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Oganov, 2007; Sakai
et al
., 2011a), and thus the
data from the shock experiments of Holmes
et al
.
have been reanalyzed and new Pt scales have been
presented (Fei
et al
., 2007; Matsui
et al
., 2009). Fei
et al
. (2007) determined the internally consistent
Pt pressure scale, based on diamond anvil cell
experiments, which was consistent with the Au,
Ne, and NaCl-B2 pressure scales. Dorogokupets
and Oganov (2007) constructed semiempirical
equations of state for Al, Au, Cu, Pt, Ta, and
W that describe the available shockwave, ultra-
sonic, X-ray, and thermochemical data within the
experimental error.
Figure 8.18 shows the compression curves of
NaCl-B2 phase based on the different Pt pres-
sure scales. The pressure differences highlight
the characteristics of these pressure scales; i.e.,
the Pt pressure scale of Holmes
et al
. (1989)
overestimates the pressure for a given volume
of NaCl-B2 phase, whereas the other pressure
scales are consistent with one another (Doro-
gokupets & Oganov, 2007; Fei
et al
., 2007; Matsui
et al
., 2009). Sakai
et al
. (2011a) summarized that
the pressure difference determined by the Birch-
Murnaghan and Vinet equations of state of the
NaCl-B2 phase based on the same Pt pressure
scale is very small, less than 2.0% in the pressure
range of 50-364 GPa. For the Birch-Murnaghan
equation of state, the pressure difference between
Holmes
et al
.'s and Matsui
et al
.'s Pt scales is
+
37
GPa (
10%); that between Fei
et al
.'s and Mat-
sui
et al
.'s Pt scales is
+
1.1%); and that
between Dorogokupets and Oganov's and Matsui
et al
.'s Pt scales is
−
4GPa (
−
+
12 GPa (3.4%) at 364 GPa
(Sakai
et al
., 2011a).
8.4.2 Crystal structures and phase relations
of the inner core materials
The structure of the inner core phase is a matter
of debate. The stability of iron alloys has been
studied up to the core conditions using several
different methods such as static high pressure
experiments, shock experiments, and
ab initio
calculations. Several authors suggested that the
hcp-iron is stable in the inner core (e.g., Mao
et al
.,
1990; Nguyen & Holmes, 2004; Asanuma
et al
.,
2008; Tateno
et al
., 2010), whereas the stability of
the bcc-iron was also suggested by some authors
(Belonoshko
et al
., 2003 and Dubrovinsky
et al
.,
2007).
The bcc-iron phase has been suggested both
theoretically and experimentally. Brown and Mc-
Queen (1986) determined the Hugoniot of iron un-
der the core conditions, and reported a kink in the
P-wave velocity at high pressures and tempera-
tures below the stability conditions of liquid iron,
as shown in Figure 8.19, although the difference
in bulk sound velocity is not clearly observed. Al-
though the structure of the high-pressure phase
was not determined by the shock experiments,
Brown and McQueen (1986) assumed that the
unknown phase had the fcc structure (
γ
−
350
Matsui
et al
.'s Pt scale
Fei
et al
.'s Pt scale
Dorogokupets and Oganov's Pt scale
Holmes
et al
.'s Pt scale
300
250
200
150
iron).
Belonoshko
et al
. (2003) and Vo cadlo
et al
. (2003)
suggested that bcc-iron is stable at the core con-
ditions based on
ab initio
calculation and also
suggested the existence of a strong anisotropy of
bcc-iron at 360 GPa and 6000 K (e.g., Belonoshko
et al
., 2008). Dubrovinsky
et al
. (2007) experi-
mentally showed that the Fe-Ni alloy possesses
the bcc structure at 230 GPa and 3500 K based
on the high pressure and temperature
in situ
X-ray diffraction using a laser-heated diamond
anvil cell.
100
50
14
16
18
20
22
24
Volume, Å
Fig. 8.18
Compression curves of NaCl-B2 phase based
on different Pt pressure scales (Holmes
et al
., 1989;
Dorogokupets & Oganov, 2007; Fei
et al
., 2007; Matsui
et al
., 2009). The pressure by Holmes
et al
. (1989)
overestimates the pressure compared to the other
pressure scales.
