Insight into the Hydrogenation Properties of Mechanically Alloyed

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Insight into the Hydrogenation Properties of Mechanically Alloyed Mg50Co50 from the Local Structure Hyunjeong Kim,*,† Jin Nakamura,† Huaiyu Shao,†,|| Yumiko Nakamura,† Etsuo Akiba,†,|| Karena W. Chapman,‡ Peter J. Chupas,‡ and Thomas Proffen§,^ †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States § Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

bS Supporting Information ABSTRACT: The local structure of mechanically alloyed Mg50Co50 and its deuteride, Mg50Co50D75, was investigated using the atomic pair distribution function (PDF) analysis of neutron and synchrotron X-ray total scattering data. The purpose of our study was to obtain the structural information of these amorphous alloys and understand their hydrogen storage properties. We found that the body centered cubic (bcc) structural model with the uniform distribution of Mg and Co atoms proposed by earlier studies was not able to explain neutron and X-ray PDFs at the same time. Our data suggest that the local environment of Mg is different from that of Co. A twophase model composed of MgCo2 and Mg2CoD5 (Mg2Co for the alloy sample) qualitatively reproduced the main features of experimental PDFs and explained the amount of hydrogen absorbed by MgxCo100-x.

’ INTRODUCTION Mg is considered as one of promising hydrogen-storage materials for its high hydrogen-storage capacity. It can reversibly absorb and desorb 7.6 mass % of hydrogen,1 which is more than the requirement for hydrogen-fueled vehicles.2 However, there still remain several material challenges; its high hydrogenation and dehydrogenation temperature (573 K) and poor kinetics are two major drawbacks for its on-board applications.3 Numerous studies have been conducted to meet these challenges and the development of new Mg-based alloys and compounds comes to the forefront in the fields of current hydrogen-storage research. In 2005, Zhang et al. successfully synthesized MgxCo100-x binary alloys using mechanical alloying,4 a widely adopted method for preparing novel nanostructured or amorphous materials.5 These metastable alloys were formed in the composition range of 20 e x e 63. In the Mg
Co binary phase diagram, MgCo2 (=Mg33Co66) is the only known stable phase, but there is no hydride form of MgCo2. Nevertheless, mechanically alloyed MgxCo100-x with x > 33 absorbed hydrogen. For instance, it was initially reported that Mg50Co50 absorbed ∼2.1 mass % of hydrogen at 373 K under 6 MPa of hydrogen gas pressure,4 but later a much higher hydrogen absorption capacity of ∼2.67 mass % at 258 K under 8 MPa was observed.6 The amount of absorbed hydrogen increased with Mg content. Although hydrogen absorption in MgxCo100-x occurred at more favorable temperatures (ambient) than in Mg, hydrogen desorption was r 2011 American Chemical Society

not observed at 373 K, even under vacuum,4 indicating the formation of a stable hydride. One broad peak spreading over 35 < 2θ < 55 was observed in the X-ray diffraction pattern of Mg50Co50 using Cu Kα radiation.4 This is a good indication of its amorphous nature. Moreover, high-resolution transmission electron microscope (TEM) images revealed less than 50 Å sized crystals embedded in amorphous matrix.7 Lattice spacing of these crystals was 2.8
3.0 Å. Selected area electron diffraction patterns (SAEDP) showed a set of broad rings whose distances were explained by a body-centered cubic (bcc) structure. On the basis of these observations together with the energy dispersive X-ray spectroscopy (EDS) analysis results, it was suggested that Mg and Co were homogeneously distributed over the Mg50Co50 sample, forming a bcc structure.7 The most unusual characteristic of Mg50Co50 is its virtually unaltered X-ray diffraction pattern after hydrogenation at ambient temperatures.4 However, TEM images and SAEDP studies showed bcc crystals grown more than 100 Å in size.7 In addition, the face-centered cubic (fcc) Co phase and a phase with a composition close to Mg2Co were also found in hydrogenated samples. It was reported that when Mg50Co50 was heated above 413 K under 4 MPa of hydrogen pressure Mg2CoH5 and Co Received: July 28, 2011 Revised: September 1, 2011 Published: September 07, 2011 20335

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The Journal of Physical Chemistry C peaks started to appear on top of broad amorphous intensities in X-ray diffraction patterns.4 A further increase in temperature above 481 K led to the formation of an additional phase, MgCo2.6 It is probable that intense electron beams promoted crystal growth observed in TEM images, since there is no report on the appearance of any extra peak in X-ray diffraction data of Mg50Co50 hydrogenated at 373 K. Even though structural information of both alloy and hydride phases is essential for understanding hydrogenation properties of Mg50Co50, only little has been known. This is mainly because featureless broad diffraction peaks prevent conventional crystallographic analysis. Alternatively, we have applied atomic pair distribution function (PDF) analysis on synchrotron X-ray and neutron total scattering data. The PDF is a local structural probing technique that gives the probability of finding atom pairs separated by distance r.8 Since the PDF makes use of both Bragg and diffuse intensities (total scattering) and does not require long-range periodicity, it has long been applied to study liquids, glasses, and amorphous materials and, more recently, to investigate nanocrystalline and disordered materials.9
11 Using the PDF technique, we estimated the milling-time-dependent crystallite sizes of initial constituent phases and investigated the local structural evolution and the formation of Mg50Co50 during mechanical milling earlier.12 In this paper, we examine the bcc structural model proposed by earlier studies,4
6 discuss its insufficiency for the local structure of Mg50Co50, and explore another structural model that explains main features of experimental X-ray and neutron PDFs rather well. Finally, we discuss hydrogenation properties of MgxCo100-x based on the proposed model.

’ EXPERIMENTAL AND ANALYSIS METHODS Sample Preparation. Mg50Co50 sample was synthesized using a Fritsch P5 planetary ball mill. Two grams of a Mg (purity >99.9%, 100 mesh) and Co (purity >99.9%, 300 mesh) powder mixture was sealed with stainless steel milling balls (10 mm in diameter) in a stainless steel pot under Ar atmosphere. The ball-to-sample weight ratio was 20:1. Mechanical alloying was carried out for 100 h with a rotation speed of 200 rpm. Because of neutron experiments, Mg50Co50 sample was deuterated by measuring the pressure
composition isotherm (PCT) at 273 K. The maximum deuterium (D2) gas pressure used was 6 MPa. The final product after the PCT measurement was Mg50Co50D75 containing 1.77 mass % of deuterium. Even though we used deuterium instead of hydrogen in this study, the words “hydrogen” and “deuterium” and “hydrogenation” and “deuteration” would be used interchangeably in this paper unless it is specified. Synchrotron X-ray Experiment. Synchrotron X-ray total scattering experiments were conducted at the 11-ID-B beamline at the Advanced Photon Source at Argonne National Laboratory. Powder samples of Mg50Co50 and Mg50Co50D75 were loaded in kapton capillaries with diameter of 1.0 mm. Data were collected at 300 K using the rapid acquisition pair distribution function (RA-PDF) technique13 with an amorphous silica area detector14 manufactured by General Electric Healthcare. The 2-D detector was mounted orthogonal to the incident X-ray beam of 90.486 keV (λ = 0.137 02 Å). The sample-to-detector distance was 220 mm. Series of frames were collected for a data set to achieve good statistics.

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Table 1. bibj/Æbæ2 Values for All Possible Pairs in Mg50Co50a Mg
Mg

Mg
Co

Co
Co

X-ray

0.379

0.853

1.918

neutron

1.869

0.866

0.401

a

For X-ray, bMg = 12 and bCo = 27 were used. For neutron, bMg = 5.375 and bCo = 2.49 were used.

Table 2. bibj/Æbæ2 Values for All Possible Pairs in Mg50Co50D75a Mg
Mg

Mg
Co

Co
Co

D
Mg

D
Co

D
D

X-ray

1.075

2.420

5.444

0.090

0.202

0.007

neutron

1.108

0.513

0.238

1.375

0.637

1.707

a

For X-ray, bMg = 12, bCo = 27, and bD = 1 were used. For neutron, bMg = 5.375, bCo = 2.49, and bD = 6.671 were used.

Neutron Experiment. Time-of-flight (TOF) neutron total scattering experiments were carried out on the NPDF instrument15 at the Lujan Neutron Scattering Center at Los Alamos National Laboratory. Mg50Co50 and Mg50Co50D75 powder samples were packed in cylindrical vanadium cans of 6.35 mm in diameter. Each data set was collected at 300 K for 26 h to improve statistics at high Q. Data Processing and Modeling. Series of X-ray image data were combined and integrated using the software FIT2D.16 The signal from an empty container (a kapton capillary for X-ray and a vanadium can for neutron data) was subtracted from the raw data, and various other corrections were made.8 The PDF is obtained by a sine Fourier transformation of the powder diffraction data according to eq 1: Z 2 Qmax Q ½SðQ Þ
1 sinðQrÞ dQ ð1Þ GðrÞ ¼ π Qmin

where Q is the magnitude of the momentum transfer and S(Q) is the total scattering structure function.8 Because of the unfavorable signal-to-noise ratio at the high-Q regions, Q[S(Q)
1] was truncated at Qmax = 25 Å
1 for both X-ray and neutron data before the transformation. Data processing programs PDFgetX217 and PDFgetN18 were used for obtaining X-ray and neutron PDFs, respectively. Real space modeling was carried out using the program PDFgui19. The PDF is simply a bond length distribution, and therefore, the PDF of a given structure can be calculated using the following equation: " # bi bj 1 δðr
rij Þ
4πrF0 ð2Þ GðrÞ ¼ r i j Æbæ2

∑∑

where bi is the scattering power of atom i (X-ray form factors or neutron scattering lengths), Æbæ is the average scattering power of the sample, rij is the distance between atoms i and j, and F0 is the number density.20 The sums go over all the atoms in the structural model. It is worth noting that the intensity of a PDF peak locating at r is proportional to the number of atom pairs separated by distance r. The contribution of each pair of atoms is determined by bibj/Æbæ2. Tables 1 and 2 show these values for all possible atom pairs in Mg50Co50 and Mg50Co50D75, respectively. In the case of Mg50Co50, due to a large contrast in X-ray and neutron scattering powers of Mg and Co, signals from Co
Co pairs appear predominantly in the X-ray PDF but insignificantly 20336

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Figure 1. Experimental PDFs of Mg50Co50 and Mg50Co50D75 obtained from (a) neutron and (b) X-ray total scattering data. Scaled X-ray PDFs are shown in the inset of part b for an easy comparison.

in the neutron PDF and those from Mg
Mg pairs appear the other way around. Meanwhile, the contribution of Mg
Co pairs to both PDFs is on a similar level.

’ RESULTS AND DISCUSSION Alloy vs Deuteride PDFs. Neutron and X-ray PDFs of

Mg50Co50 and Mg50Co50D75 are shown in Figure 1. For all four PDFs, peaks are broad (even the second peaks) and their profiles die out quickly within 10
15 Å due to their poorly defined medium-to-long-range structural order. These are common characteristics of amorphous rather than nanocrystals whose PDF peaks are usually much sharper than those of amorphous and mediumrange structural correlation exists.12,21,22 Deuteration induces significant changes in neutron data (Figure 1a); PDF peaks become less intense and broader, and most notably, extra intensities arise at ∼2 and ∼3.8 Å. However, other than scale, no apparent change was observed in X-ray PDFs (Figure 1b). Change in PDF scale is due to change in the average scattering power Æbæ by introducing deuterium. The inset of Figure 1b shows scaled Mg50Co50 and Mg50Co50D75 PDFs. They overlap each other amazingly well. This is surprising because even though D signals are negligible in X-ray data, the volume expansion of a metal sublattice due to absorbed D usually causes noticeable PDF peak-shifts. However, except very small changes in intensities, peak positions and overall shapes stay the same. This suggests that Mg50Co50 bears large spaces which can accommodate D atoms without influencing the metal substructure significantly. This is especially applicable to the configuration of Co atoms, because Co-related signals predominate in X-ray data. On the other hand, pairs associated with D and Mg atoms (D
D and D
Mg) are probably responsible for newly appearing intensities in neutron data, since the neutron scattering length of D is comparable to that of Mg (Table 2) and such new intensities are not seen in the X-ray PDF. The Bcc Model for Mg50Co50. Let us consider the bcc model proposed by earlier studies for the local structure of Mg50Co50.4,6,7 We simulated the homogeneous distribution of Mg and Co by placing both types of atoms on each bcc site with half-occupancy. Structural parameters (lattice parameters, isotropic atomic displacement parameters, scale factors, and correlated motion parameters8)

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Figure 2. PDF refinement results using the bcc model. Structural parameters were refined until the model gave the best fit to (a) neutron and (b) X-ray PDFs of Mg50Co50 simultaneously. Experimental and calculated PDFs are plotted as blue open circles and red solid lines, respectively, and their difference is shifted down for clarity.

Table 3. Refined Structural Parameters of Bcc (space group Im3m) and C14 (space group P63/mmc) Modelsa bcc model a (Å)

3.0586(72)

c (Å)

C14 model 5.058(24) 7.921(68)

z (Mg)

0.561(5)

Uiso for Mg (Å2) Uiso for Co (Å2)

0.072(13) 0.138(25)

0.058(5) 0.0609(55)

particle diameterb (Å)

11(4)

15(4)

Rwpc (%)

59

33

a The refinement range was 1.5 < r < 10.0 Å for both cases. b The spherical particle form factor was used to simulate rapidly decaying PDF profiles. c Rwp indicates the goodness of a fit.

were refined until the model gave the best fit to both X-ray and neutron PDFs simultaneously. In order to model the limited structural coherence we used the spherical particle form factor in PDFgui. Details of the spherical particle form factor and its applications can be found elsewhere.8,12,21 The refinement range was 1.5 Å < r < 10 Å. Results are shown in Figure 2, and refined structural parameters are summarized in Table 3. It is evident from overall fits that the model tried to cover as much intensity as possible by means of large isotropic atomic displacement parameters, Uiso (Table 3). Broad intensity fluctuations in PDFs were explained to some degree but obviously detailed features, especially the first peak shape, were ignored. The insufficiency of the simple bcc model for the local structure of Mg50Co50 becomes apparent when experimental X-ray and neutron PDFs are plotted on top of each other and compared to calculated PDFs (Figure 3). If Mg and Co atoms were randomly distributed over bcc sites without any preferred local environment, no difference between X-ray and neutron PDFs would be expected, as shown in Figure 3a.23 However, this is not what we see from the experimental data. There is a clear distinction between two PDFs (Figure 3b). For instance, the 20337

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Figure 3. Comparison between X-ray and neutron PDFs of Mg50Co50. (a) Calculation based on the bcc model indicates that no difference should be found between X-ray and neutron PDFs if Mg and Co atoms were randomly distributed over bcc sites. However, (b) the experimental X-ray and neutron PDFs are quite different. Vertical dashed lines indicate the positions of the intensity maxima of the first X-ray and neutron PDF peaks. For calculation, a = 3.0586 Å and Uiso = 0.012 Å2 were used.

intensity maximum of the first X-ray PDF peak occurs at 2.47 Å followed by a shoulder at 2.8 Å (open triangles), whereas that of the first neutron PDF peak appears at 2.96 Å (open squares). Moreover, two peaks merely overlap over a small area at ∼2.7 Å. Another huge deviation is found at ∼4.2 Å. At first glance it might be hard to perceive that these two data sets are from the same sample. However, discrepancies in X-ray and neutron PDFs are indeed strong indications of the presence of the distinct local environment of each element. As we discussed earlier, the contribution of each pair of atoms to the PDF peak intensity is determined by bibj/Æbæ2. Taking this into account (please refer to Table 1), we could infer that the sharp X-ray peak at ∼2.47 Å (Figure 3b) arises from Co
Co pairs, since such intensity is diminished in neutron data. For the same reason, Mg
Mg pairs may be responsible for the neutron peak at 2.96 Å. The overlapped intensity at ∼2.7 Å is probably due to Mg
Co pairs. Therefore, our experimental data suggests that Mg50Co50 include 2.47 Å Co
Co, 2.7 Å Mg
Co, and 2.96 Å Mg
Mg interatomic distances. These distances are hardly yielded by the simple bcc model and can only be explained by a model containing element specific local environments. The MgCo2 Model for Mg50Co50. MgCo2, the only known stable phase in the Mg
Co binary phase diagram, has a MgZn2type structure.24 Three shortest interatomic distances found in this C14 hexagonal structure are 2.4 Å Co
Co, 2.84 Å Mg
Co, and 2.98 Å Mg
Mg distances, which are close to what we found from the neutron and X-ray PDFs. Therefore, we attempted to fit experimental PDFs using the MgCo2 model. The refinement procedure was the same as for the bcc model. Results are shown in Figure 4, and refined structural parameters are given in Table 3. It is evident that the MgCo2 model provides substantially improved fits to both neutron and X-ray data over the bcc model. The model successfully captured main features of neutron and X-ray PDFs simultaneously, especially their first peak shapes,

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Figure 4. PDF fits of MgCo2 data using the MgCo2 model. The model was refined until the best fits to (a) neutron and (b) X-ray PDFs of Mg50Co50 were simultaneously obtained. Blue open circles, red solid lines, and green solid lines correspond to experimental PDFs, calculated PDFs, and difference curves, respectively. The difference curves are offset for clarity.

with much smaller Uiso values (Table 3). It is likely that the local atomic arrangement in Mg50Co50 is closely related to the MgCo2 structure. Although the MgCo2 model explains X-ray and neutron PDFs reasonably well, it lacks Mg of being Mg50Co50. In addition, despite of the fact that there is no known hydride phase of MgCo2 our Mg50Co50 sample absorbs hydrogen. Therefore, to make a physically reasonable model, it is necessary to introduce a hydrogen (deuterium in our case) absorbing phase consisting of Mg or Mg and Co. There are three candidates: MgH2,25 Mg2CoH5,26 and Mg6Co2H11.27 Mg50Co50 models composed of MgCo2 and one of these hydride phases are MgCo2 + MgH2, MgCo2 + Mg2CoH5, and MgCo2 + 0.25Mg6Co2H11. These models can contain a maximum 1.2, 2.0, and 1.3 mass % of hydrogen, respectively. Since Mg50Co50 is known to absorb more than 2.1 mass % of hydrogen,4,6 we consider the MgCo2 + Mg2CoH5 model for the local structure of Mg50Co50. The MgCo2 + Mg2CoH5 Two-Phase Model for Mg50Co50. The easiest way of testing this model is fitting neutron and X-ray PDFs simultaneously using MgCo2 and Mg2CoH5 structural models. This requires two independent sets of phase weighting factors, one for neutron and the other for X-ray data (please see the Supporting Information). However, this is difficult using PDFgui, and therefore, it is necessary to construct a model containing the structural features of both MgCo2 and Mg2CoH5 phases. The two-phase model considered was constructed in the following way: a large empty volume of a tetragonal shape (45  45  50 Å3) was prepared. Unit cells of MgCo2 and Mg2CoD5 were replicated into supercells in such a way that each supercell could be fit in one-half of the tetragonal volume. Then, supercells were converted into P1 space group and placed in the empty tetragonal volume. In order to avoid artificial signals from phase boundaries, two phases were separated by more than 8 Å, the upper limit of the calculation range. Fractional coordinates were properly normalized to keep all the structural correlations of original phases. The final model contains the same number of Mg 20338

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Figure 5. Comparison between experimental and calculated PDFs of Mg50Co50 and Mg50Co50D75. The MgCo2 + Mg2CoD5 two-phase model was used for calculation. (a) Deuteration of Mg50Co50 did not alter X-ray PDFs significantly and (b) this characteristic was reproduced by the model. Meanwhile, (c) considerable changes in neutron PDFs after deuteration were observed and (d) the model qualitatively captured such changes.

and Co atoms. For the alloy model, D atoms were simply removed from the Mg2CoD5 part. Changes in lattice parameters due to the removal of D atoms were not considered. This is because there is no available structural information of Mg2Co yet. The issue in the stability of Mg2Co will be mentioned later. Because it was difficult to refine the two-phase model, only qualitative analysis was carried out by calculating several PDFs and comparing them with experimental data. A rapid falloff of PDF peaks with increasing r was modeled using a particle diameter, as mentioned earlier. Various Uiso values were tested, and calculated PDFs describing the main features of experimental PDFs best are shown in Figure 5. For calculation, Uiso(Mg) = 0.03 Å2 and Uiso(Co) = 0.01 Å2 for MgCo2 and Uiso(Mg) = 0.07 Å2, Uiso(Co) = 0.03 Å2, and Uiso(D) = 0.06 Å2 for Mg2CoH5 were used. Because the lattice types of MgCo2 and Mg2CoD5 structures are hexagonal and tetragonal, respectively, void spaces are present in the combined model. Such periodically repeated void spaces usually lead to an unfavorable background in calculated PDFs, along with a small scale, which we found in our calculated PDFs. Since our interest is whether the model can reproduce changes in PDFs before and after deuteration, we simply subtracted a constant from calculated PDFs to handle the background. As is shown in Figure 5a,b, the model performed well, explaining almost identical X-ray PDFs of alloy and deuteride samples; only the scales of two PDFs are different and the patterns of their intensity fluctuations remain unchanged. Meanwhile, changes in neutron PDF peaks after deuteration, such as peak broadening and extra intensities appearing at 2 and 3.8 Å (Figure 5c), are also qualitatively reproduced by the two-phase model (Figure 5d). Although there is a small difference in deuterium content between this model (Mg50Co50D83) and our sample (Mg50Co50D75), it does not alter calculated PDFs significantly. At any rate, the MgCo2 + Mg2CoD5 model seems to work reasonably well on qualitatively explaining experimental PDFs. This suggests that the structure of Mg50Co50 is mainly comprised of MgCo2 and Mg2Co like atomic arrangements whose correlations die out within 10
15 Å since no prominent peak was observed in experimental PDFs beyond that. Hydrogen is absorbed by Mg2Co like parts. However, it is highly probable that a small amount of other phases, such as fcc Co, whose

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Figure 6. The crystal structure of Mg2CoD5. (a) In Mg2CoD5, a Co atom is surrounded by five D atoms forming a square-pyramidal complex ion CoD54-. (b) Only the metal substructure is shown. Mg atoms are connected with light gray lines to show a partly bcc metal lattice.

medium range structural order was observed for a long time period during mechanical alloying,12 may exist. The presence of extra phases or impurities may influence the amount of MgCo2 and Mg2Co formation and consequently lead to different hydrogenation properties, such as the different amount of hydrogen absorption. It is worth examining the structure of Mg2CoD5 briefly. It crystallizes in a tetragonal structure with space group P4/nmm.26 Five D atoms surround a Co atom, forming a square-pyramidal complex ion CoD54- (Figure 6a). The shortest metal to deuterium interatomic distances are 1.515
1.59 Å of Co
D and 2.17
2.402 Å of Mg
D. D
D distances range from 2.124 to 2.465 Å. One interesting aspect of the Mg2CoD5 structure is its type of metal sublattice. It has a tetragonally distorted CaF2-type structure,26 as shown in Figure 6b. In this figure, adjacent Mg atoms are connected with light gray lines for guidance. It is evident that the metal lattice is partly body centered; pseudocubic cells of Mg (a = b = 3.16, c = 3.3 Å) with and without a Co atom at the body center are alternately placed. Such Mg2Co like local atomic arrangement allows us to account for TEM observations of the bcc structure (less than 50 Å in size) embedded in amorphous matrix.7 A possible reason why our PDFs do not show prominent structural correlations above 15 Å is that most bcc or bcc-like phases span over only a several Å range, and one extended more than 50 Å, as one observed by TEM, is probably rare or they may be plate-like shaped so correlations along one direction become very weak. The best calculated PDFs using the MgCo2 + Mg2CoD5 two-phase model in Figure 5 require larger Uiso values of Mg and Co atoms for the Mg2Co phase than MgCo2. This may be an implication of the weak structural correlation of Mg2Co in Mg50Co50. Although the existence of the stable Mg2Co phase is still controversial, there are a few studies reporting observations of the Mg2Co phase from samples prepared by mechanical alloying and by dehydrogenation of Mg2CoH5.28
30 Therefore, observation of the metastable Mg2Co phase in our mechanically milled Mg50Co50 is not surprising. There is also a reported structure for MgCo,31 but this model was not able to explain the difference in X-ray and neutron PDFs of Mg50Co50. Finally, we discuss the amount of hydrogen stored in MgxCo100-x. In Figure 7, the mass percent of absorbed hydrogen by MgxCo100-x at 373 K is plotted as a function of Mg content, x (blue solid circles). Vertical dashed lines indicate the lower and upper limits of alloy formation. Experimental values were taken from literature.4 As is seen, only compositions with x > 33 can 20339

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. )

Present Addresses

International Institute for Carbon-Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819
0395, Japan. ^ Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6475, United States.

Figure 7. The hydrogen capacity of MgxCo100-x. Experimental values are plotted with calculated values using the two-phase model. The alloy formation range is specified with vertical dashed lines.

absorb hydrogen. The total number of absorbed hydrogen increases with Mg content. To test the two-phase model further, we considered several different compositions consisting of Mg2Co and MgCo2 with different fractions and estimated their maximum hydrogen content. These are shown as red open squares in Figure 7. Calculated values are in good agreement with measured values, except for the composition range close to the upper Mg limit. Furthermore, the model predicts alloys would start absorbing hydrogen when Mg content (x) exceeds 33. This is consistent with the experimental observation mentioned above. Disagreement in hydrogen content between the model and measurement near x = 63 may be closely related to the instability of Mg2Co (=Mg66Co33) because MgxCo100-x stops forming before x reaches to 66. The suppression of Mg2Co formation probably leads to a reduction in the amount of absorbed hydrogen near x = 63. Mg2CoH5 is a stable hydride at ambient temperature, and this is probably why dehydrogenation of Mg50Co50 does not occur at 373 K, even under vacuum.4

’ CONCLUSIONS We have studied the local structure of mechanically alloyed Mg50Co50 and its deuteride, Mg50Co50D75, via PDF analysis on neutron and synchrotron X-ray total scattering data. X-ray PDFs of Mg50Co50 and Mg50Co50D75 were similar in appearance, but their neutron PDFs were quite different. These characteristics were hardly explained by the simple bcc model suggested by earlier studies. We found that overall features of experimental PDFs were relatively well explained by the two-phase model made of MgCo2 and Mg2CoD5 (D was removed for alloys). This implies that the structure of Mg50Co50 could be mainly seen as a mixture of two atomic configurations similar to MgCo2 and Mg2Co, whose structural correlation is extended no more than 10
15 Å. Furthermore, the two-phase model provided the hydrogen content for other MgxCo100-x compositions close to the experimental value and possible explanations for various observations from previous studies. Our results provide a useful insight into the hydrogenation properties of mechanically alloyed MgxCo100-x. Moreover, this study also demonstrated how we could benefit from analyzing both neutron and X-ray PDFs simultaneously to obtain the structural information of materials with limited structural correlation. ’ ASSOCIATED CONTENT

bS Supporting Information. Two-phase refinement on X-ray and neutron PDFs using PDFgui. This information is available free of charge via the Internet at http://pubs.acs.org.

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