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Acta Materialia 58 (2010) 2510–2519

LaMg11 with a giant unit cell synthesized by hydrogen metallurgy:Crystal structure and hydrogenation behavior

Roman V. Denys a,b, Andrey A. Poletaev a,c, Jan Ketil Solberg c, Boris P. Tarasov d,Volodymyr A. Yartys a,c,*

a Institute for Energy Technology, P.O. Box 40, Kjeller, NO 2027, Norwayb Physico-Mechanical Institute/National Academy of Sciences of Ukraine, 5 Naukova Street, Lviv 79601, Ukraine

c Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim, NO 7491, Norwayd IPChPh Russian Academy of Sciences, Chernogolovka 142432, Russia

Received 26 October 2009; received in revised form 13 December 2009; accepted 18 December 2009Available online 22 January 2010

Abstract

We have successfully applied a hydrogen metallurgy synthesis to yield LaMg12�x intermetallic at 500 �C. During a vacuum thermaldesorption from the LaH3–MgH2 nanocomposite prepared by reactive ball milling in H2, hydrogen desorption from the La dihydrideoccurred via a mechanism of cooperative phase transformation at temperatures being 400� lower than the desorption from pureLaH2. The crystal structure of the intermetallic was determined by synchrotron X-ray diffraction (SR XRD). The orthorhombicLaMg12�x has a giant unit cell, the volume of which exceeds 8000 A3. In situ SR XRD studies showed the fine details of the hydrogendesorption process with several parallel and consecutive transformation steps.� 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Powder processing; Synchrotron X-ray diffraction; Hydrides; Magnesium alloys; Hydrogen storage

1. Introduction

Applications of magnesium hydride as hydrogen storagematerial suffer from the poor hydrogenation kinetics ofmagnesium, caused by the high activation energy barrierfor the surface dissociation of hydrogen molecules on Mg(�50 kJ mol�1 [1]), and the low diffusion rate of hydrogenin the formed MgH2. A significant improvement in therates of hydrogen exchange can be attained by the nano-structuring of the Mg-based materials [2–7]. Such alloysreadily absorb and desorb �5 wt.% hydrogen during low-temperature cycling around 200 �C. The H storage behav-ior of Mg-based alloys can, furthermore, be improved byforming binary alloys of Mg. The hydrogenated Mg-based

1359-6454/$36.00 � 2009 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2009.12.037

* Corresponding author. Address: Institute for Energy Technology, P.O.Box 40, Kjeller, NO-2027, Norway. Tel.: +47 63806453; fax: +4763812905.

E-mail address: volodymyr.yartys@ife.no (V.A. Yartys).

intermetallics studied so far include compounds with Ni[8,9], Cu [10], Al [11–16] and rare earth metals [17]. Alloy-ing with Al, Cu, Ni and Ca [15] causes: (i) destabilization ofthe formed hydrides as compared to MgH2 (intermetallicsof Al, Cu or Ni); (ii) catalytic facilitation of the reactionMg + H2 () MgH2 by the secondary intermetallicsMg2Ni and Mg2Cu; (iii) a reversible (Mg2Cu, Mg2Al3 orMg17Al12) or irreversible (Mg2Ca) disproportionation ofthe intermetallic alloy proceeding during hydrogen chargeand discharge.

For the compounds of Mg with rare earth metals,hydrogen absorption and desorption properties have beenstudied for the Mg-rich LaMg12 [18–20] and La2Mg17

[19]. The storage capacity of LaMg12 after the first chargingat 400 �C was rather low, at just 1.8 wt.% H, and thedesorption rate was very slow. However, after severalhydrogen absorption/desorption cycles the hydrogen con-tent increased to 3.7 wt.% H. A detailed in situ XRD study[19] showed no formation of the interstitial type LaMg12Hx

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R.V. Denys et al. / Acta Materialia 58 (2010) 2510–2519 2511

and La2Mg17Hx hydrides; instead, LaMg12 and La2Mg17

decompose in hydrogen gas at, respectively, 290 and270 �C to form LaH3 and MgH2. Ball milling in argon ofLaMg12 and Ni powder leads to the amorphization of theintermetallic alloy [20]. The milled material has a dischargecapacity of 3.77 wt.% H.

The LaMg12±x intermetallic has a broad homogeneityrange, from x = �1.2 to x = 1.0 at 600 �C [21]. Its crystalstructure has not yet been determined. This structure, how-ever, seems to be related to the structures of the phases formedin the binary system Ce–Mg. Two types of crystallographicstructures have been reported for CeMg12 [22,23], namely, atetragonal ThMn12 type structure crystallizing with spacegroup I4/mmm and a body-centred orthorhombic structurewith space group Immm. A single crystal X-ray diffractionstudy of the Ce-containing alloy of composition CeMg12.59–

12.71 [22] concluded that it has an orthorhombic structure witha = 10.33; b = 10.33 and c = 77.5 A (group of symmetry wasnot identified). The stoichiometry of the compound is shiftedtowards a higher Mg content compared to the ideal CeMg12

compound having a tetragonal ThMn12 type of structure.The Ce sublattice of the structure of CeMg�12.6 can be builtas a stacking of 13 CeMg12 cells with a slip of the cells by y/2 occurring at z = 4/26; 10/26; 17/26 and 23/26. Mg sublatticewas not identified and no suggestions were made concerningpossible reasons for the nonstoichiometric CeMg12+x compo-sition. In Ref. [19] the crystallographic unit cell of LaMg�12

was described as being orthorhombic with parametersa = 10.344(3), b = 10.365(15) and c = 77.586(14) A, similarto the data for CeMg�12.6 [22]. However, no atomic crystalstructure data was revealed.

The present work focused on studying the synthesis andhydrogen absorption–desorption properties of nanocom-posites of La and Mg hydrides prepared from a stoichiome-tric 1:12 mixture of La and Mg by application of reactive ballmilling (RBM) in hydrogen gas. Time-resolved synchrotronX-ray diffraction (SR XRD), offering the benefits of signifi-cantly high time resolution of the processes, sensitivity indetermining the formation of the phase constituents andexcellent accuracy in yielding the crystallographic data,was applied to monitor the mechanism and kinetics of thephase-structural transformations and microstructural evo-lution of the samples during hydrogen uptake and release.Finally, formation of LaMg12 during thermal decomposi-tion of the alloy hydrides was observed, and its crystal struc-ture was determined from an analysis of the SR XRDpowder pattern.

2. Experimental

2.1. RBM in hydrogen

The hydrogenated LaMg12 was prepared from a stoichi-ometric mixture of the high-purity metals, Mg grit 50–100mesh (99.8%) and La (99.98%) by RBM in hydrogen gas ina Fritsch Pulverisette P6 planetary mill. The metals were

placed into a custom-built Duplex SS 2377 (austenitic–fer-ritic steel) vial. The sample-to-balls weight ratio was �1:80.The vial was initially evacuated and filled with 30 bar H2.The milling was performed at 500 rpm. The hydrogenationwas accomplished during 2 h of continuous milling. A pureMg hydride sample was prepared as a reference material,and for this sample the hydrogenation process of Mg metallasted for 6 h under the same conditions of RBM.

After the milling, the synthesized hydrides were presentas fine powders having a yellow–brown (La + 12Mg) orbrown (Mg) colour.

2.2. Hydrogen absorption/desorption and temperature

desorption spectroscopy (TDS)

Hydrogen desorption–absorption experiments were per-formed in a conventional Siverts-type apparatus. Determi-nations of the reversible hydrogen storage capacity and thereaction kinetics were done by measuring the pressurechange in a closed system with a constant volume. TDSwas applied to determine the onset temperature for hydro-gen release during dehydrogenation in dynamic vacuum.During TDS, the sample was heated at a constant rate.

2.3. In situ SR XRD experiments

In situ SR XRD studies of the samples from theLa + 12Mg–H2 system were performed at the Swiss–Nor-wegian Beam Lines at station BM01A, European Synchro-tron Radiation Facility (ESRF), Grenoble, France.

The measurements were performed using a set-updesigned for in situ studies of the chemical processes occur-ring in hydrogen gas or in vacuum (see Ref. [24] and refer-ences therein). A small amount of the sample was put into a0.5 mm quartz glass capillary (0.01 mm wall thickness), fill-ing approximately 3–5 mm of the capillary. The capillarywas connected to the gaseous system and sealed using acarbon ferrule and a T-piece; the latter was attached tothe goniometer head. Vacuum was created using a turbo-molecular vacuum pump.

Experimental data at BM01A were collected using aMAR2300 image plate detector. The wavelength (k =0.7350(1) A) and sample-to-detector distance (D =200 mm, giving a usable range of d-spacing of 1.07–21 A(2h = 2–40�)) were calibrated using LaB6 as a referencematerial. Each diffraction data set was collected in 10 s; thetime of the processing of the image-plate data was 53 s, giv-ing in total a time of 63 s for a single measured data set.

Powder diffraction data were analysed by the Rietveldwhole-profile refinement method using the General Struc-ture Analysis System (GSAS) software [25]. The peakshapes were described using the Thompson–Cox–Hastingspseudo-Voigt type function [26]. The plots of the SR XRDdiffraction pattern were prepared with the Powder3D pro-gram [27]. The background was refined using the shiftedChebyshev function (#1 in GSAS).

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3. Results

3.1. Reactive ball milling of La + 12Mg in hydrogen

(Fig. S1 Supplementary material) presents the increasein hydrogen content in the material during the RBM. Threeconsecutive steps are evident:

(1) an immediate hydrogenation of La metal to formLaH3 proceeding after admission of hydrogen gasinto the vial with the initial mixture of metals;

(2) fast hydrogenation of Mg metal, lasting for 60 min,during which 95% of the total storage capacity isreached; and

(3) a slow increase in the storage capacity to reach thefinal saturation hydrogen concentration (60–120 minof the process).

From volumetric measurements, the quantity of hydro-gen absorbed during the RBM is 2.04(3)H/M. This value isslightly lower than the theoretical limit of 2.077H/M corre-sponding to the complete hydrogenation of both metals toLaH3 and MgH2. The kinetics of the hydrogenation duringthe RBM was successfully fitted by using the Avrami–Ero-feev equation:

f ¼ 1� exp�ðktÞn ð1Þwhere f is the reacted fraction, k is the rate constant, t istime and n is the factor defining the rate-limiting process(Fig. 1).

From Fig. 1 it is evident that, under the same milling con-ditions, hydrogenation of Mg proceeds much faster in thepresence of La. Indeed, k(Mg + LaH3)/k(Mg) = 4.7. Thisindicates that La hydride has a catalytic influence on thehydrogen uptake by Mg.

Fig. 1. Hydrogenation kinetics for pure Mg and Mg–La composite duringthe RBM at 30 bar H2. Experimental data are shown by points; Avrami–Erofeev fits of the data are presented as lines. The hydride fraction isMgH2/Mg as La trihydride is available from the beginning of the process.R2 = 0.9967 for Mg and R2 = 0.9987 for Mg + LaH3.

3.2. Hydrogen vacuum thermal desorption from the RBM

materials

Thermal desorption spectra of the Mg, La andLa + 12Mg samples ball milled in hydrogen are pre-sented in Fig. 2a–c and show that hydrogen desorptionspans a broad range of temperatures, from 200 to900 �C. For the magnesium dihydride that was synthe-sized in such a way, a single desorption step wasobserved with a peak at 304 �C. X-ray diffraction analy-sis showed a complete transformation of MgH2 into theMg metal after the TDS experiment.

The highest thermal stability with two events ofhydrogen evolution was observed for La trihydride.These events are associated with the followingtransformations:

(1) LaH3 ? LaH2 + ½H2 occurs from room tempera-ture to 550 �C; a peak is observed at 215 �C; and

(2) the decomposition LaH2 ? La + H2 starts at 600 �Cand finishes at �880 �C; a corresponding peak isobserved at 800 �C.

Fig. 2. Hydrogen thermal desorption traces (heating rate 2 �C min�1,dynamic vacuum) from the hydrides synthesized by RBM in hydrogen gas:(a) MgH2 (RBM for 6 h); (b) LaH3 (RBM for 2 h); and (c) LaH3 + 12MgH2

hydride composite (RBM for 2 h).

Fig. 3. Dehydrogenation (a) and rehydrogenation (b) of the hydridecomposite at 350 �C. Des1, desorption from the ball milled sample; des2and des3, desorption from the rehydrogenated sample.

R.V. Denys et al. / Acta Materialia 58 (2010) 2510–2519 2513

Such a behavior resembles the thermal stability propertiesof the trihydrides of other rare earth elements [28]. Indeed,the published data on R–H2 (R = Y, Ce, Pr, Nd, Sm, Gd,Tb) systems report two-step decomposition processes, withthe transformation RH3 ? RH2 occurring in the tempera-ture range 200–350 �C and the RH2 ? R decompositionoccurring at temperatures well above 600 �C.

It was interesting to compare the reference data availablefor MgH2 and LaH3 with the hydrogen desorption tracesfrom LaH3 + 12MgH2. These traces appeared to be ratherunusual. Two distinct peaks of hydrogen desorption arepresent, a strong peak at 283 �C and a second, much weaker,peak at 390 �C. The amount of hydrogen released during theTDS experiment corresponds to�2.065 H/M, which is closeto both the experimental observations (2.04(3); see the RBMexperiment) and the theoretical limiting hydrogen storagecapacity of the composite. A comparison of the relativequantities of hydrogen released during the first and the sec-ond events (�92% and �8%, respectively) is consistent withthe hypothesis that the first peak is mainly associated withdecomposition of nanocrystalline MgH2 hydride, whereasthe second peak can be attributed to decomposition ofLaH3�x hydride. Thus, a complete dehydrogenation of thesample occurring below 450 �C may be suggested.

Indeed, desorption was already completed at 410 �C,nearly 400� lower than the higher desorption peak ofLaH3. The nature of the hydrogen desorption events fromthe mixture LaH3 + 12MgH2 and the mechanism of thetransformations occurring during the hydrogen desorptionwere revealed by application of SR XRD.

We note in addition that the positions of the desorptionpeaks clearly depend upon the crystallite size in the hydrides.For the polycrystalline hydrides (our data for which are notpresented in this paper), the peaks are shifted towards thehigher temperatures while for the nanocrystallite hydrideshydrogen desorption takes place at lower temperatures.

3.3. Hydrogen desorption and rehydrogenation properties of

the LaH3–MgH2 composite

Nanostructuring significantly improves the hydrogenabsorption–desorption performance of the composites.The desorption proceeding at 350 �C is very fast andlasts just a few minutes (see Fig. 3). In a closed volumesystem, it started at 0.2 bar and finished at 1.9 bar,releasing 5.4 wt.% H. Rehydrogenation was studied at20 bar H2 and 350 �C. Only 1 min was sufficient to satu-rate the sample with hydrogen gas. Around 5 wt.% ofhydrogen was absorbed; however, some decrease in thestorage capacity was observed during the cycling, possi-bly because of the progressed sintering of the sample.

3.4. Ex situ synchrotron X-ray diffraction studies

A SR XRD study of the Mg and La + 12Mg samplesthat were ball milled in hydrogen gas indicated the com-

pleteness of the hydrogenation of the initial metals andshowed the formation of nanocrystalline elementalhydrides (see Fig. 4).

For pure Mg, SR XRD showed the formation of a rutiletype a-MgH2 (�70%) and a high-pressure c-MgH2 (�30%)modifications, both with a crystallite size of �7 nm. Thisobservation agrees well with our earlier data [29], whereRMB of Mg under slightly different milling conditionsyielded a 70/30 mixture of a-MgH2/c-MgH2 having crys-tallite sizes of 6.5 and 7.5 nm, respectively. A small amountof Fe impurities (�1 wt.%) is observed and is caused bywear of the milling tools.

In the case of RBM La + 12Mg, LaH3 (BiF3-type)hydride was identified in addition to two MgH2 allotropes.The possible formation of the binary La–Mg or ternaryLa–Mg–H compounds under the applied conditions ofthe RBM in hydrogen gas was ruled out from the analysisof the diffraction pattern.

An important feature of the X-ray powder diffractiondata of the RBM La + 12Mg material is the presence ofthe profound diffuse halos in the regions of the strongestpeaks from the LaH3 hydride (see Fig. 4b), indicating sub-stantial amorphization of LaH3 during the ball milling.Quantification of the crystalline products gives the follow-ing data: �90 wt.% of MgH2 (with a a- to c-ratio of�85:15) and �10 wt.% of LaH3. The amount of amor-phous phase can be estimated from a comparison of the

Fig. 4. Experimental (crosses), calculated (upper line) and difference(lower line) SR XRD patterns of Mg (a) and La + 12Mg mixture (b)hydrogenated during RBM in hydrogen. Positions of the Bragg peaks ofconstituent phases are shown by bars.

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fractions of crystalline hydrides obtained from the Rietveldrefinements, with the hydride fractions corresponding tothe complete hydrogenation of the initial metal mixture(69 wt.% of MgH2 and 31 wt.% of LaH3). Such an estima-tion shows that the main (�3=4) part of LaH3 is amorphous;the remaining 1=4 part is nanocrystalline, with an averagecrystallite size of �9 nm.

Formation of amorphous hydride of rare earth metals(RE) has also been observed during RBM of the Mg–Mm–Ni alloy [29].

We note that, because of a strong peak overlap and thepresence of the diffuse peaks from the glass capillary (withthe strongest peak at 2h � 10�), it was not possible to quan-tify the diffraction contribution from the amorphous phaseand to refine its fraction directly from the Rietveld refine-ment. The contribution from an amorphous phase wasincluded into the background subtraction (see Fig. 4b).The results of the Rietveld refinement of the SR XRD dataare summarized in Table 1.

3.5. In situ SR XRD study of hydrogen thermal desorption

from the MgH2–LaH3 nanocomposite

An in situ SR XRD study of the mechanism of hydrogenvacuum thermal desorption from the MgH2–LaH3 (12:1)nanocomposite was performed at beam station BM1A of

the Swiss–Norwegian Beam Lines, ESRF, Grenoble,France. The sample was heated under dynamic vacuumconditions in the temperature range 40–500 �C with a heat-ing rate of 2 �C min�1.

A plot of the in situ SR XRD data is presented inFig. 5. As can be seen from Fig. 5, the initial materialsynthesized by the hydrogen RMB contains three crystal-line phases: two allotropic modifications of MgH2, andLaH3. An additional diffuse peak at 2h � 14� indicatesthe presence of an amorphous phase. The intensity ofthe diffuse scattering decreases with temperature, com-pletely vanishing above 300 �C. Simultaneously, Rietveldrefinements show a gradual increase in the weight fractionof crystalline LaH3 in the material following the tempera-ture increase. These observations support the conclusionthat the diffuse scattering contribution in the diffractionprofile from the original as-synthesized sample originatesfrom amorphous LaH3.

At �300 �C, a complete decomposition of MgH2 is evi-dent from the diffraction pattern. In the range 350–450 �C,a two-phase region Mg + LaH3�x is seen. At �450 �C, thepattern completely changes; instead of Mg- and La-basedhydrides, new peaks emerge, indicating the formation ofthe LaMg�11 intermetallic compound.

From Rietveld refinements of the in situ diffraction data,the following detailed mechanism of the phase-structuralchanges takes place:

(1) Transformation a-MgH2 ? c-MgH2. It starts at�120 �C and proceeds without changes in hydrogencontent in the material. This observation agrees withour previous study of the RBM of MgH2 [29].

(2) Recrystallization of amorphous LaH3 starts at�150 �C and is completed at �315 �C.

(3) Decomposition of magnesium hydride proceeds from200 �C up to 340 �C, with two parallel processes takingplace: a-MgH2 ? Mg + H2 and c-MgH2? Mg + H2.The peak of decomposition is observed at 285 �C (seeFig. 6).

(4) Continuous transformation LaH3 ? LaH2 + ½ H2

proceeding in the temperature range 280–450 �C.This can be seen from the changes in unit cellparameter for the lanthanum hydride (see Fig. S2in Supplementary material). It should be noted thatin a nanocomposite containing MgH2 such a trans-formation starts at a higher temperature than inthe pure nanocrystalline LaH3 (see TDS for LaH3

in Fig. 2b).(5) LaH2 + �11 Mg ? LaMg�11 + H2 starts at 430 �C.

We note that the decomposition of LaH2 (and forma-tion of LaMg�11) during the thermal desorption spec-troscopy experiment in our laboratory set-up tookplace in the temperature range 360–410 �C, whereasduring the in situ SR XRD experiment this processwas shifted upwards by �70 �C. The maximum trans-formation rate was observed at 455 �C. The apparent

Table 1Crystallographic data for the RBM materials obtained from the Rietveld refinements of the SR XRD data (20 �C, Ar atmosphere).

Material Phase Spacegroup

Unit cell parameters (A) Phasecontent(wt.%)

Crystallite size (DV);microstrain (e)

Mg RBM (milled in 30 bar H2 for 6 h) Rwp = 4.4%;Rp = 3.2%; v2 = 3.2

a-MgH2 P42/mnm

a = 4.5212(3)c = 3.0183(4)

67.6(1) DV = 7.1(1) nme = 0.44(6)%

c-MgH2 Pbcn a = 4.537(1)b = 5.424(1) c = 4.949(2)

31.2(2) DV = 7.1(2) nme = 0.1(1)%

a-Fe Im �3 m a = 2.8858(8) 1.2(1)

La + 12Mg RBM composite (milled in 30 bar H2 for 2 h)Rwp = 2.7%; Rp = 2.0%; v2 = 2.7

a-MgH2 P42/mnm

a = 4.5200(4)c = 3.0243(4)

75.90(6){58.6}b

DV = 7.3(1) nme = 0.6(1)%

c-MgH2a Pbcn a = 4.537(–)

b = 5.424(–) c = 4.949(–)12.9(2){9.9}b

DV = 7(–) nm

LaH3

crystalline

Fm �3 m 5.6066(4) 10.36(4){8.0}b

DV = 8.5(2) nm

LaH3

amorphous

{22.8}b

a-Fe Im �3 m 2.883(1) 0.89(4){0.7}b

La + 12Mg RBM composite after TDS Rwp = 6.5%;Rp = 5.4%; v2 = 2.4

LaMg12�x Immm a = 10.3391(5)b = 10.3554(5)c = 77.484(4)

85.18(5) Polycrystalline

La2Mg17 P63/mmc

a = 10.359(1)c = 10.251(2)

14.8(2) Polycrystalline

a The lattice parameters and profile coefficients (size and microstrain) of c-MgH2 phase were fixed to the values obtained for this phase in the RBM Mg.b The fraction of amorphous LaH3 phase is estimated from the theoretical content of LaH3 (31 wt.%) and MgH2 (69 wt.%) hydrides corresponding to

the complete hydrogenation of the La + 12Mg mixture. Phase abundances are corrected by accounting for the fraction of amorphous LaH3, which is givenin braces.

R.V. Denys et al. / Acta Materialia 58 (2010) 2510–2519 2515

reason for this temperature shift is the lower effi-ciency of the evacuation in the thin capillary usedduring the SR XRD measurements.

Obviously, some of the processes overlap. These includedecomposition of MgH2 and recrystallization of amorphousLaH3 occurring in the same temperature range. The finalsteps of both processes overlap with the onset of hydrogendesorption from LaH3. We note that LaH3�x remains nano-crystalline (crystallite size below 30 nm) before transforma-tion to LaMg11, whereas Mg has much larger crystallitesizes, well in excess of 100 nm.

For the LaMg�11 intermetallic, all diffraction peaks canbe indexed according to the ThMn12-type tetragonal crys-tal structure (a = 10.44 A; c = 6.038 A) at the beginningof the formation process, i.e. at the lowest transition tem-perature of 430 �C. Above 470 �C, the superstructure peaksappeared in the pattern, associated with the formation ofthe orthorhombically deformed supercell which is closelyrelated to the parent ThMn12-type structure. Later in thispaper we will present detailed crystal structure data forthe orthorhombic LaMg�11 having a “giant” unit cell.

During the refinements it was concluded that the recom-bined sample contained both orthorhombic and tetragonalmodifications of LaMg�11. Formation of LaMg�11 occursin the following sequence: LaH2 + �11 Mg ? tetragonalLaMg�11 ? orthorhombic LaMg�11. The temperature var-iation of the ratio between different two modifications ofLaMg�11 is given in Fig. 7.

The data from the refinements of the SR XRD data canbe transformed into the amount of hydrogen evolutionfrom the sample, as shown in Fig. 8. This spectrum con-tains the following steps:

(1) MgH2(a + c) ? Mg + H2 at T = 200–350 �C(2) LaH3 ? LaH2 + ½H2 at T = 280–450 �C(3) LaH2 + �11 Mg ? LaMg�11 + H2 at T = 430–

500 �C.

A shoulder at �330 �C results from an overlap betweenprocesses (1) and (2).

3.6. The crystal structure of orthorhombic LaMg�11

A Le Bail profile fitting of the SR XRD pattern showedan excellent fit (see Fig. S3 in Supplementary material),suggesting the presence of a mixture of two constituents,LaMg12�x with an orthorhombic unit cell (space groupImmm; a = 10.3376(2); b = 10.3597(2); c = 77.523(1) A)and La2Mg17 (space group P63/mmc; a = 10.3657(2);c = 10.2631(5) A).

The dimensions of the orthorhombic LaMg12�x unit cellpoint towards a possible relation to the ThMn12 tetragonalstructure type; aorth � atetr; borth � atetr; corth � 13 � ctetr.

Interestingly, different synthesis conditions, i.e.

(a) thermodesorption up to 500 �C at a heating rate of5 �C/min in a laboratory Sieverts set-up; and

Fig. 5. Evolution of the in situ SR XRD pattern of the RBMMgH2 + LaH3 (12:1) during hydrogen thermal desorption into dynamicvacuum. The asterisks show superstructure peaks of orthorhombicLaMg�11. (Other peaks from LaMg�11 can be indexed according to atetragonal ThMn12-type structure.) The diffuse peak at �10� originatesfrom the quartz capillary.

Fig. 6. Evolution of the phase composition during the hydrogenthermal desorption in vacuum. Weight fractions of the constituentphases were obtained from the Rietveld refinements of the in situ SRXRD data. The weight fraction of the amorphous LaH3 was foundfrom the following equation: fc(amorphous) = {f(a-MgH2) + f(c-MgH2) + f(Mg)} � 0.3313/0.6687 � f(LaH3). Here, f(a-MgH2), f(c-MgH2), f(Mg) and f(LaH3) are weight fractions of correspondingcrystalline phases obtained from the Rietveld refinements, and 0.3313/0.6687 is the average ratio between crystalline LaH3�x and Mg phasesin the temperature region 350–430 �C. Note that small amounts ofuntransformed LaH2 and Mg are present in the sample at the end ofthe experiment at 500 �C. The weight content of LaMg�11 at 500 �Cwas 82.3%. Complete hydrogen desorption was, however, reachedduring the TDS experiment (heating in vacuum up to 500 �C at a rateof 5 �C min�1), where a two-phase sample, LaMg12�x 85.18(5) wt.% andLa2Mg17 14.8(1) wt.%, was formed.

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(b) hydrogen release into vacuum in the set-up for thein situ SR XRD studies at temperatures up to500 �C at a heating rate of 2 �C min�1.

gave different results. A formation of an orthorhombicphase took place for experiment (a) while a mixture of bothtetragonal and orthorhombic structures was formed in case(b).

Crystal structure data for the tetragonal LaMg12 wereobtained from the refinements of the SR XRD datadescribed in the previous section. The LaMg12 crystalstructure can be presented as an altered stacking along[0 0 1] of two different plain nets, one with compositionLaMg8 and one pure Mg 44 net (see Fig. 9). The latternet is composed of Mg rectangles ½a � ½b. Thus, slideshifts within the plane Mg nets, ½a, ½b or their combina-tion, will transform the 44 nets into themselves, conse-quently allowing building the structure of the initialtetragonal unit cell blocks without affecting the overall stoi-chiometry of the system. One such transformation is shownin Fig. 9. In the upper of the two unit cells shown, a trans-lation by 1/2a of the top LaMg8 (LaMg24Mg34) and Mg44

nets of the original ThMn12-type structure takes place, cre-ating a derivative structure from the original tetragonalone, without changes in stoichiometry.

A single crystal X-ray diffraction study of CeMg12 (onlythe Ce sublattice was described) [22] concluded that for thestacking of 13 tetragonal ThMn12 type cells, a translationby ½a occurs at z = 4/26, 10/26, 17/26 and 23/26. Assum-ing that the RE sublattice of the La compound has similarcrystallographic features as that of CeMg12, the structuremodel for LaMg12 shown in Fig. 10 can be tentativelyadvanced. The superstructure cell contains 42 atomic sites:7 for RE (26 atoms per unit cell) and 35 for Mg (312 atomsper unit cell). This structure model was used in the Rietveldfitting of the experimental SR XRD data. A reasonably

Fig. 7. Formation of tetragonal and orthorhombic modifications ofLaMg�11 during the in situ SR XRD experiment. Phase fractions wereobtained from the Rietveld refinements. Refined at 500 �C, the unit cellparameters are: a = 10.4531(3); c = 6.0440(5) A (tetragonal modifica-tion) and a = 0.442(3); b = 10.446(2) c = 78.37(2) A (orthorhombicmodification).

Fig. 8. Thermal desorption spectrum derived from the results of theRietveld refinements of the SR XRD data.

Fig. 9. Tetragonal LaMg12 structure of the ThMn12 type (space group I4/mmm; a�10.34, c�5.96 A; La in 2a: 0, 0, 0; Mg1 in 8f: 1=4, 1=4, 1=4; Mg2 in 8i:0.361, 0, 0; Mg3 in 8j: 0.277, ½, 0). Two unit cells are shown. La�, Mg2�

and Mg3� are atoms in the translated LaMg24Mg34 net.

Fig. 10. La sublattice in the crystal structure of LaMg12�x. For simplicity,only Mg atoms belonging to the connecting different layers Mg4 plain netsare shown.

R.V. Denys et al. / Acta Materialia 58 (2010) 2510–2519 2517

good fit to the experimental SR XRD data (Rwp = 11.9;Rp = 9.6; v2 = 6.2) was obtained for this model.

However, Rietveld refinements of the atomic structure,in combination with difference Fourier analysis, revealedthat two Mg2 dumbbell sites are partially (�30%) substi-tuted by La atoms. Thus, the final structure model wasextended to 44 atomic sites: 9 for La (7 initial sites and 2sites obtained by a partial substitution Mg2 ? La) and 35for Mg. Refinements of all atomic positions and of theMg/La occupancies of the mixed dumbbell sites yielded asignificant improvement in the fit (Fig. 11). The finalrefined crystallographic data for the LaMg�11 structureare listed in Table S1 Supplementary material. TheMg2 ? La substitutions result in a substantial deviationfrom the ideal LaMg12 stoichiometry to the Mg-deficientLaMg10.85(7).

The atoms in the LaMg�11 structure preserve the typesof coordination that are typical for the parent ThMn12

phase: 20-vertex polyhedron [Mg20] for La (ordered La

sites, La1–La7); icosahedron [RE2Mg10] for Mg1 andMg3; and 14-vertex polyhedron [REMg13] for Mg2. Thecoordination of the La atoms in the partially occupied sitesLa8 and La9 are 18-vertex equatorially capped hexagonalprisms. It is interesting that both types of the coordinationpolyhedra of La (20- and 18-vertex) are present in therelated Th2Ni17 type compound [30].

Coordination characteristics and interatomic metal–metal distances are listed in Table S2 Supplementary mate-rial. The La–Mg and Mg–Mg interatomic distances arewithin the ranges typical for Mg-rich RE–Mg compounds.The shortest Mg–Mg separation, 2.84(6) A, is close to thevalues of 2.86 A observed in the structure of La2Mg17

[30] and 2.83 A observed in CeMg10.3 [31]. The minimumLa–Mg distance (La1–Mg2e = 3.38(7) A) is close to theshortest La–Mg distances in La2Mg17 [30] and La5Mg41

[32], 3.45 and 3.42 A, respectively.

3.7. Discussion of the crystal chemistry aspects

Substitution of a larger A atom by a B2 dumbbell in theAB5 compound of the hexagonal CaCu5-type produces alarge number of the related intermetallics, including AB12

Fig. 11. Experimental (crosses), calculated (upper line) and difference(lower line) SR XRD pattern of the orthorhombic LaMg10.85 (space groupImmm; a = 10.3376(2); b = 10.3597(2); c = 77.523(1) A). Positions of theBragg peaks from the two phase constituents, LaMg10.85 and La2Mg17, areshown by bars. The inset shows the most intensive resolved “superstruc-ture” peaks, which do not overlap with the peaks of the tetragonal subcell.In addition, individual, non-overlapping peaks from the hexagonalLa2Mg17 (Th2Ni17 type) phase are shown.

2518 R.V. Denys et al. / Acta Materialia 58 (2010) 2510–2519

and A2B17, crystallizing with ordered ThMn12, Th2Zn17

and Th2Ni17 types or AB11 and AB7 compounds with dis-ordered SmZn11 and TbCu7 types. For example, a disor-dered substitution RE M Mg2 dumbbell has beenobserved in the structure of CeMg10.3 (disordered Th2Ni17

type) [31]. Other examples of the defect structures derivedfrom the CaCu5-type are reviewed in [33].

The parent tetragonal ThMn12 structure is obtained fromfour hexagonal AB5/CaCu5-type subcells (atetr = 2chex, ctetr =ahex; ctetr = atetr/

p3) by ordered replacement of half of the

larger A atoms with pairs of smaller B atoms, which are alignedalong the hexagonal axis (see Fig. 7 in Ref. [34]).

The partial substitution of Mg2 dumbbells by Laobserved in the present work for LaMg10.85 takes placeon two different types of the Mg2 dumbbells sites. How-ever, taking into account the complexity of the structurederived from the powder diffraction experiment, we cannotexclude that some degree of a substitution La M Mg2

dumbbell may occur for the other Mg sites as well. Fromour refinements, however, we conclude that such a substi-tution should be very rare, if it ever happens.

The observed partial La M Mg2 replacement makes itstructurally possible to explain the significant variation ofMg content in LaMg12±x within the homogeneity rangespanning from LaMg10.8 to LaMg13 observed in Refs.[21,35].

A similar variation in the Mg/Ce ratio has also beenreported for the related compound CeMg12±x. A recentstudy of the Mg-rich side of the Ce–Mg system showed thatthe composition of CeMg12±x is CeMg�11 rather thanCeMg12. The composition range CeMg10.75–11.31 at 530 �Cwas determined by electron probe microanalysis [36]. Theauthors postulated that this phase is a defect structure withvacancies on the Mg sites. However, assuming structuralsimilarity between the LaMg�11 and CeMg�11 compounds,partial substitution of Mg dumbbells with Ce atoms

(x = �1) appears to be a more plausible explanation forthe substantial deviation from the ideal CeMg12 stochiom-etry to form CeMg11.

4. Summary and conclusions

In summary, the present work has focused on the syn-thesis and properties of nanocomposites of LaH3 andMgH2 hydrides, which were prepared from elemental met-als by applying RBM in hydrogen gas. Complete hydroge-nation was observed (30 bar H2; milling time 2 h), yieldingthe formation of crystallites of binary hydrides with sizesbelow 10 nm. The nanostructuring resulted in the facilita-tion of the hydrogen desorption and absorption processes.

A complete hydrogen vacuum thermal desorption fromthe nanocomposite LaH3 + 12MgH2 was observed atrather low temperatures (below 500 �C), resulting in theformation of the binary intermetallic compound LaMg

12�x.The detailed mechanism of the transformation of the

LaH3–MgH2 hydride composite into intermetallic com-pounds during the hydrogen desorption process wasrevealed from in situ SR XRD studies and includes severalparallel and/or consecutive transformation steps, whichwere identified as:

(a) c-MgH2 ? a-MgH2;(b) c-MgH2 ? Mg;(c) a-MgH2 ? Mg;(d) LaH3 ? LaH2;(e) LaH2 + Mg ? LaMg10.85.

Interestingly and unexpectedly, the observed low-tem-perature decomposition of LaH2 proceeds at temperaturesthat are about 400� lower than the temperature of hydro-gen release from the individual LaH2 hydride (approxi-mately 800 �C) under the same experimental vacuumconditions. This cooperative transformation phenomenon,where a lowering of the thermal stability of LaH2 is triggedand facilitated by the formation of the thermodynamicallyfavourable intermetallic LaMg12�x, is caused by increaseddiffusivity of the metal atoms in the presence of hydrogen.

The crystal structure of the LaMg10.85 intermetallic com-pound synthesized by the hydrogen metallurgy route wasdetermined from an SR XRD study. Its orthorhombic struc-ture is characterized by a giant unit cell with a volumeexceeding 8000 A3 (space group Immm; a = 10.3391(5) A,b = 10.3554(5) A, c = 77.484(4) A). It is built from thealtered layers of the ThMn12-type structure, where 13 subcelllayers are stacked along the [0 0 1] direction. These layers areperiodically slipped by ½a (at z = 0.1345; ½ � z; ½ + z and1 � z) along the common Mg planar nets. A substantial devi-ation from the ideal LaMg12 stoichiometry towards a lowerMg content, down to Mg/La = 10.85(7), takes place and isassociated with the partial substitution of Mg pairs by Laatoms at two different crystallographic positions. Such par-

R.V. Denys et al. / Acta Materialia 58 (2010) 2510–2519 2519

tial RE ? Mg2 replacement accounts for a broad homoge-neity range of the LaMg12�x compound.

The present study has demonstrated that hydrogen met-allurgy has a great potential in the synthesis of intermetalliccompounds of Mg and rare earth metals via nanostruc-tured elemental hydrides.

Acknowledgements

This work has received support from the Norwegian Re-search Council. The authors are grateful to the staff of theSwiss–Norwegian Beam Lines, Grenoble, for their skillfulassistance in the SR XRD experiments. Dr. Jan PetterM�hlen is sincerely thanked for the help in the work.

Appendix A. Supplementary material

Details on RBM of the La + 12Mg mixture in hydro-gen; changes in unit cell parameter of La hydride duringin situ SR XRD study of vacuum thermodesorption; LeBail fit of the SR XRD pattern of the LaMg�11; a tableof crystallographic data of the LaMg�11 compound; a tableof coordination characteristics of the atoms and inter-atomic distances in the structure of LaMg�11; crystallo-graphic information file (CIF) for the LaMg�11

compound. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.actamat.2009.12.037.

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