Pressure-induced structural transformations in the Mott insulator FeI₂

A full-profile refinement of the layered antiferromagnetic FeI₂ crystallographic structures at pressures up to 70 GPa were performed combined with ab initio calculations to particularly elucidate the structural aspects of the recently observed pressure-induced quenching of the orbital term of the Fe²⁺ moment and of the Mott transition. Synchrotron powder XRD diffraction studies have shown that at ∼17 GPa a substantial alteration of the lattice parameters takes place which is attributed to the quenching of the orbital term. Starting at P∼20 GPa and completed at ∼35 GPa, a sluggish structural phase transition takes place which can be attributed to the onset of a Mott transition as has been previously observed by resistance and MS studies. In agreement with ab initio calculations, the doubling of lattice parameters and the formation of a new Fe sublattice replacing the original CdI₂-type structure, can explain this structural transition. The latter alterations in the Fe sublattice may indicate a trend of the Fe sites to disorder in the new high pressure phase. This first-order phase transition is characterized by a significant change of the unit cell parameters, a reduction in volume, and a change of the Fe-I distances. The substantial reduction of the Fe-I distances with minimal changes in the Fe-Fe bond lengths at the transition, suggests a charge-transfer-type gap closure mechanism involving the Ip-Fed bands. At P>40 GPa a overturn of the structural transition is observed resulting in the return of the original, CdI₂-type structure.