Origin of the hardest c-axis and softest a-axis and lowest transition pressure of Mo2Ga2C from first principles

摘要

The structural properties of Mo2Ga2C are simulated by first-principles under pressure. At 40 GPa, the axial compressibilities of c-and a-axes are 0.9763 and 0.9264, respectively, which are the known largest and smallest values in the M(n+1)AX(n) (n = 1) compounds. A phase transition at 48 GPa is observed with an abrupt increase of c-axis and a rapid decrease of a-axis. The elastic properties and phonon imaginary frequencies confirmed the structural transition at 48 GPa, which probably should be the lowest-pressure transition in the M(n+1)AX(n) (n = 1, 2, 3, etc.) phases. The anti-expansion of Mo-Mo bond length is responsible for the c-axis ultra-incompressibility as well as the structural transition at 48 GPa.