Journal of Alloys and Compounds 287 (1999) L4–L6
L
Letter
Structure of the high pressure phase g-MgH 2 by neutron powder
diffraction
b
¨
M. Bortz a , B. Bertheville a , G. Bottger
, K. Yvon a , *
a
` , 24, Quai Ernest-Ansermet,1211 Geneva 4, Switzerland
Laboratoire de Cristallographie, Universite´ de Geneve
b
¨ Neutronenstreuung, ETH Zurich
¨
and Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Labor f ur
Received 4 January 1999
Abstract
The high-pressure phase g-MgH 2 was formed by heating the low-pressure phase a-MgH 2 in a multianvil press at 2 GPa pressure to
1070 K for 120 min and successive rapid quenching. Investigation by X-ray and neutron powder diffraction on the deuteride at ambient
conditions revealed that it crystallises with the orthorhombic a-PbO 2 type structure (space group Pbcn, Z54, a54.5213(3), b55.4382(3),
˚ (hydride); a54.5056(3), b55.4212(3), c54.9183(3) A
˚ (deuteride) at T5295 K). The deuterium atoms surround
c54.9337(3) A
˚ The rutile structure of
magnesium in a distorted octahedral configuration with bond distances Mg–D51.915(3), 1.943(3) and 2.004(3) A.
a-MgH 2 was re-evaluated.  1999 Elsevier Science S.A. All rights reserved.
Keywords: g-MgH 2 ; a-MgH 2 ; High-pressure phase; Magnesium dihydride; Neutron powder diffraction
1. Introduction
The orthorhombic high-pressure modification of magnesium dihydride, g-MgH 2 , coexists with the tetragonal
low-pressure modification, a-MgH 2 , in samples which
undergo high-pressure high-temperature treatments at 2.5–
8 GPa and 250–9008C, respectively [1]. Its cation substructure was stated to be that of a-PbO 2 , but the hydrogen
positions were not determined. Since g-MgH 2 often occurs
as a by-product in the high-pressure synthesis of magnesium based ternary metal hydrides (see for example [2]),
its complete structural characterisation became desirable.
Here we present structural parameters at ambient conditions as refined from X-ray and neutron powder diffraction data on the deuteride. Parameters of improved accuracy are also presented for the rutile structure of a-MgD 2 .
2. Experimental and results
2.1. Synthesis
Powder samples of a-MgH 2 were pressed into pellets
and put into boron nitride crucibles. They were surrounded
*Corresponding author.
E-mail address: [email protected] (K. Yvon)
by graphite heating elements, introduced into pyrophyllite
cubes having the dimensions 20320320 mm 3 , and placed
into a multianvil high-pressure high-temperature device
[3]. After heating the samples to 1070 K for 120 min under
a pressure of 2 Gpa, the g-phase formed as a greyish
powder with yields between 20 and 50%. A complete
conversion of the a-phase into the g-phase was never
achieved. The deuteride was prepared in the same way.
The starting materials MgH 2 and MgD 2 were obtained by
hydrogenation (deuteration) of magnesium powder
(CERAC 99.6, -400 mesh).
2.2. X-ray powder diffraction
Data were collected at room temperature on a Huber
diffractometer (G600, CuKa 1 radiation, internal standard
silicon). The patterns were indexed on an orthorhombic
lattice with refined cell parameters of a54.5213(3), b5
˚ for the hydride, and a5
5.4382(3), c54.9337(3) A
˚
˚ for the
4.5056(3) A, b55.4212(3), c54.9183(3) A
deuteride (T5295 K). The diffraction intensities were
consistent with a cation substructure of the a-PbO 2 type.
2.3. Neutron diffraction
Data were collected at 260 K on the powder diffractometer D1A at ILL (Grenoble) by using a cylindrical
0925-8388 / 99 / $ – see front matter  1999 Elsevier Science S.A. All rights reserved.
PII: S0925-8388( 99 )00028-6
M. Bortz et al. / Journal of Alloys and Compounds 287 (1999) L4 –L6
L5
Table 1
Refinement results on neutron diffraction data for g-MgD 2 and for a-MgD 2 ; T5260 K; e.s.d.s in parentheses
z /c
˚ 2]
Biso [A
˚ space group Pbcn (no 60); Z54
g-MgD 2 : a54.5051(2), b55.4197(2), c54.9168(2) A;
Mg
4c
0
0.3313(6)
D
8d
0.2727(5)
0.1089(4)
R Bragg 5 4.0%, R F 5 3.4%
0.25
0.0794(4)
0.18(6)
1.30(6)
˚ space group P4 2 / mnm (no 136); Z52
a-MgD 2 : a54.5010(1), c53.0100(1) A;
Mg
2a
0
D
4f
0.3040(2)
R Bragg 5 4.3%, R F 5 2.9%
0
0
0.56(5)
1.69(4)
Atom
Site
x /a
y /b
0
x
MgD 2 [1]. For the structure refinement the atomic coordinates of orthorhombic a-PbO 2 and tetragonal a-MgD 2
[4] were taken as starting parameters. The refinements
were performed with the programme FULLPROF [5] and
converged with R p 5 12.1, R wp 5 10.5 and x 2 5 2.8. The
results are listed in Table 1, a plot of the observed,
calculated and difference patterns is shown in Fig. 1, and a
list of bond distances and bond angles is given in Table 2.
The data for a-MgD 2 are about ten times as accurate as
those reported in a previous study [4]; thus they are
included for comparison.
3. Discussion
Fig. 1. Observed, calculated and difference neutron diffraction patterns of
˚
MgD 2 ( l 51.911 A).
˚
vanadium container of 6 mm inner diameter ( l51.911 A;
2Q range 8 –1578; step size 0.058; sample weight 1 g).
The sample contained about equal amounts of g-MgD 2 and
a-MgD 2 and traces of MgO. There was no evidence for
the presence of another high-pressure phase such as bTable 2
˚ and angles (8); e.s.d. values in parentheses
Selected distances (A)
g-MgD 2
Mg
2D
2D
2D
D–Mg–D
D–Mg–D
D–Mg–D
D–Mg–D
D–Mg–D
1.915(3)
1.943(3)
2.004(3)
101.9(1)
97.6(1)
94.5(1)
169.7(1)
87.8(1)
D
Mg
Mg
Mg
D
1.915(3)
1.943(3)
2.004(3)
2.489(3)
a–MgD 2
Mg
2D
4D
D–Mg–D
D–Mg–D
1.9351(9)
1.9549(6)
90.0(2)
180.0(2)
D
Mg
2 Mg
D
1.9351(9)
1.9549(6)
2.495(1)
The structures of g-MgH 2 and a-MgH 2 are similar.
Both are built up by magnesium centred hydrogen octahedra that are linked by edges along one direction and by
corners in two other directions (Fig. 2). The chains are
straight in the tetragonal a-phase and run along the
fourfold axis of the rutile structure, while they are zigzag
shaped in the g-phase and run along a twofold screw axis
in the orthorhombic a-PbO 2 type structure. The octahedra
in g-MgH 2 are strongly distorted (Mg–D bond lengths
˚ D–Mg–D bond angles between 88
1.92, 1.94 and 2.00 A,
and 1028) compared to those in tetragonal a-MgH 2 (Mg–
˚ D–Mg–D590 and 1808). The
D51.94 and 1.95 A,
pressure induced a⇒g phase transition induces a rearrangement in the cation and anion substructures which is
reconstructive since bonds are broken and re-established.
Fig. 2. Structures of orthorhombic g- MgD 2 (left) and tetragonal a-MgD 2
(right) , both viewed approximately along [001].
L6
M. Bortz et al. / Journal of Alloys and Compounds 287 (1999) L4 –L6
The occurrence of a similar phase transition in PbO 2
suggests the importance of thermal activation or high shear
[6]. Surprisingly, the cell dimensions of the two modifications are closely related. The a⇒g phase transition
results in a doubling of the translation period along c and
an expansion along b, while a remains nearly constant.
The density increases by about 1.6%. A reconversion of
the g-phase into the a-phase occurs during a heat treatment
at 570 K in one bar hydrogen atmosphere.
Acknowledgements
We thank A. Naula for the maintenance of the anvil
press, J.-L. Lorenzoni for technical assistance, and Mrs. B.
¨
Kunzler
for help with the drawings. This work was
supported by the Swiss Federal Office of Energy and the
Swiss National Science Foundation.
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