Abstract
Magnetic and structural phase relations in FeO and MnO were studied in situ using Mössbauer spectroscopy, X-ray and neutron powder diffraction at high pressures and over a broad temperature range. Two principal transition boundaries were mapped: antiferromagnetic ordering (the Néel transition) and the structural distortion from the cubic B1 to a rhombohedral rB1 lattice. These two transitions are not necessarily coupled and the relations between the structural and magnetic properties of transition metal monoxides are discussed.