First-principles study on electronic structures and phase stability of MnO and FeO under high pressure

The electronic structures and the phase stability of MnO and FeO under ultrahigh pressure were studied by the first-principles plane-wave basis pseudopotential calculations. Different crystal structures combined with different spin structures were studied systematically for both MnO and FeO with full structure optimization. The present calculations based on generalized gradient approximation (GGA) account well for the properties of MnO and FeO especially in the high-pressure region. For the low pressure regime, where the electron correlation is very strong, we performed the LDA+U calculations with the electron correlation and the spin-orbit coupling taken into account to supplement the GGA results. Our results predict that the high-pressure phase of MnO should take the metallic normal NiAs (nB8) structure rather than the B2 structure, and that a metastable nonmagnetic B1 structure with stretched distortion along the [111] direction can be realized for MnO in the intermediate pressure range. A unique antiferromagnetic inverse NiAs (iB8) structure as the high-pressure phase of FeO was discussed in detail, and the uniqueness was made clearer by comparing with the FeS case. The distortions and the magnetic moments for different phases were characterized. The larger $c/a$ ratios for both nB8 MnO and iB8 FeO can be explained based on our analysis of the cation-radius/anion-radius ratio versus $c/a$ for series of related materials.