Enhancement of electrical conductivity to metallization of Mn3-xFexO4 spinel and postspinel with elevating pressure

Abstract

Electrical conductivity enhancement from semiconductor to metallization in Mn3-xFexO4 postspinel under extreme conditions is discussed herein. Neutron diffraction experiments have allowed precise analysis of the Mn3-xFexO4 structure by virtue of the significant difference in coherent scattering lengths between Mn (−3.73 fm) and Fe (9.54 fm). An Mn3-xFexO4 spinel solid solution transforms into an orthorhombic postspinel phase. Neutron diffraction studies have proved that cubic MnFe2O4 spinel and tetragonal Mn2FeO4 transform into a high-pressure postspinel structure (CaMn2O4-type marokite) above pressures of 18 GPa and 14 GPa, respectively. The transition pressure decreases with increasing Mn content.Synchrotron X-ray Mössbauer experiments have revealed the effects of high pressures on the distribution of Fe2+ and Fe3+ at the tetrahedral and octahedral sites in the spinel structure. MnFe2O4 and Mn2FeO4 are ferrimagnetic under ambient conditions, and they show sextet spectral features with hyperfine structure elicited by internal magnetic fields. The high-pressure postspinel polymorph shows paramagnetic character.Electron hopping persists as the charge-transport mode. The temperature dependence of resistivity indicates that the spinel phases show semiconductor properties. Electrical conduction is derived from electron hopping between cations at the tetrahedral (A) and octahedral (B) sites. A shortened B–B distance promotes higher conduction during compression and greater electron mobility between adjacent B cations. The occupancies of Fe2+ and Fe3+ at the B sites of MnFe2O4 are much higher than in the case of Mn2FeO4. The high-pressure postspinel polymorph of transforms into a phase with metallic-like character due to band conduction in the high-pressure region. Theoretical approaches have revealed the densities of state of these manganese ferrites. To verify the metallic behavior of postspinel Mn2FeO4 under high pressures, we have applied a combined approach of density functional theory and dynamical mean field theory. The spectral function clearly shows metallic character. Fe d orbitals are strongly renormalized compared to Mn d orbitals.