First-Principle Study Of Perovskite And Post-perovskite Polymorphs Of Pure End-member FeSiO₃

The D" layer (~2700-2890 kilometer ) is about 3% of Earth's mass, is 125 to 188 miles (200 to 300 kilometers) thick and covers about 4% of the mantle-crust mass. Many anomalous seismic observations that puzzled the mineral physics and seismology communities have been found near the bottom of lower mantle (D"layer), such as a discontinuity at the top in some parts, shear-wave splitting and anti-correlation of bulk and shear sound velocity. What deserves to be mentioned is an abrupt reduction in seismic velocity at the base of D"layer, the Earth's core mantle boundary (CMB) (~2890 kilometer depth) which lies between the Earth's silicate mantle and molten iron core. This region was only 5- to 40-km thick, also called ultra-low velocity zones (ULVZ). Undoubtedly, these could have important implications for the global dynamics and evolution of planet.A solid-solid phase transition with a Clapeyron slope of about 6 megapascal per kelvin could occur in this region was first proposed by Sidorin to explain the shear-wave discontinuity at the top of the D"layer instead of heterogeneity. As the dominant constituent of the lower mantle, iron-bearing magnesium silicate perovskite (Mg,Fe)SiO3 has attracted considerable attention over the past few decades. Though, experimental results were discrepant regarding its stability and structure. Knittle have confirmed the conservation of (Mg,Fe)SiO3 perovskite up to 127GPa while Shim suggested a subtle change above 83 GPa and 1700K. It's essential to know the phase relations and crystal structures of lower mantle materials for evaluating seismic observations and their geochemical and geodynamical implications. High pressure-temperature experiments and first principle simulations proved that iron free MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase similar to the conditions of D"layer. This new phase could explain some seismic features successfully except for the large reduction in seismic velocity.In view of this, the iron-rich post-perovskite crystal layer might exist at the bottom of the D"layer as a new model have been proposed recently. The iron-rich ferromagnesian silicate phase (FexMg1-x)SiO3 with different iron content have been synthesized experimentally. (FexMg1-x)SiO3 with iron content as much as 80% transformed from perovskite to post-perovskite without trace of silicate perovskite or mixed oxides. A large amount of experimental results show us that iron-rich ppv silicate is more stable than pv silicate. The high iron content greatly decreases wave velocity and provides a plausible explanation to the ultra-low velocity. Obviously, iron as the most abundant transition-metal element in the mantle plays an important role in the properties of the Earth. Pressure induced spin crossover which may affect the iron partitioning or partial melting have been found in magnesiowustite, silicate perovskite and perhaps in post-perovskite.In order to gain a comprehensive look into the high pressure behavior of iron in pure end-member FeSiO3, here systematic calculations of elastic properties and electronic structures are performed by the first-principles approach. We obtain several original results in below.(1) We fit equation of states of ferromagnetic and nonmagnetic FeSiO3 and find that the ferromagnetic state of post-perovskite silicate FeSiO3 is stable at all pressures with respect to silicate perovskite. This agrees well with Wendy L.Mao's experiment who finds that iron enters into post-perovskite preferentially relative to perovskite MgSiO3.(2) Both perovskite and post-perovskite phase FeSiO3 are mechanically stable at 130GPa which corresponding to the pressure at the Core-Mantle Boundary. Calculated seismic velocities of FeSiO3 are close to those in the ULVZ at high temperature. Our calculations confirm that iron-rich post-perovskite silicate phase might be one important reasons leading to the ultra low velocity, theoretically.(3) The pressure-induced volume decrease and corresponding band widening result in the spin cross over occur in perovskite phase FeSiO3. (4) We correlate the electronic structure and elastic properties of perovskite and post-perovskite phase FeSiO3. The weak coupling of the strong covalent-ionic layer and metallic layer along [001] and [010] directions is consistent with the small C33 and C22 for FeSiO3 in perovskite and post-perovskite structure, respectively.