Compressed MgCl 2 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.