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ABSTRACT: Many rheological and transport properties of rocks are determined by the grain boundary structures of their constituent minerals.
These grain boundaries often also hold a high concentration of dopant ions. Here, as a first step towards modelling the transport
and rheological behaviour of the lower mantle, we report the results of lattice static simulations on the surface structures
of Fe2+ and Ca2+-doped orthorhombic MgSiO3-perovskite. For all the surfaces we studied, the energies of the doped structures are lowered, sometimes by more than 1J/m2, with respect to the pure surfaces. From our calculated crystal morphologies, we predict that the grains become more tabular
as the concentration of Fe2+ ions increases, while under equilibrium conditions the grains are cubic. By calculating the replacement energies of Mg2+ by Fe2+ and Ca2+ ions in the six outermost surface layers, we conclude that these divalent ions would tend to segregate onto the crystal surfaces.
We suggest, therefore, that the grain boundary structure and rheology of MgSiO3-perovskite dominated rocks will be strongly affected by the presence of minor elements in the lower mantle.
Physics and Chemistry of Minerals 04/2012; 32(5):379-387. · 1.73 Impact Factor
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ABSTRACT: The pressure as a function of volume and temperature has been investigated for B2 -type NaCl over the pressure range of 20–360 GPa and at temperatures between 300 and 3000 K . The simulations were performed using ab initio molecular dynamics method within the density-functional theory framework. A Vinet equation of state fitted to the 300 K data yielded a bulk modulus of B<sub>Ta</sub>=128.66 GPa and a pressure derivative of B<sub>Ta</sub><sup>′</sup>=4.374 at standard state pressure of 30 GPa . The thermal pressure contribution was determined to be of the form ΔP<sub> th </sub>=[αB<sub>T</sub>(V<sub>a</sub>)+(∂B<sub>T</sub>/∂T)<sub>V</sub> ln (V<sub>a</sub>/V)]ΔT . When αB<sub>T</sub>(V<sub>a</sub>) is assumed to be constant, the fit to the data yielded αB<sub>T</sub>(V<sub>a</sub>)=0.0033 GPa / K at standard volume, corresponding to the pressure of 30 GPa . In contrast, the volume dependence of the thermal pressure was very small, and fitting yielded (∂B<sub>T</sub>/∂T)<sub>V</sub>=0.000 87 .
Journal of Applied Physics 02/2008; · 2.17 Impact Factor
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ABSTRACT: The most abundant mineral on Earth has a perovskite crystal structure and a chemistry that is dominated by MgSiO3 with the next most abundant cations probably being aluminum and ferric iron. The dearth of experimental elasticity data for this chemically complex mineral limits our ability to calculate model seismic velocities for the lower mantle. We have calculated the single crystal elastic moduli (cij) for (Mg, Fe3 +)(Si, Al)O3 perovskite using density functional theory in order to investigate the effect of chemical variations and spin state transitions of the Fe3+ ions. Considering the favored coupled substitution of Mg2+–Si4 + by Fe3+–Al3+, we find that the effect of ferric iron on seismic properties is comparable with the same amount of ferrous iron. Ferric iron lowers the elastic moduli relative to the Al charge-coupled substitution. Substitution of Fe3+ for Al3+, giving rise to an Fe/Mg ratio of 6%, causes 1.8% lower longitudinal velocity and 2.5% lower shear velocity at ambient pressure and 1.1% lower longitudinal velocity and 1.8% lower shear velocity at 142 GPa. The spin state of the iron for this composition has a relatively small effect (< 0.5% variation) on both bulk modulus and shear modulus.
Earth and Planetary Science Letters.
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Li Li,
John P Brodholt,
Stephen Stackhouse,
Donald J Weidner, Maria Alfredsson,
G David Price,
J P Brodholt,
S Stackhouse,
D J Weidner,
M Alfredsson,
G D Price
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ABSTRACT: 1] We investigate the effect of pressure on the electronic spin state of ferric iron on Al-bearing MgSiO 3 -perovskite using first-principle computations. Ferric iron (6.25 mol%) and Al (6.25 mol%) substitute for Mg and Si respectively. Five substitution models on different atomic position pairs are examined. Our results show that spin state transition from high spin (HS) to low spin (LS) occurs on the Fe 3+ ions at high pressure, while there is no stability field for the intermediate spin state. Fe 3+ alone can be responsible for the spin state transition. The five models witness a transition pressure ranging from 97– 126 GPa. Differential stress can change the pressure for the spin collapse. The lowest pressure spin state transition occurs where Al 3+ and Fe 3+ are in adjacent sites. These results are one explanation to the reported experimental observations that the spin transition occurs over a wide pressure range. This finding may have important implications for the dynamics and seismic signature of the lower mantle. Citation: (2005), Electronic spin state of ferric iron in Al-bearing perovskite in the lower mantle, Geophys. Res. Lett., 32, L17307, doi:10.1029/2005GL023045.
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ABSTRACT: Doping is a common way to activate the behavior of ceramics. Its effect is not limited to the bulk: segregation of dopants to the surfaces also yields a way to modify, and ultimately control the crystal morphology. We propose a model that allows us to calculate the surface energy beyond the Langmuir isotherm for doped and defective surfaces from atomic-level simulations. The model also allows us to account for different compositions between the bulk and surface. Computational materials design can thus be applied to optimize simultaneously the crystal behavior at the atomic (surface structure and composition) and mesoscopic (crystal size and shape) length scales. We exemplify the model with orthorhombic CaTiO3 perovskite doped with Mg2+, Fe2+, Ni2+, Sr2+, Ba2+ and Cd2+ ions, by predicting the effect that different dopants and dopant concentrations have on the crystal morphology. We find that a higher proportion of reactive {0 2 1} and {1 1 1} surfaces are exposed with the presence of divalent Mg2+, Fe2+ and Ni2+ ions than in the undoped material and in perovskite doped with Ba2+ and Sr2+. Cd2+ has only minor effects on crystal morphologies. These findings have important implications for predicting the reactivity of crystals doped with different ions and we show how this can be related to a simple parameter such as the ionic radius. We have tested our newly derived model by comparison with laboratory flux grown single crystals of CaTiO3, (Ni, Ca)TiO3 and (Ba, Ca)TiO3 and find excellent agreement between theory and experiment.
Surface Science. 601(21):4793-4800.
