Zhang et al. present an original study of the Lamb-Mossbauer factor of γ- and ε-Fe at high pressures and high temperatures. The measurements of the Lamb-Mossbauer factor of Fe were carried out by combining laser-heated diamond anvil cells, synchrotron Mossbauer spectroscopy, and a recently developed unique fast temperature readout spectrometer. The vibrational behavior of Fe under these conditions is consistent with quasiharmonicity, therefore the quasiharmonic Debye model was used to calculate the stiffness of the γ- and ε-Fe at elevated pressure-temperature conditions from the Lamb-Mossbauer factor and the atomic mean-square displacement. From the stiffness data, the equilibrium isotopic fractionation β-factor of iron is calculated at high pressures and high temperatures. Calculations based on their experimental data demonstrate that the quasiharmonic correction would lower ln βFe57/54 by 0.1 per mil; compared to the typicallyused harmonic extrapolation of room temperature nuclear resonant inelastic X-ray scattering data. The quasiharmonic correction to the ln βFe57/54 for Fe determined by this method is five times the upper bound of previous estimates and implications of this result to iron isotope fractionation inside the earth is discussed.

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