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

Powder x-ray diffraction, electrical resistance, and ⁵⁷Fe Mössbauer 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, T_N 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 H_hf 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 Mössbauer component characterized by H_hf=32 T and e²q_zzQ(3 cos² θ-1)=0, where the moments tilt to 55°, and T_N 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 Mössbauer data.