New insights into the melting behavior of MgO from molecular dynamics simulations: The importance of premelting effects

We provide a plausible resolution of a long-standing controversy relevant to the geophysics community, namely, that the experimental slope of the melting curve T-m(P) of MgO at low pressures is about 3 times smaller than that obtained from computer simulation of the melting of the normal rock-salt-structured crystal. With increasing temperature at zero pressure, our simulations predict a solid-solid phase transition (from a rock salt to a wurtzite crystalline lattice) to occur just before melting. The coexistence of wurtzite and liquid phases at low pressures is found to be described by a Clapeyron slope which is in much better agreement with the experimental results of Zerr and Boehler [Nature (London) 371, 506 (1994)] than the calculated melting line for the rock salt structure. We also show that the existence of a certain concentration of lattice defects in the rock salt phase cannot provide an alternative explanation.