Journal of Alloys and Compounds 353 (2003) 175–179

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Synthesis and structure of an orthorhombic low-pressure polymorph of caesium magnesium hydride, CsMgH 3 1

G. Renaudin, B. Bertheville , K. Yvon* ` , 24 quai Ernest Ansermet, CH-1211 Geneve ` 4, Switzerland Laboratoire de Cristallographie, Universite´ de Geneve Received 7 November 2002; accepted 18 November 2002

Abstract CsMgH 3 was synthesized by heating mixtures of the binary hydrides in an autoclave at 600 K and a hydrogen pressure of 150 bars. Synchrotron X-ray and neutron powder diffraction reveals a new structure having orthorhombic symmetry (space group Pmmn, ˚ for the hydride). In contrast to the trigonal high-pressure polymorph, magnesium centred a59.9958(1), b56.13271(6), c58.57364(9) A [MgH 6 ] octahedrons are condensed into triangular rather than linear [3MgH 3 ] 32 trimers, and these trimers are connected via four corners to two-dimensional slabs rather than via six corners to a three-dimensional network. The Mg–D distances in the deuteride range from 1.94 ˚ at the periphery to 2.00–2.08 A ˚ at the centre of the trimers. The Mg displacements within the octahedrons suggest repulsive to 1.96 A 21 21 Mg –Mg interactions.  2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen storage materials; Metal hydrides; Crystal structure; Neutron diffraction

1. Introduction Ternary metal hydrides having saline bonding character are of interest for hydrogen storage at high temperature. The Cs–Mg–H system, for example, contains two ternary hydride phases of composition Cs 3 MgH 5 and Cs 2 MgH 4 [1] that form at a gas pressure of |100 bar, and three phases of composition CsMgH 3 , Cs 4 Mg 3 H 10 and Cs 2 MgH 4 [2] that form under hydrostatic pressure of |30 kbar. While the former crystallize with tetragonal Cs 3 CoCl 5 and orthorhombic b-K 2 SO 4 type structures, respectively; the latter crystallize with rhombohedral BaRuO 3 , orthorhombic Cs 4 Mg 3 F 10 and tetragonal K 2 NiF 4 type structures, respectively. The phases formed under hydrostatic pressure contain octahedral [MgH 6 ] 42 anions and have generally better volume efficiencies than those formed at lower gas pressure that contain tetrahedral [MgH 4 ] 22 anions. In this paper we report on the existence and structure of a new low-pressure polymorph of CsMgH 3 . It crystallises with a novel orthorhombic struc-

*Corresponding author. Tel.: 141-22-702-6231; fax: 141-22-7026864. E-mail address: [email protected] (K. Yvon). 1 ´ Present address: Groupe materiaux et conception, HEVs, 1950 Sion, Switzerland.

ture and provides an example of a ternary hydride phase forming octahedral [MgH 6 ] 42 anions without the application of very high pressure.

2. Experimental

2.1. Synthesis Mixtures of the binary hydrides (deuterides) CsH (CsD) and a-MgH 2 (a-MgD 2 ) were pressed to pellets and sintered in an autoclave at 600 K under an hydrogen (deuterium) pressure of 150 bar for 4 days. The starting materials were prepared by hydrogenation (deuteration) of metallic caesium (STREM 99.9%) and magnesium powder (CERAC 99.6%, 2400 mesh) and handled in an argonfilled glove box since they were sensitive to air. An excess of MgH 2 (MgD 2 ) compared to the molar ratio CsH / MgH 2 51:1 was required to obtain the CsMgH 3 (CsMgD 3 ) phase with satisfactory yield. The reaction products were of white colour, pyrophoric and extremely sensitive to air and moisture.

2.2. Structure analysis A hydride sample of nominal composition Cs / Mg51:1

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0925-8388(02)01314-2

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G. Renaudin et al. / Journal of Alloys and Compounds 353 (2003) 175–179

was measured by synchrotron X-ray powder diffraction (Swiss–Norwegian beam lines at ESRF, Grenoble, Debye– Scherrer geometry, six Si(111) analysers in the diffracted ˚ 2u range 3.1–33.48, step size 2u 5 beam, l 50.50012 A, 0.0048, glass capillary of 0.3 mm diameter). The data showed the presence of a new ternary metal hydride phase having an unknown structure of orthorhombic symmetry ˚ with a (a59.9958(1), b56.13271(6), c58.57364(9) A) small excess of MgH 2 and a third unidentified impurity phase. The systematic extinctions (hk0: h 1 k 5 2n, with h00: h 5 2n and 0k0: k 5 2n) indicated two possible space groups: Pmn2 1 or Pmmn. The metal atom substructure was solved in space group Pmmn by using the program FOX [3] and found to consist of two Cs sites (2a and 4f ) and two Mg sites (2b and 4f ). The hydrogen positions were determined from neutron data as collected on a deuteride sample of nominal composition Cs / Mg51:3 by using the HRPT [4] powder diffractometer at PSI (Villigen) under the following experimental conditions: wavelength l 5 ˚ 2u range 4.95–164.908; step size 2u 50.058; 1.8856 A; temperature 293 K; high intensity mode; cylindrical vanadium container (6 mm inner diameter). As expected, the refined cell parameters (a59.9734(5), b56.1222(3), c5 ˚ were slightly smaller than those refined for 8.5562(4) A) the hydride with the synchrotron data. Magnesium dideuteride and magnesium oxide were found as impurity phases. Five deuterium sites (one 2b, one 4c, two 4e and one 4f ) were located by FOX [3], and a structure refinement was performed by using Fullprof.2000 [5]. The following

Table 1 Neutron structure refinement results on deuteride CsMgD 3 Atom

Site

x

y

z

˚ 2) B (A

Cs1 Cs2 Mg1 Mg2 D1 D2 D3 D4 D5

2a 4f 2b 4f 2b 4c 4e 4e 4f

0.25 0.476(1) 0.25 0.608(1) 0.25 0 0.25 0.25 0.550(1)

0.25 0.25 0.75 0.25 0.75 0 0.990(2) 0.953(2) 0.25

0.182(2) 0.665(2) 0.642(2) 0.067(1) 0.098(2) 0 0.494(1) 0.835(2) 0.300(2)

1.3(2) 5B Cs1 0.9(1) 5B Mg1 3.5(1) 5B D1 5B D1 5B D1 5B D1

Space group Pmmn (No. 59, origin at 1¯ ); T5295 K; a59.9734(5), ˚ V5522.44(8) A ˚ 3 , all site occupancies b56.1222(3), c58.5562(4) A, 100%; e.s.d. values in parentheses. Agreement factors of Rietveld refinement: R p 53.8%; R Bragg 512.7%, R F 56.9% (413 reflections, 36 variables).

36 parameters were allowed to vary: one zero shift, three scale, eight profile, six cell and 18 atomic (13 positional and five displacement) parameters. No indication for a partial occupancy of the deuterium sites was found, and the displacement parameters of the D sites were constrained to be equal. The observed and calculated neutron diffraction pattern are shown in Fig. 1, the refinement results are summarised in Table 1, and interatomic distances are listed in Table 2. Attempts to perform joint structure refinements by using the synchrotron data of the hydride together with the neutron data of the deuteride did not improve the accuracy of the results. The structure of orthorhombic

Fig. 1. Observed (top), difference (middle) and calculated with Bragg positions (bottom) neutron powder diffraction patterns of deuteride sample ˚ containing the orthorhombic CsMgD 3 phase ( l 51.8856 A).

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Table 2 ˚ (only distances less than 3.5 A; ˚ e.s.d. values in Interatomic distances (A) parentheses) Cs1–2D3 –2D1 –2D5 –4D2 –2D4

3.11(2) 3.145(6) 3.16(1) 3.31(1) 3.48(2)

Cs2–2D5 –2D3 –D5 –2D4 –2D2 –2D3 –D1

3.086(2) 3.12(1) 3.21(2) 3.24(1) 3.26(1) 3.39(1) 3.41(2)

Mg1–2D3 –2D5 –2D4 –2Mg2

1.94(2) 2.06(1) 2.07(2) 2.87(2)

Mg2–2D2 –D1 –2D4 –D5 –Mg2 –Mg1

1.955(8) 2.00(1) 2.06(1) 2.08(2) 2.84(2) 2.87(2)

D1–2Mg2 –2D4 –4D2 –2Cs1 –2Cs2

2.00(2) 2.57(2) 3.043(4) 3.145(6) 3.41(2)

D2–2Mg2 –2D4 –2D5 –2D1 –2D2 –2Cs2 –2Cs1

1.955(8) 2.877(8) 3.03(1) 3.043(4) 3.0611(1) 3.26(1) 3.31(1)

D4–2Mg2 –Mg1 –D4 –D1 –2D5 –2D2 –D3 –2Cs2 –Cs1

2.06(1) 2.07(2) 2.49(1) 2.57(2) 2.62(1) 2.878(8) 2.93(2) 3.24(1) 3.48(2)

D3–Mg1 –D4 –D3 –2D5 –Cs1 –2Cs2 –D3 –2Cs2

1.94(2) 2.93(2) 2.94(2) 3.04(1) 3.11(2) 3.12(1) 3.18(2) 3.39(1)

D5–Mg1 –Mg2 –2D4 –2D2 –2D3 –2Cs2 –Cs1 –Cs2

2.06(1) 2.08(2) 2.62(1) 3.03(1) 3.04(1) 3.086(2) 3.16(1) 3.21(2)

CsMgH 3 is represented in Fig. 2, and a comparison with its rhombohedral polymorph is given in Fig. 3.

3. Results and discussion Orthorhombic CsMgH 3 is the sixth known ternary hydride phase in the Cs–Mg–H system. In view of its formation it can be considered as a low-pressure (LP) polymorph of the trigonal high-pressure (HP) phase [2]. Both polymorphs are built up by magnesium centred [MgH 6 ] 42 octahedrons. In contrast to HP-CsMgH 3 the octahedrons in LP-CsMgH 3 are condensed into triangular rather than linear [3MgH 3 ] 32 trimers, and the latter are connected via four corners to 2-dimensional slabs (see Fig. 3) rather than via six corners to a 3-dimensional network (see Fig. 3 in Ref. [2]). The Mg–D distances in LP˚ for peripheral, 2.00–2.08 A ˚ for CsMgD 3 (1.94–1.96 A inner distances in the trimers) are similar to those in ˚ and other phases forming at HP-CsMgD 3 (2.00–2.07 A)

Fig. 2. Orthorhombic structure of low-pressure modification of CsMgD 3 . 32 Spheres represent caesium atoms and polyhedra [3MgH 3 ] trimers of condensed Mg-centred octahedra.

˚ and HPhigh-pressure such as Cs 4 Mg 3 D 10 (2.01–2.11 A) ˚ Cs 2 MgD 4 (2.01–2.16 A) but longer than those in lowpressure phases containing tetrahedral [MgH 4 ] 22 anions ˚ Interestsuch as Cs 2 MgD 4 and Cs 3 MgD 5 (1.82–1.88 A). 32 ingly, the [3MgD 3 ] trimers in both CsMgD 3 polymorphs show clear indications for repulsive Mg 21 –Mg 21 interactions as can be seen in Fig. 3 from the displacements of the Mg 21 cations away from off-centre positions in the [MgH 6 ] 42 octahedrons. Note that the octahedrons are also considerably distorted with respect to bond angles (LPCsMgD 3 : D–Mg–D573.9–102.98; HP-CsMgH 3 : D–Mg– D577.1–99.88). Both caesium sites in LP-CsMgD 3 have anti-cuboctahedral deuterium configurations with Cs–D ˚ The deuterium bond distances in the range 3.09–3.48 A. atoms occupy octahedron-like interstices having Mg / Cs coordination ratios of 1:5 (D3), 2:4 (D1, D2, D5) and 3:3 ˚ As with most (D4). The shortest D–D distance is |2.5 A. other saline metal hydrides the molar volumes of the ternary CsMgH 3 polymorphs are smaller than the weighted sum of the corresponding binary hydrides. As shown in Table 3 the volume contraction upon ternary hydride formation is 8.7% for the LP- and 12.3% for the HP modification, i.e. CsMgH 3 undergoes a 4% volume increase from the HP to the LP modification. Finally, it is interesting to note that LP-CsMgH 3 does not crystallise with a perovskite type related structure in contrast to HP-CsMgH 3 (9R variant of hexagonal perovskite) and other ternary hydrides such as RbCaH 3 [6], CsCaD 3 [7] or KMgH 3 [8] (cubic perovskites), SrLiH 3 [9], EuLiD 3 [10] or BaLiD 3 [11] (cubic inverse perovskites),

G. Renaudin et al. / Journal of Alloys and Compounds 353 (2003) 175–179

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] Fig. 3. Comparison between linear [3MgH 3 ] 32 trimers (point symmetry 3m) in rhombohedral HP-CsMgD 3 (a) and triangular [3MgH 3 ] 32 trimers (point ˚ symmetry mm2) in orthorhombic LP-CsMgD 3 (b). Bond lengths between magnesium (large white spheres) and deuterium (small black spheres) in A. Arrows indicate corner linkage to other trimers.

Table 3 Molar volumes and hydrogen storage efficiencies of CsMgH 3 polymorphs ˚ 3) V (A

Hydrides

CsH1MgH 2 LP-CsMgH 3 HP-CsMgH 3 a b

a

95.97 87.60 84.19

DV (%)

– 28.7 212.3

b

Hydrogen storage efficiencies g l 21

D(g l 21 ) (%)b

wt%

51.9 56.9 59.2

–

1.87 1.87 1.87

19.6 114.1

Weighted sum of binary hydrides. Relative difference compared to weighted sum of binary hydrides.

that assumes values in the ranges 0.89,t,1 for cubic and 0.8,t,0.89 for distorted cubic stackings, and t.1 for hexagonal (trigonal) stacking variants in oxides and fluorides [13]. As shown in Table 4, the calculated tolerance factors for the six ternary metal hydrides known so far are consistent with these ranges. The values for hexagonal RbMgH 3 (t51.04) and rhombohedral HP-CsMgH 3 , (t5 1.09), in particular, are well above unity.

Acknowledgements NaMgD 3 [12] (orthorhombic distorted perovskite) and RbMgD 3 [7] (6H variant of hexagonal perovskite). This is born out by the Goldschmidt tolerance factor, t, for perovskite structures ABX3 : r(A) 1 r(X) t 5 ]]]]] Œ]2 f r(B) 1 r(X) g

The help of the staff of the Swiss–Norwegian beamline (BM1) at ESRF (Grenoble) and by the instrument responsibles P. Fischer and D. Sheptyakov of the HRPT diffractometer at SINQ, PSI (Villigen) is gratefully acknowledged. This work was supported by the Swiss National Science Foundation and the Swiss Federal Office of Energy.

Table 4 Tolerance factor, t, for the ternary hydrides ABH 3 having perovskite-related structures at room temperature (ionic radii used are those indicated by Shannon [14]) Hydrides

˚ r(A) (A) C.N. 12

˚ r(B) (A) C.N. 6

˚ r(H) (A) C.N. 6

t

Space group

Perovkite type

RbCaH 3 a a SrLiH 3 NaMgD 3 EuLiD 3 CsCaD 3 BaLiD 3 KMgH 3 a RbMgD 3 HP-CsMgD 3

1.72 1.44 1.39 1.45 1.88 1.61 1.64 1.72 1.88

1.00 0.76 0.72 0.76 1.00 0.76 0.72 0.72 0.72

1.40 1.40 1.40 1.40 1.40 1.40 1.40 1.40 1.40

0.919 0.930 0.930 0.933 0.966 0.985 1.014 1.041 1.094

¯ Pm3m ¯ Pm3m Pnma ¯ Pm3m ¯ Pm3m ¯ Pm3m ¯ Pm3m P6 3 /mmc ¯ R3m

Normal Inverse Normal Inverse Normal Inverse Normal 6H variant 9R variant

a

Not characterised by neutron powder diffraction.

G. Renaudin et al. / Journal of Alloys and Compounds 353 (2003) 175–179

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