Phase transformations in silicates

The common minerals of the Earth s upper mantle are known all to undergo a series of phase transformations with increasing pressure. Because these transformations entail a change in physical properties, they are manifest in the stmcture of the earth s interior. 

Over the range of pressure and temperature so far explored in the laboratory, the observed structure of (Mg,Fe)Si03 perovskite is orthorhombic Pbnm (Horiuchi et al. 1987). However, because experiments have not yet accessed the entire pressure-temperature range spanned by the lower mantle, there has for some time been speculation that the structure of this phase at lower mantle conditions may differ from that experimentally observed. The perovskite structure is remarkably rich and accommodates a large variety of polymorphs, which are related by rotations of the SiOe octahedra about the three pseudo-cubic axes (Glazer 1972 Glazer 1975) (Fig. 6). Thus Pbnm may be 

Pbnm structure under static conditions. Simulations were performed in the canonical ensemble (fixed temperature and cell shape and size) temperature was maintained with a Nose (1984) thermostat. The time step was 1 fs, and the simulations were run for 1.6 ps. 

The change in stress state between 0.7 ps and 0.8 ps is caused by a change in the structure of the material. The transformation consists of a homogeneous rotation of half the SiOe octahedra that persists for the remainder of the simulation (Fig. 8). The new structure has a (—H-i-) pattern of octahedral rotation which corresponds to Pmmn symmetry. The change with respect to the Pbnm phase is more subtle than phase transformations that had been contemplated in previous theoretical and experimental work none of the three octahedral rotations vanish in the Pmmn structure and the magnitude of the octahedral rotations is similar to that in Pbnm. The increase in mean stress at the transition means that the Pmmn phase has a slightly larger volume than Pbnm at the same pressure. The anisotropy of the stress tensor reflects the differences in equilibrium axial ratios between the two phases. 

Very recently another group has performed first principles molecular dynamics simulations of MgSiOs perovskite using methods similar to ours (Oganov et al. 2001). This group finds that the Pbnm phase is stable throughout the pressure-temperature regime of their study, which overlaps the conditions at which we find a phase transformation. The reason for this discrepancy is not clear, but may be related to differences in pseudopotential construction, run time, initial conditions, or other factors. 

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