Pressure-induced structural transformations in the Mott insulator FeI2 -: art. no. 064105

A full-profile refinement of the layered antiferromagnetic FeI(2) 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 Fe(2+) moment and of the Mott transition. Synchrotron powder XRD diffraction studies have shown that at similar to17 GPa a substantial alteration of the lattice parameters takes place which is attributed to the quenching of the orbital term. Starting at Psimilar to20 GPa and completed at similar to35 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 CdI(2)-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, CdI(2)-type structure.