The stability of polymorphs of MgCl2 — an ab initio study

Periodic Hartree-Fock calculations show that the layered structures of α- and β-MgCl2 are stable at the HF level; earlier work has suggested this was not so for β-MgCl2. The neglect of correlation leads to a large overestimate of the ClCl interlayer separation and hence of the c lattice parameter. Structures optimised using density functional theory and the same basis set are in better agreement with experiment. α- and β-MgCl2 are close in energy at HF and DFT levels, and vibrational effects are likely to be important in determining their relative stability. Neglecting possible distorted variants of the more ionic rutile and fluorite structures, we estimate transition pressures of ∼ 17 GPa for β-MgCl2 → rutile and ∼ 77 GPa for rutile → fluorite.