High-pressure X-ray diffraction, Raman, and computational studies of MgCl$_{2}$ up to 1 Mbar: Extensive pressure stability of the $\beta $-MgCl$_{2}$ layered structure.

Magnesium chloride with the rhombohedral layered CdCl$_{2}$-type structure ($\alpha $-MgCl$_{2})$ has been studied using x-ray diffraction and Raman spectroscopy up to 1 Mbar. The results reveal a second-order phase transition to a hexagonal layered CdI$_{2}$-type structure at 0.7 GPa. This phase transition affects the stacking of the Cl anions, resulting to a shorter $c$-axis. An anisotropic compression along $c$-axis was observed during initial compression; altered above 10 GPa due to the repulsion between adjacent Cl-layers. According to previous theoretical studies, a series of phase transitions towards, initially, the 3D rutile (6-fold Mg cations) at 17 GPa and to fluorite structure (8-fold Mg cations) at 70 GPa are proposed. According to our experimental study MgCl$_{2}$ remains in a 2D layered structure up to 1Mbar keeping the 6-fold coordination of Mg cations. This observation contradicts with the general structural behavior of compressed AB$_{2}$ compounds; we conducted \textit{ab-initio} calculations to elucidate the mechanisms that extend the remarkable structural stability of MgCl$_{2}$.