Synthesis of single crystal (Mg 1−xFe x) 1−δO ( x=0.001–1.00) solid-solution and electrical conduction mechanism at high temperature and pressure

Abstract

The (Mg,Fe) 1− δ O rock salt-type solid solution is believed to be one of the major constituents in the Earth's lower mantle. The rock salt-type (Mg 1− x Fe x ) 1− δ O ( x=0.001–0.43) single crystals (∼8 mm in diameter) were made by a melt-growth method using high temperature graphite heater (up to 3300 K) in the Ar atmosphere and by a FZ method in the CO 2–H 2 atmosphere ( pH 2/ pCO 2=0.1). Electrical conductivity measurements were carried out as functions of temperature and frequency by a the complex impedance method under pressure (5 GPa and ∼1800 K, and, 0.1 MPa and ∼1400 K). The ratio of Fe 3+/(Fe 2++Fe 3+) in the solid solution decreases with MgO contents under the same oxygen fugacity. Our experimental results show a change in charge transport mechanism in the (Mg 1− x Fe x ) 1− δ O solid solution at high temperature. The temperature of inflection point of the slope in Arrhenius plots depend greatly on both composition and extrinsic factors of crystals. Chemical composition and homogeneity of specimen rather than pressure greatly influence the electric conductivity. The activation energy for the (Mg 0.99Fe 0.01) 1− δ O solid solution is 2.4 eV corresponding with a migration enthalpy of O ions vacancies. It is proposed that possible dominant electrical conduction mechanism in ferropericlase under the lower mantle conditions, at least in the higher temperature region, is a super ionic conduction.