A computer modelling study of the incorporation of K+, Ca2+ and Mg2+ impurities in two Na2SO4 polymorphs: Introducing a Na2SO4 potential model

Atomistic simulation techniques were used to study two sodium sulphate (Na2SO4) polymorphs, phases III and V (thenardite). A new polarisable interatomic potential model for Na2SO4 was derived and used to calculate the uptake and segregation of Ca2+, Mg2+ and K+ impurities in the two phases and their effect on the equilibrium morphologies of the crystals. The lattice energies of the two phases reflect their experimental relative stabilities and the hydrated morphologies express the experimentally found surfaces. In agreement with experiment, calcium is preferentially substituted for sodium in phase III rather than phase V, as is potassium but to a lesser extent. Magnesium substitution is energetically unfavourable in both polymorphs due to the considerably smaller size of the magnesium ion compared to sodium. In general, the impurity ions remain at the surfaces of both polymorphs, rather than segregate into the bulk material, unless the impurities are adsorbed into very stable surfaces, when they do tend to segregate into the bulk.