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Spin transition in ferrous iron in MgSiO 3 perovskite under pressure

Spin transition in ferrous iron in MgSiO 3 perovskite under pressure. Koichiro Umemoto. Minnesota Supercomputing Institute and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. Spin transition of Fe 2+

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Spin transition in ferrous iron in MgSiO 3 perovskite under pressure

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  1. Spin transition in ferrous iron in MgSiO3 perovskite under pressure Koichiro Umemoto Minnesota Supercomputing Institute and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA • Spin transition of Fe2+ Displacement of low-spin Fe Change of electronic structure • Transition pressure dependence on: Fe concentration Fe configuration • Gradual spin transition of Fe2+

  2. Collabolators • Renata Wentzcovitch (University of Minnesota) • Yonggang Yu (University of Minnesota) • Ryan Requist (Friedrich Alexandre University, Germany) • Acknowledgments • Supported by NSF/EAR-0135533, EAR-0230319, ITR-0426757 (VLab) • Computations were performed at Minnesota Supercomputing Institute • and Indiana University‘s BigRed system

  3. Spin transition of Fe in MgSiO3 pv Experiments • J. Badro et al., Science 305, 383 (2004) • (Fe0.1Mg0.9)SiO3 • Two distinct spin transitions at 70 GPa (HS-mixed S) and 120 GPa (mixed S-LS) • J. Li et al., PNAS 101, 14027 (2004) • (Fe0.09Mg0.92)SiO3 • Gradual spin transition at wide pressure range up to 100 GPa • intermediate spin states for Fe2+ at A site, Fe3+ at A and B sites • J. Jackson et al., Am. Mineral. 90, 199 (2005) • (Fe0.1Mg0.9)SiO3 • Continuous spin transition in Fe3+ ends around 70 GPa.

  4. Spin transition of Fe in MgSiO3 pv First-principles calculations • R. E. Cohen et al., Science 275, 654 (1997) • FeSiO3 : HS-LS transition in Fe2+ at 1 TPa • Li Li et al., Geophys. Res. Lett. 32, L17307 (2005) • (FeMg15)(AlSi15)O48 : HS-LS transition in Fe3+ at 97-126 GPa • F. Zhang and A. R. Oganov, Earth Planet. Sci. Lett. 249, 436 (2006) • (FeMg31)(FeSi31)O96 : HS-LS transition in Fe3+ at 76 GPa • S. Stackhouse et al., Earth Planet. Sci. Lett. 253, 282 (2007) • (Mg0.9375Fe0.0625)SiO3, (Mg0.8750Fe0.1250)SiO3,(Mg0.9375Fe0.0625)(Si0.9375Fe0.0625)O3 : HS-LS transitions in Fe2+ and Fe3+ at 130-145 GPa and 60-160 GPa, respectively. According to these first-principles studies, Fe3+ is responsible for spin transition in the lower mantle pressure range. This study will investigate the spin transition in Fe2+ with effects of Fe concentration and structural and magnetic ordering.

  5. Method • LDA (Ceperlay-Alder) and GGA (Perdew-Burke-Ernzerhof) • Vanderbilt ultrasoft pseudopotentials for Fe, Si, and O Von-Barth & Car pseudopotential for Mg • Plane wave cut-off energy : 40 Ry • Variable Cell Shape Molecular Dynamics for structural search • Supercell (up to 160 atoms) • Quantum-ESPRESSO package (www.pwscf.org)

  6. Effect of Fe concentration Atomic configurations of Fe and Mg The largest distance between Fe atoms in the smallest unit cell for each Fe concentration. Fe 6.25% 12.5% 25% 80 atoms 40 atoms 20 atoms 50% 75% 100% 20 atoms 20 atoms 20 atoms High-spin state: Ferromagnetic : Fe : Mg

  7. Calculated enthalpies w.r.t. the HS state LDA HS: 4mB/Fe IS: 2mB/Fe LS: 0mB/Fe DH(Ry/Fe) Spin transition • Spin transition from HS (4mB/Fe) to LS (0mB/Fe) • No transition to Intermediate spin state (2mB/Fe)

  8. HS(FM)-LS transition in Fe2+ Transition Pressure (GPa) Fe concentration (%)

  9. Displacement of A-site Featom by spin transition Fe 12.5%, 120 GPa, LDA HS LS 2.186 Å 1.830 1.878 1.765 2.060 1.903 1.869 1.781 1.918 1.855 1.943 1.878 2.060 2.416 2.186 2.561 : Fe : O Number: Fe-O bond length (Å)

  10. Displacement of A-site Featom by spin transition Fe 12.5%, 120 GPa, LDA HS LS Bicapped trigonal prism (8-coordinated Fe) Distorted octahedron (6-coordinated Fe) : Fe : O

  11. Electronic DOS at 120 GPa (LDA) HS LS xz,xy,yz x2-y2, z2 z2,yz,xy xz x2-y2 DOS (states/eV/spin/Fe) xy xz z2,yz x2-y2 LS: t2g and eg-derived state, wider gap (blue shift)

  12. : Fe : Mg Effect of ordering Fe 50% (Fe0.5Mg0.5SiO3) • Atomic ordering 20 atoms 20 atoms 20 atoms 40 atoms 40 atoms • Magnetic ordering for HS state: Ferro- and Antiferro-magnetic

  13. Spin ordering for AFM-HS states H: AFM1<AFM2 AFM1<AFM2 AFM2<AFM3<AFM1 AFM1<AFM2 AFM2~<AFM1

  14. FM Fe0.5Mg0.5SiO3 (LDA) DH (Ry/Fe0.5Mg0.5SiO3)

  15. AFM Fe0.5Mg0.5SiO3 (LDA) Conf 2 Conf 3 Conf 5 DH (Ry/Fe0.5Mg0.5SiO3) Conf 4 Conf 1

  16. LS Fe0.5Mg0.5SiO3 (LDA) DH (Ry/Fe0.5Mg0.5SiO3)

  17. HS-LS transition in Fe0.5Mg0.5SiO3 (LDA) Conf4_FM Conf3_FM Conf4_AFM Conf1_AFM DH (Ry/Fe0.5Mg0.5SiO3) Conf. 4 Conf4_LS

  18. Atomic structure of Configuration 4 • Fe atoms are placed on the (110) plane. • The same kind of cations prefer to be in the same column.

  19. Fe/Mg configurations in Fe0.125Mg0.875SiO3 (12.5% Fe) 80 atoms/unit cell 2 Fe + 14 Mg

  20. 80 atoms/unit cell 2 Fe + 14 Mg 80 atoms/unit cell 2 Fe + 14 Mg 160 atoms/unit cell 4 Fe + 28 Mg similar to Conf5 in Fe50% similar to Conf4 in Fe50% similar to Conf3 in Fe50%

  21. Enthalpies of Fe0.125Mg0.875SiO3 at 0 GPa (LDA) ~0.03 Ry/Fe ~0.01Ry/Fe

  22. Enthalpies of Fe0.125Mg0.875SiO3 at 150 GPa (LDA) ~0.02Ry/Fe ~0.04Ry/Fe Conf 10 is the lowest-enthalpy configuration for AFM-HS and LS.

  23. HS-LS transition in Fe0.125Mg0.875SiO3 (LDA) FM10 FM11 DH (Ry/Fe0.125Mg0.875SiO3) AFM10 LS10 56 GPa P (GPa)

  24. HS-LS transition in Fe2+ With Fe-(110) plane configurations Transition Pressure (GPa) Fe concentration (%)

  25. HS-LS transition in Fe2+ With separated-iron configurations Highest transition pressure Transition Pressure (GPa) With Fe-plane configurations Lowest transition pressure Fe concentration (%)

  26. Configuration vs HS-LS transition pressure in Fe0.125Mg0.875SiO3 (LDA) With separated-iron configurations With Fe-plane configurations

  27. At high temperature… • All configurations with different transition pressures should appear locally. • Fe planes (conf. 10) with different sizes should exist locally. The larger (smaller) size of Fe plane gives the lower (higher) spin transition pressure. Possibility of gradual spin transition in Fe2+ at the A site

  28. Electronic DOS of Fe 12.5% at 150 GPa (LDA) With separated-iron configuration With Fe-plane configuration FM AFM LS LS

  29. Summary • FexMg1-xSiO3 shows tendency to display atomic and magnetic (AFM) order at 0 K. This tendency decreases with temperature. • Spin transition in Fe2+ occurs at 0 K in the pressure range found experimentally, i.e., at lower mantle pressures. At high temperature, this transition should be broad and pass through mixed spins states. • In highly-ordered structures, where Fe‘s are close to each other and they are on the (110) plane, the spin transition pressure is the lowest. This is consistent with the transition pressure dependence on Fe concentration. • In the LS state, electronic structure of Fe at the A site becomes similar to that of Fe at the octahedron (t2g and eg-derived states) with a large gap.

  30. Spin transition of Fe in MgSiO3 pv Experiments • J. Badro et al., Science 305, 383 (2004) • (Fe0.1Mg0.9)SiO3 • Two distinct spin transitions at 70 GPa (HS-mixed S) and 120 GPa (mixed S-LS) • J. Li et al., PNAS 101, 14027 (2004) • (Fe0.09Mg0.92)SiO3 • Spin transition at wide pressure range up to 100 GPa • intermediate spin states for Fe2+ at A site, Fe3+ at A and B sites • J. Jackson et al., Am. Mineral. 90, 199 (2005) • (Fe0.1Mg0.9)SiO3 • Continuous spin transition in Fe3+ ends around 70 GPa. Calculations • R. E. Cohen et al., Science 275, 654 (1997) • FeSiO3 : HS-LS transition in Fe2+ at 1 TPa • Li Li et al., Geophys. Res. Lett. 32, L17307 (2005) • (FeMg15)(AlSi15)O48 : HS-LS transition in Fe3+ at 97-126 GPa • F. Zhang and A. R. Oganov, Earth Planet. Sci. Lett. 249, 436 (2006) • (FeMg31)(FeSi31)O96 : HS-LS transition in Fe3+ at 76 GPa • S. Stackhouse et al., Earth Planet. Sci. Lett. 253, 282 (2007) • (Mg0.9375Fe0.0625)SiO3, (Mg0.8750Fe0.1250)SiO3,(Mg0.9375Fe0.0625)(Si0.9375Fe0.0625)O3 : HS-LS transitions in Fe2+ and Fe3+ at 130-145 GPa and 60-160 GPa, respectively. This study will investigate the spin transition in Fe2+with effects of Fe concentration and structural and magnetic ordering.

  31. DH (Ry/f.u.) P (GPa) LDA-CA, 3s&3p:valence

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