On the heat of formation of ye'elimite Ca₄Al₆O₁₂.SO₄ using density functional theory

The heat of formation of ye'elimite Ca₄Al₆O₁₂.SO₄ was calculated using density functional theory (DFT). The crystallographic ground state of ye'elimite is the orthorhombic Pcc2 modification in agreement with experimental information. Three different many-body interaction functionals were used for DFT calculations: the generalised gradient approximation (GGA), the Armiento/Mattsson 2005 functional (AM05) and the strongly conditioned and appropriately normed functional (Scan). The best results were obtained with the Scan functional and a heat of formation at 298 K of −35·5 kJ/mol referred to pure calcium oxide (CaO), aluminium oxide (Al₂O₃) and calcium sulfate (CaSO₄) was found. All other reported modifications of ye'elimite (cubic, tetragonal, monoclinic) are unstable with endothermic heat of transformations ranging from +18 to +78 kJ/mol.