Structure and Deuterium Desorption from Ca3Mg2Ni13 Deuteride: A

Article pubs.acs.org/JPCC

Structure and Deuterium Desorption from Ca3Mg2Ni13 Deuteride: A Neutron Diffraction Study Qingan Zhang,† Dalin Sun,*,‡ Junxian Zhang,§ Michel Latroche,§ Liuzhang Ouyang,∥ and Min Zhu*,∥ †

School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China Department of Materials Science, Fudan University, Shanghai 200433, China § Institut de Chimie et des Matériaux de Paris Est, CMTR, UMR 7182, CNRS-UPEC, Thiais 94320, France ∥ School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China ‡

S Supporting Information *

ABSTRACT: The Ca3Mg2Ni13 unit cell can be viewed as the stacking of three blocks along the c axis. Each block is composed of two sub-blocks; one sub-block contains one layer of a [CaMgNi4] unit, and the other sub-block consists of one layer of a [CaMgNi4] unit and one layer of a [CaNi5] unit. To understand the deuterium release from the Ca3Mg2Ni13 deuteride, crystal structures of Ca 3 Mg 2 Ni 1 3 D 1 5 . 6 , Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3 corresponding to before, during, and after deuterium desorption were determined by neutron diffraction. In Ca3Mg2Ni13D15.6, D atoms occupy interstitial sites within [CaNi5] and [CaMgNi4] units as well as sites at two unit borders. Upon deuterium desorption, the D atoms located at unit borders are released first. Then D atoms located within [CaNi5] and [CaMgNi4] units are simultaneously released which leads to the coexistence of Ca3Mg2Ni13D5.9 and a deuterium-poor solid solution phase. With further desorption, Ca3Mg2Ni13D5.9 transforms into Ca3Mg2Ni13D0.3 where D atoms reside in [CaNi5] units only. system.32 Indeed, ternary compounds such as La2MgNi9 (m = 1, n = 1), La3MgNi14 (m = 1, n = 2), and La4MgNi19 (m = 1, n = 3) have been synthesized experimentally, and crystal structures of these compounds have the same structure as the corresponding binary structures, LaNi3, La2Ni7, and La5Ni19, respectively.23 Similarly, in the Pr−Mg−Ni system, the Pr1.5Mg0.5Ni7 (m = 1, n = 2) and Pr3.75Mg1.25Ni19 (m = 1, n = 3) compounds derived from binary Pr2Ni7 and Pr5Ni19 demonstrate much improved thermodynamic properties for hydrogen storage.33 Note that only compounds with m = 1 have been reported for these ternary systems. Recently, we found a new compound, Ca3Mg2Ni13 (m = 2, n = 1), in the Ca−Mg−Ni system;34 however, its corresponding binary compound Ca5Ni13 does not exist in the Ca−Ni system. To our knowledge, Ca3Mg2Ni13 is the first compound reported to date with m = 2. X-ray diffraction (XRD) studies show that Ca3Mg2Ni13 crystallizes in a space group of R3̅m with cell parameters: a = 4.978(2) Å and c = 36.180(2) Å and Z = 3. The Ca3Mg2Ni13 crystal structure is illustrated in Figure 1 and shows stacking along the c axis of three blocks (A, B, C). Each block consists of two sub-blocks (I, II) composed of a layer of [CaMgNi4] for the first block and a layer of [CaMgNi4] and

1. INTRODUCTION Compared to highly H-containing chemical hydrides such as NaAlH4, LiBH4, and BH3NH3 and their derivatives,1−6 advantages of using hydride-forming intermetallics as hydrogen storage materials are a simpler chemical process, better reversibility, higher hydrogen purity, and a lower working temperature.7 However, to meet the latest demands for practical onboard application, the development of new intermetallics with higher hydrogen capacity is imperative, and this goal can be achieved by the substitution of light metals for heavy metals. For instance, ternary X−Mg−Ni (X = Ca, Y, and rare earths) compounds have been developed from corresponding binary X−Ni compounds.8−11 Like binary X− Ni compounds, ternary X−Mg−Ni compounds have layered structures where [XMgNi4] and [XNi5] units stack along the caxis alternatively according to different patterns.12−20 To reflect the stacking structure, the chemical formula for this series of compounds can be expressed as: m[XMgNi4]·n[XNi5]; i.e., Xn+mMgmNi5n+4m where m and n represent numbers of [XMgNi4] and [XNi5] layers, respectively.21−23 As a prototype of X−Mg−Ni systems, the La−Mg−Ni system has attracted extensive attention which is motivated for both fundamental interests and potential applications.24−31 Using density functional theory calculations, Crivello et al. predicted the structural stability of various compounds with m = 1 and n = 1, 2, 3 that may be formed in the La−Mg−Ni © 2014 American Chemical Society

Received: December 18, 2013 Revised: February 10, 2014 Published: February 16, 2014 4626

dx.doi.org/10.1021/jp412378r | J. Phys. Chem. C 2014, 118, 4626−4633

The Journal of Physical Chemistry C

Article

ensure good homogeneity and was then annealed at 550 °C for 2 days and subsequently at 850 °C for 3 days under an argon atmosphere. The as-received sample was crushed mechanically into powders of 38 μm in a glovebox under a dry argon atmosphere and was characterized by XRD to identify phase components. XRD was conducted using a Rigaku D/Max 2500VL/PC diffractometer with Cu Kα radiation at 50 kV and 150 mA. The XRD pattern of the prepared Ca3Mg2Ni13 sample was refined by the Rietveld method. The result obtained is displayed in Figure S1 (Supporting Information) and reveals that the sample consists of four phases with Ca3Mg2Ni13 as the major phase and Ca2MgNi9, Ni, and CaO as minor phases. The relative amounts of each phase are 85, 12, 2, and 1 wt %, respectively. Moreover, as confirmed in Table S1 (Supporting Information), crystallographic data obtained for Ca3Mg2Ni13 are in good agreement with previous values reported by us.34 2.2. Pressure−Composition Isotherm. Prior to neutron diffraction experiments, the pressure−composition isotherm of the Ca3Mg2Ni13−D2 system was measured at 25 °C and compared further to that of the Ca3Mg2Ni13−H2 system for consistency. This measurement was taken by the volumetric method in a typical Sievert-type apparatus which facilitates accurate determination of the deuterium D content. Figure 2

Figure 1. Structure of a unit cell for a Ca3Mg2Ni13 compound which illustrates the stacking of three blocks (A, B, C) along the c axis and sub-blocks of I and II consisting of [CaNi5] and [CaMgNi4] units.

[CaNi5] for the second block. Ca3Mg2Ni13 demonstrates fast kinetics but unfavorable thermodynamics for hydrogen storage. The pressure−composition isotherm for the Ca3Mg2Ni13−H2 system at 25 °C clearly shows that the desorption plateau is very low, and the desorbed hydrogen is less than the absorbed hydrogen.34 To determine different phases formed during desorption and to better understand the thermodynamic properties of this system, two questions must to be clarified: (i) what interstitial sites are occupied by hydrogen during the absorption process, and (ii) which sites are still occupied after the desorption process? To answer these questions, for this research, we employed neutron diffraction to study the change in the Ca3Mg2Ni13 hydride crystal structure during the course of hydrogen desorption. Because hydrogen is not suitable for neutron diffraction, deuterium is substituted for hydrogen in this research. First, the Ca3Mg2Ni13 compound was fully charged by deuterium gas and was then transferred to the neutron facility to determine the crystal structure of the full deuteride, including the sites occupied by deuterium, site occupancy, and chemical bonding length. Then the fully charged deuteride was subjected stepwise to partial dedeuteration and examined again by neutron diffraction to determine which sites are occupied or no longer occupied by deuterium. Finally, the mechanism of deuterium desorption from the Ca3Mg2Ni13 deuteride was discussed on the basis of the structural data obtained.

Figure 2. Pressure−composition isotherm for the Ca3Mg2Ni13−D2 system at 25 °C; deuterium absorption (■) and deuterium desorption (□).

illustrates the pressure−composition isotherm where the composition (i.e., the x-axis) is expressed in terms of two scales: one scale is the number of D atoms per formula unit of Ca3Mg2Ni13 (D/f.u.), and the other scale is the weight percentage of hydrogen content (wt % H) for comparison. Note that among the three minor phases in the prepared Ca3Mg2Ni13 sample Ni and CaO do not absorb deuterium or hydrogen, but Ca2MgNi9 is able to absorb deuterium to form a deuteride-like La2MgNi9D13.35 Therefore, the amounts of deuterium absorbed by the sample issue from the two deuterides of Ca3Mg2Ni13 and Ca2MgNi9. Usually, a layered X−Mg−Ni compound absorbs deuterium up to a concentration of around 1.0 D/M (the ratio of the number of D atoms to metal atoms);29 thus, 97% of the measured deuterium shown in Figure 2 is estimated to belong to the Ca3Mg2Ni13 deuteride. This estimation is further proven by the following analyses of the neutron diffraction data.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. As described elsewhere,34 Ca3Mg2Ni13 was prepared by induction melting of appropriate amounts of pure metals under an argon atmosphere. Losses of Ca and Mg due to evaporation were determined from preliminary experiments, and extra 10 wt % of Ca and 16 wt % of Mg were therefore added to compensate for losses during induction melting. The sample was remelted three times to 4627



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values obtained from XRD data (see Table S1 in Supporting Information). Meanwhile, the structural model for the minor Ca2MgNi9 deuteride was taken from La2MgNi9D13.35 To limit the number of refined parameters, the isotropic thermal factors (B values) for all D atoms were constrained at 1.5 Å2. The outcomes showed that the influence caused by these constraints on the structural parameters of the major phase is not significant. As illustrated in Figure 3, the calculated diffraction pattern of Sample 1 fits the observed diffraction pattern very well. Crystal

Figure 2 confirms that the deuterium isotherm is consistent with hydrogen absorption and desorption properties of the Ca3Mg2Ni13−H2 system.34 To extract and compare structural features at various stages of the deuterium desorption of the Ca3Mg2Ni13 deuteride, samples for neutron diffraction were carefully chosen in terms of the measured pressure− composition isotherm shown in Figure 2. 2.3. Neutron Diffraction. Three deuteride samples, Ca3Mg2Ni13D15.6, Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3, corresponding to before, during, and after deuterium desorption at 25 °C were selected for neutron diffraction. The deuterium pressures in equilibrium with the three deuterides were 2.9720, 0.0022, and 0.0007 MPa, respectively. For clarity, the status of the three samples was labeled as Sample 1, Sample 2, and Sample 3 in Figure 2, respectively. Neutron diffraction patterns were obtained at the Laboratoire Léon Brilloin in Saclay (France) on a 3T2 instrument in the range of 4.5° < 2θ < 121° by a step of 0.05° (λ = 1.225 Å). Samples were measured at room temperature using a tight cylindrical stainless steel container. During neutron diffraction measurements, samples were under constant deuterium pressure to avoid any desorption effect. Rietveld analysis was performed using the RIETAN-2000 program for refinement of neutron diffraction data.36 For the neutron data refinement, diffraction peaks originating from the stainless steel container were excluded. It should be noted that there is 2 wt % of Ni in the prepared sample and three Ni diffraction peaks in the range of 4.5−70° which overlap those of major phases, thus when we tried to refine the profile in the range of 4.5−70°, the calculated Ni abundance was variable with the structural parameters of the major phases. This suggests that neither the Ni abundance nor the structural parameters of the main phase could be determined accurately if the neutron data in the range of 4.5−70° were used for refinement. Fortunately, the diffraction data of Ni exceeding 70° had good resolution, thus the data within the range of 4.5−110° were used for the refinement.

Figure 3. Rietveld refinement of the observed power neutron diffraction pattern for the Ca3Mg2Ni13D15.6 sample. Reflection markers (from above) are for Ca3Mg2Ni13D15.6 (85 wt %), Ca2MgNi9D13 (12 wt %), Ni (2 wt %), and CaO (1 wt %) phases, respectively.

structural data obtained for the Ca3Mg2Ni13 deuteride are given in Table 1 with lattice parameters of a = 5.2974(8) Å and c = 39.320(6) Å. Thirteen interstitial sites (denoted as D1, D2, D3, ..., D13) are partially filled by D atoms. The Ca3Mg2Ni13 deuteride crystal structure is illustrated in Figure 4 where the following features can be seen: (i) D atoms occupy both [CaNi5] and [CaMgNi4] units. Within the [CaNi5] unit, two sites (tetrahedral Ni4 [D1 site] and tetragonal pyramidal Ca2Ni3 [D2 site]) are occupied which is in agreement with the reported La2MgNi9D13 crystal structure.35 (ii) D3 and D4 occupy two Ca(Ca/Mg)Ni2 tetrahedra at the [CaNi5]/ [CaMgNi4] border which is defined by the z position of the Ni4 atomic position (z = 0.445(1)). If the D3 and D4 atoms are supposedly equally shared between [CaNi5] and [CaMgNi4] units, each [CaNi5] unit contains 4.8(2) D (i.e., CaNi5D4.8(2)) which is identical to the deuterium quantity in β-CaNi5D4.8, although the deuterium distribution is somehow different.40 (iii) Within the [CaMgNi4] unit, D atoms occupy eight sites (D5 in Ni4, D6 in (Ca/Mg)Ni3, D7 in (Ca/Mg)2Ni2, D8 in (Ca/Mg)Ni3, D9 in (Ca/Mg)2Ni2, D10 in (Ca/Mg)Ni3, D11 in (Ca/Mg)2Ni2, and D12 in (Ca/Mg)2Ni2). At the border between the two [CaMgNi4] units defined by the z atomic position of the Ni3 atom (z = 0), D13 is located in the (Ca/ Mg)2Ni2 tetrahedron. As a result, each [CaMgNi4] unit has a deuterium content of 5.4(3) D (i.e., CaMgNi4D5.4(3)). Therefore, based on the sum of the deuterium occupancies of the 13 sites, the deuteride can be described as Ca3Mg2Ni13D15.6(8) (CaNi5D4.8(2) + 2CaMgNi4D5.4(3)). In this case, the calculated D content of 15.6(8) D/f.u. agrees with the measured value of 15.8 D/f.u. by the volumetric method (see Figure 2). Interatomic distances between D atoms and surrounding metal atoms are listed in Table S3 (Supporting Information). Ca−D distances are in the range of 2.63−2.70 Å which agrees fairly well with distances in Ca4Mg3Fe22D22 (2.39−2.76 Å).41

3. RESULTS 3.1. Crystal Structure of Ca3Mg2Ni13D15.6. As revealed by neutron diffraction, for Mg-free deuterides like La2Ni7D6.5, Ce2Ni7D4, and CeNi3D2.8, D atoms reside only in [XNi2] units, not in [XNi5] units.37,38 On the other hand, for Mg-containing deuterides such as La4MgNi19D21.8, La1.5Mg0.5Ni7D9.1, and La2MgNi9D13, both [XMgNi4] and [XNi5] units are occupied by D atoms.29,31,35 Depending on the types of local metal atoms, various interstitial sites are identified for deuterium occupation: Ni4, (X/Mg)Ni3, (X/Mg)2Ni2, and (X/Mg)3Ni2 in [XMgNi4] units and Ni4, XNi3, X2Ni2, and X2Ni4 (or X2Ni3 derived from X2Ni4) in [XNi5] units.29,31,35,39 On the basis of this structural information and our XRD data in Table S1 (Supporting Information), we have identified 18 sites that might be available for deuterium occupation in the Ca3Mg2Ni13 structure: five sites (two Ni4, two CaNi3, and one Ca2Ni3 sites) within [CaNi5] units and ten sites (two Ni4, four (Ca/Mg)Ni3, and four (Ca/Mg)2Ni2 sites) within [CaMgNi4] units, two Ca(Ca/Mg)Ni2 sites at the [CaNi5]/[CaMgNi4] border, and one (Ca/Mg)2Ni2 site at the [CaMgNi4]/[CaMgNi4] border (see Table S2 in Supporting Information). All these probable sites have been considered and tested for Rietveld refinement of Ca3Mg2Ni13 deuteride neutron data. The starting model for the structure was based on the atomic structure of a Ca3Mg2Ni13 compound derived from XRD analysis. During the refinement, Ca/Mg occupancy ratios for the two 6c sites were fixed to the 4628



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Table 1. Atomic Coordinates, Isotropic Thermal Parameters (B Values), and Occupation Numbers (g Values) for Ca3Mg2Ni13D15.6 Refined from Neutron Diffraction Data of Sample 1a atom

site

g

x

y

z

B (Å2)

Ca1 Ca2/Mg2 Ca3/Mg3 Ni1 Ni2 Ni3 Ni4 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13

3b 6c 6c 6c 6c 9e 18h 6c 18h 18h 18h 6c 6c 18h 18h 18h 18h 36i 36i 18h

1 0.45/0.55b 0.53/0.47 1 1 1 1 0.37(1) 0.56(2) 0.12(1) 0.11(1) 0.08(1) 0.49(2) 0.08(1) 0.14(1) 0.25(1) 0.19(1) 0.20(1) 0.18(1) 0.09(1)

0 0 0 0 0 1/2 0.499(1) 0 0.174(1) 0.851(1) 0.149(1) 0 0 0.783(1) 0.217(1) 0.213(1) 0.787(1) 0.625(1) 0.946(1) 0.851(1)

0 0 0 0 0 0 −x 0 −x −x −x 0 0 −x −x −x −x 0.035(1) 0.697(3) −x

1/2 0.045(1) 0.404(1) 0.163(1) 0.279(1) 0 0.445(1) 0.205(2) 0.847(2) 0.443(2) 0.443(2) 0.680(3) 0.098(2) 0.978(1) 0.978(1) 0.578(2) 0.580(2) 0.038(2) 0.262(2) 0

2.8(3) 1.1(2) 2.9(3) 1.3(1) 2.4(2) 0.3(1) 2.3(2) 1.5c 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

a Space group R3̅m (No. 166); cell parameters: a = 5.2974(8) Å and c = 39.320(6) Å; Z = 3. Rwp = 2.42%, Rp = 1.93%, RI = 1.02%, S = 1.21. bg values for Ca2/Mg2 and Ca3/Mg3 were determined by the XRD pattern of Ca3Mg2Ni13 (see Table S1 in Supporting Information). cB values for all D atoms were constrained as 1.5.

Supporting Information) and can be only partially filled by D atoms. 3.2. Occupancies of D Sites in Ca3Mg2Ni13D5.9. To understand the deuterium unloading mechanism from interstitial sites of Ca3Mg2Ni13D15.6, Sample 1 was subjected to partial desorption. Figure 5 shows the refined diffraction

Figure 4. Crystal structure of Ca3Mg2Ni13D15.6, showing the stacking of [CaNi5D4.8] and [CaMgNi4D5.4] units. Thirteen interstitial sites are filled by D atoms; two sites are located within the [CaNi5] unit; two sites are at the border of [CaNi5] and [CaMgNi4] units; eight sites are within the [CaMgNi4] unit; and one site is at the border of [CaMgNi4] and [CaMgNi4] units.

Figure 5. Rietveld refinement of the observed power neutron diffraction pattern for the Ca3Mg2Ni13D5.9 sample. Reflection markers (from above) are for phases Ca 3 Mg 2 Ni 13 D 5.9 (65 wt %), Ca3Mg2Ni13D0.4 (20 wt %), Ca2MgNi9D7.2 (9 wt %), Ca2MgNi9D0.8 (3 wt %), Ni (2 wt %), and CaO (1 wt %), respectively.

pattern of the partially dedeuterated sample (Sample 2 in Figure 2) which reveals the coexistence of a Ca3Mg2Ni13 deuteride and a deuterium-poor Ca3Mg2Ni13 solid solution. To limit the number of refined parameters, atomic coordinates and isotropic thermal parameters for the deuterium-poor solid solution were fixed to Ca3Mg2Ni13D15.6 parameters (see Table S5 in Supporting Information). For a minor Ca2MgNi9 deuteride and deuterium-poor Ca2MgNi9 solid solution, atomic coordinates were fixed to those of La2MgNi9D13,35 and

Ca/Mg−D distances are in the range of 2.03−2.24 Å. Shorter distances should correspond to Mg−D bonds because they are close to the Mg−D distance (1.96 Å) observed for MgD2.42 Larger distances are attributed to Ca−D distances since they are close to the Ca−D distances (2.24−2.63 Å) in CaD2.43 Ni− D distances (1.61−1.68 Å) are in good agreement with LaNi5D7 distances (1.53−1.71 Å).44 Note that some neighboring D sites are less than 2.0 Å in distance (see Table S4 in 4629



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site

g

x

y

z

B (Å2)

Ca1 Ca2/Mg2 Ca3/Mg3 Ni1 Ni2 Ni3 Ni4 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13

3b 6c 6c 6c 6c 9e 18h 6c 18h 18h 18h 6c 6c 18h 18h 18h 18h 36i 36i 18h

1 0.45/0.55b 0.53/0.47 1 1 1 1 0.13(1) 0.16(1) 0 0 0 0.39(2) 0.05(1) 0.12(1) 0.10(1) 0.08(1) 0.15(1) 0 0

0 0 0 0 0 1/2 0.498(1) 0 0.173(1)    0 0.783(1) 0.217(1) 0.213(1) 0.785(2) 0.625(1)  

0 0 0 0 0 0 −x 0 −x    0 −x −x −x −x 0.035(1)  

1/2 0.045(1) 0.404(1) 0.165(1) 0.277(1) 0 0.445(1) 0.206(2) 0.847(2)    0.098(2) 0.976(2) 0.977(1) 0.578(2) 0.580(2) 0.038(2)  

2.2(2) 3.8(2) 1.0(3) 2.1(3) 2.7(2) 1.7(1) 2.2(2) 1.5c 1.5    1.5 1.5 1.5 1.5 1.5 1.5  


Figure 2). The refined diffraction pattern for Sample 3 is shown in Figure 6, and the deuterium-poor solid solution was

isotropic thermal parameters for metal and D atoms were constrained as 1 and 1.5 Å2, respectively (see Tables S6 and S7 in Supporting Information). Rietveld refinement confirmed that Sample 2 consists of Ca 3 Mg 2 Ni 13 D 5.9 (65 wt %), Ca 3Mg2Ni13D0.4 (20 wt %), Ca 2MgNi9D7.2 (9 wt %), Ca2MgNi9D0.8 (3 wt %), Ni (2 wt %), and CaO (1 wt %). Hence, the deuterium content in the sample can be calculated as 0.54 wt % H (here the D weight content is converted into the H weight content for the purpose of comparison) which is close to the measured value of 0.68 wt % H by the volumetric method outlined in Figure 2. Ca3Mg2Ni13D5.9 crystallographic parameters are listed in Table 2. Compared to the initial Ca3Mg2Ni13D15.6, the following changes in D occupancies are observable: (i) D3 and D4 sites in the tetrahedron Ca(Ca/Mg)Ni2 at the [CaNi5]/ [CaMgNi4] border and D13 atoms in the tetrahedron (Ca/ Mg)2Ni2 at the [CaMgNi4]/[CaMgNi4] border are empty; i.e., D atoms are completely released from these sites; (ii) D occupancies at sites within [CaNi5] and [CaMgNi4] units are generally reduced which indicates the partial release of D atoms from these sites; (iii) the refined D content can be expressed as Ca3Mg2Ni13D5.9(5) (CaNi5D1.22(8) + 2CaMgNi4D2.34(20)) which indicates that nearly 40% of D atoms remain in interstices of [CaNi5] and [CaMgNi4] units relative to Ca3Mg2Ni13D15.6. Interatomic distances between D atoms and surrounding metal atoms in Ca3Mg2Ni13D5.9 are listed in Table S8 (Supporting Information). Ca−D, Ca/Mg−D, and Ni−D distances are 2.62, 1.96−2.12, and 1.54−1.63 Å, respectively. These values are very close to the individual values of CaD2 or MgD2 deuterides, which suggests that these sites are energetically favorable for the D atoms. 3.3. Occupancies of D Sites in Ca3Mg2Ni13D0.3. To determine which sites are still occupied in the structure of the deuterium-poor Ca3Mg2Ni13 solid solution, Sample 2 was subjected to additional desorption to obtain Sample 3 (see

Figure 6. Rietveld refinement of the observed power neutron diffraction pattern for the Ca3Mg2Ni13D0.3 sample. Reflection markers (from above) are for Ca3Mg2Ni13D0.3 (85 wt %), Ca2MgNi9D0.7 (12 wt %), Ni (2 wt %), and CaO (1 wt %) phases, respectively.

determined to be Ca3Mg2Ni13D0.3(1). Sample 3 is composed of Ca3Mg2Ni13D0.3 (85 wt %), Ca2MgNi9D0.7 (12 wt %), Ni (2 wt %), and CaO (1 wt %); thus, the total amount of deuterium in the sample can be estimated at around 0.04 wt % H which complies with the measured value by the volumetric method in Figure 2. Table 3 contains structural data for Ca3Mg2Ni13D0.3 which indicate that a few D atoms remain in the tetrahedral Ni4 and tetragonal pyramidal Ca2Ni3 sites within [CaNi5] units. Similar deuterium occupation is also observed in the minor phase Ca2MgNi9D0.7 (see Table S9 in Supporting Information). In Ca3Mg2Ni13D0.3, the Ca−D (2.55 Å) and Ni−D (1.50−1.57 Å) distances (see Table S10 in Supporting Information) further suggest that the Ni4 and Ca2Ni3 interstices within the [CaNi5] 4630



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site

g

x

y

z

B (Å2)

Ca1 Ca2/Mg2 Ca3/Mg3 Ni1 Ni2 Ni3 Ni4 D1 D2

3b 6c 6c 6c 6c 9e 18h 6c 18h

1 0.45/0.55b 0.53/0.47 1 1 1 1 0.05(1) 0.03(1)

0 0 0 0 0 1/2 0.499(1) 0 0.174(1)

0 0 0 0 0 0 −x 0 −x

1/2 0.044(1) 0.404(1) 0.164(1) 0.278(1) 0 0.443(1) 0.206(1) 0.849(2)

2.0(3) 0.4(1) 1.1(1) 0.5(1) 1.0(1) 1.0(1) 0.9(1) 1.5c 1.5


Table 4. Lattice Expansions of Ca3Mg2Ni13D15.6, Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3 a (Å) c (Å) V (Å3) Δa/a (%) Δc/c (%) ΔV/V (%) c[CaNi5] (Å) c[CaMgNi4] (Å) Δc/c[CaNi5] (%) Δc/c[CaMgNi4] (%) V[CaNi5] (Å3) V[CaMgNi4] (Å3) ΔV/V[CaNi5] (%) ΔV/V[CaMgNi4] (%)

Ca3Mg2Ni13

Ca3Mg2Ni13D15.6

Ca3Mg2Ni13D5.9

Ca3Mg2Ni13D0.3

4.9615(5) 36.066(3) 768.9(1)

5.2974(8) 39.320(6) 955.6(2) 6.8 9.0 24.3 4.33 (3) 4.38(2) 7.4 9.6 105.2(5) 106.5(5) 22.4 25.0

5.1369(5) 37.732(4) 862.3(2) 3.5 4.6 12.1 4.15(3) 4.21(2) 2.9 5.4 94.8(5) 96.2(5) 10.3 13.0

4.9762(4) 36.358(6) 779.7(2) 0.3 0.8 1.4 4.14(3) 3.99(2) 2.6 −0.1 88.8(5) 85.6(5) 3.3 0.5

4.032(7) 3.995(4)

85.96(1) 85.17(1)

[CaMgNi4]. However, this deuterium distribution is somewhat different from the case of individual (Ca0.67Mg0.33)Ni2D2 deuteride where only (Ca/Mg)Ni3 and (Ca/Mg)2Ni2 tetrahedra are occupied.46 The difference may be caused by the constraint on the [CaMgNi4] unit coming from the adjacent [CaNi5] unit in Ca3Mg2Ni13D15.6. Such constraint is necessary to adjust the Ni4 tetrahedra size suitable for D occupation. D-occupied sites in the [CaNi5] unit of Ca3Mg2Ni13D15.6 are identical to the [LaNi5] unit of La2MgNi9D1335 but are somewhat different from the [LaNi5] unit in La4MgNi19D21.8 and La1.5Mg0.5Ni7D9.129,31 which may be related to the number of [XNi5] units. For Ca3Mg2Ni13D15.6 and La2MgNi9D13, there is one [XNi5] unit per block, whereas La4MgNi19D21.8 has three adjacent [LaNi5] units per block and La1.5Mg0.5Ni7D9.1 two adjacent [LaNi5] blocks. Likewise, the difference in D distribution in their [CaMgNi4] units may also be linked to the number of [XMgNi4] units. Ca3Mg2Ni13D15.6 has two adjacent [CaMgNi4] units per block, but La4MgNi19D21.8, La1.5Mg0.5Ni7D9.1, and La2MgNi9D13 have only one [XMgNi4] unit per block.29,31,35 4.2. Mechanism of Deuterium Desorption. The analysis of the D-site occupancies in Ca3Mg2Ni13D5.9 clearly shows that D3 and D4 sites at the [CaNi5]/[CaMgNi4] border and the D13 site at the [CaMgNi4]/[CaMgNi4] border are fully released (see Table 2) which suggests that D atoms at unit borders are released first during deuterium desorption. This result may be related to the strain caused by the mismatch between two adjacent units.29 Moreover, compared to

units are suitable for D atom accommodation. Note that the D atom distribution in the Ca3Mg2Ni13D0.3 is different from La1.64Mg0.36Ni7D0.56 where D atoms occupy only the LaNi3 tetrahedron within [LaMgNi4] units.44 This difference in D distribution may be related to the interstice size that is strongly dependent on Mg/X ratios in [XMgNi4] units (with 1:1 for Ca3Mg2Ni13D0.3 and 9:16 for La1.64Mg0.36Ni7D0.56) and atomic radii of X.45

4. DISCUSSION 4.1. Deuterium Distribution in Subunits of Ca3Mg2Ni13D15.6. Although Mg atoms reside only in the [CaMgNi4] unit in Ca3Mg2Ni13,34 cell parameters of both [CaMgNi4] and the [CaNi5] units decreased with the introduction of Mg atoms compared to CaNi5 and (Ca,Mg)Ni2 compounds. Hence, site occupancies of D atoms in [CaNi5] and [CaMgNi4] units of Ca3Mg2Ni13D15.6 are distinct from units of CaNi5 and (Ca,Mg)Ni2 compounds, respectively. For Ca3Mg2Ni13D15.6, D atoms occupy Ni4 and Ca2Ni3 sites within the [CaNi5] unit and two Ca(Ca/Mg)Ni2 tetrahedra at the [CaNi5]/[CaMgNi4] border and thus reach a deuterium content of 4.8 D/[CaNi5]. In a binary CaNi5 compound, D atoms initially occupy octahedral Ca2Ni4 and tetrahedral Ca2Ni2 sites to form a solid solution CaNi5D0.3. Further deuterium absorption leads to structural transitions from α′, β, to γ phase up to a deuterium content of 6.1 D/f.u.40 Similarly, D atoms occupy the Ni4, (Ca/Mg)Ni3, and (Ca/Mg)2Ni2 tetrahedra in the [CaMgNi4] unit up to a content of 5.4 D/ 4631



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Ca3Mg2Ni13D15.6, all D site occupancies within [CaNi5] and [CaMgNi4] units are decreased in Ca3Mg2Ni13D5.9 which implies a simultaneous deuterium desorption from both [CaNi5] and [CaMgNi4] units and corresponds to the accommodation of mismatch between two units with a same a-axis length. The deuterium content is significantly reduced by 9.7 D/f.u. as Ca3Mg2Ni13D15.6 transforms into Ca3Mg2Ni13D5.9, which means that a large solid solution domain exists between the two deuterides Ca3Mg2Ni13D5.9 and Ca3Mg2Ni13D15.6 and results in an increased branch in the pressure−composition isotherm. With further desorption, Ca3Mg2Ni13D5.9 directly transforms into a hydrogen-poor solid solution phase in the two-phase coexistence state. However, the two-phase region of the pressure−composition isotherm is very narrow (about 5.6 D/ f.u.) in the Ca3Mg2Ni13−D2 system. The relatively narrow twophase region together with a relatively wide single-deuteride domain is identical to the scenario as previously reported for the NdNi4Mg−H2 system.47 As Ca3Mg2Ni13D5.9 transforms totally into Ca3Mg2Ni13D0.3, D atoms occupy Ni4 and Ca2Ni3 sites in the [CaNi5] units only (see Table 3). 4.3. Changes in Lattice Expansion with Deuterium Desorption. Since D atoms are distributed in both [CaMgNi4] and [CaNi5] units, Ca3Mg2Ni13D15.6 shows a rather isotropic lattice expansion (Δa/a = 6.8%, Δc/c = 9.0%, and ΔV/V = 24.3%) (see Table 4). In addition, the [CaNi5] unit lattice expansion is smaller than the [CaMgNi4] unit lattice expansion. Considering that the a-axis expansion is constrained to be the same for the [CaNi5] and [CaMgNi4] units, slight differences in expansion and D content may be related to the accommodation of the mismatch between the two units. When Ca3Mg2Ni13D15.6 is dedeuterated into Ca3Mg2Ni13D5.9 and the deuterium-poor solid solution, expansions of [CaNi5] and [CaMgNi4] units simultaneously are smaller but are still isotropic. When the two phases transform into the solid solution phase, Ca3Mg2Ni13D0.3, the volume expansion occurs mainly in the [CaNi5] units (ΔV/V[CaNi5] = 3.3%). The expansion along the c-axis (Δc/c[CaNi5] = 2.6%) is larger than the expansion along the a-axis (Δa/a[CaNi5] = 0.3%) which may be related to the constraint of the adjacent [CaMgNi4] units. However, this small expansion along the a-axis still causes a slight volume expansion (ΔV/V[CaMgNi4] = 0.5%) of the [CaMgNi4] units, even though there is no deuterium occupation in this unit and the c-axis (3.99(2) Å) maintains nearly the same value (3.995(4) Å) as the initial compound, Ca3Mg2Ni13, before deuterium absorption.

Article

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic parameters of Ca3Mg2Ni13 in as-prepared alloys, Ca3Mg2Ni13D0.4, Ca2MgNi9D7.2, and Ca2MgNi9D0.8 in Sample 2, and Ca2MgNi9D0.7 in Sample 3; and interatomic distances of the occupied interstices in Ca3Mg2Ni13D15.6, Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3 structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.S.). *E-mail: [email protected] (M.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 50925102 and 51271002) and the Ministry of Science and Technology of China (No. 2010CB631302). M.L. and J.Z. wish to thank Mrs F. Porcher for her assistance in neutron data acquisition at LLB (Saclay, France).

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REFERENCES

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5. CONCLUSIONS Neutron diffraction studies of Ca3Mg2Ni13D15.6 confirmed that D atoms occupy the interstitial sites of [CaNi5] and [CaMgNi4] units as well as sites at unit borders. After deuterium desorption, D atoms located at the unit border are first released. D atoms in [CaNi5] and [CaMgNi4] units are then simultaneously released which reveals the coexistence of Ca3Mg2Ni13D5.9 and a deuterium-poor solid solution phase. With further desorption, the Ca3Mg2Ni13D5.9 transforms into Ca3Mg2Ni13D0.3 where D atoms are located only in the [CaNi5] units. These findings provide not only important information on the mechanism of hydrogen desorption but also some useful hints for tailoring intermetallic hydride properties with layered features in crystal structures. 4632



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Structure and Deuterium Desorption from Ca3Mg2Ni13 Deuteride: A

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