Abstract
Electrical conductivity enhancement from semiconductor to metallization in Mn 3- x Fe x O 4 postspinel under extreme conditions is discussed herein. Neutron diffraction experiments have allowed precise analysis of the Mn 3- x Fe x O 4 structure by virtue of the significant difference in coherent scattering lengths between Mn (−3.73 fm) and Fe (9.54 fm) . An Mn 3- x Fe x O 4 spinel solid solution transforms into an orthorhombic postspinel phase. Neutron diffraction studies have proved that cubic MnFe 2 O 4 spinel and tetragonal Mn 2 FeO 4 transform into a high-pressure postspinel structure (CaMn 2 O 4 -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 Fe 2+ and Fe 3+ at the tetrahedral and octahedral sites in the spinel structure. MnFe 2 O 4 and Mn 2 FeO 4 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 Fe 2+ and Fe 3+ at the B sites of MnFe 2 O 4 are much higher than in the case of Mn 2 FeO 4 . 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 Mn 2 FeO 4 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. • Site occupancy and magnetic structure of Mn 3-x Fe x O 4 spinel and postspinel are studies by neutron and Mössbauer experiments under high-pressure. • The tetragonal phase of Mn 2 FeO 4 shows the tetragonal-to-cubic transition with c/a>1, but many spinels show high-pressure transition with c/a<1. • MnFe 2 O 4 and Mn 2 FeO 4 spinels transform to the postspinel phase above pressures 18.4 GPa and 14 GPa, respectively, and decreasing with Mn content. • Eectron hopping persists as the charge transport from spinel to postspinel transition. The resistivity falls down to metallic values. • Theoretical density of state (DOS) calculation indicates the metallic behavior of post-spinel Mn 2 FeO 4 under high pressure at 26.8 GPa.
Published Version
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