First principles investigation of the postspinel transition in Mg₂SiO₄

We have investigated the dissociation of iron free ringwoodite, Mg₂SiO₄ γ-spinel, into MgO periclase and MgSiO₃ perovskite using quasi-harmonic free energy computations within the local density approximation (LDA) and generalized gradient approximation (GGA). The transition pressure, P_tr, obtained from GGA is higher and in much better agreement with experimental measurements than its LDA counterpart. This can be rationalized by close inspection of GGA and LDA functional forms. The Clapeyron slope obtained from this calculation is -2.9 - -2.6 MPa/K. Surprisingly, we find a small decrease in bulk sound velocity across this transition. Our results are consistent with 64 ± 12% ringwoodite volume fraction at the bottom of the transition zone.