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Enhancement of electrical conductivity to metallization of Mn$$_{3-x}$$Fe$$_x$$O$$_4$$ spinel and postspinel with elevating pressure

Yamanaka, Takamitsu*; Rahman, S.*; Nakamoto, Yuki*; Hattori, Takanori   ; Jang, B. G.*; Kim, D. Y.*; Mao, H.-K.*

High-pressure neutron diffraction proved that MnFe$$_2$$O$$_4$$ and Mn$$_2$$FeO$$_4$$ spinels transform into CaMn$$_2$$O$$_4$$-type structure above 18 GPa and 14 GPa, respectively. The transition pressure of Mn$$_{3-x}$$Fe$$_x$$O$$_4$$ solutions decreases with increasing Mn content. Synchrotron X-ray M$"{o}$ssbauer experiments revealed that Fe$$^{2+}$$ and Fe$$^{3+}$$ distribution at the tetrahedral (A) and octahedral (B) sites in the spinel structure changes with pressure. MnFe$$_2$$O$$_4$$ and Mn$$_2$$FeO$$_4$$ spinels are ferrimagnetic and the CaMn$$_2$$O$$_4$$-type phase is paramagnetic. The temperature dependence of resistivity indicates that both spinels are semiconductors wherein electrons hop between cations at the A and B sites. A pressure-induced shortening of B-B distance promoted conduction via greater electron mobility between adjacent B cations. The Fe$$^{2+}$$ and Fe$$^{3+}$$ occupancies at the B sites in MnFe$$_2$$O$$_4$$ are much larger than those in Mn$$_2$$FeO$$_4$$. The CaMn$$_2$$O$$_4$$-type phase is metallic. Theoretical calculation confirmed the metallic character and Fe d-orbitals strongly renormalized compared to Mn d-orbitals.

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Category:Chemistry, Multidisciplinary

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