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Thermoelastic properties of ferropericlase
Thermoelastic properties of ferropericlase
R. WentzcovitchDept. of Chemical Engineering and Materials Science,
Minnesota Supercomputing Institute
J. F. Justo, C. da Silva, Z. WuDept. of Chemical Engineering and Materials Science
T. Tsuchiya Ehime University, Japan
OutlineOutline
Ab initio calculations of Fe in (Mg1-
xFex)O
Thermodynamics of the spin transition
Thermoelastic properties of (Mg1-xFex)O
Geophysical implications
Motivation: Earth’s MineralsMotivation: Earth’s Minerals
(Mg1-yFey)SiO3
perovskite
(Mg1-xFex)O ferropericlase
+
Lower Mantle: Ferrosilicate Perovskite + ferropericlase
Low iron concentration (< 0.20) High-temperatures and high pressures Elasticity
First Principles CalculationsFirst Principles Calculations
Density Functional Theory (LDA+U) (Cococcioni and de Gironcoli, PRB, 2005)
Plane waves + Pseudopotential (Troullier-Martins, PRB, 1991, Vanderbilt,
PRB, 1990)
Structural relaxation in all configurations
Density Functional Perturbation Theory (Baroni et al., RMP, 2001)
14 16 18 20
4
5
6
7
V (Å 3/molec)
Hubbard U (eV)
XFe=3.125% XFe=12.5% XFe=18.75%
FeO (Cococcioni, 2005)
Optimized Hubbard U
HS
LS
PT = 32±3 GPaNo systematic dependence on XFe
(Tsuchiya et al., PRL, 2006)
0 50 100
-20
0
20
40
P (GPa)
Δ=H H
LS
-HHS
( / )kJ mol
3.125% 12.5% 18.75%
First Principles Calculations: HS-LS First Principles Calculations: HS-LS transitiontransition
0 50 100
-20
-10
0
10
20
30
P (GPa)
Δ ( / )H kJ mol
XFe=0.03125 XFe=0.125 XFe=0.1875
0 50 100
-20
-10
0
10
20
30
P (GPa)
Δ ( / )H kJ mol
n=0n=1/3n=2/3n=1A
B
C
0 20 40 60 80 1008
9
10
11
12
(V cm
3/ )mol
P (GPa)
n=0 n=1/3 n=2/3 n=1
Experimental: + (J.F.Lin et al., Nature, 2005)
17% Fe and room temperature
Equation of State (MgEquation of State (Mg0.810.81FeFe0.190.19)O)O(Tsuchiya et al., PRL,
2006)
∆ V ~-4%
⎥⎦
⎤⎢⎣
⎡
+=
HSLS
LS
nn
nn
Temperature Effects: n(P,T)Temperature Effects: n(P,T)
1) Magnetic entropy
2) HS/LS configuration entropy 3) Fe/Mg configurational entropy is insensitive to
spin state
4) Vibrational energy and entropy are insensitive to spin state
5) Minimization of G(P,T,n) with respect to n:
(Tsuchiya et al., PRL, 2006)
1( , )
1 (2 1)exp HS LS
Fe B
n P TH
m SX k T
−
=⎡ ⎤Δ
+ + ⎢ ⎥⎣ ⎦
Exp
LS fraction n(P,T)LS fraction n(P,T)
XFe=18.75%
Geotherm (Boehler, RG, 2000)
(Tsuchiya et al., PRL, 2006)
Volume of the mixed spin state Volume of the mixed spin state V(P,T,n)V(P,T,n) Mixed spin configuration was described by
the Vegard’s rule:
where n = low spin fraction
Iron-iron interaction is not significant for xFe=18.75%
( , , ) ( , ) (1 ) ( , )LS HSV P T n nV P T n V P T= + −
High temperature elasticityHigh temperature elasticity
( , , ) ( , ) (1 ) ( , )LS HSV P T n nV P T n V P T= + −
Compressibility:
1( ) ( ) (1 ) ( )
9LS HS
ij ij LS ij HS ij LS HST
nS n V n nS V n S V V V
Pα ∂
= + − − −∂
Compliances:
11 12 1α α= = 44 0α =
THSLS
HS
HS
LS
LS
P
nVV
K
Vn
K
Vn
nK
nV
∂∂
−−−+= )()1()()(
∑∑ ⎟⎟⎠
⎞⎜⎜⎝
⎛⎥⎦
⎤⎢⎣
⎡−−++=
qj B
qjB
qj
qj
Tk
VTk
VVUTVF
)(exp1ln
2
)()(),(
ωω hh
PVTSFG +−=TV
FP ⎥⎦
⎤⎢⎣⎡∂∂
−=VT
FS ⎥⎦
⎤⎢⎣
⎡∂∂
−=
IMPORTANT: crystal structure and phonon frequencies depend on volume alone!!
Static +vibrational free energyStatic +vibrational free energy
VDoS and F(T,V) within the quasiharmonic approximation
equilibrium structure
kl
re-optimize
Pji
Tij
GTPc
⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂∂
=
2
),(
V
jiTij
Sij C
VTTPcTPc
λλ+= ),(),(
Tii
S
λ
∂∂
=
(Wentzcovitch et al., PRL, 2004)
Thermoelastic Constant Tensor Thermoelastic Constant Tensor CCijij
purepure(P,T)(P,T)
Eulerian Strain
““Approximate” Virtual Crystal Approximate” Virtual Crystal modelmodel
MgO
(Mg0.8125Fe0.1875)O
Replace Mg mass by the average cation mass of the alloy
ω(V) = ωLS(V) = ωHS(V)
Compute CijLS(P,T) and Cij
HS(P,T)
SLS(P,T) = [CLS(P,T)]-1 and SHS(P,T) =[CHS(P,T)]-1
Calculate
Compute V(P,T,n) and Sij(P,T,n)
C(P,T,n) = [S(P,T,n)]-1
Compute K(P,T,n) and G(P,T,n)
Procedure to obtain CProcedure to obtain Cijij(P,T,n):(P,T,n):
⎥⎦
⎤⎢⎣
⎡Δ++=
+−
TkXG
Sm
TPn
BFe
vibstLSHSexp)12(1
1),(
+ Experiments (Lin et al., Nature, 2005) (xFe=17%, RT)
xFe= 18.75%
Volume V(P,T,n(P,T)) for xVolume V(P,T,n(P,T)) for xFeFe= 18.75%= 18.75%
+ 300K (exp.)
Experiments: ○ (Lin et al., GRL, 2006) xFe = 25% (NRIXS, RT)
● (Lin et al., Nature, 2005) xFe= 17% (X-ray diffraction, RT)
□ (Kung et al., EPSL, 2002) xFe = 17% (RUS, RT)
Isotropic Elastic ConstantsIsotropic Elastic Constants
Experiments: ○ (Lin et al., GRL, 2006) xFe = 25% (NRIXS, RT) □ (Kung et al., EPSL, 2002) xFe = 17% (RUS, RT)
3
4P
K GV
ρ
+=
S
GV
ρ=
xFe= 18.75%
Sound Wave VelocitiesSound Wave Velocities
Elasticity Along Mantle GeothermElasticity Along Mantle Geotherm
Geotherm (Boehler, Rev. Geophys. 2000)
6%
-15%
1150 km 1580 km
Geotherm (Boehler, GRL,2000)
Wave Velocities Along Mantle Wave Velocities Along Mantle GeothermGeotherm
6%
-9%
-15%
3%
1150 km 1580 km
Seismic Parameters (Mantle Geotherm)Seismic Parameters (Mantle Geotherm)
/
ln
lnSS P
VR
Vφ
φ
∂=∂
Geotherm (Boehler, RG, 2000)
(Kara
(Karato, Karki, JGR, 2001)
Geotherm (Boehler, GRL,2000)
Wave Velocities Along Mantle Wave Velocities Along Mantle GeothermGeotherm
6%
-9%
-15%
3%
1150 km 1580 km
SummarySummary
HS-LS transition in (Mg1-xFex)O is well reproduced theoretically
There is a strong softening in the bulk modulus across the spin transition. This effect broadens and decreases with temperature
Along a lower mantle geotherm this softening is more pronounced between 45-70 GPa, i.e., 1150-1580 km
The shear modulus increases monotonically in the same region
Transition can produce negative values of R/s in the upper part of the lower mantle
The softening will likely occur also in ferrosilicate perovskite
The Si/(Mg+Fe) ratio in the lower mantle should increase from pyrolitic values because of the spin transtions in ferropericlase and ferrosilicate perovskite