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We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy.

We compare the atomic dynamics of the glass to that of the relevant crystal. In the spectra of inelastic scattering, the boson peak of the glass appears higher than the transverse acoustic (TA) singularity of the crystal. However, the density of states shows that they have the same number of states. Increasing pressure causes the transformation of the boson peak of the glass towards the TA singularity of the crystal. Once corrected for the difference in the elastic medium, the boson peak matches the TA singularity in energy and height. This suggests the identical nature of the two features.

57 Fe-Mössbauer studies of superconducting Rb 0.8 Fe 1.6 Se 2.0 with T C = 32.4 K were performed on singlecrystalline and polycrystalline samples in the temperature range 4.2-295 K. They reveal the presence of 88% magnetic and 12% nonmagnetic Fe 2+ species with the same polarization dependence of their hyperfine spectra. The magnetic species are attributed to the 16i sites of the √ 5 × √ 5 × 1 superstructure and the nonmagnetic Fe species to a nanosized phase observed in recent structural studies of superconducting K x Fe 2−y Se 2 systems rather than to the vacant 4d sites in the √ 5 × √ 5 × 1 superstructure. The 57 Fe spectrum of a single-crystalline sample in an external field of 50 kOe applied parallel to the crystallographic c axis confirms the antiferromagnetic order between the fourfold ferromagnetic Fe(16i) supermoments and the absence of a magnetic moment at the Fe sites in the minority phase. A discussion of all spectral information and comparison with superconducting FeSe provides convincing evidence that the nanoscale phase separation is monitored by Mössbauer spectroscopy in Rb 0.8 Fe 1.6 Se 2.0 .

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