Electronic and magnetic properties of the $\varepsilon$-Fe phase at high pressures up to $241$ GPa in the temperature range of $4$–$300$ K

The magnetic and electronic states of iron in the hexagonal close-packed $\varepsilon$-Fe phase have been studied by synchrotron Mössbauer spectroscopy on Fe-57 nuclei (nuclear forward scattering method) at pressures of $P \approx 55{-} 241$ GPa in the temperature range of $4$–$300$ K in external magnetic fields up to $5$ T. It has been found that Fe atoms are in a nonmagnetic state in the entire studied $P-T$ region. Theoretically implied magnetic instability and quantum spin fluctuations, which can be stabilized by magnetic perturbation (e.g., external magnetic field), have not been confirmed by our measurements of nuclear forward scattering spectra in an external magnetic field. It has been established that the isomer shift IS(P) has a nonlinear pressure dependence and reaches a colossal value of about $-0.8$ mm/s at a maximum pressure of $241$ GPa, indicating a very high electron density on the Fe nucleus. A sharp change in the electron density on the Fe nucleus at temperatures of $100$–$200$ K indicates a phase transition with a change in the electronic structure, which can be due to an abrupt increase in the conductivity or even to the appearance of superconductivity.