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

Abstract

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.