Ab initio molecular dynamics study of CaSiO
3
perovskite at P-T conditions
of Earth’s lower mantle
Donat J. Adams* and Artem R. Oganov
Laboratory of Crystallography, Department of Materials, ETH Honggerberg, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland
Received 23 December 2005; revised manuscript received 16 February 2006; published 4 May 2006
First-principles molecular dynamics calculations were performed in order to investigate the structure and
properties of what is thought to be the third most abundant phase in the Earth’s lower mantle, CaSiO
3
perovskite. The commonly assumed cubic structure was found to be stable at high temperatures T
1000–2000 K and unstable at low temperatures at all pressures. For this structure we investigate the
thermal equation of state and the Grüneisen parameter. We predict that the ground state of CaSiO
3
perovskite
is tetragonal space group I4/ mcm. At room temperature an orthorhombic structure space group Imma is
possible, which explains puzzling experimental X-ray powder diffraction patterns. We consider the structure
relation between the Imma and the I4/ mcm structures and show that the Imma structure can be obtained by a
counterintuitive symmetry-lowering transition on increasing temperature.
DOI: 10.1103/PhysRevB.73.184106 PACS numbers: 61.50.Ks, 64.70.Kb, 66.10.Ed, 61.50.Ah
I. INTRODUCTION
CaSiO
3
perovskite is thought to comprise between 6 and
12 wt % of the lower half of the Earth’s transition zone and
lower mantle.
1,2
Its structure throughout the mantle P-T re-
gime is generally assumed to be cubic.
3–6
In early experi-
mental studies
5–7
CaSiO
3
perovskite appeared to have a cu-
bic structure up to 134 GPa at room temperature. Recently,
using higher-resolution experimental techniques, Shim et al.
showed that temperature-quenched CaSiO
3
perovskite had a
tetragonal symmetry from 20 to 46 GPa at room
temperature.
8
These findings were confirmed by Ono et al.
9
and Kurashina et al.
10
who did experiments in the range from
38 GPa to 106 GPa and 300 K to 2600 K, and 24 GPa to
75 GPa and 300 K to 2250 K, respectively. Both studies
agree that for the pure CaSiO
3
perovskite the phase transition
to the high-temperature phase at around 50 GPa takes place
between 500 K and 700 K with a Clapeyron slope of
180 MPa/K.
All-electron linearized-augmented plane wave LAPW-
local-density approximation LDA calculations indicate that
at low temperatures CaSiO
3
perovskite should have the
I4/ mcm symmetry.
11
According to other calculations
12,13
CaSiO
3
perovskite would have an orthorhombic Pbnm sym-
metry at low temperatures. Pseudopotential LDA
calculations
14,15
found the cubic Pm3m structure stable at
0 K, whereas more recent LDA pseudopotential
calculations
16
and all-electron projector-augmented wave
PAW generalized gradient approximation GGA
calculations
17
find the I4/ mcm structure to be thermody-
namically stable for CaSiO
3
perovskite at low temperatures.
However, the I4/ mcm structure is in conflict with experi-
ment. The lattice constants a and c from experiments for the
cubic subcell
18
give a c / a ratio of 0.993,
9
whereas computa-
tions find c / a = 1.014.
17
Shim et al. refined the structure in
the P4/ mmm c / a = 0.966, octahedra not tilted and the
I4/ mmm c / a 1,
0
+
+
space groups.
8
The P4/ mmm
structure can be ruled out, because in our calculations it does
not correspond to an energy minimum, while the I4/ mmm is
unlikely because it is higher in energy than the I4/ mcm by
3 meV/atom.
16
In this work using static energy minimization
and molecular dynamics MD calculations, we investigate
the ground state structure CaSiO
3
perovskite and—by con-
sidering temperature-driven phase transitions—address the
controversy between previous studies.
In addition, we explored the possibility of fast ionic con-
ductivity in CaSiO
3
perovskite. The mysterious mechanism
of the electrical conductivity in the lower mantle that is
needed to explain the modulation of the Earth’s magnetic
field is an open question.
19
Using classical MD simulations,
Matsui and Price predicted that MgSiO
3
perovskite—due to
diffusion of O
2-
ions—can become an ionic conductor at
high T when a cubic perovskite structure is formed.
20
Sub-
sequent ab initio MD simulations
21
did not confirm the exis-
tence of cubic MgSiO
3
perovskite at mantle conditions.
CaSiO
3
, on the contrary, does adopt the cubic perovskite
structure at all mantle conditions, which prompts the ques-
tion about its possible ionic conductivity due to fast ionic
diffusion.
II. METHODOLOGY
The Vienna Ab Initio Simulation Package VASP
22
has
been used for MD simulations and structural optimization,
based on the generalized gradient approximation GGA
23
and the all-electron projector-augmented wave PAW
method.
24–26
In all calculations we have used the following
PAW potentials for the atoms, all derived within the same
GGA functional:
23
core region cutoffs are 2.3 a.u. for Ca
core configuration 1s
2
2s
2
p
6
, 1.5 a.u. for silicon core con-
figuration 1s
2
2s
2
2p
6
and 1.52 a.u. for oxygen core configu-
ration 1s
2
. This gives ten, four, and six electrons for the
calcium, silicon, and oxygen atoms respectively, which have
been explicitly treated. It is very important to take into ac-
count so many electrons for Ca because semicore states in
this element 3s ,3p are known to play a significant role.
26
Partial core corrections were done for Ca and Si with partial
core radii 2.0 a.u. and 1.30 a.u. respectively. Plane wave ki-
PHYSICAL REVIEW B 73, 184106 2006
1098-0121/2006/7318/1841068 ©2006 The American Physical Society 184106-1