Journal of Alloys and Compounds 290 (1999) L8–L10
L
Letter
Ca 19 Mg 8 H 54 , a new salt-like ternary metal hydride
B. Bertheville, K. Yvon*
` , 24 quai E. Ansermet, CH-1211 Geneve
` 4, Switzerland
Laboratoire de Cristallographie, Universite´ de Geneve
Received 10 May 1999
Abstract
Ca 19 Mg 8 H 54 and its deuteride have been synthesised at high pressure (25 kbar) and high temperature (850 K) from a 1:2 mixture of the
binary Mg and Ca hydrides (deuterides). X-ray and neutron powder diffraction show a cubic Yb 19 Mg 8 H 54 type structure (space group
]
˚ (hydride) and 12.0642(8) A
˚ (deuteride). The hydrogen storage densities (5.4 wt.%, 100.9
Im3) with cell parameters a512.1457(6) A
g / litre) are among the highest found so far for ternary alkaline earth or mixed alkali–alkaline earth metal hydrides. Desorption
experiments suggest that the deuteride decomposes at |700 K and 5 bar deuterium pressure into Ca 4 Mg 3 D 14 and CaD 2 .  1999
Elsevier Science S.A. All rights reserved.
Keywords: Metal hydrides; High pressure synthesis; Neutron powder diffraction
1. Introduction
Magnesium based ternary alkaline-earth hydrides are of
interest as precursors for the synthesis of novel transition
metal hydrides [1]. In contrast to the strontium and barium
containing systems which contain respectively three
(Sr 2 MgH 6 [2], SrMgH 4 [3], Sr 2 Mg 3 H 10 [4]) and four
(Ba 2 MgH 6 [5], BaMgH 4 [6], Ba 6 Mg 7 H 26 [7], Ba 2 Mg 3 H 10
[8]) ternary hydride phases, only one such phase is known
for the calcium containing system (Ca 4 Mg 3 H 14 [9]). In
this article we report on the synthesis and crystal structure
of a second ternary hydride phase of composition
Ca 19 Mg 8 H 54 . The compound was obtained by high-pressure synthesis and found to be isostructural with
Yb 19 Mg 8 H 54 [10].
(deuteration) of magnesium and calcium powders in an
autoclave (Mg: CERAC 99.6%, –400 mesh; Ca: Alpha
99%). Mixtures having a molar ratio of Mg / Ca51:2 were
pressed into pellets which were put into a boron nitride–
graphite crucible and placed into a multi-anvil pressure cell
[11]. The reactions proceeded at 850 K under a quasihydrostatic pressure of 25 kbar during 180 min. The
reaction products were light grey and very sensitive to air
and moisture. Attempts to obtain the ternary hydride by
solid state reaction in an autoclave under hydrogen pressure failed. The phase equilibrium was investigated by
heating deuteride samples in an autoclave at 5 bar
deuterium pressure during 120 min to temperatures of
|620 K, 670 K and 720 K, and by slowly cooling to
ambient conditions.
2. Experimental
2.2. X-ray diffraction
2.1. Sample preparation
The hydride sample was enclosed in a sealed sample
holder and characterised by X-ray powder diffraction at
295 K on a Bragg-Brentano diffractometer (Philips PW
1820, CuKa radiation, internal standard Si). The pattern
showed a new hydride phase and unreacted CaH 2 and
a-MgH 2 . No traces of Ca 4 Mg 3 H 14 were detected. The
pattern was indexed on a cubic lattice with refined cell
˚ The diffraction intensities
parameter a5 12.1457(6) A.
suggested a Yb 19 Mg 8 H 54 type structure [10] of com]
position Ca 19 Mg 8 H 54 (space group Im3, Z52).
Binary alkaline earth hydrides MgH 2 and CaH 2 and the
corresponding deuterides were prepared by hydrogenation
*Corresponding author. Tel.: 141-22-702-62-31; fax: 141-22-781-2192.
E-mail address: [email protected] (K. Yvon)
0925-8388 / 99 / $ – see front matter  1999 Elsevier Science S.A. All rights reserved.
PII: S0925-8388( 99 )00243-1
B. Bertheville, K. Yvon / Journal of Alloys and Compounds 290 (1999) L8 –L10
2.3. Neutron diffraction
The deuteride sample was enclosed into a cylindrical
vanadium container (8 mm inner diameter) and placed on
the powder diffractometer D1A at ILL, Grenoble (focusing
˚ 2u range 6–1598;
Ge(115) monochromator, l 51.911 A;
2u step 0.18; T5293 K). The diffraction pattern confirmed
the presence of ternary Ca 19 Mg 8 D 54 (main phase), binary
CaD 2 , a-MgD 2 and g-MgD 2 (secondary phases) and MgO
(impurity phase). The refinements were performed with the
aid of the program FULLPROF [12] by allowing 40
parameters to vary: 1 for the zero correction, 23 for the
Ca 19 Mg 8 D 54 phase (1 scale factor, 1 cell, 15 atomic, 4
peakshape, and 2 asymmetry parameters) and 16 for the
secondary phases (10 for CaD 2 [13], 2 for a-MgD 2 [14], 1
for g-MgD 2 [14] and 3 for MgO). The observed, calculated and difference neutron powder diffraction patterns
are shown in Fig. 1, and the refinement results are
summarised in Table 1. Metal–deuterium and deuterium–
deuterium distances are listed in Table 2.
L9
Table 1
Refinement results on neutron diffraction data for Ca 19 Mg 8 D 54 (T5293
K; s.u.’s in parentheses)a
Atom
Position
x
y
z
˚ 2)
Biso (A
Ca1
Ca2
Ca3
Mg
D1
D2
D3
D4
24g
12d
2a
16f
48h
24g
24g
12e
0
0.333(3)
0
0.1604(8)
0.3109(7)
0
0
0.118(1)
0.314(1)
0
0
x
0.1109(6)
0.3985(8)
0.173(2)
1/2
0.349(1)
0
0
x
0.174(1)
0.184(1)
0.1100(9)
0
0.7(1)
0.7(1)
0.7(1)
1.3(2)
2.31(5)
2.31(5)
2.31(5)
2.31(5)
]
˚ RB 5
Space group, Im3 (N8204).Cell parameter: a512.0642(8) A.
4.25%, R P 53.36%, R wp 54.45%, R exp 52.44%, x 2 53.32.
a
3. Results and discussion
Ca 19 Mg 8 D 54 is a salt-like metal deuteride which is
isostructural to Yb 19 Mg 8 D 54 . Its metal atoms form a
distorted bcc type superstructure (see Fig. 2a of Ref. [10])
in which deuterium atoms occupy four types of more-orless distorted tetrahedral holes. Two (D2 and D4) are
surrounded by calcium atoms only, while the other two
have one (D1) or two (D3) magnesium atoms in their
coordination sphere (Fig. 2). The shortest metal deuterium
˚ and Mg–D151.92 A,
˚ are
bond distances, Ca–D252.20 A
slightly smaller than the corresponding distances in binary
˚ [13]) and a-MgD 2 (1.95 A),
˚ respectively.
CaD 2 (2.24 A
Magnesium has octahedral deuterium coordination as in
binary a-MgD 2 and other ternary alkaline earth deuterides
such as Sr 2 MgD 6 [2] and Ba 2 MgD 6 [5], but unlike
Ca 4 Mg 3 D 14 [9] in which it has a pentagonal bipyramidal
coordination. Calcium has eightfold (Ca1), tenfold (Ca2)
and twelvefold icosahedral (Ca3) deuterium coordinations
(see Fig. 2b of ref. [10]). The shortest D–D contact
Table 2
˚ in Ca 19 Mg 8 D 54 (T5293 K; s.u.’s in
Selected interatomic distances [A]
parentheses)
Fig. 1. Observed (top), difference (middle) and calculated (bottom)
neutron powder diffraction patterns of Ca 19 Mg 8 D 54 containing CaD 2 ,
˚
g-MgD 2 , a-MgD 2 and MgO as impurity phases, l 51.911 A.
Ca1–2D2
2D1
D2
D4
2D1
Ca2–2D3
2D2
2D4
4D1
Ca3–12D3
2.20(1)
2.21(2)
2.23(2)
2.27(2)
2.47(1)
2.34(3)
2.36(2)
2.46(3)
2.50(1)
2.48(2)
Mg–3D1
3D3
1.92(1)
2.032(9)
D1–Mg
Ca1
Ca1
Ca2
D2
D2–2Ca1
Ca1
Ca2
D2
D3–2Mg1
Ca2
Ca3
4D3
D4–2Ca1
2Ca2
2D2
1.92(1)
2.21(2)
2.47(1)
2.50(1)
2.51(1)
2.20(1)
2.23(2)
2.36(2)
2.45(1)
2.03(1)
2.34(3)
2.48(2)
2.59(2)
2.27(2)
2.46(3)
2.68(2)
L10
B. Bertheville, K. Yvon / Journal of Alloys and Compounds 290 (1999) L8 –L10
the highest found so far among ternary alkaline earth or
mixed alkali–alkaline earth metal hydrides. The volumetric
density exceeds that of Ca 4 Mg 3 H 14 (98.9 g / l), and is
about 5% higher than the weighted average of the binary
hydrides MgH 2 (108.7 g / l) and CaH 2 (91.7 g / l). This fact
could be related to the failure of obtaining Ca 19 Mg 8 H 54 by
solid state reaction in an autoclave, and to the absence of
the Ca 4 Mg 3 H 14 phase in the Ca 19 Mg 8 H 54 high-pressure
sample.
Acknowledgements
¨
¨
The authors thank Dr. G. Bottger
(ETH Zurich)
and Dr.
H. Kohlmann (University of Geneva) for help with the
neutron diffraction experiments, A. Naula for the maintenance of the anvil press, and J.-L. Lorenzoni for technical
assistance. This work was supported by the Swiss Federal
Office of Energy and the Swiss National Science Foundation.
References
Fig. 2. Metal configurations around four deuterium sites in Ca 19 Mg 8 D 54 .
Ca and Mg atoms shown as large and small circles, respectively; Ca sites
numbered; site symmetries 1 (D1), m (D2, D3) and mm2 (D4).
˚ As expected from the the cell parameter
distance is 2.45 A.
of Ca 19 Mg 8 H 54 which is almost identical to that of
Yb 19 Mg 8 H 54 , the average metal–deuterium distances in
the Ca compound are practically equal to those in the Yb
compound, suggesting that the Ca 21 ions in that structure
have nearly the same size as the Yb 21 ions. The same
observation also holds for Ca 4 Mg 3 H 14 and its Yb analogue
[15].
The high-temperature experiments in the autoclave
suggest that Ca 19 Mg 8 D 54 decomposes under 5 bar
deuterium pressure essentially in a two-step reaction, the
first (at about 650–700 K) yielding Ca 4 Mg 3 D 14 and CaD 2 ,
and the second (at about 50 K higher) yielding binary
CaD 2 and Mg. The hydrogen storage efficiencies of
Ca 19 Mg 8 H 54 are 5.4 wt.% and 100.9 g / l. They are among
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