Unusual energy balance between atoms in postperovskite MgSiO₃

To understand the phase transition from perovskite to postperovskite MgSiO₃, total energy is partitioned into atomic energy densities of constituent elements in the oxide, using the energy density analysis. The atomization energies, δE_Mg for Mg atom, δE_Si for Si atom, and δE^O for O atom, are then evaluated by subtracting the atomic energy density from the energy of the isolated neutral atom,Mg, Si, and O, respectively. It is found that δE_Si and δE_Mg are much larger than δE_O in the perovskite phase, but δE_O is much larger than δE _Si and δE_Mg in the postperovskite phase. This means that most of the energies partitioned into Mg and Si atoms in the perovskite phase transfer to the O atoms in the postperovskite phase during the transition. Such an extremely stable O-atom state is formed by the introduction of edge-shared SiO₆ octahedra into the postperovskite structure. This is because the edge-sharing makes the Si-O interatomic distances longer even in very high pressure conditions. The unusual energy balance between atoms is also seen in the other postperovskite, MgGeO₃ and NaMgF₃.