Compressed MgCl₂ Reveals Multiple Pathways to Cotunnite Structures

The phase transition sequences of MeX₂ compounds (Me = metal or, more generally, an electropositive element), whose constituent atoms contribute 16 valence electrons per formula unit under high pressure, are of fundamental importance in materials science, high-pressure chemistry, and mineral physics. Here, we report the first observation of trigonal prismatic coordination in this class of materials, realized in magnesium dichloride MgCl₂. We synthesized anhydrous MgCl₂ by the direct reaction of elemental magnesium with carbon tetrachloride (CCl₄) in laser-heated diamond anvil cells from 7(2) to 83(3) GPa. Single-crystal X-ray diffraction identified the known hP3-MgCl₂ polymorph at 7(2) GPa, and two previously unknown high-pressure phases: an orthorhombic oP72-MgCl₂ at 28(2) and 44(3) GPa, and a cotunnite-type oP12-MgCl₂ at 64(3), 73(3), and 83(3) GPa. The oP72 phase features distorted MgCl₆ trigonal prisms, while the oP12 phase adopts MgCl₈ bicapped trigonal prisms. This sequence of hP3 → oP72 → oP12 reveals a complex pressure-induced structural transition from layered to three-dimensional frameworks. Ab initio calculations agree well with the experimental structural data, support the stability range of the new polymorphs, provide the equation of states, and reveal their electronic properties. Our findings demonstrate several transformation pathways by which MeX₂ compounds evolve toward cotunnite-type structures under compression.