Electron spin transition causing structure transformations of earth's interiors under high pressure
Abstract
To elucidate the correlation between structure transitions and spin state is one of the crucial problems for understanding the geophysical properties of earth interiors under high pressure. High-pressure studies of iron bearing spinels attract extensive attention in order to understand strong electronic correlation such as the charge transfer, electron hopping, electron high-low spin transition, Jahn-Teller distortion and charge disproponation in the lower mantle or subduction zone [1]. Experiment Structure transitions of Fe3-xSixO4, Fe3-xTixO4 Fe3-xCrxO4 spinel solid solution have been investigated at high pressure up to 60 GPa by single crystal and powder diffraction studies using synchrotron radiation with diamond anvil cell. X-ray emission experiment (XES) at high pressure proved the spin transition of Fe-Kβ from high spin (HS) to intermediate spin state (IS) or low spin state (LS). Mössbauer experiment and Raman spectra study have been also conducted for deformation analysis of Fe site and confirmation of the configuration change of Fe atoms. Jahn-Teller effect A cubic-to-tetragonal transition under pressure was induced by Jahn-Teller effect of IVFe2+ (3d6) in the tetrahedral site of Fe2TiO4 and FeCr2O4, providing the transformation from 43m (Td) to 42m (D2d). Tetragonal phase is formed by the degeneracy of e orbital of Fe2+ ion. Their c/a ratios are c/a<1 due to dx2-y2 orbital of the electronic tetrahedral configuration. However, Fe3O4 (I), Fe2SiO4 (N), do not have a tetragonal polymorph because of no IVFe2+ ion [2]. Spin transition HS-to-LS transition starts from 15.6GPa in Fe3O4, 19.6 GPa in Fe2TiO4, 17GPa in Fe2SiO4. The transition is more capable due to VIFe2+ in the octahedral site. The extremely shortened octahedral bonds result in a distortion of 8-fold cation site. This structure change is accelerated by HS-LS transition of Fe2+ in the 8-fold coordination site. Post spinel transition The transition to orthorhombic post-spinel structure with Cmcm has been confirmed in the whole compositional range of Fe3-xTixO4 and Fe3-xCrxO4 . There are M1 and M2 in the orthorhombic phase. Fe2+ and Ti4+ are disordered in the M2 site. At pressures above 53 GPa, Fe2TiO4 structure further transforms to Pmma. This structure change results in the order-disorder transition [2]. New structure of Fe2SiO4 The spin transition exerts an influence to Fe2SiO4 spinel structure and triggers two distinct curves of the lattice constant in the spinel phase. The reversible structure transition from cubic to pseudo-rhombohedral phase was observed at about 45 GPa. This transition is induced by the 20% shrinkage of ionic radius of VIFe2+at the low sin state. Laser heating experiment at 1500 K has confirmed the decomposition from the pseudo-rhombohedral phase to two oxides of FeO and SiO2. [1] T. Yamanaka High-Pressure research 28, 203-216 (2008) [2] T. Yamanaka, T. Mine, S. Asogawa and Y. Nakamoto Physical Review B 80, 134120 (2009)
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMMR43B2304Y
- Keywords:
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- 3620 MINERALOGY AND PETROLOGY / Mineral and crystal chemistry;
- 3924 MINERAL PHYSICS / High-pressure behavior;
- 3929 MINERAL PHYSICS / NMR;
- Mossbauer spectroscopy;
- and other magnetic techniques;
- 3954 MINERAL PHYSICS / X-ray;
- neutron;
- and electron spectroscopy and diffraction