materials NaNH2 (>90 wt% purity) and NaH (>95 wt% purity) used
in this study were purchased from Sigma–Aldrich, whereas MgH2 (>98 wt% purity)
was purchased from Alfa-Aesar. Due to the air sensitivity of the starting materials
and the products, all manipulations have been carried out under a continuously
purified and monitored argon atmosphere in a glove box.
About 1 g of the intended mixture was loaded in a 50 ml hardened-steel vial
and ball milled using 20 g of steel balls (2 large balls weighing 8 g each and 4
small balls weighing 1 g each) in an 8000 M SPEX mill. The samples, henceforth
referred to as NaNH2–MgH2(sm), where ‘sm’ denotes the ‘starting mixture’, and
NaNH2–MgH2(time), were analyzed as a function of ball milling time. The reference
compound NaMgH3 was synthesized by ball milling of 1 g of 1:1 mixture of NaH and
MgH2 in a SPEX mill for 3 h.
3.1. XRD studies
Fig. 1 shows the XRD patterns collected on 2NaNH2–3MgH2(sm)
and samples milled for 1, 3, 6, 9 and 18 h. Besides the Bragg
reflections corresponding to the starting materials, the pattern of
2NaNH2–3MgH2(1 h) sample shows the presence of NaH and a new
set of reflections at 2ꢀ = 34.7◦, 40.8◦ and 59.3◦, which are consistent
with those assigned to MgNH [25,26]. With increase in the milling
time to 3 h the contribution from the starting materials diminishes
whereas the Bragg reflections corresponding to NaH and MgNH
become more pronounced. In the 2NaNH2–3MgH2(6 h) sample the
reflections due to NaH and MgNH are still present; however, the
intensities corresponding to MgNH decrease and a new set of Bragg
reflections corresponding to Mg3N2 becomes visible. At 9 h this set
becomes prominent, whereas the reflections corresponding to NaH
remain and the peaks from MgNH vanish. The milling of the mixture
for 18 h does not yield any additional reflections, which indicates
that the mechanochemical transformation for the 2NaNH2–3MgH2
system is completed in less than 9 h.
2.2. X-ray powder diffraction analysis
Reaction products were characterized by X-ray powder diffraction analysis at
room temperature on a PANalytical powder diffractometer using Cu K␣ radiation
with a 0.02◦ 2ꢀ step, in the range of Bragg angles 2ꢀ from 10◦ to 80◦. During the
measurements a polyimide (Kapton) film was used to protect the samples from air.
The use of film resulted in an amorphous like background in the XRD patterns in the
2ꢀ range of 13–20◦.
2.3. Solid-state NMR analysis
The 1H and 23Na solid-state NMR experiments were performed at 14.1 T on a
Varian NMR System spectrometer, equipped with a triple tuned 1.6 mm T3 magic
angle spinning (MAS) probe operating at a rate of 25 kHz. The ball milled samples
and reference compounds were packed in MAS zirconia rotors in a glove box under
argon atmosphere and tightly capped to avoid oxygen and moisture contamina-
tion. The one-dimensional spectra were acquired using a single pulse excitation, a
radio frequency (RF) magnetic field of approximately 80 kHz, a small flip angle of
15◦ for quantitative accuracy (in the case of 23Na) and 1H TPPM (Two-Pulse Phase-
Modulated) decoupling [24] at 104 kHz during acquisition. The recycle delays were
60 s for 1H and 1 s for 23Na. The 1H and 23Na shifts were referenced to tetramethyl-
silane (TMS) and 1.0 M aqueous solutions of NaCl, respectively, at 0 ppm.
Fig.
2 shows a similar series of XRD patterns for the
2NaNH2–MgH2 system. While the sample milled for 1 h
(2NaNH2–MgH2(1 h)) shows Bragg reflections representing
only the starting materials, the characteristic peaks corresponding
to Mg(NH2)2 and NaH appear in the 2NaNH2–MgH2(3 h) sample.
Further increase in the milling time to 6 h results in lowering of
the intensity of the Bragg peaks corresponding to the starting
materials whereas the intensities due to Mg(NH2)2 and NaH
increase. After 9, 12 and 15 h of ball milling, only the peaks
representing Mg(NH2)2 and NaH are observed. Thus, the XRD
data suggest that the mechanochemical transformation of the
2NaNH2–MgH2 system is also completed in 9 h or less, leading
to the final products Mg(NH2)2 and NaH in a single step with no
detectable intermediates.
3. Results and discussion
The transformations of 2NaNH2–3MgH2 and 2NaNH2–MgH2
mixtures were monitored by analyzing the XRD patterns and
solid-state NMR spectra of samples ball milled for up to 18 h and
15 h, respectively, which sufficed to complete the reactions. The
milling of 2:3 molar mixtures leads to the development of pres-
sure in the vial and, as confirmed by residual gas analysis, pure
hydrogen was observed as the gaseous product resultingfrom the
3.2. Solid-state NMR studies
In addition to the XRD analysis, the 23Na and 1H solid-
state NMR measurements were performedon the similar sets

326
N.K. Singh et al. / Journal of Alloys and Compounds 513 (2012) 324–327
Fig. 4. 23Na and 1H MAS NMR spectra of samples 2NaNH2-MgH2(3h) and 2NaNH2-
MgH2(12 h).
Fig. 2. The XRD patterns of the 2NaNH2–MgH2 mixture, reaction products after ball
milling and reference compounds. Only the locations (but not intensities) of the
Braggs peaks of the starting compounds and two reaction products are shown as
the vertical markers at the bottom of the plot.
The results of NMR measurements agree well with the XRD anal-
ysis. After 3 h of ball milling, sodium amide, whose 23Na spectrum
exhibits a characteristic second order powder pattern in Fig. 3e
[27,28], is completely transformed into NaH (compare the 23Na
spectra in Fig. 3f and c). Clearly, NaH represents the only final prod-
uct of the reaction that contains sodium. The analysis of 1H spectra
is more difficult due to small chemical shift range and consider-
able line broadening caused by strong 1H–1H dipolar interactions.
As expected, the 1H spectrum of the starting mixture (Fig. 3e)
is a superposition of the spectra of individual reactants (Fig. 3a
and b). However, while both 1H resonances observed in sample
2NaNH2–3MgH2(3 h) appear at similar frequencies to the start-
ing mixture, they represent different species. Indeed, the peak at
∼−2 ppm cannot result from sodium amide and thus is assigned
to MgNH, which should resonate in the same frequency range. The
newly formed NaH should yield a 1H resonance at ∼4 ppm, as is
indeed observed in Fig. 3f. A small shoulder observed on the high-
frequency side of this peak is due to background from the spacers
located inside the MAS rotor.
of samples. For the 2NaNH2–3MgH2 system, we measured
the spectra of 2NaNH2–3MgH2(3 h), 2NaNH2–3MgH2(10 h)
and 2NaNH2–3MgH2(18 h), as well as several reference com-
pounds. The spectra of the starting materials, MgH2 and
NaNH2, are shown in Fig. 3a and b, respectively. Also shown
are the results obtained for NaH (Fig. 3c), NaMgH3 (Fig. 3d),
the starting (physical) mixture of NaNH2 and MgH2 (Fig. 3e),
and sample 2NaNH2–3MgH2(3 h). The spectra of samples
2NaNH2–3MgH2(10 h) and 2NaNH2–3MgH2(18 h) were identical
with those of 2NaNH2–3MgH2(3 h), and thus are not reported.
Fig. 4a and b shows the 23Na and 1H solid-state NMR spectra of
samples 2NaNH2–MgH2(3 h) and 2NaNH2–MgH2(12 h). The dom-
inant 23Na resonances in Fig. 4a represent unreacted NaNH2 and
NaH product, as indicated in the spectrum. In addition, a small
amount of unidentified, sodium-containing intermediate is found
in this sample, represented by the 23Na peak at ∼13 ppm. Simi-
larly to the 2NaNH2–3MgH2 system, the 1H spectrum of sample
2NaNH2–MgH2(3 h) is dominated by two peaks at ∼4 ppm and
∼−1.5 ppm. The first peak can be easily assigned to NaH, whereas
the one at ∼−1.5 ppm is most likely a superposition of resonances
representing Mg(NH2)2 and unreacted NaNH2. (Although we were
not able to obtain a suitable Mg(NH2)2 reference, the 1H shift in
this compound should be similar to one observed for NaNH2 in
Fig. 3b.) The dominant 1H resonances in Fig. 4b are consistent
with NaH and Mg(NH2)2 being the final products of decompo-
sition. The sharp 1H peaks around 0.5 ppm originate from the
silicon rubber in the O-rings used to seal the MAS rotor, which
were not used while collecting the spectra of Fig. 3. Again, the
broad shoulder on the high-frequency side of the spectrum is
due to the MAS rotor background. The contribution to this peak
from intermediate species cannot be ruled out, at least in sample
2NaNH2–MgH2(3 h).
3.3. Transformation mechanism
The mechanochemical transformation of NaNH2 and MgH2
taken in a 2:3 molar ratio appears to proceed in two steps. In the
first step, which is similar to the LiNH2 + MgH2 system [17], NaNH2
and MgH2 react to form MgNH and release hydrogen through (R5).
Fig. 3. 23Na and 1H MAS NMR spectra of the reference compounds, the physical
mixture NaNH2-MgH2(sm), and sample 2NaNH2-3MgH2(3h).

N.K. Singh et al. / Journal of Alloys and Compounds 513 (2012) 324–327
327
In the second step MgNH reacts with the remaining MgH2, and
Mg3N2 is formed through (R6):
2NaNH2 + 3MgH2 → 2NaH + Mg3N2 + 4H2. In case of the 2:1 mix-
ture, no gaseous hydrogen is released during the milling, and the
mechanochemical transformation involves a metathesis reaction
2NaNH2 + MgH2 → Mg(NH2)2 + 2NaH.
NaNH2 + MgH2 → NaH + MgNH + H2
2MgNH + MgH2 → Mg3N2 + 2H2
(R5)
(R6)
Acknowledgements
In the abovementioned reaction path, only MgNH was observed
as an intermediate species. Since the system contains NaH and
MgH2 after the first step, the formation of NaMgH3 could be
expected during ball milling [29]. However, this possibility was
ruled out by comparing the 23Na NMR spectra of the final prod-
ucts with the specially prepared NaMgH3 reference (Fig. 3d and f).
Thus, the overall mechanochemical transformation reaction for the
2NaNH2–3MgH2 system can be written as:
This work was supported by the Office of Basic Energy Sciences
of the Office of Science of the U.S. Department of Energy under
contract No. DE-AC02-07CH11358 with Iowa State University of
Science and Technology.
References
[1] S.I. Orimo, Y. Nakamori, J.R. Eliseo, A. Zuttel, C.M. Jensen, Chem. Rev. 107 (2007)
4111–4132.
[2] B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Int. J. Hydrogen Energy 32 (2007)
1121–1140.
[3] J.J. Vajo, G.L. Olson, Scripta Mater. 56 (2007) 829–834.
[4] F. Schuth, B. Bogdanovic, M. Felderhoff, Chem. Commun. 224 (2004) 9–2258.
[5] P. Chen, Z.T. Xiong, J.Z. Luo, J.Y. Lin, K.L. Tan, Nature 420 (2002) 302–304.
[6] Y. Nakamori, S. Orimo, Mater. Sci. Eng. B 108 (2004) 48–50.
[7] W.F. Luo, J. Alloys Compd. 381 (2004) 284–287.
[8] Z.T. Xiong, G.T. Wu, H.J. Hu, P. Chen, Adv. Mater. 16 (2004) 1522–1525.
[9] W.F. Luo, S. Sickafoose, J. Alloys Compd. 407 (2006) 274–281.
[10] Y. Chen, C.Z. Wu, P. Wang, H.M. Cheng, Int. J. Hydrogen Energy 31 (2006)
1236–1240.
[11] S. Barison, F. Agresti, S. Lo Russo, A. Maddalena, P. Palade, G. Principi, G. Torzo,
J. Alloys Compd. 459 (2008) 343–347.
[12] R.R. Shahi, T.P. Yadav, M.A. Shaz, O.N. Srivastava, Int. J. Hydrogen Energy 33
(2008) 6188–6194.
[13] K. Tokoyoda, S. Hino, T. Ichikawa, K. Okamoto, H. Fujii, J. Alloys Compd. 439
(2007) 337–341.
[14] H.Y. Leng, T. Ichikawa, S. Hino, T. Nakagawa, H. Fujii, J. Phys. Chem. B 109 (2005)
10744–10748.
[15] W. Osborn, T. Markmaitree, L.L. Shaw, J. Power Sources 172 (2007) 376–378.
[16] Y.F. Liu, K. Zhong, M.X. Gao, H.H. Wang, H.G. Pan, Q.D. Wang, Chem. Mater. 20
(2008) 3521–3527.
[17] C. Liang, Y.F. Liu, K. Luo, B. Li, M.X. Gao, H.G. Pan, Q.D. Wang, Chem. Eur. J. 16
(2010) 693–702.
[18] G.T. Wu, Z.T. Xiong, T. Liu, Y.F. Liu, J.J. Hu, P. Chen, Y.P. Feng, A.T.S. Wee, Inorg.
Chem. 46 (2007) 517–521.
[19] O. Dolotko, H.Q. Zhang, S. Li, P. Jena, V. Pecharsky, J. Alloys Compd. 506 (2010)
224–230.
2NaNH2 + 3MgH2 → 2NaH + Mg3N2 + 4H2
(R7)
We note that the mechanochemical transformation of
2NaNH2 + 3MgH2 system proceeds differently than the ther-
mochemical process [20], despite the final products being the
same. During the thermal decomposition of 2NaNH2 + 3MgH2
mixture, Mg(NH2)2 and NaMgH3 are observed as intermediates,
whereas only MgNH is detected as the intermediate product dur-
ing mechanochemical reaction. These observations indicate that,
in contrast to thermochemical transformations, which proceed
in accordance with thermodynamic equilibrium, the reactions
induced by mechanical energy may drive the reaction through
nonequilibrium configurations leading to formation of different
intermediate species [19].
Based on the XRD and NMR data and the fact that no hydrogen
is released during the ball milling, the mechanochemical transfor-
mation for the 2NaNH2–MgH2 mixture can be written as
2NaNH2 + MgH2 → Mg(NH2)2 + 2NaH
(R8)
A similar metathesis reaction has been also reported for the
mechanochemical decomposition of the 2LiNH2–MgH2 system
[11,12]. However, the NMR spectra of sample 2NaNH2–MgH2(3 h)
suggest that this reaction may involve an intermediate, yet to be
identified step.
[20] O. Dolotko, N. Paulson, V.K. Pecharsky, Int. J. Hydrogen Energy 35 (2010)
4562–4568.
[21] D.A. Sheppard, M. Paskevicius, C.E. Buckley, J. Phys. Chem. C 115 (2011)
8407–8413.
[22] V.P. Balema, J.W. Wiench, K.W. Dennis, M. Pruski, V.K. Pecharsky, J. Alloys
Compd. 329 (2001) 108–114.
4. Conclusions
The effect of ball milling on 2:3 and 2:1 mixtures of NaNH2
and MgH2 has been investigated by the combined use of XRD
and SSNMR spectroscopy. For the 2:3 system, pure hydrogen
is released and the reaction proceeds in two steps. In the first
step, NaNH2 and MgH2 react to yield MgNH and NaH through
the reaction NaNH2 + MgH2 → NaH + MgNH + H2, whereas in the
second step MgNH reacts with the remaining MgH2 and trans-
forms to Mg3N2 via the reaction 2MgNH + MgH2 → Mg3N2 + 2H2.
The overall mechanochemical reaction for 2:3 mixture is
[23] O. Dolotko, H.Q. Zhang, O. Ugurlu, J.W. Wiench, M. Pruski, L.S. Chumbley, V.
Pecharsky, Acta Mater. 55 (2007) 3121–3130.
[24] A.E. Bennett, C.M. Rienstra, M. Auger, K.V. Lakshmi, R.G. Griffin, J. Chem. Phys.
103 (1995) 6951–6958.
[25] S. Hino, T. Ichikawa, Y. Kojima, J. Chem. Thermodyn. 42 (2010) 140–143.
[26] J.J. Hu, G.T. Wu, Y.F. Liu, Z.T. Xiong, P. Chen, K. Murata, K. Sakata, G. Wolf, J. Phys.
Chem. B 110 (2006) 14688–14692.
[27] A. Abragam, Principles of Nuclear Magnetism, Oxford University Press, New
York, 1986.
[28] K. Narita, J.I. Umeda, H. Kusumoto, J. Chem. Phys. 44 (1966) 2719–2723.
[29] K. Ikeda, Y. Nakamori, S. Orimo, Acta Mater. 53 (2005) 3453–3457.