Molecular dynamics simulation of MgSiO₃-Al₂O₃ perovskite

Abstract

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 (Mg_1−x,Al_x)(Al_x,Si_1−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.