II-VI oxides phase separate whereas the corresponding carbonates order: The stabilizing role of anionic groups

The formation enthalpies of isovalent, isostructural rocksalt alloys, (A,B), where X=O such as (Ca,Mg)O, are typically unfavorable (positive) for both ordered and random phases. Simple replacement of the single-atom anion, X, by a larger anionic group, such as CO₃ or SO₄, is able to induce a favorable (negative) formation enthalpy, leading to the formation of the ordered alternate monolayer, (CaCO₃)₁/(MgCO₃)₁, dolomite structure. The underlying cause of this behavior is analyzed by breaking down the formation process in a Born-Haber-like cycle into volume and cell-shape deformation, chemical exchange, and cell-internal relaxation using first-principles density-functional theory calculations in the generalized gradient approximation. It is found that when the anion is a group (CO₃), rather than a single atom (O), the energy gained from the internal relaxation overcomes the energy required to compensate the volume mismatch. This explains the general experimental trends of phase separation in isovalent, isostructural alloys without internal-anion structure, compared to ordering tendencies when the anionic group removes internal strain. The importance of obtaining structural ideality in the design of stable solid solutions is highlighted.