Methane and ethane at high pressures: structure and
stability
1
1
1,2
3
Elissaios Stavrou , Alexander Goncharov , Sergey Lobanov , Artem R Oganov , Artem
2
2
4
5
Chanyshev , Konstantin Litasov , Zuzana Konôpková , Vitali Prakapenka
1 Geophysical
2
Laboratory, Carnegie Institution of Washington, Washington, DC, United States.
V.S. Sobolev Institute of Geology and Mineralogy SB RAS , Novosibirsk, Russian Federation.
3 Department of Geosciences, State University of New York, Stony Brook, NY, United States.
4 Petra III, P02.2, DESY, Hamburg, Germany.
5 Center for Advanced Radiation Sources, University of Chicago, Chicago, IL, United States.
Methane and ethane are expected to be very promising natural energy resources in the following
years. Moreover, the broad range of thermodynamic conditions at which hydrocarbons are present
in the universe (from below 100 K to 10000 K and pressures up to 1TPa) determines the
fundamental importance of C-H physics and chemistry with respect to pressure and temperature.
Although the high pressure structural behavior of methane have attracted a lot of attention during
recent years, there are still open questions about the exact crystal structure at high pressures [1,2].
For instance, the so-called phase B of methane above 20 GPa has been indexed in a simple cubic
structure without any determination of the SG and the positional parameters. On the other hand,
there are almost no high pressure experimental results on ethane. Moreover, it has been recently
proposed that methane is thermodynamically unstable at megabar pressures and dissociates to
ethane and other hydrocarbons [3]. To throw some light on the above mentioned unanswered
questions we have performed a combined experimental, using x-ray diffraction and Raman
spectroscopy, and theoretical, using the ab-initio evolutionary algorithm, study of both methane and
ethane up to megabar pressures.
100
150
200
250
300
100
100
(b)
(a)
Phase III
10
Pressure (GPa)
10
Phase A
1
Phase II
Phase I
Fluid
0.1
100
150
1
200
250
0.1
300
Temperature (K)
Figure 1: (a) Phase diagram of ethane (see text for details) and (b) Schematic representation of the crystal
structure of phase III of ethane
For ethane we have determined the crystallization point at room temperature to be 1.7 GPa and also
the low pressure crystal structure ( Phase A). This crystal structure has orientation disorder (plastic
phase) and deviates from the known crystal structures for ethane (Phases I, II and III) at low
temperatures [4]. Moreover, a pressure induced phase transition has been indentified, for the first
time, at 18 GPa to a monoclinic phase III [4]. The high-pressure structure was determined by
evolutionary crystal structure prediction and found to match XRD. For both phases the crystal
structure, the equations of state and the positional parameters of carbon atoms are also determined.
60
2 Vpm CH4 this study
2
Vpm CH4 Sun et al.
C2H6+H2
3
Volume (Å )
40
C2H6 this study
20
H2 Loubeyre et al.
0
0
20
40
60
80
100
Pressure (GPa)
120
140
Figure 2: Comparative equations of state of: ethane (blue symbols), ethane plus hydrogen (green symbols)
and methane (red symbols). See text for details. Eos of hydrogen is also plotted.
Figure 2 shows the combined EOS of: (a) double of the volume per molecule (Vpm) of methane (b)
sum of ethane Vpm plus hydrogen Vpm according to the H2 EOS of Loubeyre et al [5]. As it can
be clearly seen there is no difference within the experimental error between these two volumes, i.e.
2Vpm(CH4) and Vpm(C2H6+H2). Moreover, with increasing pressure ethane and methane show
essentially the same compressibilities which results to no noticeable deviation of the two EOS. This
explains the fact that, there is no experimental evidence on the formation of ethane during cold
compression of methane. On the other hand, HT studies unambiguously reveal the formation of
ethane under high pressure-temperature conditions. In this case, HT is needed to overcome energy
barriers arising from the necessity of bond breaking. From our study the PV terms look identical for
methane and ethane at high pressures, or at least the difference between PV terms remains stable.
Since it is plausible to assume that internal energy terms are also similar then this implies that the
relative enthalpy ΔH difference between methane and ethane remains stable.
References
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[2]
[3]
[4]
[5]
H. Hirai et al. Chem. Phys. Lett. 454, 212 (2008)
L. Sun et al. Chem. Phys. Lett. 473, 72 (2009)
G. Gao et al. J. Chem. Phys. 133, 144508 (2010)
N. A. Klimenko et al. Low Temp. Phys. 34, 1038 (2008)
P. Loubeyre et al. Nature 383, 702 (1996)