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 Moessbauer 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.