Complexity in low-temperature phase diagrams of planetary ices: phase transition in bischofite, MgCl2.6H2O

Bischofite, MgCL₂.6H₂O is a marine evaporite that may be present also on or inside icy satellites. It has a monoclinic structure formed of [Mg(H₂O)₆]^2- octahedral cations and individual chlorine ions. The structure is quasihexagonal with a distorted hexagonal-close packed arrangement of the two structural groups. . This structure has C2/m space group. First-principles calculations based on density functional perturbation theory indicate the presence of unstable imaginary phonon modes in the zone-center. They are characterized by 153i cm^-1 and 57i cm^-1 imaginary wavenumbers. Both these modes correspond to tilting of the water molecules. We then investigate these lattice instabilities and find a new structure that is present at low temperatures. The phase transition to this new structure involves the break of the symmetry with the loss of the m symmetry plane and of the inversion center. The new structure is polar with C2 space group.