Journal of Alloys and Compounds 603 (2014) 7–13

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Journal of Alloys and Compounds

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Hydrogenation properties and crystal structure of YMgT4 (T = Co, Ni, Cu)compounds

http://dx.doi.org/10.1016/j.jallcom.2014.03.0300925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +380 50 9833506.E-mail address: zavaliy@ipm.lviv.ua (I.Yu. Zavaliy).

V.V. Shtender a, R.V. Denys a,b, V. Paul-Boncour c, A.B. Riabov a, I.Yu. Zavaliy a,⇑a Karpenko Physico-Mechanical Institute, NAS of Ukraine, 5 Naukova St., 79601 Lviv, Ukraineb Hystorsys AS, P.O. Box 45, Kjeller NO-2027, Norwayc Institut de Chimie et des Matériaux de Paris Est, CMTR, CNRS and U-PEC, 2-8 rue H. Dunant, 94320 Thiais, France

a r t i c l e i n f o

Article history:Received 20 February 2014Received in revised form 6 March 2014Accepted 7 March 2014Available online 17 March 2014

Keywords:Hydrogen storageMetal hydrideRare Earth compoundsMagnesium compoundsPressure–composition–temperaturerelationshipsCrystal structure

a b s t r a c t

New two ternary YMgCo4 and YMgCu4 and one quaternary YMgCo2Ni2 compounds have been synthe-sized by mechanical alloying with further annealing. The hydrogenation capacity of YMgCo4 reaches6.8 at. H/f.u. The Pressure-Composition-Temperature studies of YMgCo4–H2 and YMgNi4–H2 systemsrevealed that introduction of magnesium, accompanied by shrinking of the unit cell, decreases thestability of hydrides comparing to binary YCo2 and YNi2 compounds. The values of heat and entropyof the YMgCo4H6.8 hydride formation were calculated: DH = �27.9 ± 0.8 kJ mol–1 H2 and DS =�93.4 ± 2.6 J mol�1 H2 K�1. The YMgCo2Ni2–H2 system shows intermediate thermodynamic propertiescompared to the ternary hydrides (DH = �28.8 ± 0.2 kJ mol–1 H2 and DS = �117.6 ± 2.4 J mol–1 H2 K�1).The YMgCo4H6.8 and YMgCo2Ni2H4.9 hydrides keep the cubic structure of the parent compounds with acell volume expansion of 23 and 14.4% respectively. It is shown that the YMgCu4 compound does notinteract with hydrogen under normal conditions.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Recently substantial efforts of researchers have been focused onthe development of magnesium and magnesium-based materialsfor hydrogen storage technologies. There are two main directionsof such studies. The first one is the creation of nanoscale Mg-basedcomposites, which are capable of absorbing and desorbing hydro-gen at substantially lower temperatures than pure magnesium me-tal [1,2]. Another direction is the development of new ternary orpseudo-binary Mg-containing compounds, able to reversibly ab-sorb hydrogen from the gas phase or electrochemically. Amongsuch compounds one should mention recently studied Mg3TNi2

(T – Ti, Al, Mn) derivatives of Ti2Ni-type [3–5] and (R1�xMgx)nNim

intermetallic compounds (R = La, Ce or other rare earth metals)[6–8].

The hybrid R1�xMgxNi3 or (R1�xMgx)2Ni7 compounds have greatpotential for use as the new generation negative electrodes of Ni–MH batteries. Their electrochemical discharge capacity is close to400 mAh/g [7], which is 25% higher than the capacity of industrialLaNi5-based electrodes. The RMgNi4 compounds with a cubicMgCu4Sn structure type belong to the promising hydrogen absorb-ing materials also. For example, the reversible capacity (1.05 wt.%

H) and the hydride formation heat (�35.8 kJ/mol H2) of the YMgNi4

compound [9] are close to that for the LaNi5–H2 system [10], whichmakes this material attractive for hydrogen storage.

RMgNi4 ternary intermetallics (where R – rare earth metal) arecrystallized in the MgCu4Sn structure type, which is an orderedvariant of the AuBe5 (F–43m) type, where Mg occupies the positionof Au (4a) and Sn one of the Be (4c) sites (Fig. 1). They can be con-sidered also as derivative for the structure of cubic Laves phase(MgCu2 type, Fd–3m) [11]. For the first time CeMgNi4 ternary com-pound was described in [12]. After this Aono et al. [9] synthesizedYMgNi4 compound and studied its hydrogen absorption properties.RMgNi4 isostructural compounds were synthesized for a number ofother rare-earth metals [13,14].

Interaction of the RMgNi4 compounds with hydrogen was stud-ied for R = Y [9,15] and R = La, Nd [16]. They absorb the sameamount of hydrogen (H/M = 0.6�0.67). Contrary to RNi2 hydrides[17,18] these compounds are stable against hydrogen-inducedamorphisation and disproportionation at room temperature and1–2 MPa H2 pressure. Recently we have shown that the isostruc-tural compounds are formed also in R–Mg–Co systems [19]. TheCeMgCo4 compound was synthesized and its hydrogen absorptionproperties were studied. This compound readily absorbs hydrogenup to 1.0 H/M at room temperature and 10 MPa H2 pressure [19],whereas under the same conditions isostructural CeMgNi4 com-pound is unable to form intermetallic hydride [20].

Fig. 1. Comparison of the structure of RNi2 and RMgNi4 compounds. Upon the transition from RNi2 to RMgNi4 (Fd–3m ? F–43m) the site 8a (000), filled by atoms R in RNi2

Laves phases, splits into two sites: 4a (000) – R and 4c (1/4 1/4 1/4) – Mg.

20 30 40 50 60 70 80 90

Cu-Kα YobsYcalcYobs - Ycalc Bragg position

Y2O3

YMgCo2Ni2

Inte

nsity

(a.u

.)

2θ (deg.)

Fig. 2. X-ray diffraction pattern of YMgCo2Ni2 alloy. Observed (Yobs), calculated(Ycalc), difference (Yobs�Ycalc) diffraction profiles and Bragg’s peaks positions forYMgCo2Ni2 (97.1 wt.%) and Y2O3 (2.9 wt.%) phases are shown.

20 30 40 50 60 70 80 90

Cu-KαYobsYcalcYobs - Ycalc Bragg position

Inte

nsity

(a.u

.)

2θ (deg.)

Fig. 3. X-ray diffraction pattern of YMgCu4 alloy. Observed (Yobs), calculated (Ycalc),difference (Yobs–Ycalc) diffraction profiles and Bragg’s peaks positions are shown.

8 V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13

In the present work, the existence of YMgCo4 and YMgCu4 com-pounds as well as YMgCo2Ni2 intermediate compound from the Y–Mg–Ni–Co system has been shown and their structural propertiesanalyzed. Their hydrogen absorption-desorption properties wereinvestigated. The crystal structure of the YMgCo4, YMgNi4 andYMgCo2Ni2 hydrides has been determined.

2. Experimental part

As starting materials for preparation of the RMgT4 compounds we used RT4 al-loy precursors and Mg powder (Alfa Aesar, 325 mesh, 99.8%). RT4 alloys were pre-pared by arc melting from pure metals (purity P99.9%) in an atmosphere ofpurified argon. Alloys were crushed and mixed with magnesium powder in the pro-portions corresponding to RMgT4 stochiometry. The mixtures were ball-milled un-der Ar atmosphere in sealed stainless steel vials using SPEX 8000D mill for 4–8 h.After the grinding the powder alloys were annealed under argon in tantalum con-tainer, which was placed in a sealed stainless steel autoclave, at 600–800 �C for8 h. Afterwards, the alloys were quenched to room temperature.

The hydrogenation properties were measured with a Sievert type apparatus inorder to obtain the Pressure–Composition–Temperature (PCT) diagrams as well assaturated hydride. The samples were activated by heating under vacuum.

Phase analysis of the samples was carried out by X-ray powder diffraction(XRD) (DRON-3.0 diffractometer and Brucker D8) with Cu Ka radiation for Ni/Cu-containing alloy and Fe Ka radiation for RMgCo4 alloy. Crystal structures of thecompounds were refined by the Rietveld method from the diffraction data usingFullprof software [21].

In situ Synchrotron Radiation (SR) XRD study was carried out on the BM01B lineat the European Synchrotron Radiation Facility (ESRF, Grenoble, France) on high-resolution diffractometer using monochromatic X-ray beam (k = 0.5012 Å). Thinquartz capillary (diameter 0.5 mm, wall thickness 0.01 mm) was filled by the inves-tigated alloy powder (2–5 mg) and placed in a special quartz cell mounted on agoniometer head and connected to the gas system through a flexible plastic tube.The heating and cooling of the sample was performed using a programmable cryo-fluidic system with working temperature range 77–500 K. Initially the sample washeated in a dynamic vacuum to 420 K and when one reaches this temperature thehydrogen gas (99.999% purity) was introduced in the cell. Under these conditions,‘‘activation’’ of the alloy was done and a solid solution of hydrogen in IntermetallicCompounds (IMC) was formed. Further, the sample was slowly cooled (5 K/min) toroom temperature to achieve the hydride formation.

3. Results and discussions

3.1. Synthesis of YMgT4 compounds (T = Co, Ni, Cu)

In all cases, practically single phase samples were synthesizedfor the RMgT4 compounds. XRD patterns of the YMgCo2Ni2 andYMgCu4 samples were refined and shown in Figs. 2 and 3, respec-tively. XRD patterns of the YMgCo4 and YMgNi4 samples were usedfor comparison with those of the corresponding hydrides (seeFigs. 8a and 9a). Crystallographic parameters of YMgT4 compoundsobtained from X-ray diffraction data are given in Table 1.

Lattice parameters obtained in this work for YMgNi4 compound(a = 7.013(3) Å) are in good agreement with literature data(a = 7.01 Å) [9]. New compounds based on cobalt and copper arecharacterized by slightly larger lattice parameter than the Ni-con-taining analogues, because of larger atoms (rNi = 1.246 Å,rCo = 1.252 Å, rCu = 1.278 Å). As it was shown for RMgNi4 com-pounds [14], disordering of Mg and R atoms, caused by the differ-ent conditions of synthesis, leads to the increase of unit cellparameters. Positional disordering is also observed in the case ofYMgCo4 (Table 1). In all other cases, the refinement showed com-

Table 1Crystallographic parameters of YMgT4 compounds (T = Co, Ni, Cu) with the MgCu4Snstructure type (space group F–43m).

Compounds YMgNi4 YMgCo2Ni2 YMgCo4 YMgCu4

Cell parametersa (Å) 7.0129(3) 7.0247(1) 7.0596(3) 7.2301(2)V (Å3) 344.90(3) 346.64(1) 351.84(3) 377.96(1)

Atomic positionsY in 4a (000)nY/nMg

a 1.0(-) 1.0(-) 0.87(1)/0.13(1) 1.0(–)Biso, (Å2) 0.63(5) 0.57(4) 0.24(5) 1.4(5)

Mg in 4c (1/4, 1/4, 1/4)nMg/nY

a 1.0(–) 1.0(–) 0.89(1)/0.11(1) 1.0(–)Biso, (Å2) 0.6(1) 0.5(1) 1.1(1) 1.2(1)T in 16e (x, x, x) Ni 0.5Ni + 0.5Co Co Cux 0.6243(2) 0.6249(2) 0.6249(5) 0.6241(1)Biso (Å2) 0.44(3) 0.43(4) 0.37(3) 1.59(3)

a Mixed filling of the positions of R and Mg atoms, nY + nMg = 1.

Fig. 4. First hydrogenation curves of YMgNi4, YMgCo2Ni2 and YMgCo4 compounds.

0.01

0.1

1

10

0 1 2 3 4

333 K 313 K 293 K 273 K

P H2 (M

Pa)

CH (H at./f.u.)

0.0 0.5 1.0

(a)

CH (wt.% H)

Fig. 5. PCT desorption isotherms (a) and van’t

V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13 9

pletely ordered structures with 100% occupancy of site 4c by Mgand site 4a by R atoms.

3.2. Hydrogen absorption properties of YMgT4 (T = Co, Ni, Cu) alloys

In this work we investigated the hydrogen absorption proper-ties of new YMgCo4 and YMgCu4 compounds. Hydrogen absorptionproperties of the YMgNi4 compound were described in literature[9,15]. The hydrogenation properties of Ni-, Co- and Cu-based com-pounds were studied and the structure of their corresponding hy-drides compared.

3.2.1. Hydrogen absorption properties of the YMgNi4 compoundHydrogen absorption capacity of YMgNi4 compound (MgCu4Sn-

type) at 313 K and 4 MPa hydrogen pressure was equal to1.05 wt.%. A change in the structure upon the formation of inter-metallic hydride has been reported [9], however, the structure ofthis hydride has not been determined so far. In this work thehydrogen absorption capacity of YMgNi4 compound in first hydro-genated cycle reached 3.96 at. H/f.u. (1.13 wt.%) at room tempera-ture and 2 MPa H2 pressure. This value is in good agreement withliterature data [9,15]. The curves of the first hydrogenation ofYMgCo4, YMgNi2Co2 and YMgNi4 alloys are shown in Fig. 4.

After several cycles of full absorption and desorption PCT iso-therms were measured at temperatures 273, 293, 313 and 333 Kin H2 pressure range from 0.01 to 2.5 MPa. Absorption and desorp-tion of hydrogen occurs with a single plateau, which correspondsto the a-YMgNi4H�0.5 M b-YMgNi4H�4 transition and reversiblecapacity is equal to 1 wt.% H. At room temperature the equilibriumpressure is 0.54 MPa for absorption and 0.17 MPa for desorption.Desorption isotherms for the YMgNi4–H2 system were measuredand van’t Hoff plots have been built (Fig. 5). Note that the equilib-rium pressure of hydrogen desorption obtained in our work is invery good agreement with the results of [9], see Table. 2. However,the values of heat and entropy hydride formation significantlydiffer: DH = –33.1 ± 0.7 kJ mol�1 H2 and DS = –117.6 ± 2.4 J mol�1

H2 K�1, compared with DH = �35.8 ± 0.4 kJ mol�1 H2 and

3.0 3.2 3.4 3.6

0.01

0.1

1

10

(b)

P H2 (M

Pa)

1000/T (K-1)

Hoff plot (b) for the YMgNi4–H2 system.

Table 2Comparison of desorption equilibrium pressure and thermodynamic parameters forYMgNi4–H2, YMgCo2Ni2–H2 and YMgCo4–H2 systems.

YMgNi4–H2 YMgCo2Ni2–H2 YMgCo4–H2

T (K) Pdes (MPa) Pdes (MPa) Pdes (MPa)273 0.068 – –293 0.17 0.15 0.082313 0.42 (0.37 [9]) 0.31 0.17333 0.93 (0.88 [9]) 0.61 0.33353 – 1.1 –DHdes (kJ mol–1 H2) 33.1 ± 0.7 (35.8 ± 0.4 [9]) 28.8 ± 0.2 27.9 ± 0.8DSdes J mol�1 H2 K�1 117.6 ± 2.4 (106 ± 1 [9]) 101.5 ± 0.7 93.4 ± 2.6

10 V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13

DS = �106 ± 1 J mol�1 H2 K�1 [9]. The latter correspond to the val-ues of the equilibrium pressure one order lower than actually ob-served, and possibly caused by an error in the calculations ofthermodynamic parameters in [9].

3.2.2. Hydrogen absorption properties of the YMgCo4 compoundThe investigation of hydrogen absorption properties of YMgCo4

compound was performed under the same conditions as that forYMgNi4. It is interesting to note that hydrogen absorption capacityof YMgCo4 reaches 6.8 at. H/f.u. (1.92 wt.%), which is higher by 70%than the capacity of the isostructural Ni-containing compound(Fig. 4).

Thermodynamic stability of the YMgCo4H6.8 hydride was deter-mined from the equilibrium pressure of hydrogen desorption ob-tained at 293, 313 and 333 K temperatures. Desorption isothermsand van’t Hoff plot for the YMgCo4–H2 system are shown inFig. 6. The values of heat and entropy hydride formation were cal-culated: DH = �27.9 ± 0.8 kJ mol�1 H2 and DS = �93.4 ± 2.6 J mol�1

H2 K�1 (Table 2). For calculations of the thermodynamic parame-ters we assumed that there is only one hydride phase YMgCo4H6.8

in the system and have taken the values of the equilibrium pres-sures corresponding to the middle of desorption plateau(�3 H at./f.u.). However, it is possible also that the intermediatehydride phase could be formed between solid solution of hydrogenin IMC and saturated hydride YMgCo4H6.8. A faint plateau fracturefor �5 H at./f.u. content is possible evidence of this (Fig. 6a). The

0.01

0.1

1

10

0 1 2 3 4 5 6 7

(a) 333 K 313 K 293 K

P H2 (M

Pa)

CH (H at./f.u.)

0.0 0.5 1.0 1.5

CH (wt.%)

Fig. 6. PCT desorption isotherms (a) and van’t

formation of an intermediate hydride phase was observed previ-ously for the CeMgCo4 compound. At absorption–desorption iso-therms for this compound there are two distinct plateaus, whichcorrespond to the formation of b-CeMgCo4H�4 and c-CeMgCo4H�6

hydride phases [19]. Therefore, the interaction in the YMgCo4–H2

system has been studied in more detail, in particular by in situ X-ray diffraction.

3.2.3. Hydrogen absorption properties of the YMgCo2Ni2 compoundPCT desorption isotherms at different temperatures for YMgCo2-

Ni2–H2 system are shown in Fig. 7a. The feature of this interactionis gradual slope of PH2�CH curve in the range of the existence of -YMgCo2Ni2Hx (x = 3.8� � �4.9 at room temperature). The obtainedvalues of heat and entropy hydride formation(DH = �28.8 ± 0.2 kJ mol–1 H2 and DS = �117.6 ± 2.4 J mol�1 H2 -K�1) are calculated from the corresponding van’t Hoff plot (Fig. 7b).

The absorption–desorption isotherms of hydrogen interactionwith YMgNi4, YMgCo2Ni2 and YMgCo4 at room temperature arecompared in Fig. 8. For YMgCo4 we observe inclined plateau withsignificantly lower equilibrium pressure and lower hysteresis ofhydrogen absorption–desorption curves. At room temperaturethe Pabs/Pdes hysteresis ratio in the system YMgCo4–H2 is 1.5,whereas in YMgCo2Ni2–H2 it is 1.45, and 3.2 in YMgNi4–H2. Reduc-ing the equilibrium pressure can be caused by an increase in thesize of the unit cell in the transition from Ni- to Co-containingIMC, whereas a significant increase of the hydrogen absorptioncapacity probably can be explained by electronic factors. PCT mea-surements showed complete reversibility of hydride formation inthe studied YMgT4-H2 systems. Single absorption/desorption pla-teau is observed in all three systems, which corresponds to forma-tion/decomposition of one hydride phase only.

3.2.4. Hydrogenation of the YMgCu4 compoundIt was shown that in contrast to isostructural Co- and Ni-based

compounds YMgCu4 does not interact with hydrogen. We madeseveral unsuccessful attempts of hydrogenation after activationin vacuum at temperatures of 623–693 K. The compound did notabsorb hydrogen at 2 MPa (293–623 K) and at 15 MPa H2 pressure(293–473 K). This inertness to hydrogen of Cu-based compound

3.0 3.2 3.4

0.01

0.1

1

10

(b)

P H2 (M

Pa)

1000/T (K-1)

Hoff plot (b) for the YMgCo4–H2 system.

Fig. 7. PCT desorption isotherms (a) and van’t Hoff plot (b) for the YMgCo2Ni2–H2 system.

V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13 11

probably relates to the electron configuration of atomic Cu, namelythe full settlement of outer 3d electron shell. Recent calculations ofthe electronic structure of the NdMgNi4H4 hydride showed that H-atoms form covalent bonds with 3d-orbitals of Ni-atoms [22].Influence of d-element nature on hydrogen absorption propertiesof RMgT4 compounds show up brightly in differences for Ni- andCo-based compounds. In particular, hydrogen absorption capacityof RMgCo4 under the same conditions is 70% higher than that forRMgNi4. To explain these differences the additional quantum-chemical calculations of the electronic structure and thermody-namic parameters are required.

3.3. Structural studies of saturated YMgT4Hx (T = Ni, Co) hydrides

3.3.1. Crystal structure of the YMgNi4H4 hydrideIn spite of the large number of performed researches [9,15,23–

25], the crystal structure of YMgNi4H4 hydride has not been exper-imentally investigated due to its low thermodynamic stability. Atroom temperature hydrogen desorption pressure is higher than1 atm and the hydride decomposed before the structural studieswere made.

Theoretical calculations of the hydride structure were per-formed in [26,27]. Both results of quantum-chemical calculationspredicted that for YMgNi4H4 hydride the orthorhombic structure(Pmn21) is more stable than the cubic structure of parent IMC (F-43m). The lattice parameters of the rich hydride (a = 5.0033 Å,b = 5.4296 Å, c = 7.2548 Å, V = 197.08 Å3) and the heat of its forma-tion (�37.7 kJ mol�1 H2) were mentioned in the paper [26]. Thevolume of orthorhombic unit cell (197.66 Å3) and heat of formation(�26.7 kJ mol–1 H2) were theoretically calculated in [27]. It is inter-esting to note that the heat of formation of the YMgNi4H4 hydride(�33.1 ± 0.7 kJ mol–1 H2), determined in this paper from thedesorption isotherms, are between theoretically calculated valuesin [26] and [27]. The changes of cubic into orthorhombic structurewere observed upon formation of the NdMgNi4D3.6 [16] andb-LaMgNi4D3.7 [28] hydrides, whereas the metal matrix of c-LaM-gNi4D4.85 [28] and b-CeMgCo4D4.2 [19] preserves the cubicstructure of the parent IMC.

We performed in situ XRD studies to determine the YMgNi4H4

hydride structure using synchrotron radiation. Diffraction patternsof YMgNi4 obtained by in situ SR XRD are shown in Fig. 9. Diffrac-tion profile for YMgNi4H4 hydride was indexed in orthorhombicunit cell (Pmn21) with parameters correlated with the initial cubicstructure of YMgNi4 as follows: aorth � acub/

p2; borth � acub/

p2;

corth � acub; Vorth � 1/2 Vcub. It is shown that the hydrogenation ofYMgNi4 is accompanied by the predicted orthorhombic deforma-tion of the original cubic structure with volume expansion of14.9%. Hydrogen sublattice of this hydride is probably isostructuralto NdMgNi4H4 [16], where hydrogen atoms occupy three types ofinterstices: two types of trigonal bipyramids [Nd2MgNi2] (2a and4b sites) and tetrahedra [NdNi3] (2a site).

At the first hydrogenation under 2 MPa pressure we failed toachieve complete conversion of IMC into hydride (90% yield).Therefore we increased the hydrogen pressure up to 4 MPa. andheated the sample to 423 K. We observed the desorption of hydro-gen and back conversion of orthorhombic hydride into a cubic IMC.When the sample was cooled down to room temperature we gotalmost single phase hydride due to hydrogen reabsorption. The re-sults of refinement are given in Table 3. Our studies of the YMgNi4-

H4 crystal structure confirmed theoretical predictions [26,27]. Inparticular, the experimentally observed volume of the unit cell(198.23 Å3) coincides well with the value of 197.66 Å3 obtainedby quantum-chemical calculations [27]. No intermediate hydridephase was observed in the process of formation and decompositionof the YMgNi4H4 hydride.

3.3.2. Crystal structure of the YMgCo4H6.8 hydrideThe structure of the YMgCo4H6.8 hydride was studied by in situ

SR XRD using the same method as that for the YMgNi4H4 hydride(see ‘‘Experimental part’’). Diffraction patterns of initial sample(293 K, in vacuum) and after complete hydrogenation (293 K,1 MPa H2) are shown in Fig. 10. Unlike YMgNi4, the hydrogenationof YMgCo4 compound does not lead to a distortion of the cubicstructure. It should be pointed out that upon hydrogenation wedid not observe the formation of any intermediate hydrides.According to the absorption isotherm (see Fig. 6) the hydrogencontent at 293 K and 1 MPa H2 pressure is about 6.5 at. H/f.u. The

Fig. 8. PC dependences for YMgNi4, YMgCo2Ni2 and YMgCo4–H2 systems at roomtemperature.

5 10 15 20 25 30

Yobs

Ycalc

Yobs-Ycalc

Bragg position

4 MPa H2

2θ (deg.)

2 MPa H2

Inte

nsity

(a.u

.)

YMgNi4H4YMgNi4 1%

YMgNi4H4YMgNi4 10%

YMgNi4

(a)

(b)

(c)

Fig. 9. In situ SR XRD patterns of the YMgNi4 alloy: (a) parent sample, (b) hydride at2 MPa H2 pressure and (c) hydride at 4 MPa pressure. Observed (Yobs), calculated(Ycalc) and difference (Yobs–Ycalc) diffraction profiles and Bragg’s peaks positions areshown.

Table 3Crystallographic parameters of the YMgNi4H4 hydride obtained from in situ SR XRDdata (room temperature, 4 MPa H2). Space group Pmn21; a = 5.0292(4), b = 5.3996(4),c = 7.3000(6) Å; V = 198.23(3) Å3; DV/V = 14.9%.

Atoms Site x y z Biso (Å2)

Y 2a 0 0.298(1) -0.078(21) 0.91(8)Mg 2a 0 0.827(3) 0.140(21) 0.91(8)Ni1 2a 0 0.4517(9) 0.544(20) 0.45(4)Ni2 2a 0 0.987(1) 0.528(21) 0.45(4)Ni3 4b 0.755(1) 0.226(1) 0.298(21) 0.45(4)

RBragg = 2.17%. Sample contains 1.1(1)% of the YMgNi4 parent phase (a = 7.021(1) Å).

5 10 15 20 25 30

1.0 MPa H2

2θ (deg.)

(a) YobsYcalcYobs-Ycalc Bragg position

(b)Inte

nsity

(a.u

.)

Fig. 10. In situ synchrotron X-ray diffraction patterns of the YMgCo4 alloy: (a)original sample and (b) hydride at 1 MPa H2 pressure. Observed (Yobs), calculated(Ycalc) and difference (Yobs�Ycalc) diffraction profiles and Bragg’s peaks positions areshown.

Table 4Crystallographic parameters of the YMgCo4H6.8 hydride obtained from in situ SR XRDdata (293 K, 1 MPa H2). Space group F–43m; a = 7.5884(4) Å; V = 431.81(4) Å3; DV/V = 22.7%.

Atoms Site x y z Biso (Å2)

R1 4a 0 0 0 2.1(1)R2 4c 1/4 1/4 1/4 4.9(4)Co 2a 0.6251(2) x x 1.08(5)

R1 = 0.87 Y + 0.13 Mg; R2 = 0.89 Mg + 0.11 Y; RBragg = 7.06%.

12 V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13

formation of hydride leads to a cell volume increase of 23%. Specificvolume expansion for one hydrogen atom (3.1 Å3) is close to thatfor YMgNi4H4 (3.2 Å3/at.H.). Crystallographic parameters of theYMgCo4-based hydride are presented in Table 4.

Similar to the structure of CeMgCo4D4.2, we can assume that Hatoms occupy triangular faces MgCo2 (centres of R2MgCo2 bipyra-mids), 16e site [19]. Full occupancy of this position would give H-content of 6 at./f.u. From geometric considerations, another possi-ble site for H occupation is Co4 tetrahedron, 4b site. This site is theonly position distance from which to the available H atoms exceeds

2.0 Å, a lower limit for H–H distances in metal hydrides. Completeoccupancy of 16e and 4b interstices corresponds to theoretical Hcontent of 7 at. H/f.u., which agrees well with maximum capacityobtained from volumetric measurements, 6.8 at. H/f.u. Taking intoaccount that both 16e and 4b interstices are partially occupied incubic LaMgNi4D4.85 [28], such scenario of hydrogen occupancyseems to be most plausible for YMgCo4-based hydride.

3.3.3. Crystal structure of the YMgCo2Ni2H4.9 hydrideThe XRD pattern of YMgCo2Ni2H4.9 was measured by Bruker D8

diffractometer with Cu Ka radiation. Crystallographic parametersof the YMgCo2Ni2H4.9 hydride are presented in Table 5 and the re-fined pattern in Fig. 11. The hydride has the same cubic structurethan YMgCo4H6.8 but with a smaller cell parameter and volume

Table 5Crystallographic parameters of the YMgCo2Ni2H4.9 hydride obtained at roomtemperature. Space group F–43m; a = 7.3463(4) Å; V = 396.469(32) Å3; V/V = 14.4%.

Atoms Site x y z Biso (Å2)

R1 4a 0 0 0 4.9(3)R2 4c 1/4 1/4 1/4 4.7(7)Co, Ni 2a 0.6203(2) x x 2.8(1)

R1 = 0.85 Y + 0.15 Mg; R2 = 0.92 Mg + 0.08 Y; RBragg = 11.9%.

Fig. 11. X-ray diffraction pattern of YMgCo2Ni2 hydride. Observed (Yobs), calculated(Ycalc), difference (Yobs–Ycalc) diffraction profiles and Bragg’s peaks positions for 1 –YMgCo2Ni2 hydride (98 wt.%) and 2 – Y2O3 (2 wt.%) phases are shown.

V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13 13

as expected from the smaller H content. The formation of hydrideleads to increased volume by 14.4% near that observed for YMgNi4-

H4. Specific volume expansion for one hydrogen atom (3.1 Å3) issimilar to those observed for the two other hydrides.

A small occupation exchange between Y and Mg is observed onthe 4a and 4c sites, close to that observed for YMgCo4H6.8. It is veryinteresting to observe that the Co for Ni substitution allow to avoidthe orthorhombic distortion observed for YMgNi4H4 hydride,which should results from geometric constrains created by H inser-tion. The larger Co radius, increases the size of the interstitial sites,and the H atoms have therefore enough space to be inserted whithan isotropic cell expansion.

4. Conclusions

New ternary YMgCo4 and YMgCu4 and quaternary YMgCo2Ni2

compounds have been synthesized by mechanical alloying fol-lowed by further annealing. YMgCu4 does not interact with hydro-gen under normal conditions. The hydrogenation capacity ofYMgCo4 reaches 6.8 at. H/f.u., which is substantially higher thanthat for YMgNi4 (3.7 at. H/f.u.). This observation is very similar tothat for Ce-based compounds [19]. The PCT studies of YMgCo4–H2

and YMgNi4–H2 systems revealed that introduction of magnesium,accompanied by shrinking of the unit cell, decreases the stability ofhydrides comparing to binary YCo2 and YNi2 compounds. Magne-sium causes as well slight decrease in hydrogenation capacity.The YMgCo4–H2 system is characterized by one absorption/desorp-tion plateau. The values of heat and entropy of the YMgCo4H6.8 hy-dride formation were calculated: DH = �27.9 ± 0.8 kJ mol�1 H2 andDS = �93.4 ± 2.6 J mol�1 H2 K�1. The YMgCo2Ni2–H2 systemshows intermediate thermodynamic properties compared to theternary hydrides (DH = –28.8 ± 0.2 kJ mol–1 H2 and DS = �117.6 ±2.4 J mol–1 H2 K–1). The formed YMgCo4H6.8 and YMgCo2Ni2H4.9 hy-drides keep the cubic structure of the parent compound in contrastto the hydrides of isostructural YMgNi4 (as well as LaMgNi4 andNdMgNi4 compounds), which undergo orthorhombic transforma-tion around 4 H/f.u.

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