Vibrational spectrum throughout the iron spin crossover in ferropericlase (Mg<sub>1-x</sub>Fe<sub>x</sub>O)

ORAL

Abstract

Ferropericlase (Fp), (Mg1-xFex)O, is the second most abundant phase in the Earth’s lower mantle. At relevant pressure-temperature conditions, iron in Fp undergoes a high spin (HS), S=2, to low spin (LS), S=0, state change. The nature of this phenomenon is quite well understood now, but there are still basic questions regarding the structural stability and the existence of soft phonon modes during this iron state change. General theories exist to explain the volume reduction, the large elastic anomalies, and the broad nature of this HS-LS crossover. These theories make extensive use of the quasi-harmonic approximation (QHA), therefore, dynamical and structural stability are essential to the validity of these theories. Here, we investigate the vibrational spectrum of Fp throughout this spin crossover using ab initio DFT+Usc calculations. We address vibrational modes associated with isolated and (2nd) nearest neighbor Fe atoms undergoing the HS-LS state change. As expected, acoustic modes of this solid solution reproduce the well-known elastic anomalies, but optical modes display unusual features. We show that there are no soft phonon modes across this HS-LS crossover and Fp is dynamically stable at all mantle pressures and relevant iron concentrations.

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Presenters

  • Michel Marcondes

    • Department of Earth and Environmental Sciences, Columbia University
    • Lamont-Doherty Earth Observatory, Columbia University

Authors

  • Michel Marcondes

    • Department of Earth and Environmental Sciences, Columbia University
    • Lamont-Doherty Earth Observatory, Columbia University

  • Fawei Zheng

    • Institute of Applied Physics and Computational Mathematics

  • Renata Wentzcovitch

    • Department of Applied Physics and Applied Mathematics, Columbia University in the City of New York
    • Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory (LDEO), and Applied Physics and Applied Mathematics (APAM), Columbia University in the City o
    • Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, Columbia University
    • Columbia University
    • Department of Applied Physics and Applied Mathematics, Columbia University
    • Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Lamont Doherty Earth Observatory, Columbia University
    • Department of Applied Physics and Applied Mathematics, Columbia University in the City of New York, 500 W. 120th St., Mudd 200, MC 4701 New York, NY 10027, USA
    • Department of Applied Physics and Applied Mathematics, Columbia University in the City of New York, 500 W. 120th St., Mudd 200, MC 4701 New York, NY 10027, USA.
    • Department of Applied Physics and Applied Mathematics; Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory, Columbia University, 10027, USA