| Research Abstract |
(Mg, Fe, Al)(Si, Al) O3 perovskite and (Mg, Fe) O magnesio-wustite are believe to major components of the Earth's lower mantle. It is well known that the Earth's lower mantle exhibits high electric conductivity (1-10 S/m). The knowledge of the local structure is of primary importance for the understanding of the complicated physical properties. An anharmonic effective pair potential can be investigated using the EXAFS technique. The anharmonicity in potentials is directory correlated with physical properties such as the thermal expansion, thermal conductivity and ionic conductivity. We have precisely reinvestigated the conductivity and conduction mechanism of single crystals of such lower mantle minerals and analogues.
Single crystals of the Earth's internal materials were grown by various methods under high-pressure and high -temperature (e.g. 25 GPa, 2000 K and 0.1 MPa, 3000 K). Conductivity was measured by a complex impedance method under various conditions and atmospheres. Precise c
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rystal structure and thermal properties for many materials in the Earth's mantle such as majorite garnet, magnesio -wustite, MgSiO3 perovskite solid -solution and stishovite have been revealed by EXAFS spectroscopy and diffraction method.
The probability of finding mobile ions at potential barrier was estimated under high temperature and high pressure by EXAFS spectroscopy. The probability of finding mobile ions at potential barrier is larger than several percent in superionic conducting minerals at the highest temperature. Conductivities of analogues of MgSiO3 perovskite solid-solution were measured and showing that the perovskite type compound became solid electrolytes with ionic conduction near their melting points. It was clarified that activation energies for intrinsic conduction of perovskite type compounds are about 2.0 eV.The value indicates the summation of the vacancy formation energy and the vacancy migration energy, where the vacancy formation energy is about 1.0 eV.In this study, it was revealed that the high conductivity in Earth's lower mantle cannot explained by the intrinsic conduction mechanism of perovskite crystals. Possible mechanism in the lower mantle is considered as extrinsic ionic conduction of (Mg, Fe, Al)(Si, Al) O3 perovskite with eutectic reactions or as a conduction of (Mg, Fe) O magnesio-wustite by a large polaron. Less
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