High pressure studies of cobalt-hydrogen system by X-ray diffraction

Article abstract of DOI:10.1016/j.jallcom.2015.08.062

Cobalt-hydrogen system was studied in diamond anvil cell at high hydrogen pressure up to 22 GPa at room temperature by X-ray diffraction. Hydride formation from a solid solution of hydrogen in metal was observed at pressure around 4.5 GPa, and the decomposition of the hydride was observed at pressure around 3 GPa. Our studies are complementary to the earlier investigations conducted at higher temperatures, which employed technique that does not allow for the study of decomposition process.

Full text of DOI:10.1016/j.jallcom.2015.08.062

Journal of Alloys and Compounds 650 (2015) 884e886
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Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
High pressure studies of cobaltehydrogen system by X-ray diffraction
M.A. Kuzovnikov b, M. Tkacz a
a
,
, *
a
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Institute of Solid State Physics RAS, Chernogolovka, Moscow District, 2 Academician Ossipyan Str., 142432, Russia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Cobaltehydrogen system was studied in diamond anvil cell at high hydrogen pressure up to 22 GPa at
room temperature by X-ray diffraction. Hydride formation from a solid solution of hydrogen in metal was
observed at pressure around 4.5 GPa, and the decomposition of the hydride was observed at pressure
around 3 GPa. Our studies are complementary to the earlier investigations conducted at higher tem-
peratures, which employed technique that does not allow for the study of decomposition process.
Received 20 July 2015
Accepted 9 August 2015
Available online 13 August 2015
Keywords:
©
2015 Elsevier B.V. All rights reserved.
Metal hydrides, transition metal alloys and
compounds
Gasesolid reactions
Crystal structure, phase transitions
High-pressure, X-ray diffraction
1
. Introduction
VIeX. In the solutions with H/Co ꢂ 0.34, hydrogen atoms form
layered superstructures, occupying every third octahedral base
layer at H/Co ¼ 0.34 and every second layer for 0.38 ꢁ H/Co ꢁ 0.5.
High hydrogen solubility in primary solution is hardly found in
the systems where initial metal possesses cubic structure. In the
majority of such metals solid hydrogen solution has very narrow
concentration region. Good example is the palladiumehydrogen
system [6].
In the last few decades metal-hydrogen systems have been
extensively studied throughout the world due to their interesting
properties and potential applications. The main attention of re-
searchers was drawn to transition metal hydrides and systems that
can be used for hydrogen storage.
In spite of a huge amount of experimental data some systems
are still awaiting for the deeper and more comprehensive insight
into their properties. One of them is cobaltehydrogen system for
the first time investigated at high pressure by group of Ponyatovsky
Utilization of the diamond anvil cell technique allows for in situ
studies and extends available pressure range significantly. The
other members of group IX of the periodic table were recently
[1,2], and, later, by group of Fukai [3] and Takemura [15]. Their
2 3
found to form hydrides with H/Me > 1: RhH [7] and IrH [8]. So we
studies, conducted at hydrogen pressures up to 9 GPa, revealed a
continuous increase of hydrogen concentration in hcp metal lattice
followed by a formation of cubic monohydride phase at pressure
decided to revise the cobaltehydrogen system, aiming at the
determination of thermodynamic properties and looking for the
possibility of the increase of hydrogen content in cobalt at much
higher pressure than studied before.
ꢀ
around 7 GPa and T ¼ 250 C. The lattice parameter of the new
hydride phase, a ¼ 3.7124(5) Å, was almost identical to that of
nickel hydride [4,2]. Although nickel and cobalt are neighbors in the
periodic table, they possess different crystal structures e an fcc and
hcp respectively.
2. Experimental details
We used diamond anvil cell of the modified brilliant type with a
A neutron diffraction investigation at ambient pressure and
culet diameter of about 400
a rhenium gasket of ~50 m thickness after indentation of a 250
foil. Samples were made compressing high purity cobalt powder,
consisting of 5e10 m flakes. Samples were 10e20 m thick.
mm. A 200 mm hole was spark-drilled in
120 K showed [5] that hydrogen atoms in the solid solutions with
m
mm
concentration H/Co ꢁ 0.26 are randomly distributed over octahe-
dral interstices which are characteristic of all hydrides with close
packed metal lattices that are formed by transition metals of groups
m
m
Pressure was measured by a ruby fluorescence method [9], using an
excitation from 633 nm HeNe laser and a THR1000 spectrometer
for spectra recordings.
*
Corresponding author.
E-mail address: mtkacz@ichf.edu.pl (M. Tkacz).
Hydrogen was loaded into the gasket hole at initial pressure of
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M.A. Kuzovnikov, M. Tkacz / Journal of Alloys and Compounds 650 (2015) 884e886
885
about 0.4 GPa by the technique described earlier [10]. Hydrogen
was in significant excess in all experiments.
X-ray powder diffraction was measured by energy-dispersive
method with white radiation from tungsten target tube [11] colli-
mated down to about 100
material. Scattering angle was set as 2
m
m to avoid diffraction from the gasket
ꢀ
q
~ 20 , corresponding to
E*d ¼ 35 ÷ 40 keV*Å. Scattering angle was calibrated using the
diffraction pattern of hydrogen-free cobalt metal. All experiments
were conducted at room temperature.
3
. Results and discussion
The selected X-ray spectra are shown in Figs. 1 and 2 for
compression and decompression respectively. Lattice volume of
cobalt as a function of hydrogen pressure is given in Fig. 3.
Energy dispersive X-ray diffraction showed that at pressure in-
crease the lattice parameters of hcp cobalt are increasing contin-
uously up to about 4 GPa. Above this pressure a formation of a
monohydride phase with fcc structure is observed. Further
hydrogen pressure increase up to 22 GPa does not produce any
peculiarities on the V(P) dependence of the hydride. No evidence of
hydrogen content increase over H/Co ¼ 1 was observed. Unlike
rhodium, which form dihydride at hydrogen pressures less than
Fig. 2. Energy dispersive X-ray diffraction patterns for cobalt as a function of hydrogen
pressure taken during pressure decrease. Scattering angle corresponds to
E*d ¼ 34.9 keV*Å.
1
0 GPa [7], cobalt does not form hydrides with H/Co > 1 at such
pressures.
During decompression the hydride remains down to about
GPa, where a complete transformation to the hcp structure is
3
observed. This means that the reaction of hydrogen and cobalt is
reversible with a hysteresis about 1.6 GPa.
A comparison of the equations of state for cobalt in a hydrogen
atmosphere and in a 4:1 mixture of methanol and ethanol is shown
in Fig. 3. The bulk moduli of Co and CoH are 183(1) GPa and 190(20)
GPa respectively and their volumes at ambient pressure are
3
3
1
1.082(4) Å per Co atom and 13.2(1) Å per Co atom respectively.
Fig. 3. Pressureevolume relationships for the cobaltehydrogen system and pure co-
balt [12]. The solid triangles denote compression data and the empty ones indicate
decompression. The left Y axis corresponds to the volume calculated per one metal
atom, and the right axis shows molar volumes of cobalt both in pure state and in the
hydride phase.
Within the experimental error the compressibilities of both mate-
rials are the same. This behavior is typical for transition metals and
their hydrides. The difference in lattice volumes of CoH and Co is
3
2
.1(1) Å per H atom, in agreement with the estimation obtained in
Ref. [5].
Assuming that equilibrium pressure for hydride formation re-
action is close to the decomposition pressure (see Ref. [13] for
ꢀ
[
1,2] at T ¼ 250 C. However, in these works only formation process
was studied, and not the decomposition. We think that the
discrepancy is accounted for by a large baric hysteresis, accompa-
nying cobalt hydride formation. There should also be a considerable
temperature dependence of equilibrium pressure in temperature
discussion), we conclude that at room temperature the equilibrium
H
takes place at hydrogen pressureP ¼ 3:0ð5ÞGPa. This estimate is
eq
much lower comparing to 7 GPa, obtained by Ponyatovsky et al.
ꢀ
ꢀ
range 20 Ce250 C.
A standard Gibbs free energy for the reaction Co þ1/2H
2
/ CoH
at T ¼ 298 K was calculated using the following equation:
P
0
0
Z
0
DG
¼
¼
DVdP
H
H
P
eq
P
ꢀ
ꢁ
Z
1
2
H
VCoH ꢃ VCo
ꢃ
VH2 dPz ꢃ bHP
eq
H
P
eq
H
eq
P
Z
1
2
þ
VH2 dPz27ð2Þ kJ=mol H
P
0
3
where
b
H
¼ 2.1 Å /atom and the molar volume of gaseous hydrogen
was calculated using the equation of state from Ref. [14]. We
neglected the solubility of hydrogen in hcp cobalt at 3 GPa, which is
10e20 at %.
Fig. 1. Energy dispersive X-ray diffraction patterns for cobalt as a function of hydrogen
pressure taken during pressure increase. For clarity only selected spectra are presented
here. Scattering angle corresponds to E*d ¼ 34.8 keV*Å.

8
86
M.A. Kuzovnikov, M. Tkacz / Journal of Alloys and Compounds 650 (2015) 884e886
(Eds.), Problems in Solid State Physics, Mir Publishers, Moscow, 1984, pp.
4
. Conclusions
p.109e172.
[
2] V.E. Antonov, I.T. Belash, V.Yu Malyshev, E.G. Ponyatovskii, Dokl Akad, Nauk.
Formation and decomposition conditions of cobalt hydride were
SSSR 272 (5) (1983) 1147e1150.
determined at room temperature. Cobalt does not form hydrides
with H/Co > 1 at hydrogen pressures less than 22 GPa, unlike
rhodium, which is right under cobalt in the periodic table.
Our results suggest a considerable temperature dependence of
[3] Yu Fukai, S. Yokota, J. Yanagawa, J. Alloys Compd. 407 (2006) 16e24.
[4] V.E. Antonov, T.E. Antonova, V.K. Fedotov, T. Hansen, A.S. Ivanov,
A.I. Kolesnikov, J. Alloys Compd. 404e406 (2005) 73e76.
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[
[
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equilibrium
cobalt
hydride
formation
pressure
at
ꢀ
ꢀ
2
0
C < T < 250 C. We think that the phase diagram of
108 46 (2011) 18618e18621.
cobaltehydrogen system should be reconsidered for this temper-
ature range.
[8] T. Scheler, M. Marqu eꢀ s, Z. Kon o^ pkov ꢀa , ChL. Guillaume, R.T. Howie,
E. Gregoryanz, Phys. Rev. Lett. 111 (2013) 215503.
[
9] H.K. Mao, P.H. Bell, J. Shaner, D. Steinberg, J. Appl. Phys. 49 (6) (1978)
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[10] M. Tkacz, Pol. J. Chem. 69 (1995) 1205e1209.
3
Acknowledgment
[
[
[
11] M. Tkacz, S. Majchrzak, B. Baranowski, High Press. Res. 6 (2) (1990) 85e90.
12] H. Fujihisa, K. Takemura, Phys. Rev. B 54 (1996) 5e7.
The work was supported by the NCN grant 2011/03/B/ST4/02476
from the National Science Center of Poland.
13] V.E. Antonov, A.I. Latynin, M. Tkacz, J. Phys. Condens. Matter 16 (2004)
8387e8398.
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15] N. Ishimatsu, T. Shichijo, Y. Matsushima, H. Maruyama, Y. Matsuura,
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[
[
References
[1] E.G. Ponyatovsky, V.E. Antonov, I.T. Belash, in: A.M. Prokhorov, A.S. Prokhorov