MgO Phase Diagram from First Principles
Alejandro Strachan, Tahir Cagin and William A. Goddard III
Materials and Process Simulation Center, Beckman Institute (139-74)
California Institute of Technology, Pasadena, California 91125.
We use first principles methods (no empirical parameters) to develop the phase diagram for the B1(NaCl), B2(CsCO), and liquid phases of MgO. We used density functional theory (DFT) with the generalized gradient approximation (GGA) to predict the equation-of-state [volume versus pressure (V(P)] at 0K for MgO in the low density B1 (NaCl) phase and the high density B2 (CsCl) phase. We find a pressure induced phase transition at P=400GPa. We then fitted an MS-Q type force field (FF) to the quantum results. This force field, denoted as QMS-Q FF, was then used with molecular dynamics (MD) to investigate the phase coexistence curves of the B1-B2 and B1-liquid phases. This leads to a first-principles phase diagram for MgO for pressures up to 500GPa and temperatures up to 8000K.
The accuracy of the fit of the QMS-Q FF to the QM validates the functional the form of the QMS-Q FF in which the charges are obtained from charge equilibration (QEq) and the nonelectrostatic forces are described with simple two-body Morse potentials. Such QMS-Q force fields using no empirical data should be useful for MD or Monte Carlo simulations of many other materials.
We thank Dr. Ersan Demiralp for useful discussions. This research was funded by a grant from DOE-ASCI. A. S. acknowledges financial support from FOMEC (Argentina). The facilities of the MSC are also supported by grants from NSF (ASC 92-17368 and CHE 95-22179), ARO (MURI), ARO (DURIP), BP Chemical, Exxon, Owens-Corning, Avery Dennison, Chevron Petroleum Technology Co., Asahi Chemical, Chevron Chemical Co., Chevron Research Technology Co., and Beckman Institute.
MgO T=0K equation of state: DFT-GGA and QMS-Q FF
Phase diagram of MgO from MD simulations. QMS-Q/B1
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