A three-parameter version of the tetrahedron approximation in the cluster-variation method is used to model cation ordering in rhombohedral carbonate systems. The model is sufficient to calculate theoretical phase diagrams that are in complete qualitative agreement with high-temperature phase-equilibrium data and with experimental composition and temperature dependence of the excess heats that are associated with cation ordering. In addition, the many-body interaction parameters that are required to obtain appropriate phase-diagram topologies at high temperature lead to the prediction of an ordered ground-state with stoichiometry Ca3Mg(CO3)4. Cation ordering in this 3:1 phase yields a trigonally distorted analogue of the Cu3Au or Al3Ti structure.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.