MOLECULAR-DYNAMICS STUDY OF THE PEROVSKITE MGSIO3 AT HIGH-TEMPERATURE - STRUCTURAL, ELASTIC AND THERMODYNAMICAL PROPERTIES

A molecular dynamics simulation of the perovskite MgSiO3 gives evidence for a phase transition from the orthorhombic to a tetragonal phase at about 2600 K on heating the crystal under a constant pressure of 310 kbar. This phase transition is associated with a phonon acoustic mode, characterized by a precession movement of the SiO6 octahedra. In the tetragonal phase, without introducing defects in the model, we observe a rapid diffusion of oxygen, comparable with that of a solid electrolyte. This high anionic mobility may be associated with the acoustic mode observed at the phase transition. This is supported by the values calculated for oxygen mean square displacements, which become high above the phase transition temperature. More generally, our results suggest that rapid ionic conduction in perovskites is associated with phase transitions by rotations of the anionic octahedra; hence rapid anionic motion should be observed only in the perovskites undergoing ferroelastic transitions. The activation energy computed for oxygen diffusion is 6.3 +/- 0.6 eV at 310 kbar and 6.6 +/- 0.6 eV at 370 kbar. In the tetragonal phase, this cooperative substitution mechanism without stable defects should always be more efficient than the vacancy mechanism. The behaviour of the elastic and thermodynamical properties at the phase transition is investigated at 310 kbar. The shear modulus undergoes a sharp drop from 1.96 +/- 0.11 Mbar at 2400 K to 1.16 +/- 0.12 Mbar at 3075 K, whereas the bulk modulus decreases continuously with temperature; the specific heat C(v) increases slightly from 6.0 +/- 0.2 cal K-1 mol-1 in the orthorhombic structure to 6.3 +/- 0.3 cal K-1 mol-1 in the tetragonal structure; the volumetric thermal expansion coefficient, which is 2.16 (+/- 0.18) x 10(-5) K-1 at room temperature in the orthorhombic phase, becomes higher in the tetragonal phase, with a strong temperature dependence. As in other perovskites undergoing ferroelastic phase transitions, the temperature T(c) of the phase transition increases with pressure; in MgSiO3 we calculate dT(c)/dP = 5.3 +/- 1.4 K kbar-1 in the pressure range 310-370 kbar. At high temperature in the orthorhombic structure, a drop in pressure emphasizes the distortions of the structure; extrapolation of our results to high pressures suggests that at 2400 K, MgSiO3 could be cubic above 740 kbar. At low pressures, the perovskite phase is not stable; this destabilization occurs at less than 50 kbar at 300 K, and at less than 150 kbar at 2400 K. The precision of our calculations does not permit the detection of any pressure dependence of the Slater and Debye Gruneisen parameters at 300 K in the pressure range 50-310 kbar. However, the thermodynamical Gruneisen parameter gamma(th) calculated at 300 K is strongly pressure dependent: it varies from 1.5 at 310 kbar to 2.1 at zero pressure. The product of gamma(th) and crystal density is therefore lower at high pressure. The temperature dependence of gamma is estimated by calculation of the Mie-Gruneisen parameter gamma(M), which is a strongly decreasing function of temperature.