Experimental Visualization of Electron Density Across the High-Pressure Fe(II) Spin Transition
Abstract
We use high-resolution single-crystal x-ray diffraction to experimentally determine the 3-dimensional electron density distribution in (Mg0.47,Fe0.53)O as Fe(II) undergoes a pressure-induced transition from high- to low-spin at 30-50 GPa and room temperature. The density and elasticity of magnesiowüstite - one of the dominant minerals of Earth's lower mantle - are significantly affected by the spin transition, as the ionic volume of Fe(II) collapses by about forty percent. Our results are thus relevant to interpreting the geophysical properties of Earth's deep interior.
We apply two methods for inverting the diffraction measurements, after applying a Debye model to correct for atomic thermal vibrations: i) direct Fourier back-transformation of measured structure factors; and ii) a maximum entropy model that constrains the electron density to be positive. All data were collected using synchrotron radiation and a diamond-anvil cell at room temperature. The Fe(II) spin transition is typically thought of in terms of the iron 3d (eg) electrons that overlap neighboring oxygen 2p orbitals becoming energetically destabilized relative to the iron 3d (t2g) electrons that point between first-neighbor oxygens. In our experiments, we find that changes in electron density are more evident around the O atoms than the Fe/Mg site, however, and our observations upon compression are reversible on decompression. These changes include both an apparent increase in the oxygen core-electron radius, and a change in shape of the bonding electron density. Electron density is among the properties most immediately derived from first-principles quantum mechanical calculations, so our experiments may help validate and extend current theory.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR21A..01D
- Keywords:
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- 3919 Equations of state;
- MINERAL PHYSICS;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3944 Shock wave experiments;
- MINERAL PHYSICS;
- 3994 Instruments and techniques;
- MINERAL PHYSICS