First-principles-based calculations of the CaCO3-MgCO3 and CdCO3-MgCO3 subsolidus phase diagrams

Planewave pseudopotential calculations of supercell total energies were used as bases for first-principles calculations of the CaCO3-MgCO3 and CdCO3-MgCO3 phase diagrams. Calculated phase diagrams are in qualitative to semiquantitative agreement with experiment. Two unobserved phases, Cd3Mg (CO3)(4) and CdMg3(CO3)(4), are predicted. No new phases are predicted in the CaCO3-MgCO3 system, but a low-lying metastable Ca3Mg(CO3)(4) state, analogous to the Cd3Mg(CO3)(4) phase is predicted. All of the predicted lowest-lying metastable states, except for huntite CaMg3(CO3)(4), have dolomite-related structures, i.e. they are layer structures in which A(m)B(n) cation layers lie perpendicular to the rhombohedral [111] vector.