Spin transition in germanate perovskite and post-perovskite at high pressure

Princeton University, Princeton NJ

Investigators

Abstract

The geological activity at Earth's surface can profoundly affect human societies. It ultimately arises from on-going processes within the planet deep interior. These large-scale processes are in turn controlled by the properties of the constituent minerals of the deep Earth. A mineral's crystal and electronic structures are its most fundamental characteristics, from which all other physical and chemical properties follow. Due to the high pressures (millions of atmospheres) and temperatures (thousands of degrees) of the Earth's mantle, there is limited knowledge of the crystal and electronic structures of minerals residing near its base. This region is primarily composed of silicate minerals that adopt structures known as perovskite and post-perovskite. In this project, the investigator will conduct laboratory experiments on these structures to probe how incorporation of iron affects their properties. He will study magnesium germanates, a class of compounds that are close analogs for the silicate minerals of the deep Earth yet can be studied at pressures and temperatures that are more easily attainable in the laboratory. Through this work, he will provide fundamental knowledge of mineral properties that are needed to interpret geophysical observations and constrain the physical processes in Earth's interior. This project will have implications for the understanding of mantle thermal convection, which drives plate tectonics at the origin of numerous hazards for human societies, and provide insights on the effect of extreme conditions of pressure and temperature on materials properties. The project will also provide support for the training of a graduate student Iron influences key physical properties of lower mantle minerals including density, elasticity, element partitioning, and electrical and thermal conductivity. These properties in turn affect the interpretation of seismic and other geophysical observations and constrain models of the deep Earth. Perovskites and post-perovskites are major structures of the lower mantle. Iron in these materials may adopt different valences and spin states and may occupy different structural sites. High-pressure synchrotron Mossbauer spectroscopy and X-ray emission spectroscopy are the primary tools for investigating the electronic state of iron in mantle minerals. In this study the investigator will use these techniques together with conventional Mossbauer spectroscopy to explore the properties of perovskites and post-perovskites over a range of pressures and iron and aluminum contents. He will use magnesium gemanate analogs which lower pressure of formation, compared to that of Earth's silicates, will allow investigating the effect of pressure on post-perovskite properties. The expected results will provide strong constraints on the site occupancies, valences and spin states of perovskite and post-perovskite, as well as a direct test of recent theoretical calculations. They will contribute to unveiling the role of iron in perovskite and post-perovskite which is crucial for the unambiguous interpretation of Mossbauer spectra. They will also enable a more comprehensive understanding of spin-pairing behavior in major lower mantle minerals, thus better constraining the properties of Earth's deep mantle. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.