Determination of the hyperfine parameters of iron in aluminous (Mg,Fe)SiO 3 perovskite

Transcription

1 Determination of the hyperfine parameters of iron in aluminous (Mg,Fe)SiO 3 perovskite Jennifer M. Jackson Seismological Laboratory, Geological & Planetary Sciences California Institute of Technology VLab Workshop 2007 University of Minnesota

2 Collaborators Wolfgang Sturhahn and Michael Lerche Advanced Photon Source, Argonne National Laboratory, IL, USA Jie Li Department of Geology, University of Illinois at Urbana-Champaign, IL, USA Mineral Physics Group at Caltech: June Wicks Undergraduate student in Chemistry/Geochemistry Aaron Wolf Graduate student in Planetary Sciences Kirill Zhuravlev Postdoctoral Scholar in Geophysics National Science Foundation Department of Energy COMPRES California Institute of Technology

3 Earth s Interior

4 Minerals in Earth s mantle Pressure, GPa Post-Perovskite (Mg,Fe)O-Ferropericlase (Mg 1-x Fe x )(Si,Al)O 3 perovskite 0.05 x 0.15 Fe 2+ and/or Fe 3+

5 Motivation for electronic structure measurements of Iron in magnesium-silicate perovskite Iron-bearing silicate perovskite occupies a majority fraction of Earth s lower mantle Electronic charge states of iron (e.g., Fe 2+ and Fe 3+ ) and spin state may affect [e.g., Shannon & Prewitt 1969; Gaffney & Anderson 1973]: Charge balance and equilibrium defect concentration Presence of metallic iron Rheology, solubility of volatiles, partitioning of major and trace elements, and transport properties Elasticity At high-pressure, the electronic charge states of iron in silicate perovskite have been measured by: XES: Badro et al (Mg,Fe)SiO 3 Li et al (Mg,Fe)SiO 3 & (Mg,Fe)(Si,Al)O 3 SMS: Jackson et al (Mg,Fe)SiO 3 Li et al (Mg,Fe)(Si,Al)O 3 calculated by: Li. L. et al. 2005; Oganov et al (Mg,Fe)(Si,Al)O 3

6 Methods to determine and/or detect changes in electronic structure of iron in materials Mössbauer spectroscopy Conventional Mössbauer spectroscopy Synchrotron Mössbauer spectroscopy Magnetic susceptibility/resonance X-ray absorption (XAFS, XANES) and emission spectroscopy Heat capacity (enthalpy, entropy) Electrical conductivity EELS, ELNES (Fe L 2,3 edges) Theoretical investigations Vibrational spectroscopy (phonons) X-ray diffraction Many are complimentary techniques (Gütlich and Goodwin 2004)

7 Synchrotron Mössbauer spectroscopy at high-pressures and high-temperatures Advanced Photon Source (APS) Argonne National Laboratory, Chicago, IL

8 Excitation of the 57 Fe nuclear resonance: fixed, isolated nucleus nucleus & electronic interaction or external fields e,3/2> SMS e,1/2> e,-1/2> e,-3/2> g,-1/2> S(E) µev g,1/2> kev E nucleus & simple lattice excitation... NRIXS e> 2> e> 1> e> 0> S(E) Mössbauer absorption phonon side band 10 mev... g> 2> g> 1> g> 0> kev E

9 The time discrimination trick: The excited nucleus decays incoherently with its natural life time τ. log(intensity) τ = /Γ nonresonant scattering events (100 MHz) 141 ns for 57 Fe measured events (100 Hz) detector noise (0.01 Hz) time

10 Experimental setup for synchrotron Mössbauer spectroscopy at sector 3-ID of the APS x-ray pulses must be sufficiently separated in time (153 ns) detectors must have good time resolution and excellent dynamic range Argonne, IL monochromatization to mev-level required energy is stabilized at the 57 Fe nuclear transition time spectrum oscillations: 2h/

11 Conventional Mössbauer versus synchrotron Mössbauer spectroscopy: Advantages of SMS intensity and collimation control of polarization microfocusing Lower pressure gradients ph/s/ev ph/s/ev/sr ph/s/ev/mm 2 Potential difficulties data evaluation W. Sturhahn, J.Phys.: Condens.Matt. 16 (2004)

12 Synchrotron Mössbauer spectroscopy in the DAC with laser heating Laser φ30µm φ10µm Laser X-ray in mev bandwidth, focused Be-mirror (transparent for x rays) SMS signal out (Zhao et al. 2004; Lin et al. 2004) (Mg 0.88 Fe 0.09 )(Si 0.94 Al 0.10 )O 3 perovskite µm (Li et al. 2004)

13 Time spectra for (Mg0.88Fe0.09)(Si0.94Al0.10)O3 perovskite laser annealed 65 GPa 57 GPa 50 GPa 40 GPa 35 GPa 30 GPa

14 Time spectra for (Mg 0.88 Fe 0.09 )(Si 0.94 Al 0.10 )O 3 perovskite P = 1 GPa P = 35 GPa with stainless steel after laser annealing without stainless steel before laser annealing

15 Origin of the time oscillations for silicate perovskite IS quadrupole splittings ( ) and isomer shifts (IS) identify Fe valences quadrupole splitting ( ): A splitting of the excited nuclear state caused by an electric field gradient (q val & q lat ) isomer shift (IS): A shift of the nuclear states caused by the electron density in the nuclear volume Fe 3+ Fe Fe Spectra are fit using CONUSS (Sturhahn 2000)

16 Weight fraction of the iron sites in (Mg0.88Fe0.09)(Si0.94Al0.10)O3 perovskite with laser annealing at T = 1400 K for P > 30 GPa Fe3+ open symbols: Jackson et al. (2005) Fe12+ Fe22+

17 Quadrupole splitting of the iron sites in (Mg0.88Fe0.09)(Si0.94Al0.10)O3 perovskite with laser annealing at T = 1400 K for P > 30 GPa Fe12+ open symbols: Jackson et al. (2005) Fe22+ Fe3+

18 Isomer shift of the Fe3+ site relative to the Fe2+ site in (Mg0.88Fe0.09)(Si0.94Al0.10)O3 perovskite McCammon (1997 ) Li et al. (2006) Al-bearing -no annealing- This Study Al-bearing -laser annealed- Jackson et al. (2005) Al-free -no annealing-

19 Conclusions Synchrotron Mössbauer measurements on (Mg 0.88 Fe 0.09 )(Si 0.94 Al 0.10 )O 3 perovskite to 65 GPa with laser annealing at each pressure show that: Hyperfine parameters from starting material are in agreement with previous conventional Mössbauer measurements (McCammon 1997) Electronic charge states of iron (Fe 2+ and Fe 3+ ) show a gradual increase in quadrupole splitting at lower pressures, then level off at P > 35 GPa increased distortion of sites and changes in effective charges on the oxygen atoms (e.g., Bancroft, Maddock, & Burns 1967) The isomer shift decreases slightly up to 65 GPa No obvious indications of sharp changes in the spin state(s) of iron Theoretical calculations of electric field gradients and isomer shifts of iron in simple (and complex) structures under ambient and deep Earth conditions

20 Extra slides Simplified configurations of iron s 3d electrons eg t2g t2g low-spin high-spin eg (e.g. Burns 1993)