Taku Tsuchiya, Jun Tsuchiya, Koichiro Umemoto, and Renata M. Wentzcovitch

1. Post-perovskite Transition in MgSiO3 Post-perovskite Transition in MgSiO3 Taku Tsuchiya, Jun Tsuchiya, Koichiro Umemoto, and Renata M. Wentzcovitch Dept. of Chemical Engineering and Materials Science , Minnesota Supercomputing Institute UNIVERSITY OF MINNESOTA

2. MgSiO3 Perovskite ----- Most abundant constituent in the Earth’s lower mantle ----- Orthorhombic distorted perovskite structure (Pbnm, Z=4) ----- Its stability is important for understanding deep mantle (D” layer)

3. Drastic change in X-ray diffraction pattern around 125 GPa and 2500 K UNKNOWN PHASE Pbnm Perovskite (M. Murakami and K. Hirose, private communication)

4. Method • --- Density Functional Theory • (Hohenberg and Kohn, 1964) • --- Local Density Approximation • (Ceperley and Alder, 1985) • --- Plane wave basis – pseudopotential • (Troullier and Martins, 1991) • --- Variable Cell Shape Molecular Dynamics • for structural search (Wentzcovitch, 1991)

5. Perovskite SiO4 chain SiO3 layer SiO3 Mg SiO3 MgSiO3 Mg SiO3 Ab initio exploration of post-perovskite phase in MgSiO3 - Reasonable polyhedra type and connectivity under ultra high pressure -

6. Pt b c a Crystal structure of post-perovskite Lattice system: Bace-centered orthorhombic Space group: Cmcm Formula unit [Z]:4 (4) Lattice parameters [Å] a: 2.462 (4.286) [120 GPa] b: 8.053 (4.575) c: 6.108 (6.286) Volume [120 GPa] [Å3]: 121.1 (123.3) ( )…perovskite

7. A structure has lower energy than Pbnm perovskite under high pressure! Pt = 98 GPa

8. θ b’ b Si-O bonds break Share-edges form c’ Post-perovskite a’ a c Structural relation between Pv and Post-pv Perovskite Deformation of perovskite under shear strain ε6

9. Thermodynamics with QHA --- VDoS and F(T,V) --- Other thermodynamics quantities --- Density Functional Perturbation Theory for calculating phonon frequencies (Gianozzi et al., 1991)

10. Bulk modulus [GPa] [300 K, 0 GPa] B0 222 (248) dB/dP 4.2 (3.9) Ambient volume [cm3/mol] V0 24.662 (24.704) Grüneisen parameter γ0 1.6 (1.5) (∂lnγ/∂lnV)T -0.25+2.13(V/V0) (-0.32+0.86(V/V0)) Debye temperature [K] Θ0 1100 (1114) Thermodynamic properties ( )…Pv

11. 7.5 MPa/K Mantle adiabat error ~5 GPa Valley bottom Hill top ~8 GPa ~250 km High-PT phase diagram Core-mantle boundary D” layer

12. Elasticity of MgSiO3Post-perovskite Elasticity of MgSiO3Post-perovskite

13. b c Post-perovskite :layered structure large compressibility along b axis Large elastic anisotropy Si Large anisotropy as well as large heterogeneity have been observed in D” region Mg a

14. Thermoelastic constant tensor CijS(T,P) kl equilibrium structure re-optimize

15. b b a c a c Elastic Constants b a c a

16. Aggregate Elastic Moduli Bppv≈ Bpv Gppv > Gpv

17. Single crystal azimuthal anisotropy P-azimuthal: S-azimuthal: Wentzcovitch et al. (1998) Tsuchiya, Tsuchiya, Umemoto, Wentzcovitch, GRL (2004)

18. Shear wave splitting in transversely isotropic aggregates [100] [010] [001] Vertical direction// Vsh Vsv Horizontal plane

19. < Seismic velocities Longitudinal Shear Bulk Post-pv transition should produce larger anomaly in VS than in VP

20. Summary • Post-pv phase has almost the same B as pv but larger G • Across the transition at 125 GPa (2750 K) • Δρ ~ 1.5% ΔVS ~ 1.5% > ΔVP ~ 1% ΔVΦ ~ -0.7% • Post-pv has larger dV/dT’s • Lateral ΔT → ΔVppv> ΔVpv • Post-pv is very anisotropic at D” conditions • Perfect transversely isotropic aggregates • -10% < AST < 15%

21. Acknowledgements • Thanks to Murakami and Hirose for early communication of their X-ray diffraction • NSF COMPRES, NSF/EAR