Synthesis and magnetic properties of iron polyhydrides at megabar pressures

Iron polyhydrides have been synthesized at pressures of 77–157 GPa and temperatures up to 2000 K by the laser heating of an iron–borazane (ammonia borane NH$_3$BH$_3$) sample in diamond anvil cells. X-ray spectra of the synthesized products indicate the formation of several FeHx phases, in which (in two cells) reflections of FeH $_{2}$ iron hydride with the tetragonal $I4/mmm$ phase are reliably detected. The magnetic and electronic properties of FeHx compounds have been studied by nuclear forward scattering spectroscopy on Fe-57 nuclei at high pressures in the temperature range of 4–300 K in external magnetic fields up to 5 T. The nuclear forward scattering data indicate at least seven FeHx compounds with very different electronic and magnetic properties. The Néel temperature TN determined for the FeH$_2$ phase at a pressure of 82 GPa is about 174 K. One of the striking results is the observation of the FeHx phase remaining magnetically ordered at a pressure of 128 GPa in the entire temperature range of 4–300 K. Such a high pressure is characteristic of the boundary between the lower mantle and the outer core of the Earth. The existence of a magnetic phase of an iron compound at such a record high pressure is unique and has not yet been observed.