High temperature hydrogenation of CaC6

Article abstract of DOI:10.1016/j.physc.2009.08.004

The structure and superconducting properties of high temperature hydrogenated calcium-graphite intercalation compound, CaC₆ have been investigated using room temperature X-ray diffraction, and temperature and field dependence of magnetisation.

Full text of DOI:10.1016/j.physc.2009.08.004

Physica C 469 (2009) 2000–2002
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Physica C
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High temperature hydrogenation of CaC6
a
a
b
a
G. Srinivas a, , C.A. Howard , N.T. Skipper , S.M. Bennington , M. Ellerby
*
a London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London WC1H 0AH, UK
b Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure and superconducting properties of high temperature hydrogenated calcium-graphite inter-
calation compound, CaC6 have been investigated using room temperature X-ray diffraction, and temper-
ature and field dependence of magnetisation. It is found that the hydrogenation can only decompose the
CaC6 phase, and generate a mixture of CaH2 and graphite as the final compound. The hydrogenation of
CaC6 also reveals a degradation of its superconducting properties. The experimental results are discussed
in detail and it is found that the formation of stable CaH2 and deintercalation are the main source for
observed phase separation and suppression in superconductivity.
Received 23 February 2009
Received in revised form 20 May 2009
Accepted 18 August 2009
Available online 22 August 2009
PACS:
74.70.Àb
74.25.Ha
61.05.cp
71.20.Tx
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Graphite intercalation compounds
Superconductivity
Hydrogenation
1. Introduction
pressing its superconducting properties. The results have been dis-
cussed in terms of variations in structural and superconducting
Graphite intercalation compounds (GICs) are known to absorb
hydrogen either by physisorption or chemisorption depending on
the temperature [1–3]. The hydrogen reaction is found to depend
on hydrogenation pressure, temperature, the staging or ratio of
the graphite to intercalate, and the given intercalated metal [3].
Furthermore, in some of the alkali-metal-GICs, the pronounced
changes in structural and superconducting transitions were ob-
served due to hydrogenation [3–7]. The recently synthesized bulk
CaC6, with a reasonably high superconducting transition tempera-
ture, TC = 11.5 K [8] stimulates whether even higher transition tem-
peratures can be achieved by hydrogenation as happened in the
case of KC8Hx [6]. The hydrogenation in partially Ca-intercalated
graphite compound synthesized from natural graphite flakes by
Ca-metal vapour method has been reported recently [9]. Here we
present a systematic and detailed study on the effect of high tem-
perature hydrogenation on the structure and superconducting
properties of fully intercalated bulk CaC6 synthesized from highly
oriented pyrolytic graphite (HOPG) and molten Li–Ca alloy. The
X-ray diffraction (XRD) and magnetisation measurements demon-
strate that the hydrogenation leads to a segregation of the Ca-
metal atoms from the intercalated graphite and produces a
mixture of CaH2 and graphite as final compounds thereby sup-
properties which are correlated with the stability of individual
phases and the deintercalation of Ca-metal.
2. Experimental
The CaC6 samples were synthesized from molten Li–Ca alloy
and HOPG as described in detail elsewhere [8]. The hydrogenation
has been carried out in a stainless steel reactor at 500 °C under a
10 bar of high pure hydrogen. In order to avoid possible hydrogen
desorption, the sample cell was cooled down to room temperature,
then pressure was reduced to the atmospheric value. The struc-
tural and phase identification of pristine and hydrogenated CaC6
samples have been carried out using powder XRD. XRD measure-
ments were performed at room temperature using X’pert Pro, PAN-
alytical diffractometer using Mo
Ka radiation in an inert
atmosphere. Magnetization measurements as a function of tem-
perature and field were performed in a Quantum Design supercon-
ducting quantum interference device magnetometer (MPMS7),
with the applied field perpendicular to the basal plane.
3. Results and discussion
Fig. 1 shows the room temperature XRD patterns of pristine and
hydrogenated CaC6 samples, the samples are subjected to different
* Corresponding author. Tel.: +44 (0) 20 7679 3526; fax: +44 (0) 20 7679 0595.
E-mail address: g.srinivas@ucl.ac.uk (G. Srinivas).
0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2009.08.004

G. Srinivas et al. / Physica C 469 (2009) 2000–2002
2001
ond-stage ternary compound, KC8Hx (0.1 6 x 6 0.67) [3]. The dis-
tinct structural behaviour of light and heavy alkali-metal-GICs
and the present Ca-GIC due to hydrogenation clearly suggest that
the stability of alkali/alkaline-earth-hydrides plays major role
apart from the strong electronic effects suggested previously in
hydrogenated LiC6. For example, the charge transferred to the
graphene layer and the magnitude of enthalpy formation of alka-
li-hydride are lower in stage-1 KC8 compared to a stage-1 LiC6
[12,13]. Thus, in stage-1 KC8 the potassium remains intercalated
in graphite and the hydrogen atoms sit in the interstitial sites
among the K atoms in the intercalant layers for hydrogen concen-
trations up to KC8H0.1. Further hydrogen uptake leads to a full-scale
structural transition to an ordered stage-2 compound KC8H2/3 in
which the intercalants reside in a triple layer sandwich formed
by two highly electropositive K layers between which is inserted
a less electropositive H layer [3]. In CaC6, Emery et al. [14], have re-
ported the highest electron charge transfer between calcium and
graphene layer, 0.106 electrons per carbon atom, which is very
high compared to 0.07 electrons per carbon atom in LiC6 [15].
The enthalpy of formation of CaH2 (À180 kJ/mol) is twice that of
LiH (À90.6 kJ/mol) and even three times than KH (À58 kJ/mol)
[13]. Furthermore, the hydrogenation at significantly elevated tem-
perature also provokes the phase separation, because the Ca atoms
can move rapidly and rearrangement of Ca atoms can occur with
respect to the graphene planes to reduce the total free energy of
the compound. Therefore the absorbed hydrogen atoms prefer to
interact directly with Ca. Thus the CaC6 phase is no longer stable
in the hydrogenated GIC, and a stable CaH2 precipitates as hap-
pened in some of the hydrogenated intermetallics containing sta-
ble hydride forming element [16,17].
f)
5 h in H2
1h in H2
e)
d)
c)
45 min in H2
30 min in H2
15 min in H2
b)
a)
Pristine CaC6
Fig. 2 shows the temperature dependence of magnetization in
zero-field cooled (ZFC) at 100 Oe for c-axis oriented CaC6 samples
10
15
20
25
30
35
2θ (deg)
0
Fig. 1. The XRD patterns of pristine and hydrogenated CaC6 samples; the samples
subjected to different hydrogen exposure times at 500 °C and 10 bars of hydrogen
pressure. Symbol à represents the metastable higher staging compound.
pristine CaC6
15 min in H2
-6
30 min in H2
exposure times at 500 °C under 10 bar of hydrogen pressure. The
pristine CaC6 sample (Fig. 1a) shows single phase behaviour of
rhombohedral CaC6 without any identifiable unintercalated graph-
ite phase. The silver coloured pristine CaC6 sample turned into
black upon hydrogen exposure. The hydrogenated CaC6 samples
did not reveal any evidence of formation of ternary hydride-Ca-
GIC as seen in some of the hydrogenated alkali-metal-GICs [3].
The only drastic collapse of CaC6 phase and development of new
reflections corresponding to constituent graphite and orthorhom-
bic (space group: Pnma) CaH2 [10] can be seen (Fig. 1b–f) with
increasing the hydrogen exposure time. The fully hydrogenated
CaC6 sample shows only mixture of graphite and CaH2. The similar
phase separation and development of free graphite upon hydroge-
nation in the GICs was first observed in the stage-1 LiC6. The hydro-
genation of LiC6 leads to formation of LiH and graphite [3,4,11]. It
was also reported that hydrogenation in stage-1 LiC6 can produce
higher staging Li-GICs by removing fraction of Li to form LiH
[11]. Thus the existence of extra weak reflections (represented by
star) neither belonging to CaC6 nor CaH2 in the intermediate
hydrogenation state might be a metastable higher staging com-
pound. With increase in hydrogenation time the reflections be-
come weaker and finally disappear after certain hydrogenation
time. Furthermore, the position of peaks also depends on the reac-
tion time. The formation of LiH was attributed to the strong affinity
of lithium to hydrogen [3]. However, the hydrogen absorption in
stage-1 KC8 brings about changes of staging, and gives new sec-
45 min in H2
1 h in H2
5 h in H2
100 Oe
-12
-18
-24
-30
0.0
-0.5
-1.0
-1.5
8
9
10
11
12
T (K)
4
8
12
16
20
24
T (K)
Fig. 2. The temperature dependent magnetization of pristine and hydrogenated
CaC6 samples at an external applied field of 100 Oe along c-axis. Inset shows the
expanded view of superconducting transition behaviour. The samples subjected to
different hydrogen exposure times at 500 °C and 10 bars of hydrogen pressure.

2002
G. Srinivas et al. / Physica C 469 (2009) 2000–2002
hydrogenation usually proceeds from the surface of a material,
with most of the reaction nucleates at defects or grain boundaries
(or microcracks in the case of metal hydride system), in the hydro-
genated system, the fraction of remaining superconducting phases
are separated by large regions of normal conducting or insulating
material, CaH2. Further, the field dependence of magnetisation of
partially hydrogenated samples reveal the decreased lower critical
field (HC1, defined as M(H) minimum). This can be attributed to the
increased penetration depth (k), since the intercalant disorder
caused by motion of Ca atoms and inhomogeneous distribution
of remaining superconducting CaC6 phase can bring changes in
the penetration depth and in the coherence length (n).
0
-20
5 K
-40
4. Conclusion
-60
pristine CaC6
15 min in H2
30 min in H2
45 min in H2
1 h in H2
We report the first detailed study of high temperature hydroge-
nation in bulk CaC6 synthesized by molten Li–Ca alloy method. The
hydrogenation eventually regenerates the constituent graphite by
forming CaH2 as Ca is deintercalated from CaC6. The successive
suppression of superconductivity of CaC6 upon increasing hydroge-
nation time is attributed to the decrease in superconducting CaC6
phase volume fraction due to the phase separation and formation
of CaH2 at the expense of CaC6 phase.
-80
-100
-120
5 h in H2
Acknowledgements
This work was supported by the EPSRC Grant Nos. EP/F027923/
1 and EP/E003907/1 and by a Royal Society & Wolfson Foundation
Laboratory Refurbishment award. We wish thanks to Arthur Lovell
for his help.
0
500
1000
1500
2000
H (Oe)
Fig. 3. The field dependent magnetization along c-axis at 5 K for pristine and
hydrogenated CaC6 samples. The samples subjected to different hydrogen exposure
times at 500 °C and 10 bars of hydrogen pressure.
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