Title: 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.