Pressure-induced magnetic and electronic transitions in the layered Mott insulator FeI2

Powder x-ray diffraction, electrical resistance, and 57Fe Moessbauer spectroscopy at pressures to at least 40 GPa in diamond anvil cells have been employed to investigate the pressure evolution of the structural, electrical-transport, and magnetic properties of the antiferromagnetic insulator FeI2. Up to 18 GPa, the volume decreases by 25%, the resistivity decreases by eight orders of magnitude, TN increases 16-fold to 150 K, and the Fe2+ moments remain parallel to the c axis. The change in the isomer shift (IS), which is negatively proportional to the change in the s-electron density at the Fe nucleus, follows the volume reduction by continuously decreasing from 1.0 to 0.8 mm/s, the quadrupole splitting (QS) increases monotonically from 0.6 mm, peaking at 0.85 mm/s by 12 GPa, and decreases to 0.75 at 18 GPa, and the magnetic hyperfine field Hhf composed of spin and orbital terms with opposite signs increases from 8 to 12 T. At ∼18 GPa the orbital term quenches, as is evident from a Moessbauer component characterized by Hhf=32 T and e2qzzQ(3 cos2 θ-1)=0, where the moments tilt to 55 deg., and TN increases to 260 K. At 20 GPa an isostructural first-order phase transition occurs, accompanied by a discontinuous ∼5% decrease in volume and a considerably lower QS and IS. The c axis decreases by 5% with no decrease in the a axis, suggesting a considerable contraction of the Fe-I bond lengths. The high-pressure phase (HP) is diamagnetic, as characterized by a pure quadrupole-split spectrum to the lowest temperature of 5 K. The abundance of this diamagnetic phase increases with rising pressure reaching 100% by ∼38 GPa. The HP phase is also metallic, as shown by R(P,T) data. The observation of diamagnetism, metallic behavior, and the considerable reduction in volume distances establishes that-a Mott or charge-transfer transition has occurred, resulting in the total collapse of any electron correlation. The coexistence of several phases and their respective abundances were determined from the Moessbauer data