The effect of nitrogen on the compressibility and conductivity of iron at high pressure

Although nitrogen in the Earth's interior has attracted significant attention recently, it remains the most enigmatic of the light elements in the Earth's core. In this work, synchrotron X-ray diffraction (XRD) and electrical conductivity experiments were conducted on iron nitrides (Fe₂N and Fe₄N) in diamond anvil cells (DACs) up to about 70 GPa at ambient temperature. These results show that iron nitrides are stable up to at least 70 GPa. From the equation of state (EOS) parameters, iron nitrides are more compressible than iron carbides. Moreover, using the van der Pauw method and Wiedemann-Franz law, the electrical and thermal conductivity of samples were determined to be much lower than that of iron carbides. The conductivities of Fe₂N and Fe₄N were similar at 20-70 GPa, suggesting no evident effects by varying the N stoichiometries in iron nitrides. Iron nitrides are less dense and conductive but more compressible than carbides at 0-70 GPa. This study indicates that less nitrogen than carbon can explain geophysical phenomena in the deep Earth, such as the density deficit.