Abstract:
Intensive high-pressure experiments on the transition-metal monoxides have revealed that FeO [1, 2, 3, 4, 5] and MnO [6, 7, 8] undergo pressure-induced phase transition. The high-pressure phase of FeO was identified as the NiAs (B8) type [4, 5], while that of MnO is yet unclear [7, 8]. The present theoretical study predicts that the high-pressure phase of MnO is a metallic normal B8 structure (nB8), while that of FeO should take the inverse B8 structure (iB8). The novel feature of the unique high-pressure phase of stoichiometric FeO is that the system should be a band insulator in the ordered antiferromagnetic (AF) state and that the existence of a band gap leads to special stability of the phase. The observed metallicity [9, 10] of the high-pressure and high-temperature phase of FeO may be caused by the loss of AF order and also by the itinerant carriers created by nonstoichiometry. Analysis of x-ray diffraction experiments provides a further support to the present theoretical prediction for both FeO and MnO. Strong stability of the high-pressure phase of FeO will imply possible important roles in Earth’s core.
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Under normal pressure and room temperature, FeO and MnO both take the rock-salt (B1) structure with rhombohedral distortion along the < 111 > direction. This distorted B1 structure is called rB1 hereafter. It was observed that FeO undergoes a phase transition from the rB1 structure to a high pressure phase at pressures above 70GPa [1, 2], whose crystal structure was recently assigned to be the B8 by the analysis of x-ray diffraction peak positions [4, 5]. On the analogy of almost all of the compounds with the B8 structure, a natural idea for the B8 (NiAs) FeO may be such that Fe occupies the Ni site and O the As site. This structure is named nB8. However, another structure, which is named iB8, is possible by exchanging the Fe and O positions [12]. More recently, not only shock compression [7] but also static compression [8] experiments on MnO also showed the existence of a possible high-pressure phase above 90GPa. However, the crystal structure has not been determined yet.
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