Trigonal distortion of ferropericlase (Mg0.8Fe0.2)O at high pressures

Abstract

Transition-metal monoxides (MnO, FeO, CoO, NiO) are of great interest for solid-state physics in virtue of their magnetic, electron, and structural characteristics, which underlie the wide technological applications of this group of materials. These oxides are typical antiferromagnets with a cubic crystal structure of the NaCl type (structural type B1) above the Neel temperature ( T N ). Below T N , these oxides undergo a structural transition of the distortion type (corresponding to the lowering of symmetry to a trigonal or tetragonal one). The structural phase transition occurs within approximately the same temperature range in which the magnetic ordering arises. Therefore, until recently, it was commonly believed that the loss of symmetry in these oxides stems from the ordering of magnetic moments in the crystal structure [1]. The growth of pressure at room temperature leads to similar distortion-type phase transformations in MnO, FeO, CoO, and NiO. Since the Neel temperature has a positive baric coefficient (increases with pressure), it has been suggested [2] that the structural transitions in these materials under high pressures have the same nature as those occurring at low temperatures. Recent studies of the magnetic and elastic characteristics of wustite (FeO) at high pressures [3, 4] have demonstrated that the onset of magnetic ordering in wustite takes place at about 5 GPa. This pressure is significantly lower than that corresponding to the structural phase transition (about 17 GPa). In light of these results, it becomes clear that the relation between magnetic ordering and structural distortion needs a revision—for FeO at the very least.