New Pressure-Induced Polymorphic Transitions of Anhydrous Magnesium Sulfate

Abstract

The effects of pressure on the crystal structure of the three known polymorphs of magnesium sulfate have been theoretically study by means of DFT calculations up to 45 GPa. We determined that at ambient conditions gamma MgSO4 is an unstable polymorph, which decompose into MgO and SO3, and that the response of the other two polymorphs to hydrostatic pressure is non isotropic. Additionally we found that at all pressures beta MgSO4 has a largest enthalpy than alpha MgSO4. This indicates that beta MgSO4 is thermodynamically unstable versus alpha MgSO4 and predicts the occurrence of a beta alpha phase transition under moderate compression. Our calculations also predict the existence under pressure of additional phase transitions to two new polymorphs of MgSO4, which we named as delta MgSO4 and epsilon MgSO4. The alpha delta transition is predicted to occur at 17.5 GPa, and the delta epsilon transition at 35 GPa, pressures that nowadays can be experimentally easily achieved. All the predicted structural transforma ions are characterized as first order transitions. This suggests that they can be non reversible, and therefore the new polymorphs could be recovered as metastable polymorphs at ambient conditions. The crystal structure of the two new polymorphs is reported. In them, the coordination number of sulfur is four as in the previously known polymorphs, but the coordination number of magnesium is eight instead of six. In the article we will report the axial and bond compressibility for the four polymorphs of MgSO4. The pressure volume equation of state of each phase is also given. The values obtained for the bulk modulus are 62 GPa, 57 GPa, 102 GPa, and 119 GPa for alpha MgSO4, beta MgSO4, delta MgSO4, and epsilon MgSO4, respectively. Finally, the electronic band structure of these four polymorphs of MgSO4 has been calculated by the first time.