MAGNETIC COLLAPSE AND THE BEHAVIOR OF TRANSITION METAL OXIDES: FeO AT HIGH PRESSURES

Linearized augmented plane wave (LAPW) results are presented for FeO at high pressures using the Generalized Gradient Approximation (GGA) to study the high-spin low-spin transition previously predicted by LAPW with the Local Density Approximation (LDA) and Linear Muffin Tin Orbital (LMTO-ASA) methods within the GGA. We find a first-order transition at a pressure of about 105 GPa for the cubic lattice, consistent with earlier LAPW results, but much lower than obtained with the LMTO. The results are generally consistent with recent Mossbauer experiments that show a transition at about 100 GPa. We also discuss the origin of the transition, and show that it is not due to electrostatic crystal-field effects, but is rather due to hybridization and band widening with pressure. Examination of experimental data and computations suggest that the high pressure hexagonal phase of FeO is likely a polytype between the B8 NiAs and anti-B8 AsNi structures. The former is predicted to be an antiferromagnetic metal, and the latter an antiferromagnetic insulator. Implications for geophysics are discussed. At low pressures transition metal ions such as ferrous iron (Fe2+)are magnetic and highspin. In this state, ferrous iron shows complete solubilitywith Mg in most minerals, in spite of the fact that Mg2+is a simple closed shell ion, and ferrous iron is a complicated, magnetic, and non-spherical ion. In fact, FeO (wustite) is a Mott insulator, which is insulating due to the magnetic interactions and is given as a metal using band theory [1], and MgO is a simple ionic solid [2,3]. A number of transitions are expected as pressure and temperature are varied, which may occur together in groups: Mott transitions (localization transitions between band states and localized states), high spin-low spin transitions, structural phase transitions, and magnetic ordering transitions. Here we concentrate on magnetic INTRODUCTION 1700 Shock wave data / FeO 1500 ~ 1300 NIAI-type (B8) Q)