High pressure study of transition metal monoxides MnO and CoO: Structure and electrical resistance

We examined the structure of MnO and CoO up to 72 and 88 GPa, respectively, at room temperature by means of in situ X-ray diffraction using the synchrotron radiation at SPring-8. Compression was carried out with the Kawai-type apparatus equipped with sintered diamond anvils. In both MnO and CoO, the cubic B1 lattice starts distorting to the rhombohedral system at 37 and 40 GPa, respectively, which progressively proceeds under increasing pressure. Crystallographic direction of the distortion is opposite; i.e., a contraction along the diagonal [1 1 1] direction of the B1 lattice in MnO and a stretch in CoO. The rhombohedral distortion in 3d transition metal monoxides is discussed. Simultaneously measured electrical resistance of CoO showed characteristic change with pressure; i.e., the pronounced minimum at 14.9 GPa and the maximum at 49 GPa, which serve as good pressure fixed points.