Pressure-induced the formation of Mg(CH3)2 and Ca(CH3)2 studied by the first principles

Abstract

Hydrogen-rich materials have always attracted the interest for the crystal structures and electronic properties under high pressures. The crystal structures of alkaline earth compounds Mg(CH3)2 and Ca(CH3)2 have been predicted by employing a particle swarm optimization algorithm at high pressures. The P-3c1 phases of the both compounds were synthesized at about 17.5 GPa and 3 GPa, respectively. The current thermodynamical calculations reveal that the P-3c1 phases are the most stable up to 50 GPa. No imaginary phonon frequencies in the whole Brillouin zone indicate the P-3c1stuctures of Mg(CH3)2 and Ca(CH3)2 are dynamically stable. The computations of the elastic constants indicate P-3c1 structures satisfy the mechanical stability criteria under high pressures. The strong covalent bonds between C atoms and H atoms have been proved by the electronic properties and population analysis, which confirms that the CH3 groups can exist stable.