Abstract:
Planewave pseudopotential calculations of formation energies for several ordered supercells were used as bases for first principles calculations of the CaCO3}-MgCO{3~ and CdCO3}-MgCO{3~ phase diagrams. Calculated phase diagrams are in qualitative to semiquantitative agreement with experiment. Two unobserved phases Cd3/4}Mg{1/4}CO_{3~ and Cd1/4}Mg{3/4}CO_{3~ are predicted. No new phases are predicted in the CaCO3}-MgCO{3~ system, but a low lying metastable Ca3/4}Mg{1/4}CO_{3~ state, analogous to the Cd3/4}Mg{1/4}CO_{3~ phase is predicted. All of the predicted lowest lying metastable states have dolomite related structures; i.e. they are layer structures in which Am}B{n~ cation layers lie perpendicular to the rhombohedral [111] vector.
