Investigating the possibility of Spin transition in Ringwoodite using First Principles Theory

Galge, Anant (2022) Investigating the possibility of Spin transition in Ringwoodite using First Principles Theory. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Anant Galge (17MS038)) 17MS038_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

Spin crossover in Fe ions of Bridgmanite, a major mineral phase in the Earth’s lower mantle is known to be the cause behind some of the seismic velocity anomalies seen in the Earth’s lower mantle(G SHUKLA, R WENTZCOVITCH 2017). However, the pressure range over which this transition takes place is somewhat disputed. Lin et al (2012) and Mao et al (2015) suggest that the pressure range is as low as 18-25 GPa, which corresponds to the stability field of Ringwoodite, a dominant mineral phase in the Earth’s mantle transition zone (MTZ). Hence, in this project, we have investigated the possibility of spin transition in Ringwoodite (RW). The first structure we considered was that of 12.5% Fe containing RW. Our calculations suggest that even at 24 GPa, the Fe atoms are in a high spin state. Next, we tried to build in internal pressure by inserting a large cation, namely Na, which has been suggested lately to be present in abundance in the MTZ (ref). The introduction of Na⁺ into the system though compresses the polyhedron hosting the Fe ion, however, the compression was found not to be sufficient enough to cause a spin transition at pressures as high as 24 GPa. Hence, we considered doping an even larger alkali cation, namely K⁺ (Luca Bindi, Anastasia Tamarova). No spin transition in this system was found to take place in the stability field of RW. In a bid to determine the pressure beyond which the spin-crossover might theoretically take place in RW, we further increased pressure onto the system. Our calculations suggest that in RW, Fe³⁺ in the tetrahedral site goes over from a high spin to a low spin state at pressures beyond 100 GPa. In contrast, the reported pressure range over which spin-crossover takes place in Bridgmanite is well below 60 GPa. In order to understand why spin transitions occur at much higher pressures in RW as compared to Bridgmanite, we performed a structural analysis of both the minerals at 30 GPa pressure. We found that Bridmanite is a much denser phase which may be the cause behind it showing a spin-crossover at a much lower pressure as compared to RW.

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