M

1
Intermediate spin Fe2 in lower mantle perovskite
C. McCammon, I. Kantor, O. Narygina, J.
Rouquette, L. Dubrovinsky Bayerisches
Geoinstitut, Universität Bayreuth, Germany U.
Ponkratz, I. Sergueev, M. Mezouar ESRF, Grenoble,
France V. Prakapenka APS, Chicago, USA
Second VLab Workshop University of
Minnesota August 5-10, 2007
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Evidence for Fe2 spin transition XES
Mg0.9Fe0.1SiO3 perovskite
Badro et al. (2004)
decrease of intensity means reduction of S
3
Evidence for Fe2 spin transition NFS
Mg0.9Fe0.1SiO3 perovskite
Fe2 high spin
Fe3 high spin
increase in QS with P
McCammon (1998)
Li et al. (2004)
Jackson et al. (2005)
4
DAC Mössbauer spectroscopy

• Re gasket with 100 µm hole
• 250 µm culet diamonds
• starting material Fe0.12Mg0.88SiO3 and
  Mg0.86Fe0.14Si0.98Al0.02O3
• 61 enriched in 57Fe
• synthesis in multianvil press and/or by laser
  heating (LH) in DAC
• 16 different loadings of DAC
• 119 spectra collected RT 0-89 GPa
• mostly LH between measurements
• collection time 1-2 days each

5
DAC nuclear forward scattering
ESRF ID18
PRL system
image plate
Kirkpatrick-Baez multilayer mirror
DAC
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DAC nuclear forward scattering cont.

• same DAC sample as for Mössbauer experiments
• beam size 4 µm x 20 µm
• 4 bunch mode 700 ns spacing between bunches
• 15 spectra collected RT 7-110 GPa
• collection time 1-2 hours each
• high-resolution XRD collected for same DAC at
  numerous P

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P evolution of Mössbauer spectra
Fe0.12Mg0.88SiO3
high QS with narrow energy width
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P evolution of NFS spectra
Fe0.12Mg0.88SiO3
44 GPa
61 GPa
110 GPa
high QS with narrow energy width
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P evolution of hyperfine parameters
Fe0.12Mg0.88SiO3
10
High-pressure XRD experiments
? All reflections consistent with Pbnm perovskite
at 110 GPa
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Electronic structure of VIIIFe2
12
Other intermediate spin d6 compounds
e.g., Co3 in MCoO3 perovskites
thermal expansion
thermal conductivity
LaCoO3
LaCoO3
IS?HS
LS?IS
Yan et al. 2004
Murata et al. 1999
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Octahedral tilting in (Mg,Fe)SiO3 perovskite
High-resolution XRD data collected at APS on
IDD-13
Kudoh et al. 1987 Ross Hazen 1990 Funamori et
al. 1996
Mitchell 2002
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Spin number variation with pressure
S (2 Ahigh-spin Fe2) (1
Aintermediate-spin Fe2) (5/2 Ahigh-spin
Fe3)/SA
Fe0.12Mg0.88SiO3
15
Heated DAC Mössbauer spectroscopy
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Heated DAC nuclear forward scattering
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Pressure-temperature paths
Fe0.12Mg0.88SiO3
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Spectra at high P,T
Fe0.12Mg0.88SiO3
Mössbauer spectroscopy
55 GPa 800 K
Fe2 intermediate spin
? Intermediate spin stable at higher T
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Effect of Al3 and Fe3 on spin transition
Mg0.86Fe0.14Si0.98Al0.02O3

• 5 sample loadings
• 42 Mössbauer spectra
• P 0 to 70 GPa
• T 300 to 700 K

? Fe2 spin transition also occurs in the
presence of trivalent cations
20
Spin number variation with P,T
Fe0.12Mg0.88SiO3
Sturhahn et al. 2005
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Summary

• Fe2 in (Mg,Fe)(Si,Al)O3 perovskite is
  predominantly intermediate spin throughout entire
  lower mantle
• this conclusion is consistent with all existing
  data
• spin transition in Fe3 not required to explain
  data, but cannot be ruled out
• spin transition might be coupled to lattice
  distortion. Variations in sample environment
  (e.g., deviatoric stress) can change spin state
  stability (already calculated by Li et al. 2005)
• challenge to computational community to reproduce
  experimental results on spin state
• return spotlight to perovskite phase to assess
  implications of spin state change for mantle
  properties and dynamics