Molecular dynamics simulation of MgSiO_3-Al_2O_3 perovskite

概要

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The molecular dynamics method was applied to aluminous perovskite solid solution in the system MgSiO3-Al2O3. The two-body interatomic potential model CMAS94 (Matsui, 1994) was used for this purpose. The solid solution crystals (Mg1−x,Alx)(Alx,Si1−x)O3 (0≤x≤1) accommodating Al3+ ions without the production of vacant site were realized by the following two methods: (1) To replace Mg2+ and Si4+ ions with 2Al3+ ions at random disregarding the arrangement of Mg2+ and Si4+ in the crystal (Random substitution).(2) To replace a pair of (Mg2++Si4+) with 2Al3+ ions at the adjacent sites, so as to satisfy the local charge balance. Although the latter solid solution model gives slightly lower values of enthalpy, both models show virtually the same energetic and structural characteristics. Under atmospheric condition, both the unit-cell parameter c and the cell volume increase remarkably with the increase in the Al2O3 content. At around 40 - 60 GPa, however, the volume is almost insensitive to the Al2O3 content. Al2O3-containing perovskite is more compressible than aluminum-free perovskite. It is consistent with the results of recent high-pressure in-situ X-ray studies below 15 GPa. This compositional effect on compressibility becomes much smaller with increase in pressure. A positive excess enthalpy for the MgSiO3-Al2O3 binary system is observed.
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