CaCl₂-type high-pressure phase of magnesium hydride predicted by ab initio phonon calculations

A pressure-induced transition in magnesium hydride (MgH₂) from the tetragonal rutile phase to an orthorhombic CaCl₂ phase is predicted by ab initio phonon calculations. The phase transition is identified as a second-order nature, driven by the softening of the Raman active B_1g phonon mode, weakly coupling with the elastic shear modulus C_s. The newly predicted CaCl₂ phase is dynamically stable, and the calculated equation of states shows good agreement with the available experimental data. Analysis of the calculated results of Gibbs free energies and Raman active phonons for both rutile and CaCl₂ phase, and the spontaneous strains for CaCl₂ phase, further supports the existence of this new phase.