Pressure-induced magnetic and electronic transitions in the layered Mott insulator FeI2 -: art. no. 035106

Powder x-ray diffraction, electrical resistance, and Fe-57 Mossbauer 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 similar to18 GPa the orbital term quenches, as is evident from a Mossbauer component characterized by H-hf=32T and e(2)q(zz)Q(3 cos(2) theta-1) =0, where the moments tilt to 55degrees, and T-N increases to 260 K. At 20 GPa an isostructural first-order phase transition occurs, accompanied by a discontinuous similar to5% 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 similar to38 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 Mossbauer data.