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تسجيل مستخدم جديدTheoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earths D layer
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The Earths lower mantle is believed to be composed mainly of (Mg,Fe)SiO3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3). But it has not been possible to explain many unusual properties of the lowermost 150 km of the mantle (the D layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D layer: its seismic anisotropy, the strongly undulating shear-wave discontinuity at its top and possibly the anticorrelation between shear and bulk sound velocities.
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The post-perovskite phase of (Mg,Fe)SiO3 is believed to be the main mineral phase of the Earths lowermost mantle (the D layer). Its properties explain numerous geophysical observations associated with this layer - for example, the D discontinuity, it
s topography and seismic anisotropy within the layer. Here we use a novel simulation technique, first-principles metadynamics, to identify a family of low-energy polytypic stacking-fault structures intermediate between the perovskite and post-perovskite phases. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the phase transition, and as a likely mechanism for plastic deformation of perovskite and postperovskite. In particular, the predicted slip planes are (010) for perovskite (consistent with experiment) and (110) for postperovskite (in contrast to the previously expected (010) slip planes). Dominant slip planes define the lattice preferred orientation and elastic anisotropy of the texture. The (110) slip planes in post-perovskite require a much smaller degree of lattice preferred orientation to explain geophysical observations of shear-wave anisotropy in the D layer.
The temperature anomalies in the Earths mantle associated with thermal convection1 can be inferred from seismic tomography, provided that the elastic properties of mantle minerals are known as a function of temperature at mantle pressures. At present
, however, such information is difficult to obtain directly through laboratory experiments. We have therefore taken advantage of recent advances in computer technology, and have performed finite-temperature ab initio molecular dynamics simulations of the elastic properties of MgSiO3 perovskite, the major mineral of the lower mantle, at relevant thermodynamic conditions. When combined with the results from tomographic images of the mantle, our results indicate that the lower mantle is either significantly anelastic or compositionally heterogeneous on large scales. We found the temperature contrast between the coldest and hottest regions of the mantle, at a given depth, to be about 800K at 1000 km, 1500K at 2000 km, and possibly over 2000K at the core-mantle boundary.
Because substitutions of BH4- anion with Br can stabilize the hexagonal structure of the LiBH4 at room temperature, leading to a high Li-ion conductivity, its thermodynamic stability has been investigated in this work. The binary LiBH4-LiBr system ha
s been explored by means of X-ray diffraction and differential scanning calorimetry, combined with an assessment of thermodynamic properties. The monophasic zone of the hexagonal Li(BH4)1-x(Br)x solid solution has been defined from x=0.30 to x=0.55 at room temperature. Solubility limits have been determined by in-situ X-ray diffraction at various temperatures. For the formation of the h-Li(BH4)0.6(Br)0.4 solid solution, a value of the enthalpy of mixing has been determined experimentally equal to 1.0 kJ/mol. In addition, the enthalpy of melting has been measured for various compositions. Lattice stabilities of LiBH4 and LiBr have been determined by ab initio calculations, using CRYSTAL and VASP codes. Combining results of experiments and theoretical calculations, the LiBH4-LiBr phase diagram has been determined in all composition and temperature range by the CALPHAD method.
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